Fast T2-weighted and diffusion-weighted chirp CPMG sequences

Abstract

Various systems and methods for T2-weighted chirp CPMG sequences and / or diffusion-weighted chirp CPMG sequences are disclosed herein. In one aspect, the method can include projecting a magnetic field toward a target object along a longitudinal axis and transmitting a radio frequency pulse sequence to a radio frequency coil assembly configured to selectively excite magnetization in the target object. The radio frequency pulse sequence can include a frequency-swept excitation pulse and a series of frequency-swept refocusing pulses following the excitation pulse. In some aspects, the radio frequency pulse sequence can include a series of refocusing pulses followed by a frequency-swept recovery pulse. In some aspects, the method can include transmitting a preparation radio frequency pulse sequence to the radio frequency coil assembly.

Description

[Background technology]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of and priority under 35 U.S.C. § 120 of U.S. Patent Application No. 18 / 153,175, entitled "FAST T2-WEIGHTED AND DIFFUSION-WEIGHTED CHIRPED-CPMG SEQUENCES," filed January 11, 2023, which is incorporated herein by reference in its entirety.

[0002] The present disclosure relates to magnetic resonance imaging (MRI), medical imaging, medical intervention, and surgical intervention. MRI systems often involve large, complex machines that generate very high magnetic fields and pose significant constraints on the feasibility of certain surgical interventions. Constraints can include limited physical access to the patient by surgeons and / or surgical robots, and / or restrictions on the use of certain electrical and mechanical components in the vicinity of the MRI scanner. Such limitations are inherent in the underlying designs of many existing systems and are difficult to overcome. Summary of the Invention [Means for solving the problem]

[0003] According to one aspect of the present disclosure, a method is disclosed. The method can include projecting a magnetic field along a longitudinal axis toward a target object located within a field of view and transmitting a radio frequency pulse sequence to a radio frequency coil assembly configured to selectively excite magnetization in the target object within the field of view. The radio frequency pulse sequence can include an excitation pulse, a series of refocusing pulses following the excitation pulse, and a recovery pulse following the series of refocusing pulses. The excitation pulses can be frequency swept across a frequency offset range. Each of the refocusing pulses can be frequency swept across a frequency offset range. Each of the refocusing pulses can be half the duration of the excitation pulse. The recovery pulses can be frequency swept across a frequency offset range. The method can further include receiving an output signal detected by the radio frequency coil assembly midway between two of the refocusing pulses.

[0004] According to another aspect of the present disclosure, a system is disclosed. The system may include a magnet array, a radio frequency coil assembly, and a control circuit. The magnet array may be configured to generate a magnetic field of low field strength toward a target object located within a field of view. The radio frequency coil assembly may be configured to selectively excite magnetization in the target object within the field of view. The control circuit may include a processor and a memory. The memory may store instructions executable by the processor to transmit a preparation radio frequency pulse sequence to the radio frequency coil assembly and a primary radio frequency pulse sequence to the radio frequency coil assembly. The primary radio frequency pulse sequence may include an excitation pulse and a series of refocusing pulses following the excitation pulse. The excitation pulse may be frequency swept across a frequency offset range at a first rate. The excitation pulse may be a 90° pulse. Each of the refocusing pulses may be frequency swept across a frequency offset range at a second rate that is twice the first rate. Each of the refocusing pulses may be a 180° pulse and have half the duration of the excitation pulse. The memory may further store instructions executable by the processor to receive an output signal detected by the radio frequency coil assembly midway between two of the refocusing pulses. [Brief explanation of the drawings]

[0005] The various aspects described herein, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:

[0006] [Figure 1] FIG. 1 illustrates components of an MRI scanning system including a dome-shaped housing for a magnetic array in accordance with at least one aspect of the present disclosure, the dome-shaped housing enclosing a region of interest therein and further illustrating the dome-shaped housing positioned to receive at least a portion of a patient's head lying on a table within the region of interest.

[0007] [Figure 1A] FIG. 1A shows a patient's head positioned in the region of interest of the MRI scanning system of FIG.

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

[0009] [Figure 3] FIG. 3 is a perspective view of an alternative dome-shaped housing for a magnetic array for use in the MRI scanning system of FIG. 1 in accordance with at least one aspect of the present disclosure, the dome-shaped housing having an access opening and an adjustable gap defined therein.

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

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

[0012] [Figure 6] FIG. 6 illustrates a control schematic of an MRI system in accordance with at least one aspect of the present disclosure.

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

[0014] [Figure 8]FIG. 8 illustrates an MRI scanning system and a robotic system in accordance with at least one aspect of the present disclosure.

[0015] [Figure 9] FIG. 9 is a pulse sequence diagram illustrating a chirped Carr-Purcell Meiboom-Gill (CPMG) sequence in accordance with various aspects of the present disclosure.

[0016] [Figure 9A] FIG. 9A is a pulse sequence diagram illustrating an exemplary implementation of the chirp CPMG sequence of FIG. 9 in accordance with various aspects of the present disclosure.

[0017] [Figure 10] FIG. 10 is a pulse sequence diagram illustrating a chirp CPMG sequence in accordance with various aspects of the present disclosure.

[0018] [Figure 10A] FIG. 10A is a pulse sequence diagram illustrating an exemplary implementation of the chirp CPMG sequence of FIG. 10 in accordance with various aspects of the present disclosure.

[0019] [Figure 11] FIG. 11 is a pulse sequence diagram illustrating a T2 weighted imaging (T2w) preparation sequence in accordance with various aspects of the present disclosure.

[0020] [Figure 11A] FIG. 11A is a pulse sequence diagram illustrating an exemplary implementation of the T2w preparation sequence of FIG. 11 in accordance with various aspects of the present disclosure.

[0021] [Figure 12] FIG. 12 is a pulse sequence diagram illustrating a preparation sequence for diffusion weighted imaging (DWI) according to various aspects of the present disclosure.

[0022] [Figure 12A]FIG. 12A is a pulse sequence diagram illustrating an exemplary implementation of the DWI preparation sequence of FIG. 12 in accordance with various aspects of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0023] Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set forth herein are illustrative of various disclosed embodiments and are in one form only, and such exemplifications should not be construed as limiting the scope thereof in any way.

[0024] The applicant of the present application owns the following patent applications, each of which is incorporated herein by reference in its respective entirety: -International Patent Application No. PCT / US2022 / 72143, filed May 5, 2022, entitled "NEURAL INTERVENTIONAL MAGNETIC RESONANCE IMAGING APPARATUS." -U.S. Patent Application No. 18 / 057,207, filed 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 December 28, 2022, entitled "MODULARIZED MULTI-PURPOSE MAGNETIC RESONANCE PHANTOM." -U.S. Patent Application No. 18 / 147,542, filed 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." -U.S. Patent Application No. 18 / 147,556, filed January 11, 2023, entitled "ACCELERATING MAGNETIC RESONANCE IMAGING USING PARALLEL IMAGING AND ITERATIVE IMAGE RECONSTRUCTION."

[0025] Before describing various aspects of the neurointerventional magnetic resonance imaging device in detail, it should be noted that the illustrative examples are not limited in application or use to the details of construction and arrangement of parts shown in the accompanying drawings and description. The illustrative examples may be implemented or incorporated with other aspects, variations, and modifications, and may be practiced or carried out in various ways. Furthermore, unless otherwise indicated, the terms and phrases used herein have been chosen to describe the illustrative examples for the convenience of the reader, and not to limit them. It will also be understood that one or more of the aspects, aspect expressions, and / or examples described below may be combined with any one or more of the other aspects, aspect expressions, and / or examples described below.

[0026] Various aspects relate to MRI devices that enable the integration of surgical intervention and guidance by neurointerventional magnetic resonance imaging (MRI). This includes allowing not only physical access to the area around the patient, but also access to the patient's head with one or more access openings. Furthermore, neurointerventional MRI devices may enable the use of robotic-guided tools and / or conventional surgical instruments. In various instances, neurointerventional MRI can be used intraoperatively to obtain scans of a patient's head and / or brain during a surgical intervention, such as a surgical procedure such as a biopsy or neurosurgery.

[0027] 1 illustrates an MRI scanning system 100 including a dome-shaped housing 102 configured to receive a patient's head. The dome-shaped housing 102 may further include at least one access opening configured to allow access to the patient's head to enable neurointervention. 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 magnetization fields / pulses, as further described herein, to obtain imaging data representative of the target tissue.

[0028] 1A , a patient can be positioned such that his / her head is positioned within a region of interest within the dome-shaped housing 102. The brain can be positioned entirely within the dome-shaped housing 102. In such cases, to facilitate intracranial intervention (e.g., neurosurgery) in coordination with MR imaging, the dome-shaped housing 102 can include one or more openings that provide access to the brain. The openings can be spaced around the dome-shaped housing.

[0029] The MRI scanning system 100 may include an auxiliary cart (see, e.g., auxiliary cart 540 in FIG. 6 ) that houses certain conventional MRI electrical and electronic components, such as, for example, a computer, a programmable logic controller, a power distribution unit, and amplifiers. The MRI scanning system 100 may also include a magnet cart that holds the dome-shaped housing 102, gradient coils, and / or transmission coils, as described further herein. Additionally, the magnet cart may be attached to a receive coil in various instances. Referring primarily to FIG. 1 , the dome-shaped housing 102 may further include an RF transmission coil, a gradient coil 104 (shown external to the housing), and a shim magnet 106 (shown internal to the housing). Alternative configurations of the gradient coil 104 and / or shim magnet 106 are also contemplated. In various instances, the shim magnet 106 may be adjustably positioned on a shim tray within the dome-shaped housing 102, allowing a technician to granularly configure the magnetic flux density of the dome-shaped housing 102.

[0030] Various structural housings for receiving a patient's head and enabling neurointervention may be utilized with MRI scanning systems, such as MRI scanning system 100. In one aspect, MRI scanning system 100 may be equipped with an alternative housing, such as dome-shaped housing 202 (FIG. 2) or a two-part housing 302 (FIG. 3) configured to form a dome shape. Dome-shaped housing 202 defines multiple access openings 203; two-part housing 302 also defines multiple access openings 303 and further includes an adjustable gap 305 between the two parts of the housing.

