Iterative shimming for low-field head-optimized MRI

Abstract

The present invention provides various devices, systems, and methods for iterative shimming in magnetic resonance imaging (MRI). The method may include generating a field map of a magnetic field B0 from an MRI system. The MRI system may include a shim tray and an array of permanent magnets for generating the magnetic field B0. The shim tray may include slots for receiving shim magnets. The method may include iteratively shimming the MRI system by applying the field map to a genetic algorithm to determine a set of slots for receiving a set of shim magnets, arranging the set of shim magnets within the set of slots, generating the next field map of the magnetic field B0, and deciding whether to perform the next iteration based on the next field map, or whether to perform the next iteration based on the next field map.

Description

[Background technology]

[0001] (Cross-reference of related applications) This application claims the benefit of U.S. Patent Application No. 18 / 450,010, filed on 15 August 2023, which is incorporated herein by reference in its entirety.

[0002] (background) This disclosure relates to magnetic resonance imaging (MRI), medical imaging, medical interventions, and surgical interventions. MRI systems often involve large, complex machines that generate significantly higher magnetic fields, imposing significant constraints on the feasibility of certain surgical interventions. Limitations may include limitations on physical access to the patient by surgeons and / or surgical robots and / or limitations on the use of certain electrical and mechanical components in the vicinity of the MRI scanning device. Such limitations are inherent in the underlying design of many existing systems and are difficult to overcome. [Overview of the project] [Means for solving the problem]

[0003] In one aspect, the present disclosure provides a method. The method includes the step of generating a first field map of a magnetic field B0 from a magnetic resonance imaging (MRI) system. The MRI system may include an array of permanent magnets in a modified Halbach dome array and a shim tray. The array may include one open end, sized to receive a patient's head, and may be configured to generate a magnetic field B0. The shim tray may include slots configured to receive shim magnets. The first field map corresponds to a magnetic field based on each of the empty slots. The method further includes the steps of applying the first field map to a genetic algorithm to determine a first set of slots for receiving a first set of shim magnets, and arranging the first set of shim magnets within the first set of slots. The method further includes the step of generating a second field map of the magnetic field B0. The second field map may correspond to a magnetic field based on the first set of shim magnets arranged within the first set of slots. The method further includes the steps of applying a second field map to a genetic algorithm to determine a second set of slots to receive a second set of shim magnets, and arranging the second set of shim magnets within the second set of slots. The second set of slots is different from the first set of slots.

[0004] In another aspect, the present disclosure provides a system for magnetic resonance imaging (MRI). The system includes an array of permanent magnets and a shim tray. The array of permanent magnets is in a modified Halbach array forming one open end. The open end can be sized to receive a patient's head. The array of permanent magnets can be configured to generate a low-field-intensity magnetic field B0 toward an object of interest located in the field of view. The shim tray is positionable within the array of permanent magnets and includes an array of shim tray slots. The shim tray can include an opening that is sized to receive a patient's head. Each shim tray slot can be configured to receive a shim magnet in a first configuration and a second configuration. In the first configuration, the polarity of the shim magnets can be oriented in a first direction. In the second configuration, the polarity of the shim magnets can be oriented in a second direction opposite to the first direction.

[0005] In yet another aspect, the present disclosure provides a method. The method includes the step of generating a field map of a magnetic field B0 from a magnetic resonance imaging (MRI) system. The MRI system may include an array of permanent magnets in a modified Halbach dome array and a shim tray. The array may include one open end that is sized to receive a patient's head and may be configured to generate a magnetic field B0. The shim tray may include slots configured to receive shim magnets. The method further includes the step of iteratively shimming the MRI system by applying the field map to a genetic algorithm to determine a set of slots for receiving a set of shim magnets; arranging the set of shim magnets within the set of slots; generating a next field map of the magnetic field B0; and deciding whether to perform the next iteration based on the next field map or not based on the next field map. [Brief explanation of the drawing]

[0006] The various aspects described herein, along with their further objectives and advantages, concerning both the organization and operation methods, can be best understood by referring to the following description, which is considered in conjunction with the accompanying drawings below.

[0007] [Figure 1] Figure 1 depicts a component of an MRI scanning system, which includes a dome-shaped housing for a magnetic array, representing at least one aspect of the present disclosure, the housing, which encloses a region of interest within the housing, and which is positioned to receive at least a portion of the head of a patient lying on a table within the region of interest.

[0008] [Figure 2] Figure 2 is a perspective view of an alternative dome-shaped housing for a magnetic array for use with the MRI scanning system of Figure 1, with an access opening defined within the dome-shaped housing, according to at least one aspect of the present disclosure.

[0009] [Figure 3] Figure 3 is a perspective view of an alternative dome-shaped housing for a magnetic array for use with the MRI scanning system of Figure 1, with access openings and adjustable gaps defined within the dome-shaped housing, representing at least one aspect of the present disclosure.

[0010] [Figure 4] Figure 4 depicts a dome-shaped housing for use with an MRI scanning system, having an access opening in the form of a centrally defined hole, according to at least one aspect of the present disclosure.

[0011] [Figure 5] Figure 5 is a cross-sectional view of the dome-shaped housing of Figure 4, showing at least one aspect of the present disclosure.

[0012] [Figure 6] Figure 6 illustrates a schematic control diagram for an MRI system according to at least one aspect of this disclosure.

[0013] [Figure 7] Figure 7 is a flowchart illustrating a method for acquiring imaging data from an MRI system according to at least one aspect of the present disclosure.

[0014] [Figure 8] Figure 8 depicts an MRI scanning system and a robotic system according to at least one aspect of the present disclosure.

[0015] [Figure 9] Figure 9 is a perspective view of a shim tray for passive shimming according to at least one aspect of the present disclosure.

[0016] [Figure 10] Figure 10 is an exploded perspective view of the shim tray of Figure 9 according to at least one aspect of the present disclosure.

[0017] [Figure 11] Figure 11 is a side view of the shim tray of Figure 9 according to at least one aspect of the present disclosure.

[0018] [Figure 12] Figure 12 is another side view of the shim tray of Figure 9 according to at least one aspect of the present disclosure.

[0019] [Figure 12A] Figure 12A is another side view of the shim tray of Figure 9 with a subset of shim tray slots into which shim magnets are selectively incorporated according to at least one aspect of the present disclosure.

[0020] [Figure 13] Figure 13 depicts an exemplary panel of the shim tray of Figure 9 according to at least one aspect of the present disclosure.

[0021] [Figure 14] Figure 14 depicts a flowchart of a method for passive shimming for an MRI system according to at least one aspect of the present disclosure.

[0022] [Figure 15] Figure 15 illustrates a flowchart of the genetic algorithm according to at least one aspect of this disclosure.

[0023] [Figure 16] Figure 16 illustrates a flowchart of a passive shimming method for an MRI system, according to at least one aspect of this disclosure.

[0024] [Figure 17A] Figure 17A depicts a graph showing both simulated and measured principal magnetic field B0 uniformity throughout a two-step passive shimming process for an exemplary MRI system, according to at least one aspect of the present disclosure.

[0025] [Figure 17B] Figure 17B depicts histograms of the principal magnetic field intensity distribution for the exemplary MRI system of Figure 17A, measured before shimming, after the first stage of shimming, and after the second stage of shimming, according to at least one aspect of the present disclosure.

[0026] [Figure 18A] Figure 18A depicts a graph showing both simulated and measured principal magnetic field B0 uniformity throughout a two-step passive shimming process for another exemplary MRI system, according to at least one aspect of the present disclosure.

[0027] [Figure 18B] Figure 18B depicts histograms of the principal magnetic field intensity distribution for the exemplary MRI system of Figure 18A, measured before shimming, after the first stage of shimming, and after the second stage of shimming, according to at least one aspect of the present disclosure.

[0028] Corresponding reference letters indicate the corresponding parts throughout some of the figures. The examples presented herein illustrate various disclosed embodiments and are of a type, and such examples should not be construed as limiting their scope in any way. [Modes for carrying out the invention]

[0029] (Detailed explanation) The applicant of this application also owns the following patent applications, each of which is incorporated herein by reference in their respective wholes: - International patent application PCT / US2022 / 72143, filed on May 5, 2022, titled "NEURAL INTERVENTIONAL MAGNETIC RESONANCE IMAGING APPARATUS". - U.S. Patent Application No. 18 / 057,207, filed on November 19, 2022, titled "SYSTEM AND METHOD FOR REMOVING ELECTROMAGNETIC INTERFERENCE FROM LOW-FIELD MAGNETIC RESONANCE IMAGES". - U.S. Patent Application No. 18 / 147,418, filed on December 28, 2022, and titled "MODULARIZED MULTI-PURPOSE MAGNETIC RESONANCE PHANTOM". - U.S. Patent Application No. 18 / 147,542, filed on December 28, 2022, titled "INTRACRANIAL RADIO FREQUENCY COIL FOR INTRAOPERATIVE MAGNETIC RESONANCE IMAGING". - U.S. Patent Application No. 18 / 147,556, filed on December 28, 2022, titled "DEEP LEARNING SUPER-RESOLUTION TRAINING FOR ULTRA LOW-FIELD MAGNETIC RESONANCE IMAGING". -Filed on January 11, 2023, U.S. Patent Application No. 18 / 153,111, titled "ACCELERATING MAGNETIC RESONANCE IMAGING USING PARALLEL IMAGING AND ITERATIVE IMAGE RECONSTRUCTION" -Filed on January 11, 2023, U.S. Patent Application No. 18 / 153,175, titled "FAST T2-WEIGHTED AND DIFFUSION-WEIGHTED CHIRPED-CPMG SEQUENCES" -Filed on March 2, 2023, U.S. Provisional Patent Application No. 63 / 488,102, titled "A METHOD OF MERGING A CO-OPERATIVE MR-COMPATIBLE ROBOT AND A LOW-FIELD PORTABLE MRI SYSTEM"

[0030] Before detailing various aspects of interventional magnetic resonance imaging devices, it should be noted that the illustrative examples are not limited to the details of the component configurations and arrangements illustrated in the accompanying drawings and descriptions, and their applications or uses may be implemented or incorporated into other aspects, variations, and modifications, and may be practiced or performed in various ways. Furthermore, unless otherwise indicated, the terms and expressions used herein have been selected for the purpose of illustrating the illustrative examples for the convenience of the reader, and not for the purpose of limitation. It should also be understood that one or more of the aspects, expressions of aspects, and / or examples described below may be combined with any one or more of the other aspects, expressions of aspects, and / or examples described thereafter.

[0031] Various aspects concern neuro-assisted magnetic resonance imaging (MRI) devices that enable the integration of surgical intervention and guidance with MRI. This includes providing physical access to the area surrounding the patient, as well as access to the patient's head, using one or more access openings. In addition, neuro-assisted MRI devices may enable the use of robotic guidance instruments and / or conventional surgical instruments. In various cases, neuro-assisted MRI can be used in surgery to obtain scans of the patient's head and / or brain during surgical interventions such as brain biopsies or neurosurgeries.

[0032] Figure 1 depicts an MRI scanning system 100, including a dome-shaped housing 102 configured to receive a patient's head. The dome-shaped housing 102 may further include at least one access opening configured to allow access to the patient's head and enable neurological intervention. The space within the dome-shaped housing 102 forms a region of focus for the MRI scanning system 100. Target tissue within the region of focus is subjected to a magnetization field / pulse to acquire imaging data representing the target tissue, as further described herein.

[0033] For example, a patient can be positioned such that their head is located within the area of ​​interest within the dome-shaped housing 102. The brain can be positioned entirely within the dome-shaped housing 102. In such cases, to facilitate intracranial interventions (e.g., neurosurgery) coordinated with MR imaging, the dome-shaped housing 102 may include one or more openings that provide access to the brain. The openings may be spaced apart around the periphery of the dome-shaped housing.

