Method and apparatus for passive shimming in MRI systems using hollow shim portions

Abstract

The present invention relates to methods and devices for passively shimming magnetic resonance imaging (MRI) magnets to achieve optimal magnetic field homogeneity. The invention introduces hollow shim portions, which are shim components with one or more voids formed within a ferromagnetic body. These voids are designed to selectively reduce the magnetic field strength in targeted regions, allowing for finer adjustments in the magnetic field and addressing the challenges posed by traditional solid shims, particularly in compact magnet systems. The invention also leverages an L0-norm-based optimization method to minimize the number of shim cup and shim tray changes required during the shimming process.

Description

[0001] Method and Apparatus for Passive Shimming in MRI Systems Using Hollow Shim Portions

[0002] TECHNICAL FIELD

[0003] [1] The present disclosure relates to passive shimming for magnetic resonance imaging (MRI) magnets.

[0004] [2] In particular, the present disclosure relates to hollow shim portions and methods for selecting shim portions to achieve optimal homogeneity in magnets and MRI systems.

[0005] BACKGROUND

[0006] [3] Any references to methods, apparatus, or documents of the prior art are not to be taken as constituting any evidence or admission that they formed, or form part of the common general knowledge.

[0007] [4] Passive shimming is a technique used to enhance the homogeneity of the magnetic field within the region of interest in an MRI magnet. Clinical MRI relies heavily on the generation of strong, uniform magnetic fields to produce high-quality images. However, the static magnetic field generated by the main superconducting magnet often deviates from its ideal design due to factors such as manufacturing imperfections and the influence of the surrounding ferromagnetic environment during installation. These deviations can result in significant inhomogeneities, often ranging from hundreds to thousands of parts per million (ppm) peak-to-peak, which can adversely affect image quality. Therefore, a precise shimming process is required to correct these magnetic field deviations, ideally achieving a homogeneity level of less than 10 ppm or even lower. [5] Shimming techniques are generally classified into passive shimming (PS) and active shimming. Active shimming involves generating compensatory magnetic fields using current-carrying coils, typically wound with low-temperature superconductors or room-temperature copper wires. Passive shimming, on the other hand, modifies the magnetic field by strategically placing ferromagnetic materials (shims) within the magnet bore. High-end MRI systems often require a combination of both shimming techniques to achieve the extremely uniform magnetic fields necessary for high- performance imaging. Despite its simplicity and widespread use, passive shimming presents several challenges, particularly when dealing with compact magnet systems or highly localized field inhomogeneities.

[0008] [6] Traditionally, passive shimming has been implemented using linear programming (LP) methods, as described in U.S. Patent Nos. 5,045,794 and 5,677,854 by Dorri et al. These methods formulate the shimming problem as a linear optimization model, balancing the need for field homogeneity with the efficient use of ferromagnetic materials. While the LP-based approach can minimize the total thickness of iron shims and address thermal issues associated with eddy currents, it has significant drawbacks. The process often involves multiple iterations, each requiring changes to numerous shim cups and trays, making it time-consuming and prone to errors, especially when manual adjustments are needed due to system or human inaccuracies.

[0009] [7] In compact magnet systems, where passive shims are positioned close to the imaging zone, the sensitivity of the shimming process increases. Traditional solid shims, when used at full size, can lead to overcompensation and stronger magnetization in these systems compared to whole-body MRI systems. As shimming progresses and the magnetic field errors are reduced to just a few ppm, using large solid shims for fine-tuning becomes increasingly challenging. Correcting these small errors with relatively large-sized solid shims often leads to prolonged shimming processes and difficulty in achieving the shimming target. Using smaller sized shim portions could enhance the shim solution. However, this approach will significantly increase the number of shim cups and shim trays and require tedious shim adjustments, leading to a higher possibility of human errors and a considerable reduction in shimming efficiency. Therefore, it’s preferable to use shim portions of a size that is practically acceptable, instead of using an overly large number of shim cups filled with extremely small-sized shim portions.