[0031] In various instances, the housings 202 and 302 may include a bonding agent 308 (e.g., epoxy resin, etc.) that holds the plurality of magnetic elements 310 in a fixed position. The plurality of magnetic elements 310 may be bonded to a structural housing 312 (e.g., a plastic substrate, etc.). In various aspects, the bonding agent 308 and the structural housing 312 may be non-conductive or diamagnetic materials. Referring primarily to FIG. 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 three or more sub-parts. An access opening 303 in the structural housing 312 provides a direct passageway to the patient's head and is unobstructed by the structural housing 312, the bonding agent 308, or the magnetic elements 310. The access opening 303 may be positioned in the open space of the housing 302, for example.

[0032] There are many possible configurations of neurointerventional MRI devices that can achieve improved access for surgical intervention. Many configurations are based on two main designs, commonly known as Halbach cylinders and Halbach domes, as described in the following papers by 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 a genetic algorithm." IEEE Transactions on Magnetics, 54(1), 5100112. The paper by Cooley et al., "Design of sparse Halbach magnet arrays for portable MRI using a genetic algorithm," published in 2018 in IEEE Transactions on Magnetics, 54(1), 5100112, is incorporated herein by reference in its entirety.

[0033] In various cases, a dome-shaped housing for an MRI scanning system such as system 100 can include, for example, a Halbach dome that defines the dome shape and is configured based on several factors, including main magnetic field B strength, magnetic field size, magnetic field uniformity, device size, device weight, and patient access for neurointervention. In various aspects, the Halbach dome includes an outer radius and an inner radius at the base of the dome. The Halbach dome can 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 continues from the base of the dome for a predetermined length at a constant radius. In another aspect, the elongated cylindrical portion includes different outer and inner radii than the base of the dome (see, for example, FIGS. 2 and 3). In such cases, the different outer and inner radii of the elongated cylindrical portion can merge with the base radius in a transition region.

[0034] 4 illustrates an exemplary Halbach dome 400 for an MRI scanning system, such as system 100, defining an access opening, e.g., in the form of a hole or access opening 403, configured to receive a head and brain B of a patient P within a region of interest therein, the access opening 403 configured to allow access to the patient P and neurointervention with medical instruments and / or robotically controlled surgical tools. The Halbach dome 400 can be constructed with a single access opening 403 on a top surface 418 of the dome 400, allowing access to the top of the skull while minimizing impact with the magnetic field. Additionally or alternatively, the dome 300 can be configured with multiple access openings around the structure 416 of the dome 400, as shown in FIGS. 2 and 3.

[0035] Diameter D of access opening 403 hole can be small (e.g., about 2.54 cm) or very large (substantially the outer diameter r of the dome 400). ext) As the access opening 403 becomes larger, the dome 400 begins to resemble, for example, a Halbach cylinder. The access opening 403 is not limited to being at the apex of the dome 400. The access opening 403 may be located anywhere on the surface or structure 416 of the dome 400. In various instances, the entire dome 400 may be rotated so that the access opening 403 is co-located with a desired physical location on the patient P.

[0036] FIG. 5 shows the diameter D of the access opening 403. hole , the length L of the dome 400, and the outer radius r of the dome 400 ext and the inner radius r in 4. The Halbach dome 400 includes a plurality of magnetic elements, which are arranged in a Halbach array to form a magnet assembly. The plurality of magnetic elements ... Halbach dome 400 includes a plurality of magnetic elements, which are arranged in a Halbach array to form a magnet assembly. The plurality of magnetic elements are arranged in a Halbach array to form a magnet assembly. The Halb ext and the inner radius r in In one aspect, exemplary dimensions can be defined as follows: r in =19.3cm;r ext =23.6cm;L=38.7cm;2.54cm≦D<19.3cm.

[0037] Based on the above exemplary dimensions, the Halbach dome 400 with access opening 403 may be configured with a magnetic flux density B of approximately 72 mT and an overall mass of approximately 35 kg. It will be appreciated that dimensions may be selected based on the particular application to achieve the desired magnetic flux density B, total weight of the Halbach dome 400 and / or magnet cart, and geometry of the neurointerventional access opening 403.

[0038] In various aspects, the Halbach dome 400 can be configured to define a plurality of access openings 403 disposed about the structure 416 of the dome 400. These plurality of access openings 403 can be configured to allow access to the patient's head and brain B using tools (e.g., surgical tools) and / or a surgical robot.

[0039] In various aspects, the access opening 403 can be adjustable. An adjustable configuration can be achieved, for example, by adjusting the diameter D of the access opening 403. hole The ability to adjust the access opening 403 using either a motor, mechanical assistance, or a manual system with a mechanical iris configuration to adjust the access opening 403 can be provided. This allows for a configuration of the dome without the access opening 403, an imaging scan to be performed, and then allows for adjustment of the dome 400 and its mechanical iris configuration to include the access opening 403, thus allowing for surgical intervention.

[0040] Halbach domes and magnetic arrays thereof for facilitating neurointervention 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.

[0041] Referring now to FIG. 6, a schematic diagram of an MRI system 500 is shown. For example, the MRI scanning system 100 (FIG. 1) and various dome-shaped housings and their magnetic arrays described further herein may be incorporated into the MRI system 500. The MRI system 500 includes a housing 502 that may be similar in many aspects to the dome-shaped housings 102 (FIG. 1), 202 (FIG. 2), and / or 302 (FIG. 3), for example. The housing 502 is dome-shaped and configured to form a region of interest or field of view 552 therein. For example, the housing 502 may be configured to receive a patient's head in various aspects of the present disclosure.

[0042] The housing 502 includes a magnet assembly 548 having a plurality of magnets (e.g., a Halbach arrangement of magnets) disposed therein. In various aspects, the main magnetic field B generated by the magnet assembly 548 extends within a field of view 552 that includes an object (e.g., a patient's head) being imaged by the MRI system 500.

[0043] 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, to acquire imaging data, the RF transmit coil may be incorporated into the housing 502 and the RF receive coil may be positioned within the housing 502.

[0044] The housing 502 also includes one or more gradient coils 504 configured to generate gradient magnetic fields to facilitate imaging of objects within a field of view 552 generated by, for example, a magnet assembly 548 surrounded by a dome-shaped housing and a dome-shaped array of magnetic elements therein. A shim tray adapted to receive shim magnets 506 may also be incorporated into the housing 502.

[0045] During the imaging process, a main magnetic field B0 extends within the field of view 552. The direction of the effective magnetic field (B1) changes in response to RF pulses and associated electromagnetic fields transmitted by the RF transmit / receive coil 550. For example, the RF transmit / receive coil 550 may be configured to selectively transmit RF signals or pulses to objects within the field of view 552, such as tissue in a patient's brain. These RF pulses may change the effective magnetic field experienced by spins within the sample tissue.

[0046] The housing 502 is in signal communication with an auxiliary cart 530 configured to provide power to the housing 502 and send / receive control signals to / from the housing 502. The auxiliary cart 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 instances, the housing 502 can be in signal communication with multiple auxiliary carts, and each cart can support one or more of the power distribution unit 532, the computer 542, the spectrometer 544, the transmit / receive switch 545, the RF amplifier 546, and / or the gradient amplifier 558.

[0047] The computer 542 is in signal communication with the spectrometer 544 and is configured to transmit and receive signals between the computer 542 and the spectrometer 544. When an object within the field of view 552 is excited with RF pulses from the RF transmit / receive coil 550, the precession of the object results in an induced current, i.e., an MR current, which is detected by the RF transmit / receive coil 550 and transmitted to an RF preamplifier 556. The RF preamplifier 556 is configured to boost or amplify the excitation data signals and transmit them to the spectrometer 544. The spectrometer 544 is configured to transmit the excitation data to the computer 542 for storage, analysis, and image construction. The computer 542 is configured, for example, to combine multiple stored excitation data signals to create an image. In various instances, the computer 542 is in signal communication with at least one database 562 that stores a reconstruction algorithm 564 and / or a pulse sequence 566. The computer 542 is configured to utilize the reconstruction algorithm to generate an MR image 568.

[0048] From the spectrometer 544, the signal may also be relayed to an RF transmit / receive coil 550 within the housing 502 via an RF power amplifier 546 and a transmit / receive switch 545 positioned between the spectrometer 544 and the RF power amplifier 546. From the spectrometer 544, the signal may also be relayed to a gradient coil 560 within the housing 502 via a gradient power amplifier 558. For example, the RF power amplifier 546 is configured to amplify the signal and transmit it to the RF transmit coil 560, and the gradient power amplifier 558 is configured to amplify the gradient coil signal and transmit it to the gradient coil 560.

[0049] In various cases, the MRI system 500 can include a noise cancellation coil 554. For example, the auxiliary cart 530 and / or the computer 542 can be in signal communication with the noise cancellation coil 554. In other cases, the noise cancellation coil 554 can be optional. For example, certain MRI systems disclosed herein may not include additional / auxiliary RF coils for detecting and canceling electromagnetic interference, i.e., noise.

[0050] A flowchart illustrating a process 570 for acquiring an MRI image is shown in FIG. 7. This flowchart may be implemented, for example, by the MRI system 500. In various instances, in block 572, a target object (e.g., a portion of a patient's anatomy) is positioned in a main magnetic field B0 within a region of interest (e.g., region of interest 552), such as within a dome-shaped housing (e.g., magnet assembly 548) of various MRI scanners described further herein. The main magnetic field B0 is configured to magnetically polarize hydrogen protons (H-protons) in the target object (e.g., all organs and tissues), known as net longitudinal magnetization M0. It is proportional to the tissue's proton density (PD) and increases exponentially in time with a time constant known as the tissue's longitudinal relaxation time, T1. The T1 value of an individual tissue depends on many factors, including, for example, the individual tissue's microstructure, 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 its age and health.

[0051] In block 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 pulsed B1 field is set to the resonant frequency of the 1H-protons, which causes the longitudinal magnetization to flip and rotate from its equilibrium longitudinal direction, resulting in a magnetization vector that can generally have transverse and longitudinal magnetization components, depending on the flip angle used. Common B1 pulses include inversion pulses, or 180-degree pulses, and 90-degree pulses. The 180-degree pulse reverses the direction of the 1H-proton magnetization in the longitudinal axis. The 90-degree pulse rotates the 1H-proton magnetization by 90 degrees so that the magnetization is in the transverse plane. The MR signal is a time-varying current proportional to the transverse component of the magnetization and is detected with an appropriate RF coil. These MR signals decay exponentially in time with a time constant known as the transverse relaxation time, T2, which also depends on, for example, the fine tissue structure, water / lipid content, and the strength of the magnetic field used.