[0034] The MRI scanning system 100 may include an auxiliary cart (see, for example, auxiliary cart 540 in Figure 6) for housing conventional MRI electrical and electronic components such as a computer, a programmable logic controller, a power distribution unit, and an amplifier. The MRI scanning system 100 may also include a magnet cart for holding a dome-shaped housing 102, gradient coils, and / or transmission coils, as further described herein. In addition, the magnet cart may be attached to the receiving coil in various cases. Referring mainly to Figure 1, the dome-shaped housing 102 may further include an RF transmission coil, gradient coils 104 (depicted outside of them), and shim magnets 106 (depicted inside of them). Alternative configurations for the gradient coils 104 and / or shim magnets 106 are also conceivable. In various cases, the shim magnets 106 may be adjustably positioned within a shim tray in the dome-shaped housing 102, which may allow the technician to configure the magnetic flux density of the dome-shaped housing 102 with finer granularity.

[0035] Various structural housings for receiving the patient's head and facilitating neurological intervention can be used with an MRI scanning system, such as the MRI scanning system 100. In one aspect, the MRI scanning system 100 may be equipped with alternative housings, such as a dome-shaped housing 202 (Figure 2) or a two-part housing 302 (Figure 3) configured to form a dome shape. The dome-shaped housing 202 defines multiple access openings 203, and the two-part housing 302 also defines multiple access openings 303 and further includes an adjustable gap 305 between the two parts of the housing.

[0036] In various cases, the housings 202 and 302 may include a bonding agent 308, such as epoxy resin, which holds a plurality of magnetic elements 310 in fixed positions. The plurality of magnetic elements 310 may be bonded to a structural housing 312, such as a plastic substrate. In various aspects, the bonding agent 308 and the structural housing 312 may be made of non-conductive or diamagnetic materials. Referring mainly to Figure 3, a housing 302 consisting of two parts comprises two structural housings 312. In various aspects, a structural housing for receiving a patient's head may be formed from two or more sub-parts. An access opening 303 within the structural housing 312 provides a direct passage to the patient's head and is not obstructed by the structural housing 312, the bonding agent 308, or the magnetic elements 310. The access opening 303 may be located, for example, within the open space of the housing 302.

[0037] There are many possible configurations for neuro-interventional MRI devices that can achieve improved access for surgical interventions. Many of these configurations are established on two main designs, commonly known as Halbach cylinders and Halbach domes, as described in the following paper, namely, Cooley et al. (e.g., Cooley, CZ, Haskell, MW, Cooley, SF, Sappo, C., Lapierre, CD, Ha, CG, Stockmann, JP, and Wald, LL (2018)), *Design of sparse Halbach magnet arrays for portable MRI using a genetic algorithm*, IEEE Transactions on Magnetics, 54(1), 5100112. The paper by Cooley et al., *Design of sparse Halbach magnet arrays for portable MRI using a genetic algorithm*, published in IEEE Transactions on Magnetics, 54(1), 5100112 (2018), is incorporated herein by reference in its entirety.

[0038] In various cases, for example, a dome-shaped housing for an MRI scanning system such as System 100 may include a Halbach dome, which defines the dome shape and is constructed based on several factors including the main magnetic field B0 strength, field size, field uniformity, device size, device weight, and patient access for neurological intervention. In various aspects, a Halbach dome includes an outer radius and an inner radius at the base of the dome. A Halbach dome may include an elongated cylindrical portion extending from the base of the dome. In one aspect, the elongated cylindrical portion includes the same outer radius and inner radius as the base of the dome and continues from the base of the dome to a predetermined length at a constant radius. In another aspect, the elongated cylindrical portion includes a different outer radius and inner radius from the base of the dome (see, for example, Figures 2 and 3). In such cases, the different outer and inner radii of the elongated cylindrical portion can merge with the base radius in a transition region.

[0039] Figure 4 illustrates an exemplary Halbach dome 400 for an MRI scanning system, such as system 100, defining an access opening in the form of a hole or access opening 403, according to at least one aspect of the present disclosure, wherein the dome 400 is configured to receive the head and brain B of a patient P within a region of interest therein, and the access opening 403 is configured to allow access to the patient P to enable neurological interventions using surgical instruments controlled by medical instruments and / or robots. The Halbach dome 400 can be established with a single access opening 403 on the upper side 418 of the dome 400, which allows access to the upper part of the skull while minimizing the impact on the magnetic field. In addition, or alternatively, the dome 300 can be configured with multiple access openings around the structure 416 of the dome 400, as shown in Figures 2 and 3.

[0040] Diameter D of access opening 403 hole This can be small (e.g., about 2.54 cm) or very large (essentially, the outer diameter r of the dome 400). ext) This is also acceptable. As the access opening 403 becomes larger, the dome 400 begins to resemble, for example, a Halbach cylinder. The access opening 403 is not limited to being at the apex of the dome 400. The access opening 403 can be installed anywhere on the surface of the dome 400 or the structure 416. In various cases, the entire dome 400 can be rotated so that the access opening 403 can be co-set with a desired physical location on the patient P.

[0041] FIG. 5 shows the diameter D of the access opening 403 hole and the length L of the dome 400 and the outer radius r of the dome 400 ext and the inner radius r in and depicts the relative dimensions of the Halbach dome 400. The Halbach dome 400 is configured in a Halbach array and includes a plurality of magnetic elements that constitute a magnetic assembly. The plurality of magnetic elements may be enclosed by their structure 416 or housing with an outer radius r ext and an inner radius r in On one side, exemplary dimensions may be defined as r in = 19.3 cm, r [[ID=I7]] ext = 23.6 cm, L = 38.7 cm, and 2.54 cm ≦ D < 19.3 cm.

[0042] Based on the above exemplary dimensions, a Halbach dome 400 with an access opening 403 may be configured with a magnetic flux density B0 of approximately 72 mT and an overall mass of approximately 35 kg. It should be understood that the dimensions can be selected based on the desired magnetic flux density B0, total weight, and the geometry of the neurointerventional access opening 403 of the Halbach dome 400 and / or the magnet cart for a particular application.

[0043] In various aspects, the Halbach dome 400 may be configured to define a plurality of access openings 403 located around the structure 416 of the dome 400. These plurality of access openings 403 may be configured to allow access to the patient's head and brain B using instruments (e.g., surgical instruments) and / or surgical robots.

[0044] In various aspects, the access opening 403 may be adjustable. An adjustable configuration may, for example, be the diameter D of the access opening 403. hole To adjust this, the access opening 403 may be provided with the ability to be adjusted using either a motor, mechanical assist, or manual system with a mechanical iris configuration. This would allow for a dome configuration without the access opening 403, where an imaging scan is performed and then the dome 400 and its mechanical iris configuration are adjusted to include the access opening 403, and thus enable surgical intervention through it.

[0045] Halbach domes and their magnetic arrays for facilitating neural interventions are further described in International Patent Application PCT / US2022 / 72143, filed on 5 May 2022 and titled “NEURAL INTERVENTIONAL MAGNETIC RESONANCE IMAGING APPARATUS” (which is incorporated herein by reference in its entirety).

[0046] Referring here to Figure 6, a schematic diagram for the MRI system 500 is shown. For example, an MRI scanning system 100 (Figure 1) and various dome-shaped housings and magnetic arrays therefor, as further described herein, can be incorporated into the MRI system 500. The MRI system 500 includes a housing 502, which in many aspects may be similar to the dome-shaped housings 102 (Figure 1), 202 (Figure 2), and / or 302 (Figure 3). The housing 502 is dome-shaped and configured to form a field of interest or field of view 552 therein. For example, in various aspects of this disclosure, the housing 502 may be configured to receive a patient's head.

[0047] The housing 502 includes a magnetic assembly 548 (e.g., a Halbach array of magnets) having a plurality of magnets arranged therein. In various aspects, the main magnetic field B0 generated by the magnetic assembly 548 extends into the field of view 552, which contains an object being imaged by the MRI system 500 (e.g., a patient's head).

[0048] The MRI system 500 also includes an RF transmission / reception coil 550. The RF transmission / reception coil 550 is combined with an integrated transmission / reception (Tx / Rx) coil. In other cases, the RF transmission coil can be separate from the RF reception coil. For example, the RF transmission coil can be incorporated into the housing 502, and the RF reception coil can be located within the housing 502 to acquire imaging data.

[0049] The housing 502 also includes one or more gradient coils 504 configured to generate a gradient field to facilitate imaging of an object in the field of view 552, which is generated by a magnet assembly 548, for example, enclosed by a dome-shaped housing and a dome-shaped array of magnetic elements therein. A shim tray, adapted to receive shim magnets 506, can also be incorporated into the housing 502.

[0050] During the imaging process, the main magnetic field B0 extends within the field of view 552. The direction of the effective magnetic field (B1) changes in response to RF pulses transmitted by the RF transmission / receiving coil 550 and the associated electromagnetic field. For example, the RF transmission / receiving coil 550 may be configured to selectively transmit RF signals or pulses to objects within the field of view 552, such as tissue in the patient's brain. These RF pulses can alter the effective magnetic field, which is affected by spins within the sample tissue.

[0051] The enclosure 502 signals to an auxiliary cart 530, which is configured to supply power to the enclosure 502 and to transmit / receive control signals to and from the enclosure 502. The auxiliary cart 530 includes a power distribution unit 532, a computer 542, a spectrometer 544, a transmission / reception switch 545, an RF amplifier 546, and a gradient amplifier 558. In various cases, the enclosure 502 can signal to multiple auxiliary carts, each cart which can support one or more of the power distribution unit 532, computer 542, spectrometer 544, transmission / reception switch 545, RF amplifier 546, and / or gradient amplifier 558.

[0052] Computer 542 is configured to signal-communicate with the spectrometer 544 and to send and receive signals between computer 542 and the spectrometer 544. When an object in the field of view 552 is excited using RF pulses from the RF transmission / receiving coil 550, the precession of the object results in an induced current, i.e., an MR current, which is detected by the RF transmission / receiving coil 550 and transmitted to the RF preamplifier 556. The RF preamplifier 556 is configured to enhance or amplify the excitation data signals and transmit them to the spectrometer 544. The spectrometer 544 is configured to transmit the excitation data to computer 542 for storage, analysis, and image reconstruction. Computer 542 is configured to combine multiple stored excitation data signals, for example, to generate an image. In various cases, computer 542 signals-communicate with at least one database 562 that stores the reconstruction algorithm 564 and / or pulse sequence 566. Computer 542 is configured to utilize the reconstruction algorithm to generate an MR image 568.

[0053] The signal from the spectrometer 544 can also be relayed to the RF transmission / receiving coil 550 in the housing 502 via the RF power amplifier 546 and the transmission / receiving switch 545 positioned between the spectrometer 544 and the RF power amplifier 546. The signal from the spectrometer 544 can also be relayed to the gradient coil 560 in the housing 502 via the gradient power amplifier 558. For example, the RF power amplifier 546 is configured to amplify the signal and transmit it to the RF transmission coil 560, and the gradient power amplifier 558 is configured to amplify the gradient coil signal and transmit it to the gradient coil 560.

[0054] In various cases, the MRI system 500 may include a noise reduction coil 554. For example, an auxiliary cart 530 and / or a computer 542 may communicate with the noise reduction coil 554. In other cases, the noise reduction coil 554 may be optional. For example, some MRI systems disclosed herein may not include a complementary / auxiliary RF coil for detecting and reducing electromagnetic interference, i.e., noise.