[0010] [8] A significant challenge in this context is the production of very thin shims. Engineering limitations, manufacturing constraints, and costs make it difficult to produce shims below certain thicknesses. When thinner shims are not feasible, alternatives such as using materials with lower saturation magnetization introduce additional complexity and costs. These limitations underscore the need for more advanced shimming strategies that can achieve the desired magnetic field homogeneity without relying solely on increasingly thin shims.

[0011] [9] Given these challenges, there is a clear need to reduce the effort involved in passive shimming while improving its efficiency.

[0012] SUMMARY OF INVENTION

[0013]

[0010] In an aspect, the invention provides a hollow shim portion for adjusting magnetic field homogeneity in a magnetic resonance imaging (MRI) system, the hollow shim portion comprising: a body comprising a ferromagnetic material; and one or more voids formed in the body, wherein the voids are configured to selectively adjust the magnetic field strength in specific regions when the shim portion is positioned within a shim tray of the MRI system.

[0014]

[0011] Advantageously, the hollow shim portions are dimensioned to modify the local magnetic field strength in predetermined areas, thereby contributing to the overall homogeneity of the magnetic field in the MRI system. Additionally, a hollow shim portion having one or more voids can be treated as equivalent to a shim portion without the void as long as the volume of the two shim portions is the same.

[0015]

[0012] Preferably, the shim portions are symmetrical. Alternatively, the shim portions are asymmetrical.

[0016]

[0013] Preferably, the shim portions are rectangular. Alternatively or additionally, the shim portions are circular, ovoid or elliptical.

[0017]

[0014] Preferably, the one or more voids are at least one of elliptical, rectangular, ovoid, elliptical or an irregular shape.

[0018]

[0015] In another aspect, the invention provides a shim assembly for use in a magnetic resonance imaging system, the shim assembly comprising: a plurality of shim portions, at least one shim portion being a hollow shim portion comprising a ferromagnetic body with at least one void formed in the body, wherein the void is configured to produce the magnetic field strength in a localized area when placed in the shim tray of the MRI system.

[0019]

[0016] In another aspect, the invention provides a method of passive shimming a magnetic field in an MRI system, comprising: obtaining a magnetic field map of an MRI system; selecting a plurality of shim portions including one or more shim portions comprising a ferromagnetic body having one or more voids formed therein so as to adjust the magnetic field strength in specific regions; and placing the selected shim portions in the shim cups or trays of the MRI system.

[0020]

[0017] Preferably, the method includes determining shimming parameters based on the magnetic field map to identify areas where magnetic field adjustment is necessary. Preferably, the shimming parameters include a minimum shim portion thickness and a minimum shim change.

[0021]

[0018] Preferably, selecting a plurality of shim portions includes substituting one or more solid shim portions in the MRI system with one or more hollow shim portions.

[0022]

[0019] Preferably, the method includes solving an LO-norm-based passive shimming model to determine optimal shim thicknesses and a minimized number of shim trays and cups and selecting the plurality of shim portions based on the solution to the passive shimming model.

[0023]

[0020] In another aspect, the invention provides a method for passively shimming a magnetic resonance imaging (MRI) magnet to achieve optimal magnetic field homogeneity, the method comprising: measuring a magnetic field of an MRI magnet to obtain a magnetic field map; determining shimming parameters based on the magnetic field map; solving an LO-norm-based passive shimming model to determine optimal shim thicknesses and a minimized number of shim trays and cups; selecting a plurality of solid shim portions corresponding to the optimal shim thicknesses; substituting one or more of the plurality of solid shim portions with hollow shim portions comprising ferromagnetic bodies having one or more voids formed therein; placing the selected solid and hollow shim portions into shim cups located on shim trays based on the solution to the passive shimming model; and inserting the shim trays containing the shim portions into the shim pockets within the MRI magnet.

[0024]

[0021] In another aspect, the invention provides a method for passively shimming a magnetic resonance imaging (MRI) magnet to achieve optimal magnetic field homogeneity, the method comprising: measuring a magnetic field of an MRI magnet to obtain a magnetic field map; determining shimming parameters based on the magnetic field map; selecting a plurality of hollow shim portions comprising ferromagnetic bodies having one or more voids formed therein based on the shimming parameters; solving an LO-norm-based passive shimming model to determine optimal shim thicknesses and a minimized number of shim trays and cups; placing the selected hollow shim portions into shim cups located on shim trays based on the solution to the passive shimming model; and inserting the shim trays containing the shim portions into the shim pockets within the MRI magnet.