[0052] In block 576, the MR signals are spatially encoded by exposing the target object to an additional magnetic field (known as a gradient field) generated by a gradient coil (e.g., gradient coil 560). The gradient field, which varies linearly in space, is applied for a short period in pulse form and with spatial variation in each direction. The net result is the generation of multiple spatially encoded MR signals, which can be detected in block 577 and reconstructed to form an MR image depicting a slice of the object. An RF receive 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.

[0053] In block 578, the spatially encoded signals for each slice of the scan region are digitized and mathematically spatially decoded by a computer reconstruction program (e.g., by computer 542) to generate an image depicting the internal anatomical structures of the subject. In various instances, the reconstruction program may utilize an (inverse) Fourier transform to back-transform the spatially encoded data (k-space data) into geometrically decoded data.

[0054] FIG. 8 shows a diagram of a robotic system 680 that can be used for neurointervention via an MRI scanning system 600. The robotic system 680 includes a computer system 696 and a surgical robot 682. The MRI scanning system 600 can be similar to the MRI system 500 and can include a dome-shaped housing and a magnetic array with access openings, as further described herein. For example, the MRI system 500 can include one or more access openings defined in the Halbach arrangement of magnets in a permanent magnet assembly to provide access to one or more anatomical portions of a patient to be imaged during a medical procedure. In various cases, the robotic arm and / or tool of the surgical robot 682 is configured to extend through the access openings in the permanent magnet assembly to reach the patient or target site. Each access opening can provide access to the patient and / or surgical site. For example, in the case of multiple access openings, the multiple access openings can allow access from different directions and / or proximal positions.

[0055] According to various embodiments, the robotic system 680 is configured to be disposed outside the MRI system 600. As shown in FIG. 8 , the robotic system 680 can include a robotic arm 684 configured to move with one or more degrees of freedom. According to various embodiments, the robotic arm 684 includes one or more mechanical arm portions including a hollow shaft 686 and an end effector 688. The hollow shaft 686 and the end effector 688 are configured to be translated, rotated, and / or pivoted through various ranges of motion via one or more motion controllers 690. The double-headed curved arrows in FIG. 8 indicate exemplary rotational movements generated by the motion controller 690 at various joints of the robotic arm 684.

[0056] According to various embodiments, the robotic arm 684 of the robotic system 682 is configured to access various subject anatomical portions through or around the MRI scanning system 600. According to various embodiments, the access opening is designed to take into account the size of the robotic arm 684. For example, the access opening defines a circumference configured to allow the robotic arm 684, hollow shaft 686, and end effector 688 to pass therethrough. In various cases, the robotic arm 684 is configured to access various anatomical portions of the patient from around the side of the magnetic imaging device 600. The hollow shaft 686 and / or the end effector 688 may be adapted to receive a robotic tool 692, such as a biopsy needle having a cutting edge 694, for example, for obtaining a biopsy sample from the patient.

[0057] The reader will understand that the robotic system 682 can be used in combination with various dome-shaped and / or cylindrical magnetic housings, as further described herein. Furthermore, the robotic system 682 and robotic tool 692 of FIG. 8 are exemplary. Alternative robotic systems can be utilized in connection 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 utilized in connection with the various MRI systems disclosed herein.

[0058] In various aspects of the present disclosure, the MRI systems described herein can include low-field MRI (LF-MRI) systems. In such cases, the main magnetic field B generated by the permanent magnet assembly can be, for example, 0.1 T to 1.0 T. In other cases, the MRI systems described herein can include ultra-low-field MRI (ULF-MRI) systems. In such cases, the main magnetic field B generated by the permanent magnet assembly can be, for example, 0.03 T to 0.1 T.

[0059] Higher magnetic fields, such as those exceeding 1.0 T, can prevent the use of certain electrical and mechanical components near MRI scanners. For example, the presence of surgical instruments and / or surgical robot components containing metal, especially ferrous metals, can be dangerous near higher magnetic fields because such tools may be attracted toward the magnetized source. Furthermore, higher magnetic fields often require specially designed rooms with additional precautions and shielding to limit magnetic interference. Despite the limitations of high-field MRI systems, low-field and ultra-low-field MRI systems present various challenges with regard to acquiring high-quality images with sufficient resolution to achieve the desired imaging objectives.

[0060] LF-MRI and ULF-MRI systems may generally define a relatively low overall magnetic field uniformity compared to high-field MRI systems. For example, as further described herein, a dome-shaped housing for an array of magnets, in various aspects of the present disclosure, can include a Halbach arrangement of permanent magnets that generates a magnetic field B0 with a uniformity of 1,000 ppm to 10,000 ppm within a region of interest.

[0061] The relatively low uniformity (e.g., inhomogeneity) of the magnetic field B0 and / or effective magnetic field B1 typically defined by LF-MRI and ULF-MRI systems can present various challenges. For example, typical RF pulses, such as hard pulses, sinc pulses, and / or fixed-frequency pulses, generated using LF-MRI and ULF-MRI systems may fail to excite the entire target object within the region of interest due to the limited power used to generate the RF pulses. Furthermore, typical RF pulses generated using LF-MRI and ULF-MRI systems may only excite spins with a limited bandwidth. Therefore, it may be difficult to generate images with an adequate signal-to-noise ratio (SNR) using LF-MRI and ULF-MRI systems.

[0062] Various techniques can be implemented to improve the SNR of images generated by LF-MRI and ULF-MRI systems. One technique involves generating frequency-swept pulses (sometimes called chirp pulses) as part of a modified Carr-Purcell Meiboom-Gill (CPMG) sequence. Frequency-swept pulses can be generated by modulating the RF pulse from an initial frequency to a final frequency at a specific sweep rate over the entire duration of the RF pulse. As a result, compared to typical RF pulses, frequency-swept RF pulses can excite a given bandwidth of spins using signals with lower maximum amplitudes. Combinations of frequency-swept pulses can be used to generate spin echoes, as described in "Quadrupolar nuclear magnetic resonance spectroscopy in solids using frequency-swept echoing pulses" by Bhattacharyya et al., published in The Journal of Chemical Physics, 127, 194503, 2007 (incorporated herein by reference in its entirety). And, as described in "Chirped CPMG for well-logging NMR application" by Casabienca et al., published in Journal of Magnetic Resonance, 242, 197-202, 2014 (incorporated herein by reference in its entirety), frequency sweep pulses can be implemented as part of a "chirped-CPMG sequence." Chirped CPMG sequences can be used to achieve improved SNR for proton density-weighted and T1-weighted images generated using LF-MRI and ULF-MRI systems.

[0063] However, despite the improvements provided by chirp-CPMG sequences, various challenges remain. For example, conventional T2-weighted imaging (T2w) and diffusion-weighted imaging (DWI) sequences typically rely on long echo times (TEs) (e.g., TEs in the range of 3-5 times T2) and / or long repetition times (TRs) (e.g., TRs in the range of 3-5 times greater than T1) to achieve desired image contrast. Due to the long TE, applying T2w or DWI to chirp-CPMG sequences can result in low SNR. Similarly, due to the long TR, applying T2w or DWI to chirp-CPMG sequences can result in long acquisition times. Therefore, there is a need for systems and methods for increasing the SNR and / or reducing the TR produced by chirp-CPMG sequences, such as T2w or DWI chirp-CPMG sequences.

[0064] In various aspects, the present disclosure provides systems and methods for implementing chirp CPMG sequences that reduce TR compared to conventional chirp CPMG sequences. The chirp CPMG sequences provided herein can include a recovery pulse following a spin echo train. In some aspects, the recovery pulse is a 90° frequency sweep pulse that is antiphase (e.g., ±180°) with the first 90° excitation pulse of the sequence. The recovery pulse can reverse the transverse magnetic field induced by the initial excitation pulse and subsequent refocusing pulse back to the longitudinal axis, thereby reducing TR. The recovery pulse can achieve a reduced TR without substantially compromising SNR or inducing undesirable T1 weighting. Therefore, the various chirp CPMG sequences provided herein can reduce total acquisition time compared to conventional chirp CPMG sequences, thereby improving efficiency and patient comfort.

[0065] In various aspects, the systems and methods provided herein can perform a T2w chirp CPMG sequence and / or a DWI chirp CPMG sequence by performing a T2w preparation sequence and / or a DWI chirp CPMG sequence before a primary chirp CPMG sequence. The T2w preparation sequence can include a first preparation pulse that is a 90° excitation pulse, a 180° refocus 2 preparation pulse, and a third preparation pulse that is a 90° recovery pulse. The DWI preparation sequence can be similar to the T2w preparation sequence described above, except that the DWI preparation sequence includes (i) a first diffusion gradient intermediate the first and second preparation pulses, and (ii) a second diffusion gradient intermediate the second and third preparation pulses. The various preparation sequences described herein can be performed before any of the primary chirp CPMG sequences described herein to support desired contrast for imaging.

[0066] In various aspects, the preparation sequences disclosed herein can be implemented to achieve T2w chirp CPMG sequences and / or DWI chirp CPMG sequences with improved signal quality and / or reduced total acquisition time compared to T2w chirp CPMG sequences and / or DWI chirp CPMG sequences without a preparation sequence. For example, as described above, T2 weighting can be achieved by configuring a multi-echo CPMG sequence to generate a long TE. Conventionally, there are two ways to configure a multi-echo CPMG sequence to generate a long TE. First, while maintaining the CPMG sequence conditions, the time interval between 180° refocusing pulses can be increased so that the effective TE (e.g., the time over which the center of k-space is acquired) is longer. However, this method is prone to signal artifacts and unnecessary signal loss due to the long wait time between echoes. Second, the time interval between the 90° excitation pulse and the first two 180° refocusing pulses can be increased while violating the CPMG sequence conditions. However, violating the CPMG sequence conditions can make the acquisition more prone to signal artifacts. The preparation sequence disclosed herein can induce T2w while maintaining a relatively short interval between 180° refocusing pulses for a subsequent primary chirp CPMG sequence, maintaining the CMPG sequence conditions. For example, the preparation sequence disclosed herein can generate T2 contrast by: (i) performing a 90° excitation pulse to invert longitudinal magnetization to the transverse plane; (ii) performing a 180° refocusing pulse to invert and rephase transverse magnetization during a first time interval following the 90° excitation pulse (e.g., a time interval long enough to achieve T2 contrast (exp(-t / T2)), thereby adding T2 contrast and generating spin echoes during a second time interval following the refocusing pulse that is equivalent to the first time interval between the excitation and refocusing pulses; and (iii) performing a 90° recovery pulse during the spin echo to return the transverse magnetization to the longitudinal plane.Then, T2w acquisition can be performed by performing the preparation sequence followed by the primary chirp CPMG sequence. Thus, compared with conventional chirp CPMG sequences, the systems and methods provided herein can achieve improved SNR and / or reduced TR for T2w chirp CPMG sequences and DWI chirp CPMG sequences.