[0055] A flowchart illustrating the process 570 for acquiring MRI images is shown in Figure 7. The flowchart can be implemented, for example, by an MRI system 500. In various cases, in block 572, a target object (e.g., a part of a patient's anatomical structure) is positioned within a focus area (e.g., focus area 552) within a dome-shaped housing (e.g., magnet assembly 548) of various MRI scanning devices, as further described herein. The main field B0 is configured to magnetically polarize the hydrogen protons (1H protons) of the target object (e.g., all organs and tissues) and is known as the net longitudinal magnetization M0. This is proportional to the proton density (PD) of the tissue and expands exponentially in time, along with a time constant known as the longitudinal relaxation time T1 of the tissue. The T1 value of individual tissues depends on several factors, including their microstructure with respect to water and / or lipid content, and the intensity of the polarization field. For these reasons, the T1 value of a given tissue sample depends on age and health status.

[0056] In block 574, a time-varying oscillating magnetic field B1, i.e., an excitation pulse, is applied to a magnetically polarized target object using an RF coil (e.g., an RF transmission / receiving coil 550). The carrier frequency of the pulsed B1 field is set to the resonance frequency of the 1H proton, which reverses the longitudinal magnetization from its equilibrium longitudinal direction, resulting in a rotated magnetization vector, which can generally have transverse and longitudinal magnetization components depending on the reversal angle used. A typical B1 pulse includes reversal pulses, i.e., a 180-degree pulse and a 90-degree pulse. The 180-degree pulse reverses the direction of the 1H proton magnetization along the longitudinal axis. The 90-degree pulse rotates the 1H proton magnetization by 90 degrees so that the magnetization is in the transverse plane. The MR signal is a time-varying current proportional to the transverse component of the magnetization and is detected using a suitable RF coil. These MR signals decay exponentially over time, using a time constant known as the transverse relaxation time T2, which also depends, for example, on the microscopic tissue structure, water / lipid content, and the strength of the magnetic field used.

[0057] In block 576, the MR signal is spatially encoded by exposing the target object to an additional magnetic field generated by a gradient coil (e.g., gradient coil 560), which is known as a gradient field. The gradient field, which fluctuates linearly in space, is applied in pulsed form over short periods with spatial variations in each direction. The net result is the generation of multiple spatially encoded MR signals, which can be detected in block 577 and reconstructed to form an MR image depicting a slice of the object being examined. An RF receiving coil (e.g., RF transmitting / receiving coil 550) may be configured to detect the spatially encoded RF signals. The slice may be oriented transversely, sagittally, coronally, or on any slope.

[0058] In block 578, the spatially encoded signals of each slice of the scanning region are mathematically digitized and spatially decoded using a computer reconstruction program (e.g., by computer 542) to generate an image depicting the internal anatomical structure of the object being examined. In various cases, the reconstruction program may utilize the (inverse) Fourier transform to convert the spatially encoded data (k-space data) back into geometrically decoded data.

[0059] Figure 8 depicts a graphical diagram of a robotic system 680 that may be used for neurological interventions using an MRI scanning system 600. The robotic system 680 includes a computer system 696 and a surgical robot 682. The MRI scanning system 600 may be analogous to the MRI system 500, as further described herein, and may include a dome-shaped housing and a magnetic array having access openings. For example, the MRI system 500 may include one or more access openings defined in the Halbach array of magnets in a permanent magnet assembly to provide access to one or more anatomical parts of a patient being imaged during a medical procedure. In various cases, the robotic arms and / or instruments of the surgical robot 682 are configured to extend through the access openings in the permanent magnet assembly to reach the patient or target site. Each access opening may provide access to the patient and / or surgical site. For example, in the case of multiple access openings, the multiple access openings may allow access from different directions and / or proximal locations.

[0060] According to various embodiments, the robot system 680 is configured to be installed outside the MRI system 600. As shown in Figure 8, the robot system 680 may include a robot arm 684 configured for movement with one or more degrees of freedom. According to various embodiments, the robot arm 684 includes one or more mechanical arm portions, including a hollow shaft 686 and an end effector 688. The hollow shaft 686 and end effector 688 are configured to move, rotate, and / or pivot through various ranges of motion via one or more motion controllers 690. The bidirectional curved arrows in Figure 8 represent exemplary rotational motions produced by the motion controllers 690 at various joints within the robot arm 684.

[0061] According to various embodiments, the robotic arm 684 of the robotic system 682 is configured to access various anatomical parts of interest through or around the MRI scanning system 600. According to various embodiments, the access opening is designed to take into account the size of the robotic arm 684. For example, the access opening defines a perimeter through which the robotic arm 684, a hollow shaft 686, and an end effector 688 are accommodated. In various cases, the robotic arm 684 is configured to access various anatomical parts of a patient from around the side of the magnetic imaging device 600. The hollow shaft 686 and / or end effector 688 can be adapted to receive a robotic instrument 692, such as a biopsy needle, which has a cutting edge 694 for collecting a biopsy sample from a patient.

[0062] The reader will understand that the robotic system 682 may be used in combination with various dome-shaped and / or cylindrical magnetic housings, as further described herein. The robotic system 682 and robotic instrument 692 in Figure 8 are illustrative. Alternative robotic systems may be used in conjunction with the various MRI systems disclosed herein. Handheld surgical instruments and / or additional imaging devices (e.g., endoscopes) and / or systems may also be used in conjunction with the various MRI systems disclosed herein.

[0063] In various aspects of this disclosure, the MRI systems described herein may include low-field MRI (LF-MRI) systems. In such cases, the main magnetic field B0 generated by the permanent magnet assembly may be less than or equal to 1.0T, for example, 0.1T to 1.0T. In some cases, the MRI systems described herein, such as LF-MRI systems, may include extremely low-field MRI (ULF-MRI) systems. In such cases, the main magnetic field B0 generated by the permanent magnet assembly may be less than or equal to 1.0T, for example, 0.03T to 0.1T.

[0064] Various shimming techniques are often employed with MRI systems to improve the uniformity of the main magnetic field B0. Improving the uniformity of the main magnetic field B0 results in slower MR signal decay time, which can ultimately improve overall image quality.

[0065] High-field MRI (HF-MRI) systems, such as those employing superconducting magnets capable of generating magnetic fields exceeding 1.0 T, often utilize passive and / or active shimming techniques. Passive shimming generally involves the step of selectively placing a ferromagnetic object within the MRI system's scanning bore to modify the main magnetic field B0 for improved field uniformity. Active shimming generally involves the step of generating a magnetic field by passing an electric current through special coils in the MRI system to modify the main magnetic field B0 for improved field uniformity. For example, many HF-MRI systems include a dedicated set of superconducting shimming coils (separate from, for example, gradient coils) for active shimming.

[0066] LF-MRI systems generally exhibit higher field heterogeneity (e.g., sometimes exceeding 30,000 ppm) compared to HF-MRI systems. This higher field heterogeneity can be caused by several factors. For example, LF-MRI systems typically employ an array of permanent magnets to generate the main magnetic field B0. These permanent magnets generally generate a weaker main magnetic field B0 (e.g., less than or equal to 1.0 T, less than or equal to 0.1 T) than the main magnetic field B0 generated by the superconducting coils in HF-MRI systems, which can lead to increased field heterogeneity. Furthermore, in the case of LF-MRI systems configured to image a patient's head, the LF-MRI system often employs an array of compact permanent magnets arranged in an asymmetrical modified Halbach orientation (e.g., an elongated dome shape) that is closed at one end and open at the opposite end to accommodate the patient's head. This modified Halbach dome arrangement can result in increased main magnetic field B0 heterogeneity compared to the magnetic field generated by permanent magnets arranged in a symmetric Halbach cylinder, for example. Furthermore, even if a modified Halbach arrangement of magnets can be designed to produce a main magnetic field B0 with a given non-uniformity (for example, based on computer simulations), the actual non-uniformity of the main magnetic field B0 is often greater due to manufacturing-related errors.

[0067] Due to various challenges, current LF-MRI systems are often unable to implement the active shimming technique employed by HF-MRI systems. For example, as mentioned above, HF-MRI systems typically include a dedicated set of shimming coils. Each shimming coil in an HF-MRI system can produce a magnetic field that fluctuates uniformly and linearly across a specific region of interest. Therefore, the effect of the shimming coils on the main magnetic field B0 is relatively predictable with respect to HF-MRI systems. Conversely, LF-MRI systems typically do not have dedicated coils for active shimming. Furthermore, LF-MRI systems typically include gradient coils to generate gradient fields during signal acquisition, but these gradient fields are significantly less uniform and fluctuate in a more nonlinear manner compared to the fields generated by the superconducting shimming coils in HF-MRI systems. Therefore, the effect of the gradient coils on the main magnetic field B0 in an LF-MRI system can be relatively unpredictable. Consequently, LF-MRI systems often rely on passive shimming to improve the uniformity of the main magnetic field B0.

[0068] Various passive shimming techniques have been employed in LF-MRI systems, as disclosed in McDaniel's publication entitled "Computational design and fabrication of portable MRI systems" (Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science (2020)) and Wenzel K. et al.'s publication entitled "B0-Shimming Methodology for Affordable and Compact Low-Field Magnetic Resonance Imaging Magnets" (Front. Phys. (July 28, 2021)) (their disclosures are incorporated herein by reference as separate wholes). However, these techniques generally target non-iterative, one-time optimizations of shimming arrays and, for various reasons, are generally not applicable to further iterations and / or optimizations of shimming arrays to improve the main magnetic field B0 uniformity for LF-MRI systems that utilize modified Halbach arrays of permanent magnets. Therefore, as will be further described herein, there is a need for alternative systems and methods for passive shimming of LF-MRI systems.

[0069] This disclosure provides a system and method for passive shimming of an LF-MRI system. In some aspects, shimming can be employed using an MRI system comprising a shim tray and an array of permanent magnets in a modified Halbach dome array. The array of permanent magnets may include one closed end forming a dome and one open end opposite the closed end, the open end having an opening sized to receive the patient's head. Furthermore, the array of permanent magnets may be configured to generate a principal magnetic field B0 having a low field intensity within the region of interest. The shim tray may define a dome shape and, when assembled, may be partitioned so that it sits within the dome defined by the array of permanent magnets. Furthermore, the shim tray may include shim tray slots configured to receive shim magnets.

[0070] In one aspect, each shim tray slot can be configured to orient a corresponding shim magnet received therein in a first or second configuration. For example, in the first configuration, the corresponding shim magnet can generate a magnetic field in the region of interest that is aligned with the direction of the principal magnetic field B0. In the second configuration, the corresponding shim magnet can generate a magnetic field in the region of interest that is opposite to the direction of the principal magnetic field B0. In another embodiment, in the first configuration, the poles of the corresponding shim magnet can face a first direction. In the second configuration, the poles of the corresponding shim magnet can face a second direction opposite to the first direction.

[0071] According to some aspects of this disclosure, in order to perform shimming using the system described above, a first field map of the magnetic field B0 can be measured while each shim tray slot is empty. The first field map is then applied to a genetic algorithm to determine a first set (e.g., a first subset) of shim tray slots for capturing the shim magnets. The shim magnets are then captured into the first set of shim tray slots (e.g., the shim magnets are placed within the first set of shim tray slots). A second field map of the magnetic field B0 is measured while the shim magnets are being captured into the first set of shim tray slots. The second field map is then applied to a genetic algorithm to determine a second set (e.g., a second subset) of shim tray slots for capturing the shim magnets. The shim magnets are then captured into the second set of shim tray slots. This iterative process can be repeated, for example, until a desired magnetic field B0 uniformity is achieved, until the difference in magnetic field B0 uniformity across iterations meets a predetermined threshold, and / or until a desired number of iterations are completed (e.g., measuring a third field map, determining a third set of shim tray slots, incorporating a third set of shim tray slots, measuring a fourth field map, etc.).

