Active shimming for low-field magnetic resonance imaging

Abstract

This disclosure provides various apparatuses, systems, and methods for active shimming in MRI systems. For example, the MRI system may include a first gradient coil, a second gradient coil, a third gradient coil, and a permanent magnet. The permanent magnet is configured to generate a magnetic field B0 with a low field strength. A method for an active shimming MRI system may include a set of candidate shimming configurations having a first current value associated with the first gradient coil, a second current value associated with the second gradient coil, and a third current value associated with the third gradient coil. The method further includes applying a pulse sequence to each candidate shimming configuration; acquiring a magnetic resonance (MR) signal for each pulse sequence; determining a signal bandwidth for each MR signal based on the frequency domain of the MR signal; and specifying a shimming configuration for the MRI system.

Description

[0001] Cross-referencing This application claims the benefit of U.S. Patent Application No. 18 / 459,712, filed September 1, 2023, the entire contents of which are incorporated herein by reference. Background Technology

[0002] This disclosure relates to magnetic resonance imaging (MRI), medical imaging, medical interventions, and surgical interventions. MRI systems typically comprise large, complex machines that generate significantly high magnetic fields, posing substantial limitations on the feasibility of certain surgical interventions. These limitations may include restricted physical access to the patient by the surgeon and / or surgical robot and / or limitations on the use of certain electrical and mechanical components near the MRI scanner. Such limitations are inherent in the basic design of many existing systems and are difficult to overcome. Summary of the Invention

[0003] According to one aspect, this disclosure provides an active shimming method for a magnetic resonance imaging (MRI) system. The MRI system includes a first gradient coil, a second gradient coil, a third gradient coil, and a permanent magnet. The permanent magnet is configured to generate a magnetic field B0 with a low field strength. The method includes selecting a set of candidate shimming configurations. The set of candidate shimming configurations includes a first current value associated with the first gradient coil from a first current value range, a second current value associated with the second gradient coil from a second current value range, and a third current value associated with the third gradient coil from a third current value range. The method further includes applying a pulse sequence to each candidate shimming configuration; acquiring a magnetic resonance (MR) signal for each pulse sequence; determining a signal bandwidth for each MR signal based on the frequency domain of the MR signal; and specifying a shimming configuration for the MRI system. The shimming configuration is based on the candidate shimming configuration corresponding to the narrowest signal bandwidth of the MR signal in the set of candidate shimming configurations.

[0004] According to another aspect, this disclosure provides a system. The system includes a permanent magnet array, a radio frequency (RF) coil assembly, a first gradient coil, a second gradient coil, a third gradient coil, and control circuitry including a processor and a memory. The permanent magnet array is configured to generate a low-intensity magnetic field B0 toward an object of interest located within a field of view. The RF coil assembly includes an array of RF coils locatable around the object of interest in the field of view, the RF coil array being configured to acquire magnetic resonance signals. The first gradient coil is configured to modify the magnetic field B0 in a first direction based on receiving a first current. The second gradient coil is configured to modify the magnetic field B0 in a second direction based on receiving a second current. The third gradient coil is configured to modify the magnetic field B0 in a third direction based on receiving a third current. The memory stores instructions executable by the processor for applying the first current, the second current, and the third current according to a predetermined shimming configuration of the system.

[0005] According to another aspect, this disclosure provides a method for operating a magnetic resonance imaging (MRI) system. The MRI system includes a radio frequency (RF) coil, a first gradient coil, a second gradient coil, a third gradient coil, and a permanent magnet. The permanent magnet is configured to generate a magnetic field B0 with a low field strength. The method includes modifying the magnetic field B0 by simultaneously applying a first baseline current to the first gradient coil, a second baseline current to the second gradient coil, and a third baseline current to the third gradient coil. The method further includes applying an excitation pulse to an object of interest through the RF coil; generating a gradient field by the first gradient coil by applying a current greater than the first baseline current to the first gradient coil; and detecting a magnetic resonance (MR) signal induced by the applied excitation pulse through the RF coil. Attached Figure Description

[0006] The various aspects of the organization and operation methods described herein, as well as their further purposes and advantages, can be best understood by referring to the following description in conjunction with the accompanying figures.

[0007] Figure 1 An assembly of an MRI scanning system according to at least one aspect of the present disclosure is depicted, the assembly including a dome-shaped housing of a magnetic array surrounding a region of interest therein, and further depicting the dome-shaped housing positioned for receiving at least a portion of the head of a patient lying on a table into the region of interest.

[0008] Figure 2 It is for use with at least one aspect of this disclosure Figure 1 A perspective view of an alternative dome-shaped housing for a magnetic array used in MRI scanning systems, wherein an access hole is defined in the dome-shaped housing.

[0009] Figure 3 It is for use with at least one aspect of this disclosure Figure 1 A perspective view of an alternative dome-shaped housing for a magnetic array used in MRI scanning systems, wherein access holes and adjustable gaps are defined within the dome-shaped housing.

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

[0011] Figure 5 It is based on at least one aspect of this disclosure Figure 4 A cross-sectional view of the dome-shaped shell.

[0012] Figure 6 A control schematic diagram of an MRI system according to at least one aspect of this disclosure is depicted.

[0013] Figure 7 This is a flowchart describing a method for obtaining imaging data from an MRI system according to at least one aspect of this disclosure.

[0014] Figure 8 An MRI scanning system and a robotic system according to at least one aspect of this disclosure are described.

[0015] Figure 9 This is a flowchart describing an active shimming method for an MRI system according to at least one aspect of the present disclosure.

[0016] Figure 10 An example sampling strategy for selecting candidate shimming configurations according to at least one aspect of this disclosure is shown.

[0017] Figure 11 An example pulse sequence diagram for acquiring MR signals for candidate shimming configurations according to at least one aspect of the present disclosure is shown.

[0018] Figure 12 This is a graph showing an example spectrum of an MR signal acquired without active shimming.

[0019] Figure 13 This is a graph showing an example spectrum of the MR signal acquired during active shimming.

[0020] Figure 14 A comparison of example MR images acquired with and without active shimming is depicted.

[0021] Figure 15A comparison of example MR images acquired with and without active shimming is depicted.

[0022] Figure 16 A comparison of example MR images acquired with and without active shimming is depicted.

[0023] Figure 17 This is a flowchart describing a method of using an active shimming MRI system according to at least one aspect of this disclosure.

[0024] In the various views, the corresponding reference characters represent the corresponding components. The examples listed herein illustrate various disclosed implementations and are one form; such examples should not be construed as limiting its scope in any way. Detailed Implementation

[0025] The applicant of this application also owns the following patent applications, each of which is incorporated herein by reference in its entirety: - International patent application number PCT / US2022 / 72143, filed on May 5, 2022, entitled NEURAL INTERVENTIONAL MAGNETIC RESONANCEIMAGING APPARATUS; - U.S. Patent Application No. 18 / 057,207, filed on November 19, 2022, entitled 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, entitled MODULARIZED MULTI-PURPOSE MAGNETICRESONANCE PHANTOM; - U.S. Patent Application No. 18 / 147,542, filed on December 28, 2022, entitled INTRACRANIAL RADIO FREQUENCY COIL FOR INTRAOPERATIVE MAGNETIC RESONANCE IMAGING; - U.S. Patent Application No. 18 / 147,556, filed December 28, 2022, entitled "Deep Learning Super-Resolution Training for Ultra Low-Field Magnetic Resonance Imaging"; and - U.S. Patent Application No. 18 / 153,111, filed January 11, 2023, entitled "ACCELERATING MAGNETIC RESONANCE IMAGINGUSING PARALLEL IMAGING AND ITERATIVE IMAGE RECONSTRUCTION"; and - U.S. Patent Application No. 18 / 153,175, filed on January 11, 2023, entitled FAST T2-WEIGHTED AND DIFFUSION-WEIGHTEDCHIRPED-CPMG SEQUENCES; - U.S. Provisional Patent Application No. 63 / 488,102, filed March 2, 2023, entitled "A METHOD OF MERGING A CO-OPERATIVE MR-COMPATIBLE ROBOT AND A LOW-FIELD PORTABLE MRI SYSTEM"; and - U.S. Patent Application No. 18 / 450,010, filed on August 15, 2023, entitled ITERATIVE SHIMMING FOR LOW-FIELD HEAD-OPTIMIZED MRI.

[0026] Before explaining the various aspects of interventional magnetic resonance imaging apparatus in detail, it should be noted that the illustrative examples are not limited in application or use to the details of the construction and arrangement of the components shown in the figures and description. The illustrative examples may be implemented or combined in other aspects, variations, and modifications, and may be practiced or performed in various ways. Furthermore, unless otherwise stated, the terminology and expressions used herein are chosen for the convenience of describing the illustrative examples to the reader, and not for the purpose of limiting the illustrative examples. Moreover, it will be understood that one or more aspects, expressions, and / or examples described below may be combined with any one or more other aspects, expressions, and / or examples described below.

[0027] Various aspects involve neurointerventional magnetic resonance imaging (MRI) devices that allow for the integration of surgical intervention and guidance with MRI. This includes obtaining consent for physical access to areas surrounding the patient, as well as access to the patient's head using one or more access ports. Additionally, neurointerventional MRI devices can allow for the use of robotic-guided tools and / or conventional surgical instruments. In various situations, neurointerventional MRI can be used during surgery to obtain scans of the patient's head and / or brain during surgical interventions, such as procedures like brain biopsies or neurosurgery.