[0025]

[0022] Preferably, the method includes analysing the magnetic field map to identify regions where the magnetic field strength needs to be modified, thereby guiding the specific locations for shimming and shim portion substitution.

[0023] This method seeks to minimise the total number of shim cup and tray changes required during each shimming iteration, thereby improving the overall efficiency of the shimming process.

[0026]

[0024] Preferably, determining shimming parameters based on the magnetic field map includes setting minimum shim portion thicknesses.

[0027]

[0025] Preferably, configuring an LO-norm-based passive shimming model to optimize the placement and thickness of shim portions includes minimizing the total number of shim cups and trays required.

[0028]

[0026] Preferably, a hollow shim portion is substituted for a solid shim portion where optimal shim thickness is below the minimum solid shim portion thickness, provided that the hollow shim portion has an equivalent volume.

[0029]

[0027] Preferably, the method includes: measuring the magnetic field again (preferably, to ensure that the field homogeneity meets the required specifications); and either: ending the method or; repeating the steps as necessary until the magnetic field homogeneity meets the desired specification.

[0030] BRIEF DESCRIPTION OF THE DRAWINGS

[0031]

[0028] Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows:

[0032]

[0029] Figure 1 illustrates a schematic view of shims, shim cups and shim trays, as well as the shim holder with shim pockets;

[0033]

[0030] Figures 2 and 2a illustrate the rectangular (including square) shims that can be solid shims or different types of hollow shims;

[0034]

[0031] Figure 3 illustrates the elliptical (including round) shims that can be solid shims or different types of hollow shims;

[0035]

[0032] Figure 4 is a flow chart for LO-norm-based passive shimming using solid shims and equivalent hollow shims;

[0036]

[0033] Figure 5 is a flow chart for LO-norm-based passive shimming using hollow shims;

[0037]

[0034] Figure 6 is an initial magnetic field distribution on a 40cm DSV surface with a peak-to-peak inhomogeneity of 520.63ppm.

[0038]

[0035] Figure 7 is the existing LP-based passive shimming solution that shimmed the field from 520ppm to 70ppm; and

[0039]

[0036] Figure 8 is an LO-norm-based passive shimming solution, which improving the field homogeneity from 520ppm to 70ppm.

[0040]

[0037] Figure 9 is a magnetic field distribution on a 24cm (Z) x 15cm (D) race track type of imaging volume of a compact magnet with a peak-to-peak bare inhomogeneity of 1172ppm.

[0038] Figure 10 is the after-shimmed magnetic field distribution on a 24cm (Z) x 15cm (D) racetrack type of imaging volume of the magnet with a peak-to-peak homogeneity of 9.6ppm, using mixed solid and hollow shim portions.

[0041]

[0039] Figure 11 shows the final shim solution that shims the magnet to as low inhomogeneity as 9.6ppm using the thinnest shim portion thickness up to 0.001 mm.

[0042]

[0040] Figure 12 shows localized peak and trough field points can occur during the final stages of shimming.

[0043] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0044]

[0041] Embodiments of the present invention provide for passively shimming (homogenizing) the static magnetic field (B0) in the imaging region of an MRI system. In particular, embodiments of the present invention a device, assembly and method to passively shimming an MRI magnet with minimal shimming effort. These embodiments address one or more of the challenges of achieving precise magnetic field homogeneity, particularly in compact magnet systems where traditional shimming techniques may be inadequate.

[0045]

[0042] Embodiments of the invention are directed to optimizing the substitution and placement of shim portions within the MRI magnet bore to correct inhomogeneities in the magnetic field. The method utilizes both solid and hollow shim portions having one or more voids formed therein, to provide more precise adjustments to the magnetic field. This approach is particularly effective in scenarios where manufacturing constraints limit the production of very thin shims.