[0072] In at least one aspect of this disclosure, when applying a second field map to a genetic algorithm to determine a second set of shim tray slots for shim magnet acquisition, the genetic algorithm is constrained to consider only unacquired shim tray slots (e.g., slots not included in the first set of shim tray slots) as candidates for the second set of shim tray slots. Thus, the systems and methods disclosed herein can enable a single shim tray to be selectively acquired over multiple shim iterations to achieve a gradual improvement in principal magnetic field B0 uniformity. This approach can thus avoid the potential challenges associated with using multiple shim trays, which, when stacked, gradually reduce the size of the inner bore of the MRI system (e.g., limiting the space available to receive the patient's head) and / or require a larger MRI housing in the first place to accommodate more than one shim tray therein. Furthermore, by constraining the algorithm to consider only unincorporated shim tray slots, the systems and methods disclosed herein can enable an iterative shimming approach in which adjustments made in the immediately preceding iteration (e.g., incorporating a first set of shim tray slots) are also taken into consideration in subsequent iterations.

[0073] In at least one aspect of this disclosure, the step of determining a set of shim tray slots for incorporating shim magnets by applying a field map to a genetic algorithm may include, for each of the set of shim tray slots, the step of determining whether to incorporate a shim magnet into the shim tray slot that is oriented in a first configuration (e.g., to generate a magnetic field aligned with the direction of the principal magnetic field B0 within the region of interest) or in a second configuration (e.g., to generate a magnetic field that runs counter to the direction of the principal magnetic field B0 within the region of interest). This approach can identify potential improvements to principal magnetic field B0 uniformity that may be unintentionally achieved by incorporating into one or more of the unincorporated shim trays a magnet oriented to generate a magnetic field with field lines that run counter to the direction of the principal magnetic field B0 within the region of interest (e.g., rather than considering only incorporating into the shim trays a magnet oriented to generate a magnetic field with field lines that run counter to the direction of the principal magnetic field B0 within the region of interest). In other words, this approach can identify a potential improvement to the main magnetic field B0 uniformity that can be unexpectedly achieved by incorporating magnets oriented in a direction opposite to the array of permanent magnets arranged in a modified Halbach dome orientation into one or more of the unincorporated shim trays.

[0074] In at least one aspect of this disclosure, the step of determining a set of shim tray slots for incorporating shim magnets by applying a field map to a genetic algorithm may include, for each of the sets of shim tray slots, the step of determining the corresponding intensity of one of the shim magnets for incorporating into the shim tray slot. For example, the genetic algorithm may be configured to consider incorporating either a first shim magnet (e.g., N52) having a first intensity or a second shim magnet (e.g., N42) having a second intensity into a given shim tray slot. This approach can leverage the potential to increase the uniformity of the main magnetic field B0 by incorporating magnets of different intensities into the shim tray slot. In another embodiment, the genetic algorithm may be configured to consider incorporating one of two or more shim magnets of different intensities into a given shim tray slot (e.g., one of three shim magnet options, each having a different intensity, one of four shim magnet options, each having a different intensity, etc.).

[0075] In at least one aspect of this disclosure, each shim tray slot of a shim tray can be configured to receive shim magnets of the same size. In another aspect, rather than using a custom shim tray with slots specifically selected based on shimming simulations for a particular MRI system, the shim trays and shim tray slots provided herein can include a standardized configuration that allows the same shim tray design to be applied across multiple different MRI systems. These and other aspects of this disclosure can provide manufacturing-related benefits. For example, during the production of multiple LF-MRI systems, each LF-MRI system may have the same permanent magnet array design, but due to manufacturing tolerances, each LF-MRI system may not produce a main magnetic field B0 with the same uniformity. Thus, each LF-MRI system may require unique shimming. Rather than fabricating a custom shim tray for each LF-MRI system, this disclosure can enable the same or substantially the same shim tray to be used to shim each LF-MRI system, with each shim tray selectively and / or iteratively receiving shim magnets based on the output of a genetic algorithm.

[0076] Figure 9-12 depicts the shim tray 900 in relation to at least one aspect of the present disclosure. The shim tray 900 is molded to be seated or otherwise positioned within a dome-shaped housing of an LF-MRI system, such as a dome-shaped housing similar to the dome-shaped housing 102 of the MRI system 100 described above with respect to Figure 1.

[0077] As described above, the various LF-MRI systems disclosed herein may comprise an array of permanent magnets configured to generate a principal magnetic field B0 within a region of interest. The array of permanent magnets may be arranged in a modified Halbach dome array optimized for imaging a patient's head (for example, as described in the aforementioned publication by McDaniel and / or as described in the aforementioned international patent application PCT / US2022 / 72143, filed on May 5, 2022, titled "NEURAL INTERVENTIONAL MAGNETIC RESONANCE IMAGING APPARATUS"). For example, in a modified Halbach dome array, the array of permanent magnets may comprise one closed end forming a dome and one open end opposite the closed end, the open end comprising an opening sized to receive a patient's head. A modified Halbach dome orientation can differ from, for example, a Halbach cylindrical array, which can include two open ends (e.g., open at opposing ends of the cylinder) and is defined by a stacked ring of magnets arranged in a magnetic rotation pattern to produce a substantially uniform field inside the resulting cylinder. Generally, to maintain a Halbach array, a modified Halbach dome array can include a more complex arrangement of magnets compared to a Halbach cylindrical array, as it can have open ends and dome-shaped closed ends opposite the open ends, unlike a Halbach cylinder which can be symmetrical between ends.

[0078] In various aspects, the modified Halbach dome arrangement of magnets has an outer radius r at the base of the dome. ext And the internal radius r inThe following can be defined: A modified Halbach dome array may include an elongated cylindrical portion extending from the base of the dome. On one side, the elongated cylindrical portion includes the same outer radius and inner radius as the base of the dome and continues from the base of the dome for a predetermined length at a constant radius. On another side, the elongated cylindrical portion includes a different outer radius and inner radius from the base of the dome. The different outer and inner radii of the elongated cylindrical portion merge with the base radius in the transition region.

[0079] Returning to Figure 9-12, the shim tray 900 is positioned within an array of permanent magnets oriented according to a modified Halbach dome array, and is shaped to at least partially surround them. Also, as described above, the LF-MRI system and the corresponding modified Halbach dome array of permanent magnets disclosed herein may include an opening configured for imaging a patient's head and therefore sized to receive the patient's head. As shown most in detail by Figures 9 and 12, the shim tray 900 includes an opening 906 at one end, sized to receive the patient's head. When the LF-MRI system with the shim tray 900 receives the patient's head, the shim tray 900 is positioned between the modified Halbach dome array of permanent magnets and the patient's head.

[0080] In some respects, the shim tray 900 may include a spherical and / or dome-shaped portion 908 and a substantially cylindrical portion 910 extending from the dome-shaped portion 908. The substantially cylindrical portion 910 may define an opening 906 for receiving the patient's head. The dome-shaped portion 908 and the substantially cylindrical portion 910 may be seated within or otherwise sized to correspond to the dome-shaped and substantially cylindrical portions of a modified Halbach dome array of permanent magnets in an LF-MRI system.

[0081] In some respects, as most clearly shown in Figure 10, the shim tray 900 can be constructed from multiple panels 904. Each panel 904 may be precisely sized to be insertable into an opening defined by the modified Halbach dome array of the permanent magnets of the LF-MRI system, thereby enabling the shim tray 900 to be assembled and positioned within the modified Halbach dome array of the permanent magnets. The shim tray 900 depicted by Figure 9-12 comprises panels 904 in a specific configuration, but the shim tray 900 may be constructed with any preferred number and arrangement of panels 904. In other respects, the shim tray 900 may be formed from a single piece of material and / or may not comprise separate panels 904.

[0082] Figure 13 depicts an exemplary panel 904 of the shim tray 900 according to at least one aspect of the present disclosure. The panel 904 may include holes 912. Each hole 912 may be configured to receive fasteners for fastening panel 904 to another panel 904 in order to construct or otherwise assemble the shim tray 900. The holes 912 may be configured to receive any type of fastener suitable for use with an MRI system, such as fasteners constructed from a non-ferromagnetic material.

[0083] Referring here to Figure 9-13, the shim tray 900 may include shim tray slots 902. Each of the shim tray slots 902 may be molded or otherwise configured to receive and / or support shim magnets 914. For example, Figure 12A shows the shim tray 900 of Figure 12, except that shim magnets 914 are selectively incorporated into an illustrative subset of the shim tray slots 902, according to one aspect of the present disclosure.

[0084] As described above, the shim tray 900 is compatible with and can be positioned within an LF-MRI system that includes an array of permanent magnets in a modified Halbach dome array. The array of permanent magnets can be configured to generate a principal magnetic field B0 within the region of interest. When positioned within an LF-MRI system with an array of permanent magnets, each shim tray slot 902 can be molded or otherwise configured to orient the shim magnet 914 received therein in one of a set of possible configurations. In some aspects, the set of possible configurations includes a first configuration or a second configuration. For example, if the shim magnet 914 is placed in one of the shim tray slots 902 in the first configuration, the shim magnet 914 may act to enhance the strength of the principal magnetic field B0 generated by the array surrounding the permanent magnets (for example, the magnetic field generated by the shim magnet 914 can be aligned with the direction of the principal magnetic field B0 within the region of interest). When a shim magnet 914 is placed in the same shim tray slot 902 in the second configuration, the shim magnet may act to reduce the strength of the principal magnetic field B0 generated by the array surrounding the permanent magnet (for example, the magnetic field generated by the shim magnet may be in the opposite direction to the principal magnetic field B0 in the region of interest). In one aspect, in the first configuration, the poles of the corresponding shim magnet 914 may face a first direction. In the second configuration, the poles of the corresponding shim magnet 914 may face a second direction opposite to the first direction.

[0085] In some respects, the set of possible configurations includes eight different configurations. For example, the shim tray slot 902 can be configured to receive a shim magnet 914 (e.g., a magnet carrier) having eight sides. Each of the eight different configurations can correspond to a different side of the shim magnet 914 facing the bottom edge of the shim tray slot 902 when inserted therein. Thus, each of the eight different configurations can correspond to eight different orientations of the poles of the shim magnet 914 with respect to the direction of the main magnetic field B0.

[0086] In some aspects, each shim tray slot 902 is configured to receive an end portion of a shim magnet 914. In other aspects, each shim tray slot 902 is configured to receive an end portion of the same size. For example, the rectangular opening defined by the shim tray slot 902, as depicted in Figure 9-13, may vary in size due to the various parts of the shim tray 900 having a curved internal shape. Each shim tray slot 902 can define a cavity, which may also vary in size (for example, having different sidewall depths based on where the corresponding shim tray slot 902 is formed along the internal shape of the shim tray 900). Although the cavity defined by the shim tray slot 902 may vary in size, each cavity defined by the shim tray slot 902 can be molded to receive an end portion of the shim magnet 914 of the same size. Furthermore, while the shim tray slot 902 can be molded to receive end portions of shim magnets 914 of the same size, it can nevertheless be molded to receive shim magnets 914 of different sizes. For example, the first shim magnet 914 may have a rectangular parallelepiped shape with dimensions of 10 mm × 10 mm × 5 mm. The second shim magnet 914 may have a cubic shape with dimensions of 10 mm × 10 mm × 10 mm. Both the first and second shim magnets 914 have end portions with a square cross-section of 10 mm × 10 mm. The shim tray slot 902 can be molded to accommodate these precisely sized end portions. However, when placed in one of the shim tray slots 902, the second shim magnet 914 may protrude further out of the shim tray slot than the first shim magnet 914. When the shim tray 900 is positioned within an LF-MRI system that includes an array of permanent magnets generating a main magnetic field B0, selectively incorporating the shim magnets 914 into the shim tray slot 902 (for example, according to methods 1400 and / or 1600 described below) can result in increased uniformity of the main magnetic field B0.