[0067] FIG. 9 is a pulse sequence diagram illustrating a chirp CPMG sequence 1000 in accordance with at least one non-limiting aspect of the present disclosure. In some aspects, the chirp CPMG sequence 1000 may be the same as or similar to the chirp CPMG sequence described in the aforementioned publication by Casabienca et al., entitled "Chirped CPMG for well-logging NMR application." Various MRI systems described herein may be configured to perform the chirp CPMG sequence 1000. For example, with reference to FIGS. 6 and 10, the chirp CPMG sequence 1000 may be a pulse sequence 566 stored in the database 562 of the MRI system 500. The RF transmit / receive coil 550 may be configured to generate pulses of the chirp CPMG sequence 1000 and / or detect spin echo signals of the chirp CPMG sequence 1000.

[0068] Referring again to FIG. 9 , the chirp CPMG sequence 1000 includes an excitation pulse 1002 followed by a series of refocusing pulses 1004 and 1006. The excitation pulse 1004 can be configured to induce a 90° flip angle. Each of the series of refocusing pulses 1004 and 1006 can be configured to induce a 180° flip angle. RF pulses described as being configured to induce a particular flip angle (e.g., 90°, 180°) are referred to herein as RF pulses of a particular flip angle (e.g., 90° RF pulse, 180° RF pulse). Thus, the excitation pulse 1002 can be a 90° pulse, and each of the series of refocusing pulses 1004 and 1006 can be a 180° pulse. Thus, the chirp CPMG sequence 1000 can be configured to induce a spin echo train.

[0069] 9, a series of refocus pulses 1004, 1006 following an excitation pulse 1002 can include sequentially repeating a first refocus pulse 1004 and a second refocus pulse 1006. The excitation pulse 1002 can have a first phase φ1, each of the first refocus pulses 1004 can have a second phase φ2, and each of the second refocus pulses 1006 can have a third phase φ3. The second phase φ2 can be shifted 90° (e.g., ±90°) relative to the first phase φ1, similar to a conventional CPMG sequence. The third phase φ3 can be shifted 90° (e.g., ±90°) relative to the second phase φ2.

[0070] 9A is a pulse sequence diagram illustrating a chirp CPMG sequence 1000a. The chirp CPMG sequence 1000a provides an exemplary implementation of the chirp CPMG sequence 1000 of FIG. 9. As shown in FIG. 9A, the excitation pulse 1002a has a first phase φ1 oriented in the +y direction along the horizontal axis, the first refocusing pulse 1004a has a second phase φ2 oriented in the +x direction along the horizontal axis (shifted 90° relative to the first phase φ1), and each of the second refocusing pulses 1006a has a third phase φ3 oriented in the +y direction along the horizontal axis (shifted 90° relative to the second phase φ2).

[0071] 9, the excitation pulse 1002, each of the first refocusing pulses 1004, and / or each of the second refocusing pulses 1006 may be frequency sweep pulses. In some aspects, the excitation pulse 1002, each of the first refocusing pulses 1004, and each of the second refocusing pulses 1006 are frequency swept over the same frequency offset range. The frequency offset range may be a range from an initial frequency offset O i and the final frequency offset O f The initial frequency offset O i and the final frequency offset O f may be selected based on the target resonant frequency. For example, the initial frequency offset O i and the final frequency offset O f may be selected so that the frequency sweep of the pulse is symmetric about the target resonant frequency.

[0072] The excitation pulse 1002 has an excitation pulse duration τ exe and each of the first refocusing pulse 1004 and the second refocusing pulse 1006 has a refocusing pulse duration τ ref The excitation pulse 1002 can have an excitation pulse duration τ exe initial frequency offset O over i to the final frequency offset O f Constant excitation sweep rate R up toexe Each of the first refocusing pulse 1004 and the second refocusing pulse 1006 can be swept in frequency with a refocusing pulse duration τ ref initial frequency offset O over i to the final frequency offset O f Constant refocus sweep rate R up to ref In some aspects, the refocusing sweep rate R ref is the excitation sweep rate R exe and the refocusing pulse duration τ ref is the excitation pulse duration τ exe For example, the chirp CPMG sequence 1000a of FIG. 9A illustrates an exemplary implementation of the chirp CPMG sequence 1000 of FIG. 9, where the first refocusing pulse 1004 and the second refocusing pulse 1006 each have a duration of τ exe Refocusing pulse duration τ equal to / 2 ref It has.

[0073] 9, the excitation pulse 1002 and the first refocusing pulse 1004 can cause the generation of an echo signal 1008. Further, additional echo signals 1008 can be generated by a series of refocusing pulses 1004, 1006. The echo signals 1008 can be spatially encoded by applying gradient magnetic fields, detected by an RF coil assembly, and used for image construction (e.g., according to blocks 576, 577, and 578 of the method 570 described with respect to FIG. 6). The initial echo signal 1008 is detected at a time τ after the first refocusing pulse 1004. echo In some aspects, τ echo is the excitation pulse duration τ exe , refocusing pulse duration τ ref , and / or the time τ between the excitation pulse 1002 and the first refocusing pulse 1004 δ For example, as shown by the exemplary chirp CPMG sequence 1000a in FIG. 9A, the refocusing pulse duration τ refis half the excitation pulse duration (τ ref =τ exe / 2), the echo 1008 is δ +τ exe Time τ equal to / 2 echo It can be formed in

[0074] Referring again to FIG. 9, the echo signal 1008 is obtained at an acquisition phase φ acq In some aspects, the acquisition phase φ of the echo signal 1008 acq is shifted by 90° (e.g., + / −90°) relative to the first phase φ of the excitation pulse 1002. For example, as shown by the exemplary chirp CPMG sequence 1000a in FIG. 9A , the echo signal 1008 is acquired with an acquisition phase φ oriented in the +x direction along the horizontal axis (e.g., shifted by 90° relative to the first phase φ oriented in the +y direction). acq It has.

[0075] As described above, frequency-sweep pulses can excite a wider bandwidth of spins than conventional hard pulses using the same RF peak power. In some aspects, by applying frequency-sweep pulses according to the chirped CPMG sequence 1000, the number of spins contributing to the echo signal 1008 can be more than four times the number of spins contributing to the echo signal of a conventional CPMG sequence using a hard pulse. Therefore, the chirped CPMG sequence 1000 can be implemented to achieve a higher SNR than that achievable using conventional CPMG at the same RF peak power. Further details regarding the improved SNR that can be achieved using the chirped CPMG arrangement are described in the aforementioned publication by Casabienca et al. entitled "Chirped CPMG for well-logging NMR applications."

[0076] 9 , in some aspects, the chirp CPMG sequence 1000 can generate echo signals 1010 that appear after each second refocusing pulse 1006. In some aspects, in addition to or instead of the echo signals 1008, the echo signals 1010 can be spatially encoded by applying gradient magnetic fields, detected by an RF coil assembly, and used for image construction (e.g., according to blocks 576, 577, and 578 of the method 570 described with respect to FIG. 6 ). In some aspects, the echo signals 1008 can be characterized as free induction decay (FID) echo signals, and the echo signals 1010 can be characterized as spectral echo signals.

[0077] 9 and 9A illustrate the chirp CPMG sequences 1000, 1000a as having a series of refocusing pulses 1004, 1006 including two first refocusing pulses 1004 and two second refocusing pulses, the chirp CPMG sequences 1000, 1000a may be modified to include any suitable number of first refocusing pulses 1004 and second refocusing pulses 1006, such as any number of first refocusing pulses 1004 and second refocusing pulses 1006 that are positive integers and result in detectable echo signals 1008 and / or 1010. Furthermore, those skilled in the art will understand that the chirp CPMG sequences 1000, 1000a may be repeated as desired.

[0078] As described above, conventional T2-weighted imaging (T2w) and diffusion-weighted imaging (DWI) sequences typically rely on a relatively long echo time TE and / or a relatively long repetition time TR to achieve desired image contrast. Therefore, applying T2w or DWI to a chirp CPMG sequence, such as the chirp CPMG sequences 1000 and 1000a (FIGS. 9 and 9A), may result in low SNR and / or long acquisition times. The chirp CPMG sequences 1100 and 1100a (FIGS. 10 and 10a) and / or preparation sequences 1114 (FIGS. 10 and 10A), 1200 (FIG. 11), 1200a (FIG. 11A), 1300 (FIG. 12), and 1300a (FIG. 12A) described herein may be implemented to support T2-weighted imaging and / or diffusion-weighted imaging. In some aspects, chirp CPMG sequences 1100, 1100a (FIGS. 10, 10A) and / or preparation sequences 1114 (FIGS. 10, 10A), 1200 (FIG. 11), 1200a (FIG. 11A), 1300 (FIG. 12), 1300a (FIG. 12A) may be implemented to achieve shorter TR and / or improved SNR compared to chirp CPMG sequences 1000, 1000a (FIGS. 9, 9A).

[0079] 10 is a pulse sequence diagram illustrating a chirp CPMG sequence 1100 in accordance with at least one non-limiting aspect of the present disclosure. The chirp CPMG sequence 1100 includes an excitation pulse 1102, a series of refocusing pulses 1104, 1106 (e.g., a first refocusing pulse 1104 and a second refocusing pulse 1106), and one or more echo signals 1108. In some aspects, the chirp CPMG sequence 1100 may include one or more echo signals 1110. The chirp CPMG sequence 1100 may be similar to the chirp CPMG sequences 1000, 1000a described above with respect to FIGS. 9 and 9A. For example, excitation pulse 1102 ( FIG. 10 ) may be similar to excitation pulse 1002 ( FIG. 9 ), the series of refocusing pulses 1104, 1106 ( FIG. 10 ) may be similar to the series of refocusing pulses 1004, 1006 ( FIG. 11 ), echo signal 1108 ( FIG. 10 ) may be similar to echo signal 1008 ( FIG. 9 ), and / or echo signal 1110 ( FIG. 10 ) may be similar to echo signal 1010 ( FIG. 9 ). Various details disclosed above in connection with the chirp CPMG sequences 1000, 1000 a of FIGS. 9 and 9A can be similarly applied to the chirp CPMG sequence 1100 of FIG. 10 . Various MRI systems described herein may be configured to implement the chirp CPMG sequence 1100. For example, with reference to FIGS. 6 and 10 , the chirp CPMG sequence 1100 may be a pulse sequence 566 stored in database 562 of MRI system 500. The RF transmit / receive coil 550 may be configured to generate pulses of the chirp CPMG sequence 1100 and / or detect echo signals of the chirp CPMG sequence 1100.