[0028] Figure 1 An MRI scanning system 100 is depicted, comprising a dome-shaped housing 102 configured to receive a patient's head. The dome-shaped housing 102 may further include at least one access hole configured to allow access to the patient's head for neurological intervention. The space within the dome-shaped housing 102 forms a region of interest for the MRI scanning system 100. Target tissue within the region of interest is subjected to a magnetizing field / pulse, as further described herein, to obtain imaging data representative of the target tissue.

[0029] For example, a patient can be positioned such that his / her head is located within a region of interest within the dome-shaped housing 102. The brain can be completely positioned within the dome-shaped housing 102. In such cases, to facilitate intracranial interventions (e.g., neurosurgery) in conjunction 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.

[0030] MRI scanning system 100 may include an auxiliary cart (e.g., see...) Figure 6 The MRI scanning system 100 may also include a magnet cart 540 that houses certain conventional MRI electrical and electronic components, such as a computer, programmable logic controller, power distribution unit, and amplifier. The magnet cart may also hold the dome-shaped housing 102, gradient coils, and / or transmit coils, as further described herein. Additionally, in various cases, the magnet cart may be attached to a receive coil. Main Reference Figure 1 The dome-shaped housing 102 may further include an RF transmitting coil, a gradient coil 104 (depicted on its exterior), and a shim magnet 106 (depicted on its interior). Alternative configurations of the gradient coil 104 and / or the shim magnet 106 are also contemplated. In various cases, the shim magnet 106 may be adjustably positioned in a shim tray within the dome-shaped housing 102, which allows a technician to finely configure the magnetic flux density of the dome-shaped housing 102.

[0031] Various structural housings used to receive a patient's head and enable neurointervention can be used with MRI scanning systems, such as MRI scanning system 100. In one aspect, MRI scanning system 100 can be equipped with alternative housings, such as dome-shaped housing 202. Figure 2 ) or configured to form a two-part housing 302 in a dome shape ( Figure 3 The dome-shaped housing 202 defines a plurality of access holes 203; the two-part housing 302 also defines a plurality of access holes 303, and further includes an adjustable gap 305 between the two parts of the housing.

[0032] In various cases, housings 202 and 302 may include a binder 308 (such as epoxy resin), for example, binder 308 holding a plurality of magnetic elements 310 in a fixed position. The plurality of magnetic elements 310 may be bonded to structural housing 312, for example, such as a plastic substrate. In various aspects, binder 308 and structural housing 312 may be non-conductive or antimagnetic materials. Main Reference Figure 3 The two-part housing 302 includes two structural housings 312. In various aspects, the structural housing for receiving the patient's head may be formed from more than two sub-parts. An access hole 303 in the structural housing 312 provides direct access to the patient's head and is not obstructed by the structural housing 312, the bonding agent 308, or the magnetic element 310. For example, the access hole 303 may be positioned within an open space of the housing 302.

[0033] Neurointerventional MRI devices have many possible configurations that can improve accessibility to surgical interventions. Many configurations are built on two main designs, often referred to as the Halbach cylinder and Halbach dome as described in the following articles: Cooley et al. (e.g., Cooley, CZ; Haskell, MW; Cauley, SF; Sappo, C.; Lapierre, CD; Ha, CG; Stockmann, JP; and Wald, LL (2018), Design of sparse Halbach magnet arrays for portable MRI using agenetic algorithm, IEEE transactions on magnetics , 54 (1), 5100112. Published by Cooley et al. in 2018. IEEE transactions on magnetics , 54 (1) The article “Design of sparse Halbach magnet arrays for portable MRI using a genetic algorithm” on 5100112 is incorporated herein by reference in its entirety.

[0034] In various cases, the dome-shaped housing of an MRI scanning system (such as system 100) may, for example, include a Halbach dome, which defines the dome shape and is configured based on several factors, including the strength of the main magnetic field B0, field size, field homogeneity, device size, device weight, and access for neurological intervention on the patient. In various aspects, the Halbach dome includes an outer radius and an inner radius at the base of the dome. The 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 and inner radii as the base of the dome and extends from the base of the dome by a predetermined length with a constant radius. In another aspect, the elongated cylindrical portion includes an outer and inner radius different from the base of the dome (e.g., see...). Figure 2 and Figure 3 In such cases, the different outer and inner radii of the slender cylindrical portion can be merged with the base radius in the transition region.

[0035] Figure 4 An exemplary Halbach dome 400 of an MRI scanning system (such as system 100) according to at least one aspect of the present disclosure is illustrated. For example, the exemplary Halbach dome 400 defines an access hole in the form of an aperture or access hole 403, wherein the dome 400 is configured to receive the head and brain B of a patient P in a region of interest therein, and the access hole 403 is configured to allow access to the patient P to perform neurointerventions using medical devices and / or robotically controlled surgical instruments. The Halbach dome 400 may have a single access hole 403 constructed at the top side 418 of the dome 400, which allows access to the top of the skull while minimizing the influence of magnetic fields. Additionally or alternatively, the dome 300 may be configured with a plurality of access holes around a structure 416 of the dome 400, such as Figure 2 and Figure 3 As shown.

[0036] Approaching the diameter D of hole 403 孔 It can be small (e.g., about 2.54 cm) or very large (basically the outer diameter of a dome of 400 mm). 外 (Diameter). For example, as the access hole 403 becomes larger, the dome 400 begins to resemble a Halbach cylinder. The access hole 403 is not limited to being located at the apex of the dome 400. The access hole 403 can be placed anywhere on the surface of the dome 400 or on the structure 416. In various cases, the entire dome 400 can be rotated such that the access hole 403 can be co-located with the desired physical location on the patient P.

[0037] Figure 5 The relative dimensions of Halbach Dome 400 are depicted, including the diameter D near the aperture 403.孔 The length L of dome 400 and the outer radius r of dome 400 外 and inner radius r 内 The Halbach dome 400 includes multiple magnetic elements arranged in a Halbach array and forming a magnetic assembly. These magnetic elements can be formed by the outer radius r within structure 416 or its housing. 外 and inner radius r 内 Enclosed. In one respect, the example size can be defined as: r 内 =19.3 cm; r 外 =23.6 cm; L=38.7 cm; and 2.54 cm ≤ D< 19.3 cm.

[0038] Based on the example dimensions above, the Halbach dome 400 with proximity aperture 403 can be configured to have a magnetic flux density B0 of approximately 72 mT and a total mass of approximately 35 kg. It will be understood that dimensions can be selected based on specific applications to achieve the desired magnetic flux density B0, the total weight of the Halbach dome 400 and / or the magnet trolley, and the geometry of the neural intervention proximity aperture 403.

[0039] In various aspects, the Halbach dome 400 can be configured to define a plurality of access holes 403 placed around a structure 416 of the dome 400. These plurality of access holes 403 can be configured to allow access to the patient’s head and brain B using tools (e.g., surgical instruments) and / or surgical robots.

[0040] In various respects, the proximity hole 403 can be adjustable. An adjustable configuration provides the ability to adjust the proximity hole 403 using a motor, mechanical aids, or a manual system with a mechanical iris configuration, for example, to adjust the diameter D of the proximity hole 404. 孔 This will allow the dome to be configured for imaging scans without access hole 403, and then the configuration of the dome 400 and its mechanical iris to include access hole 403, thus enabling surgical intervention.

[0041] The Halbach dome and its magnetic array for facilitating neural intervention are further described in International Patent Application No. PCT / US2022 / 72143 entitled “NEURAL INTERVENTIONAL MAGNETIC RESONANCE IMAGING APPARATUS”, filed May 5, 2022 (which is incorporated herein by reference in its entirety).

[0042] Now for reference Figure 6A schematic diagram of an MRI system 500 is shown. For example, an MRI scanning system 100 ( Figure 1 Various dome-shaped housings and their magnetic arrays, as further described herein, can be incorporated into the MRI system 500. For example, the MRI system 500 includes a housing 502, which may resemble the dome-shaped housing 102 in many respects. Figure 1 ), 202 Figure 2 ) and / or 302 ( Figure 3 The housing 502 is dome-shaped and configured to form a region 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.

[0043] The housing 502 includes a magnet assembly 548 having a plurality of magnets (e.g., a Halbach magnet array) disposed therein. In various aspects, a main magnetic field B0 generated by the magnet assembly 548 extends into a field of view 552, which contains an object (e.g., a patient's head) imaged by the MRI system 500.

[0044] The MRI system 500 also includes an RF transmit / receive coil 550. The RF transmit / receive coil 550 is combined into an integrated transmit-receive (Tx / Rx) coil. In other cases, the RF transmit coil may be separate from the RF receive coil. For example, the RF transmit coil may be integrated into a housing 502, and the RF receive coil may be positioned within the housing 502 to obtain imaging data.

[0045] The housing 502 also includes one or more gradient coils 504 configured to generate a gradient field to facilitate imaging of an object in a field of view 552 generated by the magnet assembly 548, for example, which is enclosed by a dome-shaped housing and an array of dome-shaped magnetic elements therein. A shimming tray adapted to receive a shimming magnet 506 may also be incorporated into the housing 502.