[0087] The shim tray 900 can be constructed from any material suitable for MRI. For example, the shim tray 900 may consist of non-ferrous materials such as polycarbonate, acrylonitrile butadiene styrene (ABS), polycarbonate / acrylonitrile butadiene styrene (PC-ABS), nylon, or polyoxymethylene (Delrin), or a combination thereof. Various additive and subtractive manufacturing techniques may be used to construct the shim tray 900. For example, the shim tray 900 may be formed via 3D printing, injection molding, and / or CNC machining.

[0088] Figure 14 illustrates a flowchart of a passive shimming method 1400 for an MRI system, according to at least one aspect of the present disclosure. Method 1400 can be performed using various MRI systems disclosed herein, such as an MRI system 100 (Figure 1) equipped with a shim tray 900 (Figures 9-12). As described above, the MRI system 100 may include an array of permanent magnets configured to generate a main magnetic field B0 having a low field intensity, such as about 70 mT. The shim tray 900 may include an array of shim tray slots 902 configured to receive shim magnets 914. Some aspects of Method 1400 may be performed based on instructions stored in memory and executed by a processor.

[0089] Referring primarily to Figure 14 and also to Figures 9-12, an initial field map of the main magnetic field B0 is measured according to Method 1400 (1402). Various sensors, such as Hall effect sensors (e.g., triaxial Hall probes) and / or nuclear magnetic resonance (NMR) probes, can be used to measure the field map of the main magnetic field B0 (1402).

[0090] In accordance with method 1400, the available sim tray slots 902 are identified (1406). In some aspects, all of the sim tray slots 902 of the sim tray 900 may be identified as available (1406). In other aspects, the user may designate one or more of the sim tray slots 902 as unavailable (for example, based on user preference, based on previous simming experience, etc.). In some aspects, the available sim tray slots 902 are stored in memory and automatically identified based on instructions executed by the processor (1406). For example, characteristics of the sim tray 900, such as the number and location of the sim tray slots 902, may be stored in memory and read out to identify the available sim tray slots 902 (1406).

[0091] Referring primarily to Figure 14 and Figures 9-12, it is determined according to Method 1400 whether the shim magnet 914 is taken into any of the shim tray slots 902 (1408). For example, if the initial iterations of Method 1400 have not yet been performed, it may be determined that the shim magnet 914 is not taken into any of the shim tray slots 902 (1408). Thus, all of the shim tray slots 902 may be designated as available for taking in the shim magnet 914 (1410). As will be further explained below, if one or more iterations of Method 1400 have been performed, it may be determined that the shim magnet 914 is taken into some of the shim tray slots 902 (1408), and only the undocumented shim tray slots 902 may be designated as available (1412). In some aspects, the determination 1408 of whether the shim magnet 914 is taken into one of the shim tray slots 902, and the designation 1410, 1412 of the available shim tray slots 902 may be performed automatically based on instructions stored in memory and executed by the processor. For example, instructions stored in memory may cause the processor to determine whether a previous iteration of method 1400 has been executed to determine (1408) whether the shim magnet 914 is taken into one of the shim tray slots 902.

[0092] Referring primarily to Figure 14 and Figures 9-12, according to Method 1400, the measured (1402) initial field map is added as input to the genetic algorithm (1404), and the shim tray slots 902 designated as available for the acquisition of the shim magnets 914 (1410, 1412) are added as input to the genetic algorithm (1414). It should be noted that, rather than relying on simulated field data as input, the optimization input to the genetic algorithm in 1414 can include measured field data, such as magnetic field data acquired using a magnetometer (e.g., a triaxial Hall probe and / or an NMR probe). Using measured field data as optimization input can help compensate for mounting tolerances and / or probe detection errors of the shim magnets 914, for example.

[0093] Based on the optimization input, a set (e.g., a subset) of shim tray slots 902 for incorporating the shim magnet 914 is determined using a genetic algorithm (1416). For example, the genetic algorithm can be configured to determine the combination of shim tray slots 902 that, when the shim magnet 914 is incorporated, will result in the greatest increase in the uniformity of the main magnetic field B0 (1416). The genetic algorithm can also be configured to consider whether to incorporate shim magnets 914 of different intensities into one or more of the shim tray slots 902, and / or whether to incorporate shim magnets 914 oriented in a first configuration (e.g., to generate a magnetic field aligned with the direction of the main magnetic field B0 within the region of interest) or a second configuration (e.g., to generate a magnetic field reversed with respect to the direction of the main magnetic field B0 within the region of interest) into one or more of the shim tray slots 902. In some aspects, the genetic algorithm may be implemented according to the genetic algorithm 1500 discussed below with respect to Figure 15. In some respects, the genetic algorithm may be implemented as described in the aforementioned publication by Cooley et al., published in 2018 in IEEE Transactions on Magnetics, 54(1), 5100112, entitled "Design of sparse Halbach magnet arrays for portable MRI using a genetic algorithm" (which is incorporated herein by reference as described above). In some respects, the genetic algorithm may be stored in memory as instructions that can be executed by the processor.

[0094] Still referring primarily to Figure 14 and Figures 9-12, the shim magnets 914 are physically incorporated into a set of shim tray slots 902 according to method 1400, in accordance with the above determination 1416 by the genetic algorithm (for example, the shim magnets are placed within the set of shim tray slots 902). With the shim magnets 914 incorporated into the set of shim tray slots 902, another field map of the main magnetic field B0 is measured (1420). The field map may be measured using any of the various sensors described above in relation to measuring the initial field map (1402) (1420).

[0095] Referring primarily to Figure 14 and Figures 9-12, a determination 1422 is made according to Method 1400 as to whether the reduction in non-uniformity of the main magnetic field B0 meets a predetermined threshold. For example, the most recently measured (1420) field map (e.g., the field map of the current iteration of Method 1400) can be compared with the most recently measured (1420) field map (e.g., the field map of a previous iteration or the initial field map) to determine the change in non-uniformity of the main magnetic field B0. If the change in non-uniformity of the main magnetic field B0 meets a predetermined threshold, another iteration of Method 1400 does not need to be performed (1424) (e.g., the passive shimming process is completed). If the change in the non-uniformity of the main magnetic field B0 does not meet a predetermined threshold, another iteration of method 1400 may again begin to identify available shim tray slots 902 (1406) and determine whether any shim tray slot 902 is to be taken up (1408).

[0096] As still primarily referring to Figure 14 and Figures 9-12, if a second or subsequent iteration of Method 1400 is performed according to Method 1400, the shim magnets 914 from a previous iteration will be incorporated into at least some of the shim tray slots 902. Therefore, the step 1408 of determining whether a shim magnet is incorporated into any shim tray slot may include identifying the shim tray slots 902 into which the shim magnet 914 was incorporated during a previous iteration of Method 1400. For example, if a third iteration of Method 1400 is performed, the shim tray slots 902 into which the shim magnet 914 was incorporated will include the shim tray slots 902 into which the shim magnet 914 was incorporated during a first iteration of Method 1400 and the shim tray slots 902 into which the shim magnet 914 was incorporated during a second iteration of Method 1400. If it is determined that at least some of the shim tray slots 902 have a shim magnet 914 loaded into them (1408), then only the unloaded shim tray slots 902 may be designated as available for loading in the current iteration (1412). In some aspects, the determination 1408 of whether any of the shim tray slots 902 have a shim magnet loaded into them and the designation 1412 of only the unloaded shim tray slots 902 as available for loading may be performed automatically based on instructions stored in memory and executed by the processor. For example, instructions stored in memory may cause the processor to determine whether a previous iteration of method 1400 was executed to determine whether any of the shim tray slots 902 have a shim magnet 914 loaded into them (1408).

[0097] If a second or subsequent iteration of Method 1400 is performed, still primarily referring to Figure 14 and also to Figures 9-12, Method 1400 may proceed as described above by adding (1404) a previously measured (1420) field map, adding (1414) sim tray slots 902 available as input to the genetic algorithm, determining (1416) the next set of sim tray slots to be incorporated, incorporating (1418) the next set of sim tray slots, measuring (1420) the field map, and determining whether the reduction of field heterogeneity meets a predetermined threshold (1422).

[0098] Figure 15 illustrates a flowchart of the genetic algorithm 1500 according to at least one aspect of the present disclosure. As described above, the genetic algorithm 1500 may be implemented as part of method 1400 described above with respect to Figure 14 to determine (1416) a set of shim trays 902 for capturing shim magnets 914. In some aspects, the genetic algorithm 1500 may be analogous to the genetic algorithm described in the aforementioned publication by Cooley et al., published in 2018 in IEEE Transactions on Magnetics, 54(1), 5100112, entitled "Design of sparse Halbach magnet arrays for portable MRI using a genetic algorithm" (which is incorporated herein by reference as described above). Any aspect of the genetic algorithm described by Cooley may be implemented as part of the genetic algorithm 1500.

[0099] Referring primarily to Figure 15 and Figures 9-12, a parent generation of candidate shim tray slot inclusion sets is generated (1502) according to the genetic algorithm 1500. For example, step 1502 for generating each candidate shim tray slot inclusion set for the parent generation may include a step of randomly selecting a combination of one or more shim tray slots 902 to simulate the inclusion of shim magnets 914. Each candidate shim tray slot inclusion set can be defined by a set of inclusion values, where each value corresponds to one of the shim tray slots. Slots that should be simulated as not containing shim magnets 914 are designated with a value of 0, and slots that should be simulated as containing shim magnets 914 are designated with a value of 1. Thus, two simulation options (k=2) may be available for each shim tray slot 902.

[0100] In one aspect of algorithm 1500, the step of randomly selecting one or more combinations of shim trays 902 to simulate the inclusion of a shim magnet 914 may include a step of randomly selecting whether to include either a shim magnet 914 oriented in a first configuration (e.g., to generate a field aligned with the main magnetic field B0) or a shim magnet 914 oriented in a second configuration (e.g., to generate a field opposite to the direction of the main magnetic field B0) in one or more of the shim tray slots 902. In this aspect, a slot to be simulated as not containing a shim magnet 914 is designated with a value of 0, a slot to be simulated as containing a shim magnet 914 oriented in a first configuration is designated with a value of 1, and a slot to be simulated as containing a shim magnet oriented in a second configuration is designated with a value of 2. Thus, three simulation options (k=3) may be available for each shim tray slot 902.

[0101] In one aspect of algorithm 1500, the step of randomly selecting one or more combinations of shim trays 902 to simulate the inclusion of shim magnets 914 may include the step of randomly selecting whether one or more of the shim trays 902 include magnets oriented in a first configuration having a first strength, magnets oriented in a first configuration having a second strength, magnets oriented in a second configuration having a first strength, or magnets oriented in a second configuration having a second strength. On this side, slots that should be simulated as if no shim magnets were included are specified with a value of 0, slots that should be simulated as if shim magnets oriented in the first configuration and having a first strength are included are specified with a value of 1, slots that should be simulated as if shim magnets oriented in the second configuration and having a first strength are included are specified with a value of 2, slots that should be simulated as if shim magnets oriented in the first configuration and having a second strength are included are specified with a value of 3, and slots that should be simulated as if shim magnets oriented in the second configuration and having a second strength are included are specified with a value of 4. Thus, five simulation options (k=5) can be made available for each shim tray slot 902.

[0102] Step 1502, which generates a parent generation of candidate sim tray slot import sets, may include steps 1502 that generate two or more candidate sim tray slot import sets (for example, three candidate sim tray slot import sets, four candidate sim tray slot import sets, five candidate sim tray slot import sets, ten candidate sim tray slot import sets, 100 candidate sim tray slot import sets, more than 100 candidate sim tray slot import sets, etc.).