[0043] References to solid shim portions should be taken to refer to shim portions that are materially uniform in that there are no voids in the body of the shim portion. In contrast, references to hollow shim portions should be understood to be refer to shim portions having one or more voids (or hollow portions) formed therein.

[0046]

[0044] Embodiments of this invention can significantly improve the efficiency of the shimming process by minimizing the number of shim changes required, thereby reducing the time and labour involved in achieving the desired magnetic field homogeneity.

[0047]

[0045] The use of hollow shim portions, provided with voids, allows for more precise adjustments to the magnetic field, enhancing the accuracy of the shimming process, particularly in highly localized areas.

[0048]

[0046] Furthermore, by allowing for the substitution of solid shim portions with hollow shim portions, the invention offers a cost-effective solution that overcomes the limitations associated with manufacturing very thin shims, which are often expensive and difficult to produce. These advancements make the shimming process more streamlined and accessible, ensuring that high-performance imaging can be achieved with reduced operational costs.

[0049]

[0047] In Figure 1 , a passive shimming system 01 is shown for illustrative purposes. The passive shimming system 01 includes shim portions 101 , shim cups 102, and shim trays 103 in which shim cups 102 housing shim portions 101 are located. The shim trays 103 may take the form of elongate trays that can house multiple shim cups 102 along its length.

[0050]

[0048] The shim portions 101 are placed in the shim cups 102 to smooth out the magnetic fields in the imaging volume of the magnet based on the distribution of field inhomogeneities. The shim trays 103, containing the shim portions 101 , are inserted into the shim pockets 105 within the shim tray holder 104, which can be either a dummy shim jig or a practical gradient coil.

[0051]

[0049] To achieve satisfactory and accurate field homogeneity while utilizing fewer shim cups and shim trays, two types of shim portions are provided: rectangular and elliptical shapes. However, these are only examples and other shapes can be used.

[0052]

[0050] Each of these portions can be either solid or partially hollowed (hollow shim portions) with one or more voids according to the magnetic field characteristics, as shown in Figures 2, 2a and 3.

[0053]

[0051] Shim portions 201 and 301 are solid shims that are most commonly used in the coarse shimming processes or for magnets with relatively small bare field inhomogeneities. In the cases of magnets with very high bare inhomogeneities, say, more than 1000 ppm peak-to-peak, it will be difficult to achieve satisfactory homogeneities in a limited number of shimming iterations using only solid shims. Therefore, hollow shims with lower magnetization values are critical for achieving accurate homogeneity specifications with minimal effort.

[0054]

[0052] Different types of hollow shim portions 202-211 , 302-306 can be applied, based on the homogeneity targets and the magnetic field characteristics. These hollow shim portions can range from roughly three-quarter shim portion (see hollow shim portions 202, 203, 302, 303 and 304), half shim portions (see hollow shim portions 204, 205, 206, 207 and 305), one-third shim portions (see hollow shim portion 208), through to one-quarter and smaller shim portions (see hollow shim portions 209, 210, 211 , and 306).

[0055]

[0053] Different shapes, such as rectangular or elliptical or ovoid, can be chosen based on the physical requirements and constraints of the MRI system and specific magnetic field characteristics that need correction. For example, smaller shims can target localized areas of inhomogeneity, while larger shims can provide broader corrections, albeit with less precision.

[0056]

[0054] Similarly, the voids in the hollow shim portions can take various shapes, including but not limited to circular voids, which provide a uniform reduction in magnetic field strength across a localized area. Elliptical voids may be used to create an anisotropic reduction in magnetic field strength, allowing for more precise control in elongated regions. Additionally, irregularly shaped voids can be customized to target complex magnetic field inhomogeneities, thereby providing tailored magnetic field adjustments in non-uniform areas. As such, the hollow shim portions may be symmetrical or asymmetrical.

[0057]

[0055] Furthermore, the location of the voids may vary on the body of the shim portions.

[0058]

[0056] The Inventors have found that the hollow shim portions can be treated as equivalent thickness of solid shims, so that the equivalent solid shims will have the same / similar volumes or magnetization values as the relevant hollow shims, making the shimming algorithms easier to be implemented.