[0046] During imaging, the main magnetic field B0 extends into the field of view 552. The direction of the effective magnetic field (B1) changes in response to RF pulses emitted by the RF transmit / receive coil 550 and the associated electromagnetic field. For example, the RF transmit / receive coil 550 can be configured to selectively transmit RF signals or pulses to objects in the field of view 552, such as tissue from a patient's brain. These RF pulses can alter the effective magnetic field experienced by the sample tissue via spin.

[0047] Housing 502 communicates with auxiliary trolley 530, which is configured to supply power to housing 502 and send / receive control signals to / from housing 502. Auxiliary trolley 530 includes a power distribution unit 532, a computer 542, a spectrometer 544, a transmit / receive switch 545, an RF amplifier 546, and a gradient amplifier 558. In various cases, housing 502 may communicate with multiple auxiliary trolleys, and each trolley may support one or more of the power distribution unit 532, computer 542, spectrometer 544, transmit / receive switch 545, RF amplifier 546, and / or gradient amplifier 558.

[0048] Computer 542 communicates with spectrometer 544 via signals, and computer 542 is configured to send and receive signals between computer 542 and spectrometer 544. When an object in field of view 552 is excited by an RF pulse from RF transmit / receive coil 550, the precession of the object generates an induced current or MR current, which is detected by RF transmit / receive coil 550 and sent to RF preamplifier 556. RF preamplifier 556 is configured to amplify or amplify the excitation data signals and send them to spectrometer 544. Spectrometer 544 is configured to send excitation data to computer 542 for storage, analysis, and image construction. For example, computer 542 is configured to combine multiple stored excitation data signals to create an image. In various cases, computer 542 communicates with at least one database 562 of stored reconstruction algorithm 564 and / or pulse sequence 566. Computer 542 is configured to use the reconstruction algorithm to generate MR image 568.

[0049] The signal can also be relayed from the spectrometer 544 via the RF power amplifier 546 and the transmit / receive switch 545 positioned between the spectrometer 544 and the RF power amplifier 546 to the RF transmit / receive coil 550 in the housing 502. The signal can also be relayed from the spectrometer 544 via the gradient power amplifier 558 to the gradient coil 560 in the housing 502. For example, the RF power amplifier 546 is configured to amplify the signal and send it to the RF transmit coil 560, and the gradient power amplifier 558 is configured to amplify the gradient coil signal and send it to the gradient coil 560.

[0050] In various cases, the MRI system 500 may include a noise cancellation coil 554. For example, the auxiliary cart 530 and / or the computer 542 may communicate signalically with the noise cancellation coil 554. In other cases, the noise cancellation coil 554 may be optional. For example, some MRI systems disclosed herein may not include supplemental / auxiliary RF coils for detecting and eliminating electromagnetic interference (i.e., noise).

[0051] Figure 7 A flowchart depicting the process 570 for acquiring MRI images is shown. This flowchart, for example, can be implemented by an MRI system 500. In various cases, at box 572, the target object (e.g., a portion of a patient's anatomy) is positioned within a main magnetic field B0 of a region of interest (e.g., region of interest 552), such as within the dome-shaped housing (e.g., magnet assembly 548) of various MRI scanners further described herein. The main magnetic field B0 is configured to magnetically polarize the hydrogen protons (¹H protons) of the target object (e.g., all organs and tissues) and is referred to as the net longitudinal magnetization M0. It is proportional to the proton density (PD) of the tissue and develops exponentially over time, with a time constant referred to as the longitudinal relaxation time T1 of the tissue. For example, the T1 value of a single tissue depends on many factors including its microstructure, its water and / or lipid content, and the strength of the polarizing magnetic field. For these reasons, the T1 value of a given tissue sample depends on age and health status.

[0052] At box 574, a time-varying oscillating magnetic field B1, i.e., an excitation pulse, is applied to the magnetically polarized target object using an RF coil (e.g., RF transmit / receive coil 550). The carrier frequency of the pulse B1 field is set to the resonant frequency of the 1H protons, which causes the longitudinal magnetization to flip from its equilibrium longitudinal direction, resulting in a rotating magnetization vector. Depending on the flip angle used, this vector magnetization can typically have both transverse and longitudinal magnetization components. Common B1 pulses include inversion pulses, or 180-degree pulses and 90-degree pulses. A 180-degree pulse reverses the magnetization direction of the 1H protons along the longitudinal axis. A 90-degree pulse rotates the magnetization of the 1H protons by 90 degrees, so that the magnetization lies in the transverse plane. The MR signal is proportional to the transverse component of the magnetization and is a time-varying current detected by a suitable RF coil. These MR signals decay exponentially over time, and their time constant is called the transverse relaxation time T2, which, for example, also depends on the microstructure, water / lipid content, and the strength of the magnetic field used.

[0053] At box 576, the MR signal is spatially encoded by exposing the target object to an additional magnetic field (referred to as a gradient field) generated by a gradient coil (e.g., gradient coil 560). The gradient field, which varies linearly in space, is applied in pulses over short time intervals and varies spatially in each direction. The end result is the generation of multiple spatially encoded MR signals, which are detected at box 577 and can be reconstructed to form an MR image depicting a slice of the examined object. An RF receiving coil (e.g., RF transmit / receive coil 550) can be configured to detect the spatially encoded RF signals. The slice can be oriented in a transverse, sagittal, coronal, or any oblique plane.

[0054] At box 578, the spatially encoded signal of each slice of the scanned area is digitized and mathematically spatially decoded using a computer reconstruction program (e.g., computer 542) to generate an image depicting the internal anatomical structure of the examined object. In various cases, the reconstruction program can inversely transform the spatially encoded data (k-space data) into geometrically decoded data using an (inverse) Fourier transform.

[0055] Figure 8 A graphical illustration is provided of a robotic system 680 that can be used for neurointerventions utilizing 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 similar to the MRI system 500 and may include a dome-shaped housing with access holes and a magnetic array, as further described herein. For example, the MRI system 500 may include one or more access holes defined in a Halbach magnet array within a permanent magnet assembly to provide access to one or more anatomical sites of a patient being imaged during a medical procedure. In various configurations, the robotic arm and / or tools of the surgical robot 682 are configured to extend through the access holes in the permanent magnet assembly to reach the patient or target site. Each access hole may provide access to the patient and / or surgical site. For example, in the case of multiple access holes, the multiple access holes may allow access from different directions and / or proximal positions.

[0056] According to various embodiments, the robotic system 680 is configured to be placed outside the MRI system 600. For example... Figure 8 As shown, 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 robot arm sections, including a hollow shaft 686 and an end effector 688. The hollow shaft 686 and the end effector 688 are configured to move, rotate, and / or rotate within a variety of motion ranges via one or more motion controllers 690. Figure 8 The double-headed curved arrows in the diagram represent exemplary rotational movements generated by the motion controller 690 at various joints in the robot arm 684.

[0057] According to various embodiments, the robotic arm 684 of the robotic system 682 is configured to approach various anatomical sites of interest via or around the MRI scanning system 600. According to various embodiments, the access aperture is designed to accommodate the size of the robotic arm 684. For example, the access aperture defines a circumference configured to accommodate the robotic arm 684, the hollow shaft 686, and the end effector 688 passing through it. In various cases, the robotic arm 684 is configured to approach various anatomical sites of a patient from around one side of the magnetic imaging device 600. For example, the hollow shaft 686 and / or the end effector 688 may be adapted to receive robotic tools 692, such as biopsy needles with cutting edges 694, for collecting biopsy samples from the patient.

[0058] The reader will understand that the robot system 682 can be used in conjunction with various dome-shaped and / or cylindrical magnetic housings described further herein. Furthermore, Figure 8 The robotic system 682 and robotic tool 692 described herein are exemplary. Alternative robotic systems can be used in conjunction with the various MRI systems disclosed herein. Furthermore, handheld surgical instruments and / or additional imaging devices (e.g., endoscopes) and / or systems can also be used in conjunction with the various MRI systems disclosed herein.

[0059] In various aspects of this disclosure, the MRI systems described herein may include low-field MRI (LF-MRI) systems. In such cases, for example, the main magnetic field B0 generated by the permanent magnet assembly may be less than or equal to 1.0 T, such as between 0.1 T and 1.0 T. In some cases, the MRI systems described herein may include ultra-low-field MRI (ULF-MRI) systems. In such cases, for example, the main magnetic field B0 generated by the permanent magnet assembly may be less than or equal to 0.1 T, such as between 0.03 T and 0.1 T.

[0060] For example, high magnetic fields (such as those above 1.0 T) can impede the use of certain electrical and mechanical components near an MRI scanner. For instance, the presence of surgical instruments and / or surgical robotic components, including those made of metal (especially ferrous metals), can be hazardous near high magnetic fields, as such tools may be drawn towards the magnetization source. Furthermore, high magnetic fields typically require specially designed rooms with additional precautions and shielding measures to limit magnetic interference. Despite the limitations of high-field MRI systems, low-field and ultra-low-field MRI systems present various challenges in acquiring high-quality images with sufficient resolution to achieve desired imaging goals.