[0103] Referring primarily to Figure 15 and Figures 9-12, a simulated field map is generated for each of the parent candidate sim tray inclusion sets according to the genetic algorithm 1500 (1504). Furthermore, the offspring generation of the candidate sim tray slot inclusion sets is determined from the parent generation based on a tournament selection process (1506). The tournament selection process may involve matching (e.g., comparing) the candidate sim tray slot inclusion sets from the parent generation against each other, allowing only the most suitable candidate of the selected number (e.g., the candidate sim tray slot inclusion set having the lowest heterogeneity of the simulated field map) to pass the tournament selection. The child generation of a candidate sim tray slot import set can be determined, for example, by (i) designating one or more of the most suitable parents as a child, (ii) generating a child based on the step of combining a portion of a set of values ​​that define two or more parents (e.g., two or more of the most suitable parents), and / or (iii) generating a child by introducing a random mutation of the parents (e.g., a random change in a set of values) (1506).

[0104] As still primarily referring to Figure 15 and Figures 9-12, according to the genetic algorithm 1500, it is determined whether the difference between the parent generation and the child generation of the candidate sim tray slot inclusion set satisfies a predetermined threshold (1508). For example, as the difference between the sets of values ​​defining the parent and child generations becomes similar, this may indicate that the genetic algorithm 1500 is converging to a single solution. Thus, subsequent iterations of the genetic algorithm 1500 may result in a slight reduction in the simulated field uniformity. If it is determined that the predetermined threshold is not met (1508), the child generation of the candidate sim tray inclusion set can be designated as the parent generation for the next iteration of the algorithm 1500 (1510). Alternatively, if it is determined that the predetermined threshold is met (1508), the child generation candidate sim tray slot inclusion set can be identified as the output candidate sim tray slot inclusion set of the algorithm 1500 (1512). For example, the candidate shim tray slot inclusion set for the offspring generation corresponding to the simulated field map with the lowest heterogeneity may be identified as the output candidate shim tray slot inclusion set of algorithm 1500 (1512). Referring again to Figure 14, the output candidate shim tray inclusion set identified by genetic algorithm 1500 (1512) may correspond to the set of shim tray slots for inclusion of shim magnets determined as part of method 1400 (1416).

[0105] Figure 16 illustrates a flowchart of a passive shimming method 1600 for an MRI system, according to at least one aspect of the present disclosure. Method 1600 can be performed using various MRI systems disclosed herein, such as an MRI system 100 (Figure 1) equipped with a shim tray 900 (Figure 9-12). As described above, the MRI system 100 may include an array of permanent magnets configured to generate a main magnetic field B0 having a low field intensity. The shim tray 900 may include shim tray slots 902 configured to receive shim magnets 914. Some aspects of the method may be performed based on instructions stored in memory and executed by a processor.

[0106] Referring primarily to Figure 16 and Figures 9-12, a first field map of the magnetic field B0 is measured according to method 1602 while each of the shim tray slots 902 is empty. A first set of shim tray slots 902 for capturing the shim magnet 914 is determined by applying the first field map to a genetic algorithm (1604). The shim magnet 914 is captured in the first set of shim tray slots 902 (1606). A second field map of the magnetic field B0 is measured after the shim magnet 914 has been captured in the first set of shim tray slots 902 (1608). A second set of shim tray slots 902 for capturing the shim magnet 914 is determined by applying the second field map to a genetic algorithm (1610). The shim magnet 914 is then captured in the second set of shim tray slots 902.

[0107] In one aspect, method 1600 further includes a step of determining a set of uncaptured shim tray slots 902. For example, the set of uncaptured shim tray slots 902 may be the remaining shim tray slots 902 that are not present in the first set of shim tray slots 902. Furthermore, step 1610, which determines a second set of shim tray slots 902 for capturing the shim magnets 914 by applying a second field map to the genetic algorithm, may include a step of restricting the genetic algorithm to consider only the uncaptured shim tray slots 902 for inclusion in the second set of shim tray slots 902.

[0108] According to one aspect of Method 1600, step 1604, which determines a first set of shim tray slots 902 for incorporating shim magnets by applying a first field map to a genetic algorithm, may include the step of determining a configuration for arranging the corresponding shim magnets for each of the first set of shim tray slots 902. The configuration may be selected from a set of possible configurations. In some aspects, the set of possible configurations includes a first configuration and a second configuration. For example, in the first configuration, the corresponding shim magnet 914 can generate a magnetic field in the region of interest that is aligned with the direction of the principal magnetic field B0. In the second configuration, the corresponding shim magnet 914 can generate a magnetic field in the region of interest that is reversed with respect to the direction of the principal magnetic field B0. In another embodiment, in the first configuration, the poles of the corresponding shim magnet 914 can face a first direction. In the second configuration, the poles of the corresponding shim magnet 914 can face a second direction opposite to the first direction. In other respects, the set of possible configurations includes eight different configurations. For example, the shim magnet 914 may have eight sides. In one respect, each of the eight different configurations can correspond to a different side of the shim magnet 914 that, when arranged, faces toward the bottom of the shim tray slot 902. In another respect, each of the eight different configurations can correspond to a pole of the shim magnet 914 that, when arranged within the shim tray slot 902, is oriented differently with respect to the main magnetic field B0 in the region of interest.

[0109] According to one aspect of method 1600, step 1604, which determines a first set of shim tray slots 902 for incorporating shim magnets 914 by applying a first field map to a genetic algorithm, may include a step of determining, for each of the first set of shim tray slots 902, the corresponding strength of one of the shim magnets 914 to be incorporated into the shim tray slot 902. For example, the step of determining, for each of the first set of shim tray slots 902, the corresponding strength of one of the shim magnets 914 to be incorporated into the shim tray slot 902 may include a step of determining whether to incorporate into the shim tray slot 902 either a first shim magnet having a first strength (e.g., an N42 magnet) or a second shim magnet having a second strength (e.g., an N52 magnet).

[0110] According to one aspect of Method 1600, step 1606 for loading shim magnets 914 into a first set of shim tray slots 902 may include the step of loading first shim magnets, each having a first strength, into the first set of shim tray slots 902. According to another aspect of Method 1600, step 1612 for loading shim magnets 914 into a second set of shim tray slots 902 may include the step of loading second shim magnets, each having a second strength, into the second set of shim tray slots 902. For example, the second strength may be weaker than the first strength. In another embodiment, the first strength may be weaker than the second strength. [Examples]

[0111] The inventors of this disclosure implement methods 1400 and 1600 for passively shimming an LF-MRI system, as described below in the following embodiments.

[0112] Two low-field (approximately 70 mT) head-optimized permanent magnet MRI systems (exemplary system #1 and exemplary system #2) were examined using an iterative shimming strategy. For exemplary system #1 and exemplary system #2, the principal magnetic field B0 was measured in three stages: pre-shimming, first stage, and second stage. Regions of interest (ROIs) of brain shape were used to assess field uniformity. For both exemplary system #1 and exemplary system #2, after the two-stage shimming process, both the simulated and measured field maps showed improved B0 field uniformity compared to the simulated and measured field maps generated after a single stage of shimming.

[0113] For example, Figure 17A depicts Graph 1700, which shows both simulated and measured main magnetic field B0 uniformity through a two-stage passive shimming process for exemplary system #1. Specifically, Graph 1700 shows the measured pre-shimming uniformity 1702, simulated first-stage uniformity 1704 (simulated main magnetic field B0 uniformity with magnets incorporated into the first set of shim tray slots), measured first-stage uniformity 1706 (actually measured main magnetic field B0 uniformity with magnets incorporated into the first set of shim tray slots), simulated second-stage uniformity 1708 (simulated main magnetic field B0 uniformity with magnets incorporated into the first and second sets of shim tray slots), and measured second-stage uniformity 1710 (actually measured main magnetic field B0 uniformity with magnets incorporated into the first and second sets of shim tray slots). A 31.0% improvement in field uniformity was observed from the measured uniformity of the first stage (1706) to the measured uniformity of the second stage (1710). Thus, a significant improvement in field uniformity was observed within exemplary system #1 by performing a second iteration of passive shimming.

[0114] Similarly, Figure 18A depicts Graph 1800, which shows both simulated and measured main magnetic field B0 uniformity through a two-stage passive shimming process for exemplary system #2. Specifically, Graph 1800 shows the measured pre-shimming uniformity 1802, simulated first-stage uniformity 1804 (simulated main magnetic field B0 uniformity with magnets incorporated into the first set of shim tray slots), measured first-stage uniformity 1806 (actually measured main magnetic field B0 uniformity with magnets incorporated into the first set of shim tray slots), simulated second-stage uniformity 1808 (simulated main magnetic field B0 uniformity with magnets incorporated into the first and second sets of shim tray slots), and measured second-stage uniformity 1810 (actually measured main magnetic field B0 uniformity with magnets incorporated into the first and second sets of shim tray slots). A 30.0% improvement in field uniformity was observed from the measured uniformity of the first stage 1806 to the measured uniformity of the second stage 1810, with the measured uniformity of the second stage 1810 being 8,592 ppm at an average field intensity of 70.0 mT. Again, a significant improvement in field uniformity was observed in exemplary system #2 by performing a second iteration of passive shimming.

[0115] In both Exemplary System #1 and Exemplary System #2, during the first stage of shimming, for each shim tray slot, the genetic algorithm considered either not incorporating it into the shim tray slot (value = 0) or incorporating a magnet into the shim tray slot that is oriented to generate a field matched with the main magnetic field (value = 1). In Exemplary System #2, during the second stage of shimming, for each available shim tray slot, the genetic algorithm again considered either not incorporating it into the shim tray slot (value = 0) or incorporating a magnet into the shim tray slot that is oriented to generate a field matched with the main magnetic field (value = 1). However, in exemplary system #1, during the second stage of shimming, for each available shim tray slot, the genetic algorithm considered not incorporating a magnet into the shim tray slot (value=0), incorporating a magnet into the shim tray slot that is oriented to generate a field consistent with the direction of the main magnetic field (value=1), or incorporating a magnet into the shim tray slot that is oriented to generate a field opposite to the direction of the main magnetic field (value=2).

[0116] In Exemplary System #1, some of the shim tray slots incorporated magnets oriented to generate a field reversed with respect to the direction of the principal magnetic field during the second stage of shimming, resulting in a decrease in principal magnetic field strength observed from the first stage of shimming to the second stage of shimming. For example, Figure 17B shows a histogram of principal magnetic field strength distribution measured for Exemplary System #1 before shimming 1752, after the first stage of shimming 1754, and after the second stage of shimming 1765. The field strength after the first stage of shimming 1754 is higher than the field strength after the second stage of shimming 1756. Similarly, Figure 18B shows a histogram of principal magnetic field strength distribution measured for Exemplary System #2 before shimming 1852, after the first stage of shimming 1854, and after the second stage of shimming 1865. Unlike system #1, in system #2, the field intensity at 1854 after the first stage of shimming is lower than the field intensity at 1856 after the second stage of shimming. Note

[0117] Various additional aspects of the subject matter described herein are presented in the following numbered appendices.

[0118] Note 1: A method comprising the steps of generating a first field map of a magnetic field B0 from a magnetic resonance imaging (MRI) system, wherein the MRI system is an array of permanent magnets in a modified Halbach dome array, the array having one open end sized to receive a patient's head, the array comprising an array of permanent magnets configured to generate a magnetic field B0, and a shim tray having slots configured to receive shim magnets, the first field map corresponding to the magnetic field based on each of the empty slots, and applying the first field map to a genetic algorithm and slots for receiving a first set of shim magnets A method comprising the steps of: determining a first set of slots; arranging the first set of shim magnets in the first set of slots; generating a second field map of magnetic field B0, wherein the second field map corresponds to the magnetic field based on the first set of shim magnets arranged in the first set of slots; applying the second field map to a genetic algorithm to determine a second set of slots for receiving the second set of shim magnets, wherein the second set of slots is different from the first set of slots; and arranging the second set of shim magnets in the second set of slots.