[0059]

[0057] Thus, in a first embodiment, the invention provides a hollow shim portion 202 for adjusting magnetic field homogeneity in a magnetic resonance imaging (MRI) system. The hollow shim portion 202 includes a body 202a made of a ferromagnetic material and one or more voids 202b formed in the body 202a. The voids 202b are configured to selectively reduce the magnetic field strength in specific regions when the shim portion is positioned within a shim tray of an MRI system.

[0060]

[0058] The hollow shim portions may be provided as part of a shim assembly for use in a magnetic resonance imaging system.

[0059] The shim assembly (such as the shim assembly in Figures 1 , 7 and 8) includes a plurality of shim portions. The plurality of shim portions can include a combination of solid shim portions and hollow shim portions. However, at least one shim portion is a hollow shim portion.

[0061]

[0060] Figure 4 shows a flowchart using true and equivalent solid shims (i.e. , hollow shims). Also, to achieve more accurate results, the contribution of the hollow shims can be treated without simplification in the shimming algorithms.

[0062]

[0061] Figure 5 shows the shimming flowchart directly using hollow shims.

[0063]

[0062] Embodiments of the invention use an LO-norm optimization method to minimise the total number of shim cup and shim tray changes required during each iteration of the shimming process. In mathematical terms, the LO-norm measures the number of non-zero elements in a vector. Applied to passive shimming, it reduces the number of non-zero elements in solution vectors corresponding to the shim cups and trays that need to be adjusted, thus streamlining the shimming process.

[0064]

[0063] In this invention, the LO-norm approach is used to formulate the PS model. In mathematics, the LO-norm measures the number of non-zero elements in a vector. In the context of PS, it minimizes the number of non-zero elements in the solution vectors X and Y, corresponding to the changes made in the shim cups and shim trays, respectively.

[0065]

[0064] The passive shimming problem can be mathematically expressed as follows: min w XIo + W2|Y|0(1)

[0066] Subject to:

[0067] LB < X < UB

[0068]

[0065] In which: X = {%j|j = 1,2, ..., N}, xt is the shim portion thickness in shim cup i and N is the total number of shim cups. |X|0is the number of non-zeros in vector X.

[0069] Y={yk\k = 1,2, I is the total thickness of modified shim portions in a shim tray k and Nkis the number of shim cups in shim tray k, while L is the total number of shim trays. |T |0is the number of non-zeros in vector Y. w1and w2are weighting factors that are used for regularizing the count of non-zero shim cups and non-zero shim trays.

[0070] A denotes the sensitivity matrix of the PS system and AX represents the magnetic field generated by shim portions. B refers to the magnetic field strength (in Tesla) before shimming, typically measured at various sampling points on the surface of the imaging volume. Bmdenotes the mean value of the magnetic field, s is the desired field homogeneity specification measured in parts per million (ppm). LB and UB denote the lower and upper boundary values of the shim thickness in each cup.

[0071]

[0066] While the LO-norm can handle the passive shimming optimization problem, it is a non-smooth function and known as an NP-hard problem. Therefore, a relaxation operation must be performed to use it in a gradient-based optimization framework. The following smoothed functions are adopted to make an effective approximation of the LO-norm:

[0072]

[0067] Where and / z2are sufficiently small positive values that determine the approximation property of the function to the LO-norm. This function is closely equivalent to counting the non-zero elements in vectors X and Y. It can be seamlessly integrated into standard nonlinear optimization methods due to its differentiability, allowing for gradient searching.

[0073]

[0068] Embodiments of the present invention provide a method for passively shimming a magnetic resonance imaging (MRI) magnet to achieve optimal magnetic field homogeneity. The method involves first measuring the magnetic field within the MRI magnet to obtain a magnetic field map.

[0074]

[0069] Based on this field map, the necessary shimming parameters are determined, including the minimum shim portion thicknesses.

[0075]

[0070] An LO-norm-based passive shimming model may then be configured to optimize the placement and thickness of the shim portions, with the objective of minimizing the total number of shim cups and trays required.

[0076]

[0071] The model is subsequently solved to determine the optimal shim thicknesses and the minimized number of shim trays and cups.