[0080] 10, the chirp CPMG sequence 1100 includes a recovery pulse 1112. The recovery pulse 1112 may be a 90° pulse following a series of refocusing pulses 1104, 1106. In some aspects, the recovery pulse 1112 may reverse the transverse magnetic field induced by the excitation pulse 1102 and the series of refocusing pulses 1104, 1106 back to the longitudinal axis (e.g., so that the spins are again aligned with the main magnetic field B). Thus, the recovery pulse 1112 may effectively reduce the TR compared to the chirp CPMG sequence 1000 (FIG. 9). Therefore, by including the recovery pulse 1112 following the series of refocusing pulses 1104, 1106, the total acquisition time associated with the chirp CPMG sequence 1100 (FIG. 10) may generally be shorter than the total acquisition time associated with the chirp CPMG sequence 1000 (FIG. 9).

[0081] 10, the recovery pulse 1112 can have a fourth phase φ4. As described above with respect to FIG. 9 (which describes the excitation pulse 1002 and refocus pulses 1004, 1006), the excitation pulse 1102 can have a first phase φ1, each of the first refocus pulses 1104 can have a second phase φ2, and each of the second refocus pulses 1106 can have a third phase φ3. The second phase φ2 can be shifted 90° (e.g., + / - 90°) relative to the first phase φ1, and the third phase φ3 can be shifted 90° (e.g., + / - 90°) relative to the second phase φ2. The fourth phase φ4 of the recovery pulse 1112 can be shifted 180° (e.g., + / - 180°) relative to the first phase φ1. Thus, the excitation pulse 1002 can flip the spins from alignment with the longitudinal plane (e.g., z direction) to a transverse plane (e.g., along the xy plane), and the refocusing pulses 1104, 1106 can flip the spins 180° around the transverse plane to generate spin echoes. Furthermore, the recovery pulse 1112 can flip the spins from alignment with the transverse plane back to the longitudinal plane, thereby effectively reducing the TR.

[0082] 10A is a pulse sequence diagram illustrating a chirp CPMG sequence 1100a. The chirp CPMG sequence 1100a provides an exemplary implementation of the chirp CPMG sequence 1100 of FIG. 10. As shown in FIG. 10A, the excitation pulse 1102a has a first phase φ1 oriented in the +y direction along the horizontal axis, the first refocusing pulse 1104a has a second phase φ2 oriented in the +x direction along the horizontal axis (shifted 90° relative to the first phase φ1), each of the second refocusing pulses 1106 has a third phase φ3 oriented in the +y direction along the horizontal axis (shifted 90° relative to the second phase φ2), and the recovery pulse 112a has a fourth phase φ4 oriented in the −y direction (shifted 90° relative to the first phase φ1).

[0083] 10, the recovery pulse 1112 may be frequency swept over a frequency range, such as the same frequency range as the frequency range of the excitation pulse 1102 and the refocusing pulses 1104, 1106. For example, each of the excitation pulse 1102, the refocusing pulses 1104, 1106, and the recovery pulse 1112 may be frequency swept over a frequency offset range that is symmetric about the target resonant frequency, where the frequency offset range is equal to or greater than the initial frequency offset O. i and the final frequency offset O f The recovery pulse 1112 has a recovery pulse duration τ rec The recovery pulse 1112 can have a recovery pulse duration τ rec initial frequency offset O over i to the final frequency offset O f Constant recovery sweep rate R up to rec In some aspects, the recovery sweep rate R rec is the excitation sweep rate R exe and the recovery pulse duration τ rec is the excitation pulse duration τ exeFor example, the chirp CPMG sequence 1100a of FIG. 10A shows an example implementation of the chirp CPMG sequence 1100 of FIG. 10, where the recovery pulse 1112a is equal to τ exe The recovery pulse duration τ is equal to rec It has.

[0084] 10 and 10A illustrate the chirp CPMG sequences 1100, 1100a as having a series of refocusing pulses 1104, 1106 including two first refocusing pulses 1104 and two second refocusing pulses 1106, the chirp CPMG sequences 1100, 1100a may be modified to include any suitable number of first refocusing pulses 1104 and second refocusing pulses 1106, such as any number of first refocusing pulses 1104 and second refocusing pulses 1106 that are positive integers and result in detectable echo signals 1108 and / or 1110. A recovery pulse 1112 may be performed after the series of refocusing pulses 1104, 1106 having any suitable number of first refocusing pulses 1104 and second refocusing pulses 1106. Furthermore, those skilled in the art will appreciate that the chirp CPMG sequences 1100, 1100a may be repeated as desired.

[0085] Referring again to FIG. 10 , in some aspects, a preparation sequence 1114 may be performed before each chirp CPMG sequence 1100. In various aspects, the preparation sequence 1114 may be configured to support T2w or DWI without having to increase the total acquisition time of the primary chirp CPMG sequence 1100. For example, the preparation sequence 1114 may be configured to achieve T2w or DWI using the chirp CPMG sequence 1100 without configuring (e.g., increasing) the time interval between the excitation pulse 1102 and / or the refocusing pulses 1104, 1106 to generate the TE required for T2w or DWI. The terms “primary pulse sequence” and “primary RF pulse sequence” are used herein to refer to a chirp CPMG sequence performed following a preparation sequence. Various MRI systems described herein may be configured to perform the preparation sequence 1114 and the chirp CPMG sequence 1100. 6 and 10 , the preparation sequence 1114 and the chirp CPMG sequence 1100 may be pulse sequences 566 stored in the database 562 of the MRI system 500. The RF transmit / receive coil 550 may be configured to generate the pulses of the preparation sequence 1114 and the chirp CPMG sequence 1100.

[0086] 11 is a pulse sequence diagram illustrating a T2w preparation sequence 1200. The T2w preparation sequence 1200 is configured for T2-weighted imaging and can be implemented as the preparation sequence 1114 (FIG. 10). The T2w preparation sequence 1200 includes a first preparation pulse 1202, a second preparation pulse 1204, and a third preparation pulse 1206. The first preparation pulse 1202 is a 90° excitation pulse, the second preparation pulse 1204 is a 180° refocusing pulse, and the third preparation pulse 1206 is a 90° recovery pulse. The T2w preparation sequence 1200 can induce T2-weighted imaging while allowing subsequent primary chirp CPMG sequences (e.g., chirp CPMG sequence 1000, chirp CPMG sequence 1100) to maintain relatively short intervals between 180° refocusing pulses and maintain CMPG sequence conditions, thereby reducing acquisition time compared to conventional T2-weighting methods. The T2w preparation sequence 1200 can generate T2 contrast by: (i) performing a first preparation pulse 1202 to flip the longitudinal magnetization onto the transverse plane; (ii) performing a second preparation pulse 1204 at a first time interval after the first preparation pulse 1202 long enough to achieve T2 contrast (e.g., exp(-t / T2)), thereby generating a spin echo at a second time interval after the second preparation pulse 1204 that is equivalent to the first time interval between the first preparation pulse 1202 and the second preparation pulse 1204; and (3) performing a third preparation pulse 1206 at the time of the generated spin echo to flip the transverse magnetization back onto the longitudinal plane.

[0087] For example, the first preparation pulse 1202 has a fifth phase φ, the second preparation pulse 1204 has a sixth phase φ, and the third preparation pulse 1206 has a seventh phase φ. In some aspects, the fifth phase φ of the first preparation pulse 1202 can be shifted 90° (e.g., + / −90°) relative to the first phase φ of the excitation pulse 1102 ( FIG. 10 ), the sixth phase φ of the second preparation pulse 1204 can be shifted 90° (e.g., + / −90°) relative to the fifth phase φ, and the seventh phase φ of the third preparation pulse 1206 can be shifted 180° (e.g., + / −180°) relative to the fifth phase φ. Thus, a first preparation pulse 1202 can flip the spins from alignment with the longitudinal plane (e.g., z direction) to a transverse plane (e.g., along the xy plane), a second preparation pulse 1204 can flip the spins 180° around the transverse plane, and a third preparation pulse 1206 flips the spins from alignment with the transverse plane back to the longitudinal plane.

[0088] Figure 11A is a pulse sequence diagram illustrating a T2w preparation sequence 1200a. The T2w preparation sequence 1200a provides an exemplary implementation of the T2w preparation sequence 1200 of Figure 11. As shown in Figure 11A, a first preparation pulse 1202a has a fifth phase φ5 oriented in the +x direction along the horizontal axis (shifted by -90° relative to the first phase φ1, y of the excitation pulse 1102a of Figure 10A), a second preparation pulse 1204a has a sixth phase φ5 oriented in the +y direction along the horizontal axis (shifted by 90° relative to the fifth phase φ5), and a third preparation pulse 1206 has a seventh phase φ7 oriented in the -x direction along the horizontal axis (shifted by 180° relative to the fifth phase φ5).

[0089] 11 , each of the first preparation pulse 1202, the second preparation pulse 1204, and the third preparation pulse 1206 can be frequency swept over a frequency range, such as the same frequency range as the frequency range of the excitation pulse 1102 ( FIG. 10 ) and the refocus pulses 1104, 1106 ( FIG. 10 ). For example, each of the first preparation pulse 1202, the second preparation pulse 1204, and the third preparation pulse 1206 can be frequency swept over a frequency offset range that is symmetric about the target resonant frequency, where the frequency offset range is an initial frequency offset O. i and the final frequency offset O f The first preparation pulse 1202 has a first preparation pulse duration τ prep1 and the second preparation pulse 1204 can have a second preparation pulse duration τ prep2 and the third preparation pulse 1206 can have a third preparation pulse duration τ prep3 The first preparation pulse 1202 can have a first preparation pulse duration τ prep1 initial frequency offset O over i to the final frequency offset O f The first preparation pulse sweep rate R is constant until prep1 The second preparation pulse 1204 may be frequency swept with a second preparation pulse duration τ prep2 initial frequency offset O over i to the final frequency offset O f Constant second preparation pulse sweep rate R until prep2 The third preparation pulse 1206 may be swept in frequency with a third preparation pulse duration τ prep3 initial frequency offset O over i to the final frequency offset O f The third preparation pulse sweep rate R is kept constant until prep3 The frequency can be swept by

[0090] In some aspects, the first preparation pulse sweep rate R prep1 and the third preparation pulse sweep rate R prep3 is the excitation sweep rate R exe (FIG. 10), and the first preparation pulse duration τ prep1 and the third preparation pulse duration τ prep3 is the excitation pulse duration τ exe (FIG. 10). Furthermore, the second preparation pulse sweep rate R prep2 is the excitation sweep rate R exe (Fig. 10) and the second preparation pulse duration τ prep2 is the excitation pulse duration τ exe For example, the T2w preparation sequence 1200a of FIG. 11A shows an exemplary implementation of the T2w preparation sequence 1200 of FIG. 11, in which the first preparation pulse 1202a and the third preparation pulse 1206a each have an excitation pulse duration τ exe The first preparation pulse duration τ is equal to (Fig. 10) prep1 and the third preparation pulse duration τ prep3 Furthermore, the T2w preparation sequence 1200a has a half-excitation pulse duration (τ exe / 2) the second preparation pulse duration τ prep2 12 shows a second preparation pulse 1204a having a pulse width of 1.5 .mu.m.

