A superconducting shimming control method

Abstract

The present invention relates to a superconducting field shimming control method, which can accurately determine the current direction and spatial orientation of superconducting field shimming coils. On this basis, a high-performance magnetic field fitting algorithm is applied to accurately solve the shimming currents of each superconducting field shimming coil, and the superconducting magnet is shimmed accordingly. In addition, the present invention proposes an idea of gradually iterative shimming for a single superconducting field shimming coil. As a further in-depth shimming operation after the preliminary shimming of the overall field shimming coils, the gradually iterative shimming method for a single superconducting field shimming coil can effectively reduce the coupling interference of multi-coil interaction and reduce the combined error of the shimming operation of the overall field shimming coils, thereby significantly improving the magnetic field uniformity of the superconducting magnet.

Description

Technical Field

[0001] The present invention belongs to the field of magnetic resonance, and particularly relates to a superconducting shimming control method. Background Art

[0002] Superconducting magnets for magnetic resonance not only require the magnet to have sufficient magnetic field strength, but also require the magnet to have a sufficiently high magnetic field uniformity. If the magnetic field uniformity does not meet the standard, the entire magnetic resonance instrument cannot be used normally. However, during the processes of winding, assembling, transporting, and cooling the superconducting magnet, engineering errors will inevitably be introduced, resulting in the loss of magnetic field uniformity in the central region. Such a magnetic field environment cannot be directly used for magnetic resonance detection. In order to improve the magnetic field uniformity in the central region of the magnet, a common method is to introduce additional shimming devices, that is, by comprehensively applying one or more shimming devices, the magnetic field uniformity in the central region is finally improved to the level required for magnetic resonance analysis. Generally, for conventional low- and medium-field magnetic resonance superconducting magnets, the magnetic field uniformity can be improved to an acceptable level by using iron sheets and room-temperature shimming methods. However, the method for forming a uniform magnetic field in high-field magnetic resonance superconducting magnets poses significant challenges compared to low- and medium-field superconducting magnets. Due to the high magnetic field strength and large magnetic field inhomogeneous harmonic components in high-field magnetic resonance superconducting magnets, it is difficult to effectively eliminate the large magnetic field inhomogeneous harmonic components only by using iron sheets and room-temperature shimming methods. Therefore, it is necessary to introduce superconducting shimming devices. Superconducting shimming can carry a very high operating current, has a very high sensitivity compared to iron sheets and room-temperature shimming, can effectively eliminate the high-intensity inhomogeneous harmonic components in the magnetic field of high-field superconducting magnets, and the superconducting shimming coil, like the magnet coil, is enclosed in a cryostat with a stable operating temperature and the current in the wire can continuously run. The uniformity of the main magnetic field of high-field magnetic resonance is significantly improved after being adjusted by the superconducting shimming coil, and the improved magnetic field can be stably maintained. It can be considered that the superconducting shimming system is not affected by the external environment. Therefore, a high-performance superconducting shimming method is of great significance for the application transformation of high-field superconducting magnet technology in magnetic resonance.

[0003] Chinese Invention Patent CN114236440A discloses a shimming method. The idea it adopts is based on the initial magnetic field distribution, sets a target magnetic field, and realizes the final shimming effect by adjusting the shimming coil current and structural parameters. The shimming coil used is a non-superconducting coil, the coil structure used is not a fixed structure, and the optimization strategy used does not consider the gradual optimization of a single coil under the condition of multiple coil combinations. International Invention Patent WO2021 / 109847A1 discloses a shimming control method for magnetic resonance imaging. Aiming at the problem that it is difficult to cancel high-order harmonics of the magnetic resonance, a coil array shimming method is proposed. According to the spherical harmonic function expression of the actual magnetic field distribution and the kernel function of the coil array, the target current values of each basic coil corresponding to the imaging region in the coil array are determined. The shimming coil used is a non-superconducting coil, the coil used is an array structure different from the harmonic coil structure in the present invention, and the coil current is determined according to the expansion of the spherical harmonic function of the actual magnetic field, which is different from the method of determining the shimming coil current according to the fitting solution of the designed magnetic field and the measured magnetic field of the shimming coil in the present invention. Summary of the Invention

[0004] The present invention aims to solve the problem of the current regulation accuracy when using a superconducting shimming coil for shimming a superconducting magnet of a magnetic resonance, specifically manifested in the magnetic field deviation problems caused by winding and pasting of the superconducting shimming coil, the magnetic field directionality problem when superconducting coils are connected in series, the axial position and circumferential phase deviation problems during the assembly of the superconducting shimming device, and the electromagnetic coupling and comprehensive error problems when currents are applied to multiple superconducting shimming coils. The present invention provides an idea for solving the above problems and discloses the corresponding optimization methods and operation steps.

[0005] The technical solution of the superconducting shimming regulation method of the present invention is as follows:

[0006] In the first step, each superconducting shimming coil is opened one by one, and the induced current in the superconducting shimming coil is eliminated by heating the superconducting switch.

[0007] In the second step, the magnetic field distribution on the central axis of the central region of the magnet is measured as the basic magnetic field. Then, an axial superconducting shimming coil is opened. After applying a certain intensity of current, the axial superconducting shimming coil is closed, and the magnetic field distribution on the central axis of the central region of the magnet is measured again. Then, the axial superconducting shimming coil is opened, and the current in the axial superconducting coil is removed. Such operations are performed for the measurement of other axial superconducting shimming coils. Then, the magnetic field distribution on the spherical surface of the central region of the magnet is measured as the basic magnetic field. Then, a radial superconducting shimming coil is opened. After applying a certain intensity of current, the radial superconducting shimming coil is closed, and the magnetic field distribution on the spherical surface of the central region of the magnet is measured again. Then, the radial superconducting shimming coil is opened, and the current in the radial superconducting shimming coil is removed. Such operations are performed for the measurement of other radial superconducting shimming coils.