[0077]

[0072] Shim portions, including one or both of solid shim portions a hollow shim portions, may be selected based on the solution to the model.

[0078]

[0073] If the required solid shim thicknesses are available, they are selected and placed in the shim cups. If these thicknesses are too small (i.e. , optimal shim thickness is below the minimum thickness of a solid shim portion), or unavailable, equivalent hollow shim portions with voids are substituted and placed into the shim cups. An equivalent hollow shim portion may be determined by comparing the following characteristics of the hollow and solid shim portions: (1) volume; (2) shape; (3) dimensions of length and width. The substituted hollow shim portion should contribute equivalently to the magnetic field, meaning it may have a substantially similar or equivalent volume to the solid shim portion identified by the model.

[0079]

[0074] The shim trays containing these shim portions are then inserted into the shim pockets within the MRI magnet.

[0080]

[0075] Finally, the magnetic field is measured again to ensure that the field homogeneity meets the required specifications, with the process being repeated as necessary until the desired magnetic field homogeneity is achieved.

[0081]

[0076] An example of selecting shim portions for a passive shimming process will now be provided. The passive shimming process involves several steps. The process begins by measuring the magnetic fields and obtaining a field map of the magnet at block 401 , as depicted in Figure 4.

[0082]

[0077] If the obtained homogeneity meets the specification requirement, the shimming process stops at block 402. Otherwise, the minimum shim portion thickness and, in some embodiments, other parameters, or the minimum shim portion thickness and a minimum shim change and, in some embodiments, other parameters (including, but not limited to, the maximum thickness of shim cups, the option to add / subtract shims only, the target value for field homogeneity, spherical harmonics values, etc) are set at block 403 based on the inhomogeneity values provided by the field map.

[0083]

[0078] The LO-norm passive shim model is then configured at block 404 using the relevant mathematical formulae (described above), and the model is solved at block 405 to determine the optimal shim thicknesses and the minimized number of shim trays and cups required. If the solid shim thicknesses obtained in the solution are available in the solid shim material inventory, the solid shims 406 can be chosen and placed in the shim cups. If the solid shim thicknesses are too small to be available in the material inventory, equivalent hollow shims 407 can be chosen and placed into or removed from the shim cups 408. The shim trays with added or removed shim portions are then inserted into the shim pockets, and the magnetic fields are measured again at block 401 to ensure that the field homogeneity meets the required specifications.

[0084]

[0079] The process with hollow shims is similar, starting with measuring the magnetic fields and obtaining the field map of the magnet at block 501 , as depicted in Figure 5.

[0085]

[0080] If the homogeneity obtained meets the specification requirement, the shimming process stops at block 502. Otherwise, the minimum shim portion thickness and, in some embodiments, other appropriate parameters, (such as, but not limited to, the maximum thickness of shim cups, the option to add / subtract shims only, the target value for field homogeneity, spherical harmonics values, etc), are set at block 503 based on the inhomogeneity value. Appropriate types of hollow shims are selected at block 504, and the LO-norm passive shim model is configured 505 using the formulas in the shimming model (1), (2), and (3). The model is solved to obtain the solution at block 506 for shim thicknesses and the optimized number of shim trays and cups. The hollow shim portions are placed in the shim cups according to the solution of the model, and the shim trays with added hollow shims are inserted into the shim pockets at block 507. The magnetic fields are measured again at block 501 to ensure that the field homogeneity meets the specification.

[0086]

[0081] A shimming example using solid shims is illustrated in Figures 6-8. A magnetic field with peak-to-peak inhomogeneity of 520 ppm over the DSV surface is shown in Figure 6. Initially, the field homogeneity target is set to 70 ppm. The distributions of shim portions among 24 shim trays are calculated using the existing LP and the invention’s LO-norm algorithms, respectively. Subsequently, the usage of shim cups, shim trays, and total thicknesses for each calculation has been recorded for comparison. Both approaches achieve a similar level of field uniformity. However, as depicted in Figures 7 and 8, the LP approach requires 83 shim cups in all 24 shim trays (see Figure 7), while the LO-norm approach requires only 40 shim cups in 11 trays (see Figure 8), reducing the shimming effort by 52% and 54%, respectively.