[0061] Compared to higher-field MRI systems, LF-MRI and ULF-MRI systems typically define relatively poor overall magnetic field homogeneity. For example, as further described herein, the dome-shaped housing for the magnet array may include a Halbach permanent magnet array, which, in various aspects of this disclosure, generates a magnetic field B0 with homogeneity between 1,000 ppm and 10,000 ppm in the region of interest.

[0062] To improve the homogeneity of the main magnetic field B0, MRI systems typically employ various shimming techniques. Improving the homogeneity of the main magnetic field B0 results in a slower MR signal decay time and ultimately improves overall image quality.

[0063] High-field MRI (HF-MRI) systems, such as those employing superconducting magnets capable of generating magnetic fields above 1.0 T, typically utilize passive and / or active shimming techniques. Passive shimming generally involves selectively placing a ferromagnetic object within the MRI system's scan aperture to modify the dominant magnetic field B0, thereby improving field homogeneity. Active shimming typically involves passing a current through dedicated coils within the MRI system to generate a magnetic field that alters the dominant magnetic field B0, thus improving field homogeneity. For example, many HF-MRI systems include an assembly of dedicated superconducting shimming coils (e.g., separate from gradient coils) for active shimming.

[0064] LF-MRI systems can utilize passive shimming techniques. However, due to various challenges, LF-MRI systems may not be able to achieve the active shimming techniques employed in HF-MRI systems. For example, as mentioned above, HF-MRI systems typically include a collection of dedicated shimming coils. Each of the shimming coils in an HF-MRI system is capable of generating a magnetic field that varies uniformly and linearly over a specific region of interest. For HF-MRI systems, the effect of the shimming coils on the main magnetic field B0 is relatively predictable. In contrast, LF-MRI systems typically do not have dedicated coils for active shimming. Furthermore, although LF-MRI systems typically include gradient coils for generating gradient fields during signal acquisition, these gradient fields may be more non-uniform and vary in a more non-linear manner compared to the fields generated by the superconducting shimming coils of an HF-MRI system. Therefore, the effect of the gradient coils on the uniformity of the main magnetic field B0 in an LF-MRI system can be relatively unpredictable. This unpredictability is further exacerbated by activating the gradient coils while effectively stacking the effects of the non-linear gradient fields generated by each gradient coil to modify the main magnetic field B0.

[0065] In several aspects, this disclosure provides an active shimming method for an LF-MRI system. In at least one aspect, the active shimming method for an LF-MRI system can implement additional shimming functionality using the system's gradient coils. For example, the method may include selecting a set of candidate shimming configurations for the gradient coils. Each of the candidate shimming configurations may define different combinations of current values ​​applied to each of the gradient coils respectively. The method may also include testing each of the candidate shimming configurations to identify which candidate shimming configurations improve the homogeneity of the main magnetic field B0, and / or identifying and evaluating the amount or extent of improvement provided by each candidate shimming configuration.

[0066] To achieve a desired candidate shimming configuration during image acquisition (e.g., a candidate shimming configuration that provides the best possible homogeneity of the main magnetic field B0), a baseline current can be applied to each of the gradient coils according to the desired candidate shimming current value. Any gradient field required for image acquisition can be achieved by applying a current to the gradient coil that is incrementally greater than the corresponding baseline current value. Therefore, the active shimming method disclosed herein can be applied using existing gradient coils in LF-MRI systems, while still enabling existing gradient coils to be used to generate imaging gradients. In these cases, a separate coil dedicated to active shimming is not required.

[0067] In some aspects, the active shimming method disclosed herein can be employed using an LF-MRI system comprising three gradient coils. For example, the LF-MRI system may include a first gradient coil for generating a gradient field along a first direction (e.g., along the x-axis perpendicular to the direction of the main magnetic field B0), a second gradient coil for generating a gradient field along a second direction (e.g., along the y-axis perpendicular to the direction of the main magnetic field B0), and a third gradient coil for generating a gradient field along a third direction (e.g., along the z-axis aligned with the direction of the main magnetic field B0). Each of the candidate shimming configurations may include different combinations of current values ​​applied to the first, second, and third gradient coils.

[0068] In some aspects, the active shimming method disclosed herein may include, for example, using a teslameter to test candidate shimming configurations by directly measuring the effect of each candidate configuration on the uniformity of the main magnetic field B0. In other aspects, the active shimming method disclosed herein may include testing candidate shimming configurations by indirectly determining the effect of each candidate configuration on the uniformity of the main magnetic field B0 relative to a phantom excited by a pulse sequence. For example, MR signals may be acquired while implementing each candidate configuration. Typically, when converted to the frequency domain, a narrower MR signal bandwidth is associated with a main magnetic field B0 having higher uniformity. Therefore, the frequency domain bandwidths of the acquired MR signals can be compared to identify the relative effect of the corresponding candidate configuration on the uniformity of the main magnetic field B0. For example, the candidate configuration used to acquire the MR signal with the narrowest frequency domain bandwidth may be considered the candidate configuration that provides the greatest improvement to the uniformity of the main magnetic field B0.

[0069] The active shimming method provided by this disclosure offers numerous benefits. For example, the method disclosed herein enables LF-MRI systems to achieve improved main magnetic field B0 homogeneity and ultimately improved image quality compared to existing systems (e.g., systems that only implement passive shimming). Furthermore, by indirectly determining the relative change in field homogeneity based on the frequency domain bandwidth of the MR signal acquired during the implementation of candidate shimming configurations, the active shimming method disclosed herein can be used to rapidly and efficiently identify feasible candidate shimming configurations (e.g., compared to methods involving direct measurement of field homogeneity). Moreover, by testing multiple different candidate shimming configurations (e.g., via brute-force sampling strategies), the active shimming method disclosed herein can provide a solution to the aforementioned problem concerning the unpredictable effects of the nonlinear gradient field generated by the LF-MRI gradient coil on the main magnetic field B0.

[0070] Figure 9 This is a flowchart describing an active shimming method 9000 for an MRI system according to at least one aspect of this disclosure. In some cases, the active shimming method 9000 may also be used in combination with one or more passive shimming methods. The method 9000 can be used with an LF-MRI system (e.g., the one described herein). Figure 6 The MRI system 500 described above may be used to perform this action. As described above, the MRI system 500 may include a magnet assembly 548 having a plurality of magnets (e.g., a Halbach magnet array) disposed therein for generating a main magnetic field B0 extending to a region of interest 552, which contains an object (e.g., a patient's head) being imaged by the MRI system 500. The MRI system 500 also includes one or more gradient coils 504 configured to generate a gradient field in response to the passage of a current therethrough to drive imaging of an object in the field of view 552.

[0071] Again, the main reference Figure 9 Also refer to Figure 6 According to method 9000, a set 9002 of candidate shimming configurations is selected. Each candidate shimming configuration in the set may include a current value (e.g., setting the current in amperes to apply current to each gradient coil) of each of the gradient coils 504. For example, the gradient coils 504 may include a first gradient coil for generating a gradient field along a first direction (e.g., along the x-axis), a second gradient coil for generating a gradient field along a second direction (e.g., along the y-axis), and a third gradient coil for generating a gradient field along a third direction (e.g., along the z-axis). Each candidate shimming configuration may include the current values ​​of the first, second, and third gradient coils.

[0072] According to some aspects of method 900, selecting the set of candidate shimming configurations 9002 may include selecting a set of first current values ​​spanning a first range, a set of second current values ​​spanning a second range, and a set of third current values ​​spanning a third range. The set of first current values ​​may correspond to a first gradient coil, the set of second current values ​​may correspond to a second gradient coil, and the set of third current values ​​may correspond to a third gradient coil. The set of candidate shimming configurations may be determined based on various combinations of current values ​​derived by selecting one of the first current values, one of the second current values, and one of the third current values.

[0073] For example, Figure 10 An example sampling strategy according to at least one aspect of this disclosure is illustrated, which can be used to select a set of 9002 candidate shim configurations. A set 1002 of first current values ​​1002 for the first gradient coil Gx spans current values ​​Gx1 to Gx2. n The range includes current values ​​Gx1, Gx2…Gx separated by specified intervals within that range. n The set of second current values ​​1004 for the second gradient coil Gy spans current values ​​Gy1 to Gy2. n The range includes current values ​​Gy1, Gy2…Gy separated by specified intervals within that range. n The set of third current values ​​1006 for the third gradient coil Gz spans current values ​​Gz1 to Gz2. n The range includes current values ​​Gz1, Gz2…Gz separated by specified intervals within that range. n .like Figure 10As shown, by forming each possible combination of current values ​​that can be applied to the first gradient coil Gx, the second gradient coil Gy, and the third gradient coil Gz, a set 1008 of candidate shimming configurations can be derived based on the set 1002 of first current values, the set 1004 of second current values, and the set 1006 of third current values. Therefore, the set of candidate shimming configurations selected by method 9002 can include a large number of candidate shimming configurations covering the range of currents to be applied to each gradient coil in various combinations.

[0074] Again, the main reference Figure 9 Also refer to Figure 6 According to method 9000, a set 9004 of magnetic resonance (MR) signals is acquired based on a set of candidate shim configurations. For example, in one aspect, a single MR signal for each candidate shim configuration can be acquired by applying current to gradient coil 504 according to first, second, and third current values ​​specified by the candidate shim configuration and performing a signal acquisition sequence. In another aspect, two or more different MR signals for each candidate shim configuration can be acquired (e.g., via two separate sequences). The two or more different MR signals can be used for averaging purposes, as further explained below. In yet another aspect, more than two different MR signals for each candidate shim configuration can be acquired.