[0119] Note 2: The method described in Note 1, wherein the size of the first set of shim magnets is the same as the size of the second set of shim magnets.

[0120] Note 3: The size of the first set of shim magnets is different from the size of the second set of shim magnets, as described in Note 1.

[0121] Note 4: The method according to any one of Notes 1-3, further comprising the steps of determining a set of empty slots in a sim tray, wherein the set of empty slots consists of slots excluded from a first set of slots, and restricting the genetic algorithm to consider only the set of empty slots for inclusion in a second set of slots.

[0122] Note 5: The method according to any one of Notes 1-4, further comprising the steps of applying a first field map to a genetic algorithm to determine a first set of slots for receiving a first set of shim magnets, wherein for each of the first set of slots, a configuration for arranging a corresponding one of the shim magnets in the slot is selected from a set of possible configurations, the set of possible configurations includes a first configuration and a second configuration.

[0123] Note 6: The method according to Note 5, wherein in the first configuration, the corresponding shim magnet generates a first magnetic field that is substantially aligned with the direction of the magnetic field B0, and in the second configuration, the corresponding shim magnet generates a second magnetic field that is substantially opposite to the direction of the magnetic field B0.

[0124] Note 7: The set of possible configurations further includes a second configuration, a third configuration, a fourth configuration, a fifth configuration, a sixth configuration, a seventh configuration, and an eighth configuration, as described in any of Notes 5-6.

[0125] Note 8: The method according to any of Notes 5-7, wherein in the first configuration, the poles of the shim magnet are oriented in a first direction, and in the second configuration, the poles of the shim magnet are oriented in a second direction opposite to the first direction.

[0126] Note 9: The method according to any of Notes 5-8, wherein the step of applying a second field map to a genetic algorithm to determine a second set of slots for receiving a second set of shim magnets further includes the step of determining a configuration for arranging a corresponding one of the shim magnets in each of the second set of slots, wherein the configuration is selected from a set of possible configurations.

[0127] Note 10: The method according to any of Notes 1-9, wherein each slot is configured to receive at least an end portion of a shim magnet, and each slot is configured to receive an end portion of the same size.

[0128] Note 11: The method according to any of the methods in Notes 1-10, wherein the step of applying a first field map to a genetic algorithm to determine a first set of slots to receive a first set of shim magnets includes the step of determining the intensity of a corresponding one of the shim magnets for each of the first set of slots.

[0129] Note 12: The method according to Note 11, wherein, with respect to each of the first set of shim tray slots, the step of determining the strength of one of the corresponding shim magnets includes the step of determining whether to provide a first shim magnet having a first strength or a second shim magnet having a second strength.

[0130] Note 13: The method according to Note 12, wherein the first shim magnet having a first strength includes an N42 magnet, and the second shim magnet having a second strength includes an N52 magnet.

[0131] Note 14: The method according to any of the notes 1-11, wherein the step of arranging a first set of shim magnets in a first set of slots includes arranging first shim magnets, each having a first strength, and the step of arranging a second set of shim magnets in a second set of slots includes arranging second shim magnets, each having a second strength, the second strength being weaker than the first strength.

[0132] Note 15: A system for magnetic resonance imaging (MRI), comprising: an array of permanent magnets in a modified Halbach array forming one open end, the open end being sized to receive a patient's head, and the array of permanent magnets being configured to generate a low-field-intensity magnetic field B0 toward an object of interest located in the field of view; and a shim tray positionable within the array of permanent magnets, comprising an array of shim tray slots, having an opening sized to receive a patient's head, each shim tray slot being configured to receive a shim magnet in a first configuration and a second configuration, in the first configuration the polarity of the shim magnets is oriented in a first direction, and in the second configuration the polarity of the shim magnets is oriented in a second direction opposite to the first direction.

[0133] Note 16: The system according to claim 15, wherein the array of permanent magnets comprises primary permanent magnets, and the system further comprises a plurality of shim magnets disposed within a portion of the shim tray slots, the plurality of shim magnets comprising secondary permanent magnets positioned relative to the primary permanent magnets to increase the uniformity of the magnetic field B0 generated by the permanent magnets.

[0134] Note 17: The system according to claim 16, wherein each shim tray slot is configured to receive at least an end portion of a shim magnet, and each shim tray slot is configured to receive an end portion of the same size.

[0135] Note 18: The system according to claim 17, wherein the plurality of shim magnets include a first shim magnet that defines a first magnetic field strength and a second shim magnet that defines a second magnetic field strength.

[0136] Note 19: The system according to claim 16, wherein at least one of the multiple shim magnets is arranged in the second configuration.

[0137] Note 20: The system according to claim 16, wherein multiple shim magnets are arranged in fewer than all of the shim tray slots.

[0138] Note 21: A method for shimming an MRI system, comprising the steps of: generating a field map of a magnetic field B0 from a magnetic resonance imaging (MRI) system, wherein the MRI system is an array of permanent magnets in a modified Halbach dome array, the array having one open end sized to receive a patient's head, the array comprising an array of permanent magnets configured to generate a magnetic field B0, and a shim tray having slots configured to receive shim magnets; iteratively applying the field map to a genetic algorithm to determine a set of slots for receiving a set of shim magnets; arranging a set of shim magnets in a set of slots; generating a next field map of magnetic field B0, the next field map corresponding to a magnetic field based on a set of shim magnets arranged in a set of slots; and deciding to perform a next iteration based on the next field map, or not to perform a next iteration based on the next field map.

[0139] While several forms are illustrated and described, it is not the applicant's intention to limit or restrict the scope of the attached claims to such details. Numerous modifications, variations, alterations, substitutions, combinations, and equivalents to those forms may be implemented and will be conceivable to those skilled in the art without departing from the scope of this disclosure. Furthermore, the structure of each element associated with the described forms may, as an alternative, be described as a means for providing the function performed by the element. Also, if a material is disclosed in relation to a component, other materials may also be used. Therefore, it should be understood that the foregoing description and the attached claims are intended to cover all such modifications, combinations, and variations that fall within the scope of the disclosed forms. The attached claims are intended to cover all such modifications, variations, alterations, substitutions, alterations, and equivalents.

[0140] The detailed descriptions above illustrate various forms of the device and / or process through the use of block diagrams, flowcharts, and / or embodiments. To the extent that such block diagrams, flowcharts, and / or embodiments contain one or more functions and / or operations, it will be understood by those skilled in the art that each function and / or operation within such block diagrams, flowcharts, and / or embodiments can be implemented individually and / or collectively by a wide range of hardware, software, firmware, or virtually any combination thereof. A person skilled in the art will recognize that several aspects of the forms disclosed herein can be uniformly implemented within an integrated circuit, in whole or in part, as one or more computer programs running on one or more computers (e.g., as one or more programs running on one or more computer systems), as one or more programs running on one or more processors (e.g., as one or more programs running on one or more microprocessors), as firmware, or virtually any combination thereof, and that designing the network and / or writing the code relating to the software and / or firmware would clearly be within the skill of a person skilled in the art in light of this disclosure. In addition, a person skilled in the art will understand that the mechanisms of the subject matter described herein can be distributed as one or more program products in various forms, and that the illustrative forms of the subject matter described herein apply regardless of the specific type of signal-carrying medium used to actually carry out the distribution.

[0141] Instructions used to program the logic for implementing various disclosed aspects may be stored in memory within the System, such as dynamic random access memory (DRAM), cache, flash memory, or other storage devices. Furthermore, instructions may be distributed over a network or using other computer-readable media. Thus, machine-readable media may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer), but are not limited to floppy diskettes, optical disks, compact disks, read-only memory (CD-ROMs), and magneto-optical disks, read-only memory (ROMs), random access memory (RAMs), erasable programmable read-only memory (EPROMs), electrically erasable programmable read-only memory (EEPROMs), magnetic or optical cards, flash memory, or tangible machine-readable storage devices used in the transmission of information over the Internet via electrical, optical, acoustic, or other forms of propagating signals (e.g., carrier waves, infrared signals, digital signals, etc.). Therefore, non-transient computer-readable media include any type of tangible machine-readable medium suitable for storing or transmitting electronic instructions or information in a form readable by a machine (e.g., a computer).

[0142] As used in any aspect of this Specification, the term “control circuit” can mean, for example, a wired network, a programmable network (e.g., a computer processor including one or more individual instruction processing cores, processing units, processors, microcontrollers, microcontroller units, controllers, digital signal processors (DSPs), programmable logic devices (PLDs), programmable logic arrays (PLAs), or field-programmable gate arrays (FPGAs)), a state-machine network, firmware that stores instructions executed by a programmable network, and any combination thereof. Control circuits may be embodied as a network of circuits that form part of a larger system, such as an integrated circuit (IC), an application-specific integrated circuit (ASIC), a system-on-a-chip (SoC), a desktop computer, a laptop computer, a tablet computer, a server, a smartphone, etc. Therefore, as used herein, “control circuit” includes, but is not limited to, an electrical network having at least one discrete electrical circuit, an electrical network having at least one integrated circuit, an electrical network having at least one application-specific integrated circuit, an electrical network forming a general-purpose computing device (e.g., a general-purpose computer, at least partially composed of a computer program performing the processes and / or devices described herein, or a microprocessor, at least partially composed of a computer program performing the processes and / or devices described herein), an electrical network forming a memory device (e.g., in the form of random-access memory), and / or an electrical network forming a communication device (e.g., a modem, a communication switch, or an optical-electrical device). Those skilled in the art will recognize that the subject matter described herein may be implemented in analog or digital manner or in any combination thereof.

[0143] As used in any aspect of this specification, the term “logic” may mean an app, software, firmware, and / or network configured to perform any of the operations described herein. Software may be embodied as software packages, code, instructions, instruction sets, and / or data recorded on a non-transient computer-readable storage medium. Firmware may be embodied as code, instructions, or instruction sets, and / or data hardcoded (e.g., non-volatile) within a memory device.

[0144] As used in any aspect of this specification, the terms “component,” “system,” “module,” and their equivalents may refer to any control circuit computer-related entity, whether hardware, a combination of hardware and software, software, or software in operation.

[0145] As used in any aspect of this specification, “algorithm” refers to a self-consistent sequence of steps leading to a desired result, and “step” refers to the manipulation of physical quantities and / or logical states, which may take the form of electrical or magnetic signals that can be stored, transferred, combined, compared, and otherwise manipulated, although this is not necessarily required. It is common practice to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or equivalents. These and similar terms may be associated with appropriate physical quantities and are merely convenient labels applied to these quantities and / or states.

[0146] The network may include a packet-switched network. Communication devices may be able to communicate with each other using a selected packet-switched network communication protocol. One exemplary communication protocol may include the Ethernet® communication protocol, which may enable communication using the Transmission Control Protocol / Internet Protocol (TCP / IP). The Ethernet® protocol may conform to or be compatible with the Ethernet® standard and / or later versions of this standard, published by the Institute of Electrical and Electronics Engineers (IEEE) in December 2008 and titled "IEEE 802.3 Standard". Alternatively, or in addition, communication devices may be able to communicate with each other using the X.25 communication protocol. The X.25 communication protocol may conform to or be compatible with standards published by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). Alternatively, or in addition, communication devices may be able to communicate with each other using the Frame Relay communication protocol. The Frame Relay communication protocol conforms to, or may be compatible with, the standards published by the International Telecommunications Consultative Committee (CCITT) and / or the American National Standards Institute (ANSI). Alternatively, or in addition, transceivers may communicate with each other using the Asynchronous Transfer Mode (ATM) communication protocol. The ATM communication protocol conforms to, or may be compatible with, the ATM standard and / or later versions of this standard published by the ATM Forum in August 2001, entitled "ATM-MPLS Network Interworking 2.0". Naturally, other and / or later developed connection-oriented network communication protocols are also considered equally in this specification.