[0091] FIG. 12 is a pulse sequence diagram illustrating a DWI preparation sequence 1300. The DWI preparation sequence 1300 is configured for diffusion-weighted imaging and can be implemented as the preparation sequence 1114 (FIG. 10). The DWI preparation sequence 1300 includes a first preparation pulse 1302, a second preparation pulse 1304, and a third preparation pulse 1306. The first preparation pulse 1302 is a 90° excitation pulse, the second preparation pulse 1304 is a 180° refocusing pulse, and the third preparation pulse 1306 is a 90° recovery pulse. The DWI preparation sequence 1300 further includes a first diffusion gradient 1308 and a second diffusion gradient 1310. The first diffusion gradient 1308 is applied after the first preparation pulse 1302 and before the second preparation pulse 1304. A second diffusion gradient 1310 is applied after the second preparation pulse 1304 and before the third preparation pulse 1306. Those skilled in the art will understand that the strength, duration, and time interval of the first diffusion gradient 1308 and the second diffusion gradient 1310 can be selected to achieve a desired b value for diffusion weighting.

[0092] The first preparation pulse 1302, the second preparation pulse 1304, and the third preparation pulse 1306 of the DWI preparation sequence 1300 ( FIG. 12 ) may be similar to the first preparation pulse 1202, the second preparation pulse 1204, and the third preparation pulse 1206 of the T2w preparation sequence 1200 described above with respect to FIG. 11 . Various details (e.g., pulse duration, frequency sweep, frequency sweep rate, phase) described above regarding the first preparation pulse 1202, the second preparation pulse 1204, and the third preparation pulse 1206 of the T2w preparation sequence 1200 ( FIG. 11 ) may similarly apply to the first preparation pulse 1302, the second preparation pulse 1304, and the third preparation pulse 1306 ( FIG. 12 ). Thus, the DWI preparation sequence 1300 can induce diffusion-weighted imaging while allowing subsequent primary chirp CPMG sequences (e.g., chirp CPMG sequence 1000, chirp CPMG sequence 1100) to maintain a relatively short interval between 180° refocusing pulses and maintain CMPG sequence conditions, thereby reducing acquisition time compared to conventional diffusion-weighted methods.

[0093] Figure 12A is a pulse sequence diagram illustrating a DWI preparation sequence 1300a. The DWI preparation sequence 1300a provides an implementation of the DWI preparation sequence 1300 of Figure 21. As shown in Figure 12A, a first preparation pulse 1302a has a fifth phase φ5 oriented in the +x direction along the horizontal axis (shifted by -90° relative to the first phase φ1, y of the excitation pulse 1102a of Figure 10A), a second preparation pulse 1304a has a sixth phase φ5 oriented in the +y direction along the horizontal axis (shifted by 90° relative to the fifth phase φ5), and a third preparation pulse 1306 has a seventh phase φ7 oriented in the -x direction along the horizontal axis (shifted by 180° relative to the fifth phase φ5). Furthermore, the first preparation pulse 1302a and the third preparation pulse 1306a each have an excitation pulse duration τ exe The first preparation pulse duration τ is equal to (Fig. 10) prep1 and the third preparation pulse duration τ prep3 The second preparation pulse 1304a has a half excitation pulse duration (τ exe / 2) the second preparation pulse duration τ prep2 It has.

[0094] 6 and 12 , a first diffusion gradient 1308 and a second diffusion gradient 1310 may be generated by the gradient coil 504 of the MRI system 500. A first preparation pulse 1302, a second preparation pulse 1304, and a third preparation pulse 1306 may be generated by the RF transmit / receive coil 550 of the MRI system 500. The DWI preparation sequence 1300 may be a pulse sequence 566 stored in the database 562 of the MRI system 500 and may include an RF pulse sequence corresponding to the first preparation pulse 1302, the second preparation pulse 1304, and the third preparation pulse 1306, and a gradient sequence corresponding to the first diffusion gradient 1308 and the second diffusion gradient 1310.

[0095] Any of the pulse sequences described herein (chirp CPMG sequence 1000 (FIG. 9), chirp CPMG sequence 1100 (FIG. 10), preparation sequence 1114 (FIG. 10), T2w preparation sequence 1200 (FIG. 11), DWI preparation sequence 1300 (FIG. 12)) can include standard imaging, crusher, and / or spoiler gradients. (Example)

[0096] Various additional aspects of the subject matter described herein are set forth in the following numbered clauses:

[0097] Clause 1: A method, the method including: projecting a magnetic field along a longitudinal axis toward a target object located within a field of view; transmitting a radio frequency pulse sequence to a radio frequency coil assembly configured to selectively excite magnetization in the target object within the field of view, the radio frequency pulse sequence comprising an excitation pulse that is frequency swept over a frequency offset range; a series of refocus pulses following the excitation pulse, each of the refocus pulses being frequency swept over a frequency offset range, each of the refocus pulses being half the duration of the excitation pulse; and a recovery pulse following the series of refocus pulses, the recovery pulse being frequency swept over the frequency offset range; and receiving an output signal detected by the radio frequency coil assembly midway between two of the refocus pulses.

[0098] Clause 2: The method described in Clause 1, wherein the radio frequency pulse sequence is a primary radio frequency pulse sequence, and the method further includes transmitting a preparation radio frequency pulse sequence to the radio frequency coil assembly before transmitting the primary radio frequency pulse sequence, and the preparation radio frequency pulse sequence is configured to achieve T2 weighted imaging or diffusion weighted imaging.

[0099] Clause 3: The method of claim 2, further comprising sequentially repeating transmission of the preparation radio frequency pulse sequence and the primary radio frequency pulse sequence.

[0100] Clause 4: The method of any of clauses 1 to 3, wherein the excitation pulse is a 90° pulse, each of the refocusing pulses is a 180° pulse, and the recovery pulse is a 90° pulse.

[0101] Clause 5: A method described in any of clauses 1 to 4, wherein the excitation pulse and recovery pulse are frequency swept across a frequency offset range at a first rate, and the refocusing pulse is frequency swept across the frequency offset range at a second rate, the second rate being twice the first rate.

[0102] Clause 6: A method according to any one of clauses 1 to 5, wherein the excitation pulse has a first phase φ1 and the recovery pulse has a second phase φ2, the second phase φ2 being equal to the first phase φ1 + 180°.

[0103] Clause 7: The method of any of clauses 1 to 6, wherein the series of refocusing pulses comprises first and second refocusing pulses that repeat sequentially.

[0104] Clause 8: The method of clause 7, wherein receiving an output signal detected by the radio frequency coil assembly midway between two of the refocus pulses includes receiving a plurality of output signals from the radio frequency coil assembly, each of the plurality of output signals being detected midway between one of the first refocus pulses and one of the second refocus pulses.

[0105] Clause 9: The method of clause 8, wherein each of the plurality of output signals is detected after one of the first refocusing pulses and before a corresponding one of the second refocusing pulses.

[0106] Clause 10: A method according to any one of clauses 7 to 9, wherein the first refocusing pulse has a third phase φ3, the second refocusing pulse has a fourth phase φ4, the third phase φ3 being equal to the first phase φ1 + / - 90°, and the fourth phase φ4 being equal to the third phase φ3 + / - 90°.

[0107] Clause 11: The method of clause 10, wherein the preparation radio frequency pulse sequence comprises a first preparation pulse that is frequency swept over a frequency offset range at a first rate, the first preparation pulse being a 90° pulse, a second preparation pulse that is frequency swept over a frequency offset range at a second rate, the second preparation pulse being a 180° pulse, and a third preparation pulse that is frequency swept over a frequency offset range at the first rate, the third preparation pulse being a 90° pulse.

[0108] Clause 12: The method of clause 11, wherein the first preparation pulse has a fifth phase φ5, the second preparation pulse has a sixth phase φ6, and the third preparation pulse has a seventh phase φ7, the fifth phase φ5 being equal to the first phase φ1 + / - 90°, the sixth phase φ6 being equal to the fifth phase φ5 + / - 90°, and the seventh phase φ7 being equal to the fifth phase φ5 + 180°.

[0109] Clause 13: The method described in clauses 10 to 11, wherein the preparation radio frequency pulse sequence is configured for diffusion weighted imaging, and the method further includes transmitting a gradient sequence to a gradient coil assembly configured to modify a magnetic field projected along the longitudinal axis, the gradient sequence comprising a first diffusion gradient after a first preparation pulse and before a second preparation pulse of the preparation radio frequency pulse sequence, and a second diffusion gradient after a second preparation pulse and before a third preparation pulse of the preparation radio frequency pulse sequence.

[0110] Clause 14: The method of any of clauses 1 to 13, wherein projecting a magnetic field along a longitudinal axis toward an object of interest located within the field of view includes projecting a magnetic field of low field strength.

[0111] Clause 15: A system comprising: an array of magnets configured to generate a magnetic field of low magnetic field strength toward a target object located within a field of view; a radio frequency coil assembly configured to selectively excite magnetization in the target object within the field of view; and a control circuit including a processor and a memory, the memory storing instructions executable by the processor to: transmit a preparation radio frequency pulse sequence to the radio frequency coil assembly; transmit a primary radio frequency pulse sequence to the radio frequency coil assembly, the primary radio frequency pulse sequence comprising an excitation pulse that is frequency swept across a frequency offset range at a first rate, the excitation pulse being a 90° pulse; and a series of refocus pulses following the excitation pulse, each of the refocus pulses being frequency swept across the frequency offset range at a second rate that is twice the first rate, and each of the refocus pulses being a 180° pulse and having half the duration of the excitation pulse; and receiving an output signal detected by the radio frequency coil assembly midway between two of the refocus pulses.