[0075] Figure 11 An example sequence diagram 1100 for acquiring MR signals of a candidate shimming configuration according to at least one aspect of the present disclosure is shown. Sequence diagram 1102 includes a radio frequency pulse sequence 1102, a signal acquisition sequence 1112, a first gradient coil sequence 1122, a second gradient coil sequence 1132, and a third gradient coil sequence 1142. To acquire the MR signal of the candidate shimming configuration, gradient sequences 1122, 1132, and 1142 are executed by applying current to the first, second, and third gradient coils (e.g., gradient coil 504) according to a first current value 1124, a second current value 1134, and a third current value 1144 specified by the first, second, and third current values ​​of the candidate shimming configuration. Figure 11 In the example pulse sequence diagram 1100, current values ​​1124, 1134, and 1144 are each set to zero, corresponding to a candidate shimming configuration without shimming. A candidate shimming configuration with one or more specified non-zero current values ​​for the first, second, or third gradient coil can be executed based on gradient sequences 1122, 1132, and 1142 (having a first current value 1124, a second current value 1134, and / or a third current value 1144 set to non-zero current values ​​for at least a portion of the sequence).

[0076] Still referencing Figure 11The RF pulse sequence 1102 is executed by applying excitation pulses to induce MR signals (e.g., via the transmit / receive coil 550 of the MRI system 500). Figure 11 In a non-limiting aspect, the RF pulse sequence 1102 is a spin-echo sequence with a 90-degree excitation pulse 1104 and a 180-degree refocusing pulse 1106, configured to induce a spin-echo MR signal. As an example, the RF pulse sequence 1102 can define an acquisition time of 0.004 seconds, a repetition time of 0.12 seconds, and a pulse duration of 0.0001 seconds for the excitation pulse 1104 and the refocusing pulse 1106. Alternative timeframes are also considered. In other aspects, any type of pulse sequence configured to induce an MR signal can be implemented. The acquisition sequence 1112 is performed by detecting / receiving the MR signal (e.g., via the transmit / receive coil 550 of the MRI system 500) during the signal acquisition window 1114. The MR signal acquisition shown in sequence diagram 1100 can be repeatedly performed, thereby updating the gradient sequences 1122, 1132, 1142 for each candidate shim configuration until an MR signal is acquired for each candidate shim configuration. In other words, for each candidate shimming configuration, the same pulse sequence can be repeatedly sent and applied.

[0077] Again, the main reference Figure 9 Also refer to Figure 6 According to method 9000, a set of signal bandwidths is determined based on the set of acquired MR signals. For example, the bandwidth of each signal in the set of signal bandwidths can be determined based on the different signals in the acquired signals. Each signal bandwidth can be determined by converting the acquired MR signals to the frequency domain and determining the full width half maximum (FWHM) bandwidth in the frequency domain.

[0078] As described above, two or more distinct MR signals can be acquired for each candidate shim configuration. Therefore, according to several aspects, the signal bandwidth for a given candidate shim configuration can be determined as the average of the MR signal bandwidths acquired for that candidate shim configuration. Using the average MR signal bandwidth acquired for each candidate shim configuration can help eliminate potential outliers in the data and provide a more accurate estimate of the relative impact of the candidate shim configuration on the uniformity of the main magnetic field B0.

[0079] As mentioned above, when converted to the frequency domain, a narrower MR signal bandwidth is generally associated with a main magnetic field B0 having higher uniformity. Therefore, the frequency domain bandwidths of the acquired MR signals can be compared to identify the relative impact of the corresponding candidate configurations on the uniformity of the main magnetic field B0.

[0080] For example, Figure 12Figure 1200 shows an example spectrum 1202 of an MR signal acquired without active shimming; and Figure 13 Graph 1300 shows the curves in accordance with method 9000 ( Figure 9 Example spectrum 1302 of the MR signal acquired with active shimming is shown. By comparing spectrum 1202 and spectrum 1302, it is evident that the FWHM bandwidth 1204 of spectrum 1202 is wider than the FWHM bandwidth 1304 of spectrum 1302. Therefore, this comparison can be used to infer that the shimming configuration achieved when acquiring the MR signal with spectrum 1302 results in improved main magnetic field B0 uniformity compared to the main magnetic field B0 uniformity present when acquiring the MR signal with spectrum 1202 (i.e., without active shimming).

[0081] Again, the main reference Figure 9 See also Figure 6 According to method 9000, a shimming configuration 9008 for an MRI system (e.g., MRI system 500) can be specified based on a set of signal bandwidths determined 9006. Specifying the shimming configuration 9008 for the MRI system may include selecting candidate shimming configurations from a set of candidate shimming configurations and storing the current values ​​(e.g., a first current value, a second current value, and a third current value) of the candidate shimming configurations in the memory of the MRI system (e.g., a database 562). The stored current values ​​can be set as the baseline current values ​​of the gradient coils. For example, from a manufacturing perspective, active shimming can be performed on each phantom of the generated MRI system. Furthermore, based on active shimming, a unique set of baseline current values ​​can be stored in each generated MRI system and implemented therein. Additionally, throughout the lifetime of the MRI system, the active shimming method 9000 can be repeated to update the baseline gradient current values ​​and reset the active shimming system of the MRI system.

[0082] A baseline current value can be applied at any time in an MRI system used for imaging, where any gradient field required for imaging is achieved by applying current to gradient coils at increments greater than the baseline value. If a particular acquisition sequence requires no gradient to be applied, the MRI system can actually apply an incremental gradient during the imaging sequence based on the stored baseline current value. And if a particular acquisition sequence requires a gradient to be applied by applying a specific current value, the system can actually apply the gradient based on the sum of the specific current value and the stored baseline current value.

[0083] In some aspects, the shimming configuration 9008 for the MRI system can be specified based on the candidate shimming configuration corresponding to the narrowest signal bandwidth in the set of signal bandwidths. Therefore, method 9000 may consist of only a single iteration of candidate shimming configuration selection 9002 and MR signal acquisition 9004.

[0084] In other aspects, method 9000 may include multiple iterations of candidate shimming configuration selection 9002 and MR signal acquisition 9004. For example, a set 9006 of signal bandwidths determined for an initial set of candidate shimming configurations can be used as the basis for selecting the next set of candidate shimming configurations for the next iteration of method 9000. This iterative process can be used to fine-tune the current value applied to each gradient coil to further improve the uniformity of the main magnetic field B0.

[0085] In one aspect, to select the next set of candidate shimming configurations for the next iteration of method 9002, an initial set of signal bandwidths can be plotted on a three-dimensional graph, where the three axes of the graph correspond to the ranges selected for the first, second, and third current values, from which the initial set of candidate shimming configurations is derived. The region of the three-dimensional graph showing the narrowest signal bandwidth (a subset of the ranges within the first, second, and third current values) can be used as a new range for selecting the first, second, and third current values ​​(e.g., as per the selection...). Figure 10 The next set of candidate shim configurations is obtained based on the ranges of the first set of current values ​​1002, the second set of current values ​​1004, and the third set of current values ​​1006 discussed in the previous discussion. After selecting the next set of candidate shim configurations 9002, as described above, the next iteration of method 9000 can be continued by acquiring the next set of MR signals 9004 based on the next set of candidate shim configurations and determining the next set of signal bandwidths 9006 based on the next set of MR signals. This iterative process can continue, for example, until one of the candidate shim configurations reaches a desired bandwidth threshold, or until multiple iterations have been completed (e.g., 2 iterations, 3 iterations, 4 iterations, etc.). The shim configuration 9008 for the MRI system can be specified based on the candidate shim configuration corresponding to the narrowest signal bandwidth from the last iteration.

[0086] Figure 17 This is a flowchart describing a method 9000 using an active shimming MRI system according to at least one aspect of this disclosure. Method 9000 can use an LF-MRI system (e.g., the one described herein). Figure 6The MRI system 500 described above may be used to perform this operation. As described above, the MRI system 500 may include a magnet assembly 548 having a plurality of magnets (e.g., a Halbach magnet array) disposed therein for generating a main magnetic field B0 extending to a region of interest 552, which contains an object (e.g., a patient's head) being imaged by the MRI system 500. The MRI system 500 also includes an RF transmit / receive coil 550 (e.g., a head coil optimized to receive the patient's head) and one or more gradient coils 504 (which may include a first gradient coil, a second gradient coil, and a third gradient coil).

[0087] Main Reference Figure 17 Also refer to Figure 6 According to method 1700, the magnetic field B01702 is modified by simultaneously applying a first baseline current to the first gradient coil, a second baseline current to the second gradient coil, and a third baseline current to the third gradient coil. Furthermore, an excitation pulse 1704 is applied to the object via the RF transmit / receive coil 550. A gradient field 1706 is generated by the first gradient coil by applying a current greater than the first baseline current. The magnetic resonance (MR) signal induced by the applied excitation pulse is detected 1708 via the RF transmit / receive coil 550.

[0088] According to at least one aspect of method 1700, the uniformity of magnetic field B0 is increased by simultaneously applying a first baseline current to a first gradient coil, a second baseline current to a second gradient coil, and a third baseline current to a third gradient coil during the entire pulse sequence (i.e., throughout the repetition time of the pulse sequence).