[0087]

[0082] In certain cases, as described throughout this document, solid and hollow shims can be combined to achieve optimal shimming results, as shown in Figures 9-11.

[0088]

[0083] The bare magnetic field distribution with peak-to-peak inhomogeneity as high as 1172 ppm is provided in Figure 9 for a compact magnet with a 24 cm x 15 cm racetrack-type imaging volume. Shimming the magnet to a peak-to-peak inhomogeneity of 10 ppm or less using conventional solid shims alone is challenging.

[0089]

[0084] Mixed solid and hollow shims can be used effectively to shim the magnet to as low as 9.6 ppm in the final shimming stage, as shown in Figure 10. In this case, quarter and half hollow shims with equivalent solid shim thicknesses up to 0.001 mm, in conjunction with conventional solid shims, were applied. The modifications made to the shim in the final stage are illustrated in Figure 11.

[0090]

[0085] Turning to Figure 12, it can be seen that localized peak and trough field points can occur during the final stages of shimming. A spatially configured hollow shim, with an equivalent volume to the solid shim, can more precisely target a smaller area compared with the solid counterpart. This allows for more precise shimming of the specific field region while minimizing its impact on adjacent fields, thereby enhancing the effectiveness of the shimming process. Therefore, localized material distribution represents an additional benefit of employing hollow shims.

[0091]

[0086] Embodiments of inventions described in this disclosure offer a significant advancement in passive shimming for MRI magnets, providing a method that not only reduces the complexity and effort involved but also improves the precision and effectiveness of magnetic field adjustments. By incorporating a range of shim shapes and the ability to substitute shim portions with more specialized designs, the invention addresses the challenges posed by manufacturing constraints and the need for highly localized field corrections.

[0092]

[0087] The introduction of an LO-norm-based method, as presented in this disclosure, can address at least some of the challenges expressed elsewhere by minimizing the number of shim cup and tray changes required during each iteration. It also allows for the substitution of regular shims with more specialized shim portions that feature voids, thereby enhancing the precision of magnetic field corrections. By utilizing multiple shim shapes, both solid and hollow, this approach ensures that the desired homogeneity is achieved even when manufacturing constraints limit the availability of very thin shims.

[0093]

[0088] Embodiments of the invention provide an LO-norm-based method to reduce the effort involved in passive shimming and to enhance shimming efficiency. This is achieved by minimizing the total number of shim cup and shim tray changes required during each shimming iteration and by utilizing multiple shim shapes, both solid and hollow, to achieve optimal homogeneity in the final stages of magnet shimming.

[0094]

[0089] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. The term “comprises” and its variations, such as “comprising” and “comprised of” is used throughout in an inclusive sense and not to the exclusion of any additional features.

[0095]

[0090] It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect.

[0091] The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted by those skilled in the art.

Claims

AMENDED CLAIMS received by the International Bureau on 11 February 2026 (11 .02.2026)1. A hollow shim portion for adjusting magnetic field homogeneity in a magnetic resonance imaging (MRI) system, the hollow shim portion comprising: a body comprising a ferromagnetic material; and two or more voids formed in the body, wherein the voids are configured to selectively adjust the magnetic field strength in specific regions when the shim portion is positioned within a shim tray of the MRI system, wherein at least one void of the two or more voids are a first shape and at least one void of the two or more voids are a second shape.

2. The hollow shim portion according to claim 1 , wherein the shim portion is symmetrical.

3. The hollow shim portion according to claim 1 , wherein the shim portion is asymmetrical.

4. The hollow shim portion according to any one of claims 1 to 3, wherein the shim portion is rectangular.

5. The hollow shim portion according to any one of claims 1 to 3, wherein the shim portion is circular.

6. The hollow shim portion according to any one of claims 1 to 3, wherein the shim portion is ovoid or elliptical.

7. The hollow shim portion according to any one of claims 1 to 6, wherein the one or more voids are at least one of elliptical, rectangular, ovoid, elliptical or an irregular shape.