[0147] As is evident from the foregoing disclosures, unless otherwise specifically stated, discussions throughout the foregoing disclosures using terms such as “processing,” “computing,” “calculating,” “determining,” “displaying,” or equivalents should be understood to refer to the actions and processes of a computer system or similar electronic computing device that manipulate data represented as physical (electronic) quantities in the registers and memory of a computer system and convert it into other data similarly represented as physical quantities in computer system memory or registers or other such information storage, transmission, or display devices.

[0148] One or more components may be referred to herein as “configured to,” “configurable to,” “operable / operative to,” “adapted / adaptable,” “able to,” “conformable / conformed to,” etc. Those skilled in the art will recognize that, unless the context requires otherwise, “configured to” may generally encompass active state components and / or inactive state components and / or standby state components.

[0149] The terms “proximal” and “distal” are used herein to refer to the clinician operating the handle portion of a surgical instrument. “Proximal” refers to the part closest to the clinician, and “distal” refers to the part located further away from the clinician. For convenience and clarity, spatial terms such as “vertical,” “horizontal,” “up,” and “down” may also be used herein in reference to drawings. However, surgical instruments are used in many orientations and positions, and these terms are not intended to be restrictive and / or absolute.

[0150] Those skilled in the art will generally recognize that, in particular, the terms used herein in the appended claims (e.g., the body of the appended claims) are generally intended to be “open” terms (for example, the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” and the term “includes” should be interpreted as “includes but is not limited to,” etc.). Those skilled in the art will further understand that where an enumeration of specific numbers of claims is intended, such intent is explicitly enumerated within the claims, and in the absence of such enumeration, no such intent exists. For example, for the sake of understanding, the following appended claims may contain the use of the introductory phrases “at least one” and “one or more” to introduce an enumeration of claims. However, the use of such phrases should not be interpreted as implying that the introduction of a claim enumeration by the indefinite article "a" or "an" limits any particular claim containing such introduced claim enumeration to only one such claim, even when the same claim contains the introductory phrase "one or more" or "at least one" and an indefinite article such as "a" or "an" (for example, "a" and / or "an" should typically be interpreted as meaning "at least one" or "one or more"). The same applies to the use of the definite article used to introduce a claim enumeration.

[0151] In addition, even when a claim enumeration introducing specific numbers is explicitly enumerated, a person skilled in the art will recognize that such an enumeration should typically be interpreted as meaning at least the number enumerated (for example, a literal enumeration of "two recitations" without other modifiers typically means at least two enumerations or two or more). Furthermore, in cases where a notation similar to "at least one of A, B, and C, etc." is used, such a construct is generally intended in a sense that a person skilled in the art will understand (for example, "a system having at least one of A, B, and C" would, but would not be limited, include systems having A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B and C together, etc.). In cases where notation similar to "at least one of A, B, or C, etc." is used, such constructs are generally intended in a sense that a person skilled in the art would understand the notation to mean (for example, "a system having at least one of A, B, and C" would, but would not be limited, include systems having A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B and C together). Typically, it will be further understood by a person skilled in the art that disjunctive words and / or phrases presenting two or more alternative terms, whether in a description, claim, or drawing, should be understood to assume the possibility of including one of the terms, either of the terms, or both, unless the context indicates otherwise.For example, the phrase "A or B" would typically be understood to include the possibilities of "A" or "B" or "A and B".

[0152] Those skilled in the art will understand that, with respect to the attached claims, the operations enumerated therein may generally be performed in any order. Furthermore, while various operation flowcharts are presented in a certain sequence, it should be understood that various operations may be performed in other orders or in parallel than those illustrated. Examples of such alternative orderings may include overlapping, interspersing, interrupting, reordering, incremental, preparatory, supplementary, simultaneous, reverse, or other variant orderings, unless the context indicates otherwise. Moreover, terms such as “responsive to,” “related to,” or other past tense adjectives are generally not intended to exclude such variants, unless the context indicates otherwise.

[0153] It is worth noting that any reference to “one aspect,” “an aspect,” “an exemplification,” “one exemplification,” and their equivalents means that a particular feature, structure, or characteristic described in relation to an aspect is included in at least one aspect. Therefore, the expressions “in one aspect,” “in an aspect,” “in an exemplification,” and “in one exemplification” in various places throughout this specification do not necessarily all refer to the same aspect. Furthermore, a particular feature, structure, or characteristic may be combined in any preferred manner in one or more aspects.

[0154] Any patent application, patent, non-patent publication, or other disclosure material referenced herein and / or enumerated in any application data sheet is incorporated herein by reference to the extent that the incorporated material does not conflict with this specification. Accordingly, to the extent necessary, disclosures expressly stated herein take precedence over any conflicting material incorporated herein by reference. Any material or any part thereof that is considered to be incorporated herein by reference but conflicts with existing definitions, descriptions, or other disclosure materials stated herein will be incorporated only to the extent that no conflict arises between the incorporated material and the existing disclosure material.

[0155] In summary, numerous benefits resulting from adopting the concepts described herein are described. One or more of the foregoing descriptions are presented for illustrative and explanatory purposes. It is not intended to be exhaustive or limiting to the precise forms disclosed. Modifications or variations are possible in light of the foregoing teachings. One or more forms have been selected and described to illustrate the principle and practical applications, thereby enabling those skilled in the art to utilize various forms and to consider various modifications suitable for specific uses. The claims submitted with this specification are intended to define the overall scope.

Claims

1. It is a method, Magnetic resonance imaging (MRI) system captures magnetic field B ０ The first field map is generated, and the MRI system is An array of permanent magnets in a modified Halbach dome array, the array having one open end sized to receive a patient's head, the array being in the magnetic field B ０ An array of permanent magnets configured to generate, A shim tray and a slot configured to receive shim magnets The first field map is provided with a magnetic field corresponding to each of the empty slots, Applying the first field map to a genetic algorithm to determine a first set of slots for receiving a first set of shim magnets, The first set of shim magnets is arranged within the first set of slots, The aforementioned magnetic field B ０ The means of generating a second field map, wherein the second field map corresponds to the magnetic field based on the first set of shim magnets arranged in the first set of slots, Applying the second field map to the genetic algorithm to determine a second set of slots for receiving a second set of shim magnets, wherein the second set of slots is different from the first set of slots. The second set of shim magnets is arranged within the second set of slots. Methods that include...

2. The method according to claim 1, wherein the size of the first set of shim magnets is the same as the size of the second set of shim magnets.

3. The method according to claim 1, wherein the size of the first set of shim magnets is different from the size of the second set of shim magnets.

4. Determining a set of empty slots in the shim tray, wherein the set of empty slots consists of slots excluded from the first set of slots. Constraining the genetic algorithm to consider only the set of empty slots for inclusion of the said slots into the second set, The method according to claim 1, further comprising:

5. The method according to claim 1, wherein applying the first field map to the genetic algorithm to determine a first set of slots for receiving the first set of shim magnets further comprises determining, for each of the first set of slots, a configuration for arranging the corresponding one of the shim magnets in the slot, wherein the configuration is selected from a set of possible configurations, the set of possible configurations includes a first configuration and a second configuration.

6. In the first configuration described above, the corresponding shim magnet is located in the magnetic field B ０ A first magnetic field is generated that is substantially aligned with the direction of the second configuration, and in the second configuration, the corresponding shim magnet is the magnetic field B ０ The method according to claim 5, which generates a second magnetic field that is substantially opposite to the direction of the first magnetic field.

7. The method according to claim 6, wherein the set of possible configurations further includes a second configuration, a third configuration, a fourth configuration, a fifth configuration, a sixth configuration, a seventh configuration, and an eighth configuration.

8. The method according to claim 5, wherein in the first configuration, the poles of the shim magnet are oriented in a first direction, and in the second configuration, the poles of the shim magnet are oriented in a second direction opposite to the first direction.

9. The method of claim 5, wherein applying the second field map to the genetic algorithm to determine a second set of slots for receiving the second set of shim magnets further comprises determining, for each of the second set of slots, a configuration for arranging the corresponding one of the shim magnets in the slot, the configuration being selected from a set of possible configurations.

10. The method according to claim 1, wherein each of the slots is configured to receive at least an end portion of a shim magnet, and each of the slots is configured to receive an end portion of the same size.

11. The method according to claim 1, wherein applying the first field map to the genetic algorithm to determine a first set of slots for receiving the first set of shim magnets includes determining the intensity of the corresponding one of the shim magnets for each of the first set of slots.

12. The method according to claim 11, wherein determining the strength of the corresponding one of the shim magnets for each of the first set of shim tray slots includes determining whether to provide a first shim magnet having a first strength or a second shim magnet having a second strength.

13. The method according to claim 12, wherein the first shim magnet having the first strength includes an N42 magnet, and the second shim magnet having the second strength includes an N52 magnet.

14. Placing the first set of shim magnets within the first set of slots further includes arranging the first shim magnets, each having a first strength, The method according to claim 1, wherein arranging the second set of shim magnets within the second set of slots includes arranging a second shim magnet having a second strength, wherein the second strength is weaker than the first strength.

15. A system for magnetic resonance imaging (MRI), An array of permanent magnets in a modified Halbach array forming one open end, wherein the open end is sized to receive a patient's head, and the array of permanent magnets emits a low-field-intensity magnetic field B toward an object of interest located within the field of view. ０ An array of permanent magnets configured to generate, A shim tray that can be positioned within the array of permanent magnets, the shim tray comprising an array of shim tray slots, the shim tray comprising an opening sized to receive the patient's head, each of the shim tray slots configured to receive shim magnets in a first configuration and a second configuration, in the first configuration the polarity of the shim magnets is oriented in a first direction, and in the second configuration the polarity of the shim magnets is oriented in a second direction opposite to the first direction, and A system equipped with these features.

16. The array of permanent magnets comprises primary permanent magnets, and the system further comprises a plurality of shim magnets disposed within a portion of the shim tray slot, wherein the plurality of shim magnets are generated by the magnetic field B produced by the permanent magnets. ０ The system according to claim 15, further comprising a secondary permanent magnet positioned relative to the primary permanent magnet to increase uniformity.

17. The system according to claim 16, wherein each of the shim tray slots is configured to receive at least an end portion of a shim magnet, and each of the shim tray slots is configured to receive the end portion of the same size.

18. The aforementioned multiple shim magnets are A first shim magnet that defines the first magnetic field strength, A second shim magnet that defines the second magnetic field strength and The system according to claim 17, including the system described in claim 17.

19. The system according to claim 16, wherein at least one of the plurality of shim magnets is arranged in the second configuration.

20. The system according to claim 16, wherein the plurality of shim magnets are arranged in fewer than all of the shim tray slots.

21. It is a method, Magnetic resonance imaging (MRI) system captures magnetic field B ０ The MRI system generates a field map of the MRI system, An array of permanent magnets in a modified Halbach dome array, the array comprising one open end dimensioned to receive a patient's head, the array configured to generate the magnetic field B ０ and an array of permanent magnets configured to generate the magnetic field B A shim tray and a slot configured to receive shim magnets To be equipped with, Repeatedly, The aforementioned field map is applied to a genetic algorithm to determine the set of slots for receiving the set of shim magnets, The set of shim magnets is placed within the set of slots, The aforementioned magnetic field B ０ The next field map is generated, wherein the next field map corresponds to the magnetic field based on the set of shim magnets arranged within the set of slots. To either perform the next iteration based on the following field map, or to decide not to perform the next iteration based on the following field map. By doing so, the MRI system is shimmed. Methods that include...

Images