[0089] According to at least one aspect of method 1700, method 1700 further includes determining a first baseline current, a second baseline current, and a third baseline current. Determining the first baseline current, the second baseline current, and the third baseline current may include selecting a set of candidate shimming configurations, acquiring a set of MR signals based on the set of candidate shimming configurations, determining a set of signal bandwidths based on the set of MR signals, and selecting the first baseline current, the second baseline current, and the third baseline current based on the candidate shimming configuration in the set of candidate shimming configurations that corresponds to the narrowest signal bandwidth in the set of signal bandwidths. Each of the candidate shimming configurations may include a first current corresponding to a first gradient coil, a second current corresponding to a second gradient coil, and a third current corresponding to a third gradient coil. Each of the MR signals may correspond to one of the candidate shimming configurations. Each of the signal bandwidths may be determined based on the frequency domain of the corresponding signal in the MR signals.

[0090] Example The inventors of this disclosure have performed method 9000 to specify and implement an active shimming configuration for an LF-MRI system. Figure 14 , Figure 15 and Figure 16 Each of these examples shows a comparison between example MR images acquired with active shimming (images 1400b, 1500b, and 1600b) and example MR images acquired without active shimming (images 1400a, 1500a, and 1600a). As in... Figure 14 , Figure 15 and Figure 16 Each of the images seen in the image (images 1400b, 1500b, and 1600b) acquired with active shimming shows reduced geometric distortion compared to images (images 1400a, 1500a, and 1600a) acquired without active shimming.

[0091] Terms and Conditions Various other aspects of the subject matter described herein are set forth in the following numbered clauses: Clause 1: An active shimming method for a magnetic resonance imaging (MRI) system, the MRI system including a first gradient coil, a second gradient coil, a third gradient coil, and a permanent magnet, wherein the permanent magnet is configured to generate a magnetic field B0 with a low field strength, the method comprising: selecting a set of candidate shimming configurations, wherein the set of candidate shimming configurations includes a first current value associated with the first gradient coil from a first current value range, a second current value associated with the second gradient coil from a second current value range, and a third current value associated with the third gradient coil from a third current value range; applying a pulse sequence to each candidate shimming configuration; acquiring a magnetic resonance (MR) signal for each pulse sequence; determining a signal bandwidth for each MR signal based on the frequency domain of the MR signal; and specifying a shimming configuration for the MRI system, wherein the shimming configuration is based on the candidate shimming configuration corresponding to the narrowest signal bandwidth of the MR signal in the set of candidate shimming configurations.

[0092] Clause 2: The method according to Clause 1 further includes: plotting the signal bandwidth of the set of candidate shimming configurations in a three-dimensional array, wherein the three-dimensional array includes a first dimension containing the first current value, a second dimension containing the second current value, and a third dimension containing the third current value; and determining, based on the three-dimensional array, the candidate shimming configuration corresponding to the narrowest signal bandwidth of the MR signal of the set of candidate shimming configurations.

[0093] Clause 3: The method according to any one of Clauses 1-2, wherein the set of candidate shimming configurations is a first set of candidate shimming configurations, the method further comprising: selecting a second set of candidate shimming configurations based on the candidate shimming configurations corresponding to the narrowest signal bandwidth of the MR signal of the first set of candidate shimming configurations.

[0094] Clause 4: The method according to Clause 3 further comprises: for each candidate shimming configuration in the second set of candidate shimming configurations, applying the pulse sequence, acquiring the MR signal, and determining the signal bandwidth based on the frequency domain of the MR signal; and specifying the shimming configuration for the MRI system based on the candidate shimming configuration corresponding to the narrowest signal bandwidth of the MR signal in the second set of candidate shimming configurations.

[0095] Clause 5: The method according to any one of Clauses 3-4, wherein selecting the second set of candidate shimming configurations comprises: selecting a fourth current value range, a fifth current value range, and a sixth current value range based on the candidate shimming configuration corresponding to the narrowest signal bandwidth of the MR signal of the first set of candidate shimming configurations; wherein the second set of candidate shimming configurations comprises a fourth current value associated with the first gradient coil from the fourth current value range, a fifth current value associated with the second gradient coil from the fifth current value range, and a sixth current value associated with the third gradient coil from the sixth current value range.

[0096] Clause 6: The method according to any one of Clauses 1-5, wherein the set of candidate shimming configurations includes candidate shimming configurations derived by combining each of the first current values ​​with each of the second current values ​​and each of the third current values.

[0097] Clause 7: The method according to any one of Clauses 1-6, wherein the MRI system comprises a dome-shaped housing containing a head coil, and wherein acquiring the MR signal comprises acquiring the MR signal induced in a phantom located within the head coil.

[0098] Clause 8: The method according to any one of Clauses 1-7 further includes: setting the baseline current values ​​of the first gradient coil, the second gradient coil, and the third gradient coil based on the shimming configuration specified for the MRI system.

[0099] Clause 9: The method according to any one of Clauses 1-8, wherein the MR system further includes a radio frequency (RF) coil, and wherein applying the pulse sequence to each candidate shim configuration comprises: modifying the magnetic field B0 in a first manner by applying a current to the first gradient coil according to the candidate shim configuration; modifying the magnetic field B0 in a second manner by applying a current to the second gradient coil according to the candidate shim configuration; modifying the magnetic field B0 in a third manner by applying a current to the third gradient coil according to the candidate shim configuration; and generating an RF pulse for inducing the MR signal via the RF coil.

[0100] Clause 10: The method according to Clause 9, wherein generating the RF pulse for inducing the MR signal includes generating a spin echo pulse sequence.

[0101] Clause 11: The method according to any one of Clauses 1-10, wherein applying the pulse sequence to each candidate shimming configuration includes applying at least two pulse sequences to each candidate shimming configuration, and wherein acquiring the MR signal for each pulse sequence includes acquiring at least two MR signals for each candidate shimming configuration, the method further comprising: determining an average signal bandwidth for each candidate shimming configuration based at least on the MR signals acquired for the candidate shimming configuration.

[0102] Clause 12: A system comprising: a permanent magnet array configured to generate a low-intensity magnetic field B0 toward an object of interest located within a field of view; a radio frequency (RF) coil assembly including an RF coil array, wherein the RF coils are positionable around the object of interest in the field of view and wherein the RF coils are configured to acquire magnetic resonance signals; a first gradient coil configured to modify the magnetic field B0 in a first direction based on receiving a first current; a second gradient coil configured to modify the magnetic field B0 in a second direction based on receiving a second current; a third gradient coil configured to modify the magnetic field B0 in a third direction based on receiving a third current; and control circuitry including a processor and a memory, wherein the memory stores instructions executable by the processor for applying the first current, the second current, and the third current according to a predetermined shimming configuration of the system.

[0103] Clause 13: The system according to Clause 12, wherein the instructions executable by the processor for applying the first current, the second current and the third current according to the predetermined shimming configuration include instructions for applying the first current according to a first baseline current value, applying the second current according to a second baseline current value and applying the third current according to a third baseline current value.

[0104] Clause 14: A system according to any one of Clauses 13, wherein the memory stores instructions executable by the processor for causing the RF coil assembly to apply a sequence of RF pulses simultaneously with the first current, the second current, and the third current according to the predetermined shimming configuration of the system.

[0105] Clause 15: The system according to Clause 14, wherein the memory stores instructions executable by the processor for applying a gradient field via the first gradient coil by applying the first current according to a value greater than the first baseline current value.

[0106] Clause 16: A method of operating a magnetic resonance imaging (MRI) system, the MRI system including a radio frequency (RF) coil, a first gradient coil, a second gradient coil, a third gradient coil, and a permanent magnet, wherein the permanent magnet is configured to generate a magnetic field B0 having a low field strength, the method comprising: modifying the magnetic field B0 by simultaneously applying a first baseline current to the first gradient coil, applying a second baseline current to the second gradient coil, and applying a third baseline current to the third gradient coil; applying an excitation pulse to an object of interest through the RF coil; generating a gradient field by the first gradient coil by applying a current greater than the first baseline current to the first gradient coil; and detecting a magnetic resonance (MR) signal induced by the applied excitation pulse through the RF coil.

[0107] Clause 17: The method according to Clause 16, wherein modifying the magnetic field B0 by simultaneously applying the first baseline current to the first gradient coil, applying the second baseline current to the second gradient coil, and applying the third baseline current to the third gradient coil increases the uniformity of the magnetic field B0.

[0108] Clause 18: The method according to any one of Clauses 16-17 further comprises determining the first baseline current, the second baseline current, and the third baseline current by: selecting a set of candidate shimming configurations, wherein each of the candidate shimming configurations includes: a first current corresponding to the first gradient coil; a second current corresponding to the second gradient coil; and a third current corresponding to the third gradient coil; acquiring a set of MR signals based on the set of candidate shimming configurations, wherein each of the MR signals corresponds to one of the candidate shimming configurations; determining a set of signal bandwidths based on the set of MR signals, wherein each of the signal bandwidths is determined based on the frequency domain of the corresponding one of the MR signals; and selecting the first baseline current, the second baseline current, and the third baseline current based on the candidate shimming configuration in the set of candidate shimming configurations that corresponds to the narrowest signal bandwidth in the set of signal bandwidths.