8. A shim assembly for use in a magnetic resonance imaging system, the shim assembly comprising:a plurality of shim portions, at least one shim portion being a hollow shim portion comprising a ferromagnetic body with at least two voids formed in the body, wherein the void is configured to produce the magnetic field strength in a localized area when placed in the shim tray of the MRI system, wherein at least one void of the at least two voids are a first shape and at least one void of the at least two voids are a second shape.

9. A method of passive shimming a magnetic field in an MRI system, comprising: obtaining a magnetic field map of an MRI system; selecting a plurality of shim portions including one or more shim portions comprising a ferromagnetic body having two or more voids formed therein so as to adjust the magnetic field strength in specific regions, wherein at least one void of the two or more voids are a first shape and at least one void of the two or more voids are a second shape; and placing the selected shim portions in the shim cups or trays of the MRI system .

10. The method according to claim 9, wherein the method includes determining shimming parameters based on the magnetic field map to identify areas where magnetic field adjustment is necessary.

11. The method according to claim 10, wherein the shimming parameters include a minimum shim portion thickness and a minimum shim change.

12. The method according to any one of claims 9 to 11 , wherein selecting a plurality of shim portions includes substituting one or more solid shim portions in the MRI system with one or more hollow shim portions.

13. The method according to any one of claims 9 to 12, wherein the method includes configuring an LO-norm-based passive shimming model to optimize the placement and thickness of shim portions includes minimizing the total number of shim cups and trays required.

14. The method according to claim 13, wherein the method includes solving the L0- norm-based passive shimming model to determine optimal shim thicknesses and a minimized number of shim trays and cups and selecting the plurality of shim portions based on the solution to the passive shimming model.

15. A method for passively shimming a magnetic resonance imaging (MRI) magnet to achieve optimal magnetic field homogeneity, the method comprising: measuring a magnetic field of an MRI magnet to obtain a magnetic field map; determining shimming parameters based on the magnetic field map; solving an LO-norm-based passive shimming model to determine optimal shim thicknesses and a minimized number of shim trays and cups; selecting a plurality of solid shim portions corresponding to the optimal shim thicknesses; substituting one or more of the plurality of solid shim portions with hollow shim portions comprising ferromagnetic bodies having one or more voids formed therein; placing the selected solid and hollow shim portions into shim cups located on shim trays based on the solution to the passive shimming model; and inserting the shim trays containing the shim portions into the shim pockets within the MRI magnet.

16. The method according to claim 15, wherein substituting a hollow shim portion for a solid shim portion includes substituting a hollow shim portion for a solid shim portion where optimal shim thickness is below a minimum solid shim portion thickness, provided that the hollow shim portion has an equivalent volume to the solid shim portion.

17. A method for passively shimming a magnetic resonance imaging (MRI) magnet to achieve optimal magnetic field homogeneity, the method comprising: measuring a magnetic field of an MRI magnet to obtain a magnetic field map; determining shimming parameters based on the magnetic field map; selecting a plurality of hollow shim portions comprising ferromagnetic bodies having one or more voids formed therein based on the shimming parameters; solving an LO-norm-based passive shimming model to determine optimal shim thicknesses and a minimized number of shim trays and cups; placing the selected hollow shim portions into shim cups located on shim trays based on the solution to the passive shimming model; and inserting the shim trays containing the shim portions into the shim pockets within the MRI magnet.

18. The method according to any one of claims 9 to 17, wherein the method includes analysing the magnetic field map to identify regions where the magnetic field strength needs to be modified, thereby guiding the specific locations for shimming and shim portion substitution.

19. The method according to any one of claims 1 to 17, wherein determining shimming parameters based on the magnetic field map includes setting minimum shim portion thicknesses.

20. A hollow shim portion for adjusting magnetic field homogeneity in a magnetic resonance imaging (MRI) system, wherein the hollow shim portion is asymmetrical and compromises: a body comprising a ferromagnetic material; and one or more voids formed in the body, wherein the voids are configured to selectively adjust the magnetic field strength in specific regions when the shim portion is positioned within a shim tray of the MRI system.

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