[0112] Clause 16: The system described in Clause 15, wherein the preparation radio frequency pulse sequence comprises a first preparation pulse that is frequency swept across a frequency offset range at a first rate, the first preparation pulse being a 90° pulse and having the same duration as the excitation pulse; a second preparation pulse that is frequency swept across a frequency offset range at a second rate, the second preparation pulse being a 180° pulse and having half the duration of the excitation pulse; and a third preparation pulse that is frequency swept across the frequency offset range at the first rate, the third preparation pulse being a 90° pulse and having the same duration as the excitation pulse.

[0113] Clause 17: The system described in Clause 16, wherein the primary radio frequency pulse sequence further comprises a recovery pulse following a series of refocusing pulses, the recovery pulses being frequency swept across a frequency offset range at a first rate, the recovery pulses being 90° pulses and having the same duration as the excitation pulses.

[0114] Clause 18: The system of clause 17, wherein the excitation pulse has a first phase φ1 and the recovery pulse has a second phase φ2, the second phase φ2 being equal to the first phase φ1 + 180°.

[0115] Clause 19: The system of claim 18, wherein the series of refocusing pulses comprises a first refocusing pulse and a second refocusing pulse that repeat sequentially, and the memory stores instructions executable by the processor to receive a plurality of output signals from the radio frequency coil assembly, each of the plurality of output signals being received intermediate one of the first refocusing pulses and one of the second refocusing pulses.

[0116] Clause 20: The system of Clause 19, wherein the first refocusing pulse has a third phase φ3, the second refocusing pulse has a fourth phase φ4, the first preparation pulse has a fifth phase φ5, the second preparation pulse has a sixth phase φ6, the third preparation pulse has a seventh phase φ7, the third phase φ3 is equal to the first phase φ1 + / - 90°, the fourth phase φ4 is equal to the third phase φ3 + / - 90°, the fifth phase φ5 is equal to the first phase φ1 + / - 90°, the sixth phase φ6 is equal to the fifth phase φ5 + / - 90°, and the seventh phase φ7 is equal to the fifth phase φ5 + 180°.

[0117] Clause 21: The system described in Clause 20, further comprising a gradient coil assembly configured to modify a low field strength magnetic field projected along the vertical axis, wherein the memory stores instructions executable by the processor to transmit a gradient sequence to the gradient coil assembly, the gradient sequence comprising a first diffusion gradient after a first preparation pulse and before a second preparation pulse of the preparation radio frequency pulse sequence, and a second diffusion gradient after a second preparation pulse and before a third preparation pulse of the preparation radio frequency pulse sequence.

[0118] While several embodiments have been illustrated and described, it is not the intention of the applicant to restrict or limit the scope of the appended claims to such details. Numerous modifications, variations, changes, substitutions, combinations, and equivalents to these embodiments may be made without departing from the scope of the present disclosure, and will occur to those skilled in the art. Moreover, the structure of each element associated with the described embodiments may alternatively be described as a means for providing the function performed by the element. Furthermore, where materials are disclosed for particular components, 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 that fall within the scope of the disclosed embodiments. The appended claims are intended to cover all such modifications, variations, changes, substitutions, modifications, and equivalents.

[0119] The foregoing detailed description describes various aspects of devices and / or processes through the use of block diagrams, flowcharts, and / or examples. To the extent that such block diagrams, flowcharts, and / or examples include one or more functions and / or operations, those skilled in the art will appreciate that each function and / or operation within such block diagrams, flowcharts, and / or examples can be individually and / or collectively implemented by a wide range of hardware, software, firmware, or substantially any combination thereof. Those skilled in the art will recognize that some aspects of the embodiments disclosed herein can equivalently be implemented, in whole or in part, as one or more computer programs running on one or more computers (e.g., as one or more programs running on one or more computer systems), as one or more programs running on one or more processors (e.g., as one or more programs running on one or more microprocessors), as firmware, or substantially any combination thereof, in integrated circuits, and that designing circuitry and / or writing code for the software and / or firmware is within the skill of those skilled in the art in light of this disclosure. Furthermore, those skilled in the art will understand that the subject matter mechanisms described herein may be distributed as one or more program products in a variety of forms, and that the illustrative forms of the subject matter described herein apply regardless of the particular type of signal-bearing medium used to actually effect the distribution.

[0120] The instructions used to program the logic to execute the various disclosed aspects may be stored in memory within the system, such as dynamic random access memory (DRAM), cache, flash memory, or other storage device. Additionally, the instructions may be distributed over a network or via other computer-readable media. Thus, a machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer), including, but not limited to, a floppy diskette, an optical disk, a compact disk, a read-only memory (CD-ROM), and a magneto-optical disk, a read-only memory (ROM), a random access memory (RAM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), a magnetic or optical card, a flash memory, or any tangible machine-readable storage device used to transmit information over the Internet via an electrical, optical, acoustical, or other form of propagated signal (e.g., carrier wave, infrared signal, digital signal, etc.). Accordingly, non-transitory computer-readable media includes any type of tangible machine-readable medium suitable for storing or transmitting electronic instructions or information in a form readable by a machine (e.g., a computer).

[0121] As used in any aspect of this specification, the term "control circuitry" can refer to, for example, hardwired circuitry, programmable circuitry (e.g., a computer processor including one or more individual instruction processing cores, a processing unit, a processor, a microcontroller, a microcontroller unit, a controller, a digital signal processor (DSP), a programmable logic device (PLD), a programmable logic array (PLA), or a field programmable gate array (FPGA)), a state machine circuit, firmware that stores instructions executed by the programmable circuitry, and any combination thereof. Control circuitry may be embodied, collectively or individually, as circuitry that forms part of a larger system, e.g., an integrated circuit (IC), an application specific integrated circuit (ASIC), a system on a chip (SoC), a desktop computer, a laptop computer, a tablet computer, a server, a smartphone, etc. Thus, as used herein, "control circuitry" includes, but is not limited to, an electrical circuit having at least one discrete electrical circuit, an electrical circuit having at least one integrated circuit, an electrical circuit having at least one application-specific integrated circuit, an electrical circuit forming a general-purpose computing device configured by a computer program (e.g., a general-purpose computer and / or device described herein configured by a computer program that at least partially executes a process, or a microprocessor and / or device described herein configured by a computer program that at least partially executes a process), an electrical circuit forming a memory device (e.g., a form of random access memory), and / or an electrical circuit forming a communications device (e.g., a modem, a communications switch, or an optoelectronic appliance). Those skilled in the art will recognize that the subject matter described herein can be implemented in analog or digital form or some combination thereof.

[0122] As used in any aspect of this specification, the term "logic" may refer to an application, software, firmware, and / or circuitry configured to perform any of the aforementioned operations. Software may be embodied as a software package, code, instructions, instruction sets, and / or data recorded on a non-transitory computer-readable storage medium. Firmware may be embodied as code, instructions, instruction sets, and / or data hard-coded (e.g., non-volatile) within a memory device.

[0123] When used in any aspect of this specification, the terms "component," "system," "module," and the like may refer to a controlled circuit computer-related entity that is either hardware, a combination of hardware and software, software, or software in execution.

[0124] As used in any aspect of this specification, an "algorithm" refers to a self-consistent sequence of steps leading to a desired result, and the "steps" refer to manipulations of physical quantities and / or logical states, which may, but need not, take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It is common usage to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. These and similar terms may be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities and / or states.

[0125] The network may include a packet-switched network. The communication devices may be capable of communicating with each other using a selected packet-switched network communication protocol. One exemplary communication protocol may include an Ethernet communication protocol, which may enable communication using Transmission Control Protocol / Internet Protocol (TCP / IP). The Ethernet protocol may conform to or be compatible with the Ethernet standard published by the Institute of Electrical and Electronics Engineers (IEEE) entitled "IEEE 802.3 Standard," published in December 2008, and / or later versions of this standard. Alternatively or additionally, the communication devices may be capable of communicating with each other using an X.25 communication protocol. The X.25 communication protocol may conform to or be compatible with standards published by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). Alternatively or additionally, the communication devices may be capable of communicating with each other using a frame relay communication protocol. The frame relay communication protocol may conform to or be compatible with standards published by the Consultative Committee for International Telegraph and Telephone (CCITT) and / or the American National Standards Institute (ANSI). Alternatively or additionally, the transceivers may be capable of communicating with each other using an Asynchronous Transfer Mode (ATM) communications protocol. The ATM communications protocol may conform to or be compatible with the ATM standard published by the ATM Forum entitled "ATM-MPLS Network Interworking 2.0," published in August 2001, and / or later versions of this standard. Of course, different and / or later-developed connection-oriented network communications protocols are also contemplated herein.

[0126] Unless otherwise indicated as is apparent from the foregoing disclosure, throughout the foregoing disclosure, discussions using terms such as "processing," "calculating," "computing," "determining," "displaying," and the like will be understood to refer to the operations and processes of a computer system or similar electronic computing device that manipulates and converts data represented as physical (electronic) quantities in the computer system's registers and memory into other data similarly represented as physical quantities in the computer system's memory or registers or other such information storage, transmission, or display devices.

[0127] One or more components may be referred to herein as being "configured," "configurable," "operable," "adapted," "capable," "adaptable," etc. Those skilled in the art will recognize that, unless the context requires otherwise, "configured to" may generally encompass active and / or inactive and / or standby components.

[0128] The terms "proximal" and "distal" are used herein with respect to a clinician manipulating the handle portion of a surgical instrument. The term "proximal" refers to the portion closest to the clinician, and the term "distal" refers to the portion located away from the clinician. It will be further understood that for convenience and clarity, spatial terms such as "vertical," "horizontal," "upper," and "lower" may be used herein with respect to the drawings. However, surgical instruments are used in many orientations and positions, and these terms are not intended to be limiting and / or absolute.

[0129] Those skilled in the art will recognize that, generally, the terms used in this specification, and particularly in the appended claims (e.g., the body of the appended claims), are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but not limited to," etc.). Where recitation of a specific number of introduced claim scopes is intended, such intention will be explicitly set forth in the claim, and it will be further understood by those skilled in the art that, in the absence of such recitation, no such intention exists. For example, to aid in understanding, the following appended claims include the use of the introductory phrases "at least one" and "one or more" to introduce the recitation of claim scopes. However, the use of such phrases should not be construed to mean that introducing a claim recitation with the indefinite article "a" or "an" limits any particular claim that includes such an introduced claim recitation to claims that include only one of such recitations, even when the same claim includes the introductory phrase "one or more" or "at least one" and an indefinite article such as "a" or "an" (e.g., "a" and / or "an" should typically be construed to mean "at least one" or "one or more"), and the same is true for the use of definite articles used to introduce claim recitations.