[0109] While several forms have been described and illustrated, the applicant does not intend to limit or restrict the scope of the appended claims to such details. Many modifications, variations, alterations, substitutions, combinations, and equivalents of these forms can be implemented and will now be apparent to those skilled in the art without departing from the scope of this disclosure. Furthermore, the structure of each element associated with a described form can alternatively be described as means for providing the function performed by that element. Moreover, where the materials of some components are disclosed, other materials may be used. Therefore, it should be understood that the foregoing description and the appended claims are intended to cover all such modifications, combinations, and variations falling within the scope of the disclosed forms.

[0110] The foregoing detailed description has illustrated various forms of apparatus and / or processes using block diagrams, flowcharts, and / or examples. As long as such block diagrams, flowcharts, and / or examples contain one or more functions and / or operations, those skilled in the art will understand that each function and / or operation in such block diagrams, flowcharts, and / or examples can be implemented individually and / or collectively by various hardware, software, firmware, or virtually any combination thereof. Those skilled in the art will recognize that some aspects of the forms disclosed herein can be implemented, wholly or partially equivalently, in integrated circuits, 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 as virtually any combination thereof, and those skilled in the art will recognize that designing circuits and / or writing software and / or firmware code in accordance with this disclosure will be entirely within the skill of those skilled in the art. Furthermore, those skilled in the art will understand that the mechanisms of the subject matter described herein are capable of being distributed in various forms as one or more program products, and that the illustrative form of the subject matter described herein applies regardless of the specific type of signal-bearing medium actually used to perform the distribution.

[0111] Instructions for programming logic to execute various public aspects can be stored in the system's memory, such as dynamic random access memory (DRAM), cache, flash memory, or other storage devices. Furthermore, instructions can be distributed via a network or through other computer-readable media. Therefore, machine-readable media can include any mechanism for storing or transmitting information in a machine-readable (e.g., computer-readable) form, but is not limited to floppy disks, optical disks, read-only optical disks (CD-ROMs) and magneto-optical disks, read-only memory (ROM), random access memory (RAM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), magnetic cards or optical cards, flash memory, or tangible machine-readable storage devices for transmitting information over the Internet via electrical signals, optical signals, acoustic signals, or other forms of propagation signals (e.g., carrier waves, infrared signals, digital signals, etc.). Therefore, non-transitory computer-readable media includes any type of tangible machine-readable medium suitable for storing or transmitting electronic instructions or information in a machine-readable (e.g., computer-readable) form.

[0112] As used in any part of this document, the term "control circuitry" can refer to, for example, hardwired circuitry, programmable circuitry (e.g., a computer processor, processing unit, processor, microcontroller, microcontroller unit, controller, digital signal processor (DSP), programmable logic device (PLD), programmable logic array (PLA), or field-programmable gate array (FPGA) including one or more individual instruction processing cores), state machine circuitry, firmware storing instructions executed by the programmable circuitry, and any combination thereof. Control circuitry can be embodied collectively or individually as part of a larger system, such as integrated circuits (ICs), application-specific integrated circuits (ASICs), system-on-a-chip (SoCs), desktop computers, laptop computers, tablet computers, servers, smartphones, etc. Therefore, as used herein, "control circuitry" includes, but is not limited to, circuitry having at least one discrete circuit, circuitry having at least one integrated circuit, circuitry having at least one application-specific integrated circuit, circuitry forming a general-purpose computing device configured by a computer program (e.g., a general-purpose computer configured by a computer program that at least partially executes the processes and / or devices described herein, or a microprocessor configured by a computer program that at least partially executes the processes and / or devices described herein), circuitry forming a storage device (e.g., in the form of random access memory), and / or circuitry forming a communication device (e.g., a modem, a communication switch, or an optoelectronic device). Those skilled in the art will recognize that the subject matter described herein can be implemented in analog or digital modes or some combination thereof.

[0113] As used in any aspect herein, the term "logic" can mean an application, software, firmware, and / or circuit configured to perform any of the operations described above. Software can be embodied in software packages, code, instructions, instruction sets, and / or data recorded on a non-transitory computer-readable storage medium. Firmware can be embodied in hard-coded (e.g., non-volatile) code, instructions, or instruction sets and / or data in a storage device.

[0114] As used in any part of this document, the terms “component,” “system,” “module,” etc., can refer to computer-related entities that control circuitry, and can be hardware, a combination of hardware and software, software, or software in execution.

[0115] As used anywhere in this document, "algorithm" refers to a self-consistent sequence of steps that leads to a desired result, where "step" refers to the manipulation of physical quantities and / or logical states, which, although not necessarily required, can take the form of electrical or magnetic signals capable of being stored, transmitted, combined, compared, and otherwise manipulated. These signals are typically referred to as bits, values, elements, symbols, characters, terms, numbers, etc. These terms and similar terms may be associated with appropriate physical quantities and are merely convenient labels applied to these quantities and / or states.

[0116] The network may include a packet-switched network. Communication devices may be able to communicate with each other using the selected packet-switched network communication protocol. An example communication protocol may include an Ethernet communication protocol that allows communication using Transmission Control Protocol / Internet Protocol (TCP / IP). The Ethernet protocol may conform to or be compatible with the Ethernet standard entitled "IEEE 802.3 Standard" published by the Institute of Electrical and Electronics Engineers (IEEE) in December 2008 and / or subsequent versions of that standard. Alternatively or additionally, 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 issued by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T). Alternatively or additionally, communication devices may be able to communicate with each other using the Frame Relay communication protocol. The Frame Relay communication protocol may conform to or be compatible with standards issued by the International Telegraph and Telephone Consultative Committee (CCITT) and / or the American National Standards Institute (ANSI). Alternatively or additionally, transceivers may be able to communicate with each other using the Asynchronous Transfer Mode (ATM) communication protocol. The ATM communication protocol may conform to or be compatible with the ATM standard entitled "ATM-MPLS Network Interoperability 2.0" and / or subsequent versions of that standard, published by the ATM Forum in August 2001. Of course, this document also anticipates different and / or subsequently developed connection-oriented network communication protocols.

[0117] Unless otherwise expressly stated in the foregoing disclosure, it is to be understood that throughout the foregoing disclosure, discussions using terms such as “processing,” “calculation,” “operation,” “determine,” and “display” refer to the actions and processes of a computer system or similar electronic computing device that manipulates data represented as physical (electronic) quantities in computer system registers and memories and converts that data into other data similarly represented as physical quantities in computer system memories or registers or other such information storage, transmission, or display devices.

[0118] One or more components may be referred to herein as “configured to,” “configurable to,” “operable / working for,” “adaptable / fittable,” “capable of,” “suitable / compliant,” etc. Those skilled in the art will recognize that, unless the context otherwise requires, “configured to” generally includes active state components and / or inactive state components and / or standby state components.

[0119] The terms “proximal” and “distal” are used herein by reference to the portion of a surgical instrument that is manipulated by the clinician. The term “proximal” refers to the portion closest to the clinician, and the term “distal” refers to the portion furthest from the clinician’s position. It will be further understood that, for convenience and clarity, spatial terms such as “vertical,” “horizontal,” “upward,” and “downward” may be used herein with reference to the accompanying drawings. However, surgical instruments are used in many orientations and positions, and these terms are not intended to be restrictive and / or absolute.

[0120] Those skilled in the art will recognize that, in general, the terminology used herein, and especially in the appended claims (e.g., the terms used in the subject of the appended claims), is typically intended to be “open-ended” (e.g., the term “comprising” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “at least having,” the term “comprising” should be interpreted as “including but not limited to,” etc.). Those skilled in the art will further understand that if a particular number of the introductory claim statements are intentional, such intention will be expressly stated in the claims, and without such statements, such intention does not exist. For example, to aid understanding, the appended claims may contain the use of the introductory phrases “at least one” and “one or more” to introduce the claim statements. However, the use of such phrases should not be construed as implying that a claim statement introduced by the indefinite article "a" or "an" limits any particular claim containing such an introductory claim statement to a claim containing only one such statement, even when the same claim includes the introductory phrase "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and / or "an" should generally be interpreted as meaning "at least one" or "one or more"); the same applies to the use of definite articles used to introduce claim statements.

[0121] Furthermore, even when a specific number of statements in the introductory claims is explicitly stated, those skilled in the art will recognize that such statements should generally be interpreted as referring to at least the number stated (e.g., in the absence of other modifiers, the naked statement "two statements" generally means at least two statements, or two or more statements). Moreover, when using conventions such as "at least one of A, B, and C," generally, such constructions are intended to be understood by those skilled in the art (e.g., "a system having at least one of A, B, and C" will include, but is not limited to, systems having only A, only B, only C, both A and B, both A and C, both B and C, and / or both A, B, and C). When using conventions such as "at least one of A, B, or C," generally, such constructions are intended to be understood by those skilled in the art (e.g., "a system having at least one of A, B, or C" will include, but is not limited to, systems having only A, only B, only C, both A and B, both A and C, both B and C, and / or both A, B, and C). Those skilled in the art will further understand that, generally, whether in the specification, claims, or drawings, extractive terms and / or phrases presenting two or more alternative terms should be understood to contemplate the possibility of including one, any, or both of the terms, unless the context otherwise specifies. For example, the phrase “A or B” will generally be understood to include the possibility of including “A” or “B” or “A and B”.