[0130] Furthermore, even if the recitation of a particular number of introduced claims is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two enumerations" without other modifiers typically means at least two enumerations, or more than two enumerations). Furthermore, in instances where a convention similar to "at least one of A, B, and C, etc." is used, such configuration is generally intended in the sense that one skilled in the art would understand the convention (e.g., "a system having at least one of A, B, and C" includes, but is not limited to, systems having A only, B only, C only, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). In instances where a convention similar to "at least one of A, B, or C, etc." is used, such configuration is generally intended in the sense that one of ordinary skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" includes, but is not limited to, systems having A only, B only, C only, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). It will be further understood by those of ordinary skill in the art that typically disjunctive words and / or phrases presenting two or more alternative terms in either this specification, claims, or drawings should be understood as considering the possibility of including one of the terms, either of the terms, or both terms, unless the context dictates otherwise. For example, the phrase "A or B" is typically understood to include the possibilities of "A" or "B" or "A and B."

[0131] With respect to the appended claims, those skilled in the art will understand that the operations described therein may generally be performed in any order. While various operational flow diagrams are shown sequentially, it should be understood that various operations may be performed in orders other than those shown, or may even be performed simultaneously. Examples of such alternative orderings may include overlapping, interleaved, interrupted, reordered, incremental, preliminary, supplemental, simultaneous, reverse, or other variant orderings, unless the context dictates otherwise. Furthermore, terms such as "responsive to," "related to," or other past-tense adjectives generally are not intended to exclude such variants, unless the context dictates otherwise.

[0132] It is worth noting that references to "one aspect," "aspect," "example," "one example," etc. mean that the particular features, structures, or characteristics described in connection with that aspect are included in at least one aspect. Thus, appearances of the phrases "in one aspect," "in one aspect," "in an example," and "in one example" in various places throughout this specification do not necessarily all refer to the same aspect. Furthermore, particular features, structures, or characteristics may be combined in any suitable manner in one or more aspects.

[0133] Any patent application, patent, non-patent publication, or other disclosure material referred to herein and / or listed in any Application Data Sheet is incorporated herein by reference to the extent that the incorporated material does not contradict this specification. Accordingly, to the extent necessary, the disclosure explicitly 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 that contradicts existing definitions, descriptions, or other disclosure material set forth herein is incorporated only to the extent that no contradiction arises between the incorporated material and the existing disclosure material.

[0134] In summary, many advantages have been described that arise from the use of the concepts described herein. The foregoing description of one or more embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to be limited to the precise embodiments disclosed. Modifications or variations are possible in light of the above teachings. One or more embodiments have been selected and described in order to illustrate the principles and practical applications, thereby enabling those skilled in the art to utilize various embodiments and various modifications suited to the particular use contemplated. The claims submitted herewith are intended to define the overall scope.

Claims

1. 1. A method, comprising: projecting a magnetic field along a longitudinal axis toward a target object located within a field of view; transmitting a radio frequency pulse sequence to a radio frequency coil assembly configured to selectively excite magnetization in the target object within the field of view, the radio frequency pulse sequence comprising: an excitation pulse that is frequency swept over a range of frequency offsets; a series of refocusing pulses following the excitation pulse, each of the refocusing pulses being frequency swept across the frequency offset range, each of the refocusing pulses being half the duration of the excitation pulse; a recovery pulse following the series of refocusing pulses, the recovery pulse being frequency swept across the frequency offset range; and receiving an output signal detected by the radio frequency coil assembly midway between two of the refocusing pulses; A method comprising:

2. 2. The method of claim 1, wherein the radio frequency pulse sequence is a primary radio frequency pulse sequence, the method further comprising transmitting a preparation radio frequency pulse sequence to the radio frequency coil assembly before transmitting the primary radio frequency pulse sequence, the preparation radio frequency pulse sequence configured to achieve T2-weighted imaging or diffusion-weighted imaging.

3. 3. The method of claim 2, further comprising sequentially repeating the transmission of the preparation radio frequency pulse sequence and the primary radio frequency pulse sequence.

4. 4. The method of claim 3, wherein the excitation pulse is a 90° pulse, each of the refocusing pulses is a 180° pulse, and the recovery pulse is a 90° pulse.

5. 5. The method of claim 4, wherein the excitation pulse and the recovery pulse are frequency swept across the frequency offset range at a first rate, and the refocusing pulse is frequency swept across the frequency offset range at a second rate, the second rate being twice the first rate.

6. The excitation pulse has a first phase φ １ and the recovery pulse has a second phase φ ２ wherein the second phase φ ２ is the first phase φ １ 6. The method of claim 5, wherein the angle is equal to +180°.

7. 7. The method of claim 6, wherein the series of refocusing pulses comprises sequentially repeating first and second refocusing pulses.

8. 8. The method of claim 7, wherein receiving an output signal detected by the radio frequency coil assembly midway between two of the refocusing pulses comprises receiving a plurality of output signals from the radio frequency coil assembly, each of the plurality of output signals being detected midway between one of the first refocusing pulses and one of the second refocusing pulses.

9. 9. The method of claim 8, wherein each of the plurality of output signals is detected after one of the first refocusing pulses and before a corresponding one of the second refocusing pulses.

10. The first refocusing pulse has a third phase φ ３ and the second refocusing pulse has a fourth phase φ ４ and the third phase φ ３ is the first phase φ １ + / - 90°, and the fourth phase φ ４ is the third phase φ ３ The method of claim 9, wherein the angle is equal to + / - 90°.

11. The preparation radio frequency pulse sequence comprises: a first preparation pulse swept in frequency across the frequency offset range at the first rate, the first preparation pulse being a 90° pulse; a second preparation pulse swept in frequency across the frequency offset range at the second rate, the second preparation pulse being a 180° pulse; and a third preparation pulse swept in frequency across the frequency offset range at the first rate, the third preparation pulse being a 90° pulse; and The method of claim 10, comprising:

12. The first preparation pulse has a fifth phase φ ５ and the second preparation pulse has a sixth phase φ ６ and the third preparation pulse has a seventh phase φ ７ wherein the fifth phase φ ５ is the first phase φ １ + / - 90°, and the sixth phase φ ６ is the fifth phase φ ５ the seventh phase φ is equal to + / - 90° ７ is the fifth phase φ ５ 12. The method of claim 11, wherein the angle is equal to +180°.

13. The preparation radio frequency pulse sequence is configured for diffusion weighted imaging, and the method includes: transmitting a gradient sequence to a gradient coil assembly configured to modify the magnetic field projected along the longitudinal axis; The gradient sequence is a first diffusion gradient after the first preparation pulse and before the second preparation pulse of the preparation radio frequency pulse sequence; a second diffusion gradient after the second preparation pulse and before the third preparation pulse of the preparation radio frequency pulse sequence; The method of claim 11 , comprising:

14. The method of claim 2 , wherein projecting a magnetic field along a longitudinal axis toward an object of interest located within the field of view comprises projecting a magnetic field of low field strength.

15. 1. A system comprising: an array of magnets configured to generate a low field strength magnetic field toward a target object located within a field of view; a radio frequency coil assembly configured to selectively excite magnetization in the target object within the field of view; a control circuit including a processor and a memory; Equipped with The memory includes: transmitting a preparation radio frequency pulse sequence to the radio frequency coil assembly; transmitting a primary radio frequency pulse sequence to the radio frequency coil assembly, the primary radio frequency pulse sequence comprising: an excitation pulse that is frequency swept over a frequency offset range at a first rate, the excitation pulse being a 90° pulse; a series of refocusing pulses following the excitation pulse, each of the refocusing pulses being frequency swept across the frequency offset range at a second rate that is twice the first rate, each of the refocusing pulses being a 180° pulse and having half the duration of the excitation pulse; and receiving an output signal detected by the radio frequency coil assembly midway between two of the refocusing pulses; storing instructions executable by the processor for performing the steps of:

16. The preparation radio frequency pulse sequence comprises: a first preparation pulse swept in frequency across the frequency offset range at the first rate, the first preparation pulse being a 90° pulse and of the same duration as the excitation pulse; a second preparation pulse swept in frequency across the frequency offset range at the second rate, the second preparation pulse being a 180° pulse and half the duration of the excitation pulse; a third preparation pulse that is frequency swept across the frequency offset range at the first rate, the third preparation pulse being a 90° pulse and having the same duration as the excitation pulse; and The system of claim 15, comprising:

17. 17. The system of claim 16, wherein the primary radio frequency pulse sequence further comprises a recovery pulse following the series of refocusing pulses, the recovery pulse being frequency swept across the frequency offset range at the first rate, the recovery pulse being a 90° pulse and having the same duration as the excitation pulse.

18. The excitation pulse has a first phase φ １ and the recovery pulse has a second phase φ ２ wherein the second phase φ ２ is the first phase φ １ 18. The system of claim 17, wherein the angle is equal to +180°.

19. 20. The system of claim 18, wherein the series of refocusing pulses comprises sequentially repeating first and second refocusing pulses, and the memory stores instructions executable by the processor to receive a plurality of output signals from the radio frequency coil assembly, each of the plurality of output signals being received intermediate one of the first refocusing pulses and one of the second refocusing pulses.

20. The first refocusing pulse has a third phase φ ３ and the second refocusing pulse has a fourth phase φ ４ wherein the first preparation pulse has a fifth phase φ ５ and the second preparation pulse has a sixth phase φ ６ and the third preparation pulse has a seventh phase φ ７ and the third phase φ ３ is the first phase φ １ + / - 90°, and the fourth phase φ ４ is the third phase φ ３ + / - 90°, and the fifth phase φ ５ is the first phase φ １ + / - 90°, and the sixth phase φ ６ is the fifth phase φ ５ the seventh phase φ is equal to + / - 90° ７ is the fifth phase φ ５ 20. The system of claim 19, wherein the angle is equal to + / - 180°.

21. a gradient coil assembly configured to modify the low field strength magnetic field produced by the array of magnets; The memory stores instructions executable by the processor to transmit a gradient sequence to the gradient coil assembly, the gradient sequence comprising: a first diffusion gradient after the first preparation pulse and before the second preparation pulse of the preparation radio frequency pulse sequence; a second diffusion gradient after the second preparation pulse and before the third preparation pulse of the preparation radio frequency pulse sequence; 21. The system of claim 20, comprising:

Images