[0122] Regarding the appended claims, those skilled in the art will understand that the operations described herein can generally be performed in any order. Furthermore, although the various operation flowcharts are presented sequentially, it should be understood that the various operations can be performed in any order other than those shown, or can be performed simultaneously. Unless the context otherwise specifies, examples of such alternative orders may include overlapping, interlacing, interrupted, reordered, ascending, preparatory, supplementary, simultaneous, reversed, or other variations of the order. Furthermore, unless the context otherwise specifies, terms such as “in response to,” “related,” or other past participle adjectives are generally not intended to exclude such variations.

[0123] It is important to note that any reference to "one aspect," "in one respect," "example," "an example," etc., implies that the specific feature, structure, or characteristic described in connection with that aspect is included in at least one aspect. Therefore, the phrases "in one aspect," "in one respect," "in an example," and "in an example" appearing in various places throughout the specification do not necessarily refer to the same aspect. Furthermore, specific features, structures, or characteristics can be combined in any suitable manner with one or more aspects.

[0124] Any patent application, patent, non-patent publication, or other disclosure mentioned in this specification and / or listed in any application data sheet is incorporated herein by reference, provided that the incorporated material is consistent with this document. Thus, and to the extent necessary, the disclosure expressly set forth herein supersedes any conflicting material incorporated herein by reference. Any material or portion thereof that is said to be incorporated herein by reference but conflicts with existing definitions, statements, or other disclosures set forth herein will be incorporated only if there is no conflict between the incorporated material and existing disclosures.

[0125] In summary, the numerous benefits of adopting the concepts described herein have been described. For illustrative and descriptive purposes, one or more forms of the above description have been presented. It is not intended to be exhaustive or to limit the precise forms disclosed. Modifications or variations may be made in accordance with the above teachings. One or more forms were chosen and described to illustrate the principles and practical applications, thereby enabling those skilled in the art to utilize the various forms and make various modifications to suit a particular intended use. The claims submitted herein are intended to define the overall scope.

Claims

1. An active shimming method for a magnetic resonance imaging (MRI) system, the MRI system comprising a first gradient coil, a second gradient coil, a third gradient coil, and a permanent magnet, wherein the permanent magnet is configured to generate a magnetic field B0 with a low field strength, the method comprising: Select a set of candidate shimming configurations, wherein the set of candidate shimming configurations includes a first current value associated with the first gradient coil from a first current value range, a second current value associated with the second gradient coil from a second current value range, and a third current value associated with the third gradient coil from a third current value range. Apply a pulse sequence to each candidate shim configuration; Magnetic resonance (MR) signals are acquired for each pulse sequence; The signal bandwidth for each MR signal is determined based on the frequency domain of the MR signal; as well as Specify a shimming configuration for the MRI system, wherein the shimming configuration is based on the candidate shimming configuration corresponding to the narrowest signal bandwidth of the MR signal in the set of candidate shimming configurations.

2. The method according to claim 1, further comprising: The signal bandwidth of the set of candidate shimming configurations is plotted in a three-dimensional array, wherein the three-dimensional array includes a first dimension containing the first current value, a second dimension containing the second current value, and a third dimension containing the third current value; and Based on the three-dimensional array, the candidate shim configuration is determined to correspond to the narrowest signal bandwidth of the MR signal in the set of candidate shim configurations.

3. The method according to claim 1, wherein the set of candidate shimming configurations is a first set of candidate shimming configurations, and the method further comprises: A second set of candidate shim configurations is selected based on the candidate shim configuration corresponding to the narrowest signal bandwidth of the MR signal in the first set of candidate shim configurations.

4. The method according to claim 3, further comprising: For each candidate shimming configuration in the second set of candidate shimming configurations, the pulse sequence is applied, the MR signal is acquired, and the signal bandwidth is determined based on the frequency domain of the MR signal; as well as The shimming configuration for the MRI system is specified based on the candidate shimming configurations corresponding to the narrowest signal bandwidth of the MR signals in the second set of candidate shimming configurations.

5. The method of claim 3, wherein selecting the second set of candidate shimming configurations comprises: The fourth current value range, the fifth current value range, and the sixth current value range are selected based on the candidate shimming configuration corresponding to the narrowest signal bandwidth of the MR signal in the first set of candidate shimming configurations. The second set of candidate shimming configurations includes a fourth current value associated with the first gradient coil from the fourth current value range, a fifth current value associated with the second gradient coil from the fifth current value range, and a sixth current value associated with the third gradient coil from the sixth current value range.

6. The method of claim 1, wherein the set of candidate shimming configurations comprises candidate shimming configurations derived by combining each of the first current values ​​with each of the second current values ​​and each of the third current values.

7. The method of claim 1, wherein the MRI system comprises a dome-shaped housing containing a head coil, and wherein acquiring the MR signal comprises acquiring the MR signal induced in a phantom located within the head coil.

8. The method according to claim 1, further comprising: Based on the shimming configuration specified for the MRI system, the baseline current values ​​of the first gradient coil, the second gradient coil, and the third gradient coil are set.

9. The method of claim 1, wherein the MR system further comprises a radio frequency (RF) coil, and wherein applying the pulse sequence to each candidate shimming configuration comprises: The magnetic field B0 is modified in a first manner by applying a current to the first gradient coil according to the candidate shim configuration; The magnetic field B0 is modified in a second manner by applying a current to the second gradient coil according to the candidate shim configuration; The magnetic field B0 is modified in a third manner by applying a current to the third gradient coil according to the candidate shim configuration; as well as An RF pulse is generated by the RF coil to induce the MR signal.

10. The method of claim 9, wherein generating the RF pulse for inducing the MR signal comprises generating a spin echo pulse sequence.

11. The method of claim 1, wherein applying the pulse sequence to each candidate shimming configuration comprises applying at least two pulse sequences to each candidate shimming configuration, and wherein acquiring the MR signal for each pulse sequence comprises acquiring at least two MR signals for each candidate shimming configuration, the method further comprising: The average signal bandwidth is determined for each candidate shimming configuration based at least on the MR signals acquired for the candidate shimming configuration.

12. A system comprising: A permanent magnet array configured to generate a low-intensity magnetic field B0 toward an object of interest located within the field of view; A radio frequency (RF) coil assembly comprising an RF coil array, wherein the RF coils are positionable around an object of interest in the field of view, and wherein the RF coils are configured to acquire magnetic resonance signals; A first gradient coil is configured to modify the magnetic field B0 in a first direction based on receiving a first current; A second gradient coil is configured to modify the magnetic field B0 in a second direction based on receiving a second current. A third gradient coil is configured to modify the magnetic field B0 in a third direction based on receiving a third current; as well as A control circuit includes a processor and a memory, wherein the memory stores instructions executable by the processor for applying the first current, the second current, and the third current according to a predetermined shimming configuration of the system.

13. The system of claim 12, wherein the instructions executable by the processor for applying the first current, the second current, and the third current according to the predetermined shimming configuration include instructions for applying the first current according to a first baseline current value, applying the second current according to a second baseline current value, and applying the third current according to a third baseline current value.

14. The system of claim 13, wherein the memory stores instructions executable by the processor for causing the RF coil assembly to apply a sequence of RF pulses simultaneously with the first current, the second current, and the third current according to the predetermined shimming configuration of the system.

15. The system of claim 14, wherein the memory stores instructions executable by the processor for applying a gradient field via the first gradient coil by applying the first current according to a value greater than the first baseline current value.

16. A method of operating a magnetic resonance imaging (MRI) system, the MRI system comprising a radio frequency (RF) coil, a first gradient coil, a second gradient coil, a third gradient coil, and a permanent magnet, wherein the permanent magnet is configured to generate a magnetic field B0 having a low field strength, the method comprising: The magnetic field B0 is modified by simultaneously applying a first baseline current to the first gradient coil, a second baseline current to the second gradient coil, and a third baseline current to the third gradient coil. An excitation pulse is applied to the object of interest via the RF coil; A gradient field is generated by applying a current greater than the first baseline current to the first gradient coil. as well as The magnetic resonance (MR) signal induced by the applied excitation pulse is detected by the RF coil.

17. The method of claim 16, wherein modifying the magnetic field B0 by simultaneously applying the first baseline current to the first gradient coil, the second baseline current to the second gradient coil, and the third baseline current to the third gradient coil increases the uniformity of the magnetic field B0.

18. The method of claim 16, further comprising determining the first baseline current, the second baseline current, and the third baseline current by: Select a set of candidate shimming configurations, wherein each of the candidate shimming configurations includes: The first current corresponding to the first gradient coil; The second current corresponds to the second gradient coil; as well as The third current corresponds to the third gradient coil; A set of MR signals is acquired based on the set of candidate shimming configurations, wherein each of the MR signals corresponds to one of the candidate shimming configurations. A set of signal bandwidths is determined based on the set of MR signals, wherein each of the signal bandwidths is determined based on the frequency domain of the corresponding one of the MR signals. as well as The first baseline current, the second baseline current, and the third baseline current are selected based on the candidate shimming configuration that corresponds to the narrowest signal bandwidth in the set of signal bandwidths.

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