[0008] In the third step, subtract the measured magnetic field of the superconducting shimming coil from the corresponding basic magnetic field to obtain the net magnetic field of the superconducting shimming coil. The superconducting shimming coil includes at least first-order shimming coils, namely the axial Z1 superconducting shimming coil, the radial X superconducting shimming coil, and the radial Y superconducting shimming coil. Determine the direction of the Z-axis and the current direction of the Z1 superconducting shimming coil according to the magnetic field direction of the Z1 superconducting shimming coil, determine the direction of the X-axis and the current direction of the X superconducting shimming coil according to the magnetic field direction of the X superconducting shimming coil, and determine the direction of the Y-axis and the current direction of the Y superconducting shimming coil according to the magnetic field direction of the Y superconducting shimming coil. Then, based on the Cartesian coordinate system composed of the Z-axis, X-axis, and Y-axis, determine the corresponding current directions according to the net magnetic field directions of the remaining superconducting shimming coils;

[0009] In the fourth step, open the superconducting shimming coils one by one again. Eliminate the induced current in the superconducting shimming coil by heating the superconducting switch, and measure the magnetic field distribution on the spherical surface in the central region of the magnet as the initial magnetic field. Then, according to the current directions and spatial orientations of the superconducting shimming coils determined in the third step, perform a fitting operation on the initial magnetic field by multiplying the magnetic field distribution generated by a unit current of the superconducting shimming coil by the corresponding current intensity to be solved. Pass the calculated current intensity into all superconducting shimming coils and close the superconducting shimming coils, and then measure the magnetic field distribution on the spherical surface in the central region of the magnet and evaluate the magnetic field uniformity;

[0010] In the fifth step, take the magnetic field distribution on the spherical surface in the central region of the magnet after shimming in the fourth step as the initial magnetic field, and further perform shimming using a single superconducting shimming coil. Calculate the expected magnetic field uniformity when each superconducting shimming coil is used alone, compare the expected magnetic field uniformities when all superconducting shimming coils are used alone, and select the superconducting shimming coil with the best expected magnetic field uniformity for magnetic field regulation. Pass the calculated current intensity into the superconducting shimming coil and close the superconducting shimming coil, and then measure the magnetic field distribution on the spherical surface in the central region of the magnet. Perform the magnetic field regulation calculation and test for the remaining superconducting shimming coils one by one in this way until the magnetic field uniformity is increased to the highest level.

[0011] Furthermore, the determination criterion for whether there is residual induced current in the superconducting shimming coil in the first step is: when heating the heating wire on the superconducting switch of the superconducting shimming coil, measure whether there is voltage at both ends of the incoming and outgoing lines of the superconducting shimming coil. If there is voltage, it means that the induced current has not been completely eliminated, and continue to heat the heating wire of the superconducting switch until the voltage at both ends of the incoming and outgoing lines of the superconducting shimming coil completely disappears.

[0012] Further, in the second step, before each current loading on the superconducting shim coil, the basic magnetic field is measured, and the number of sampling points for measuring the magnetic field distribution is set according to actual needs; for the axial superconducting shim coil, since the magnetic field is circularly distributed, the required number of sampling points is sufficient to determine the current direction of the coil; for the radial superconducting shim coil, since the magnetic field is non-circularly distributed, the required number of sampling points is sufficient to determine both the current direction of the coil and the spatial orientation of the coil in the circumferential direction.

[0013] Further, in the third step, the measured net magnetic field distribution of the superconducting shim coil is compared with the magnetic field distribution of the superconducting shim coil obtained by theoretical calculation to find the phase difference between the two, and based on this, the current direction and spatial orientation of the superconducting shim coil are determined.

[0014] Further, in the fourth step, the magnetic field of the coil in the imaging region is calculated according to formula (1):

[0015]

[0016] where μ0 is the magnetic permeability of vacuum, M is the total number of turns of the coil, N is the number of units per turn of the coil, l ij is the position coordinate of the j-th unit of the i-th turn of the coil, dl ij is the direction vector of the line segment l ij r is the position coordinate of the magnetic field point, I k is the current intensity of the superconducting shim coil, and B(r) is the magnetic field intensity generated by the superconducting shim coil at the magnetic field point r;

[0017] The solution of the superconducting shim coil current uses the least squares method, that is, to solve the optimal current combination to minimize the magnetic field non-uniformity in the central region. The corresponding optimization equations are shown in formulas (2) and (3):

[0018] Minimize:

[0019] Constraint condition: -I max ≤I≤I max (3)

[0020] where b n is the magnetic field intensity at the n-th sampling point on the spherical surface in the central region of the magnet. There are a total of S sampling points. A is the magnetic field transfer coefficient matrix, that is, the calculated value after removing the current I k in formula (1). I is the superconducting shim coil current vector, x is the central magnetic field intensity to be solved, and I max is the maximum current that the superconducting shim coil is allowed to pass through;

[0021] The calculation of the magnetic field uniformity is shown in formula (4):

[0022] (max(b) - min(b)) / mean(b) (4)

[0023] Where b is the magnetic field intensity vector on the spherical surface of the magnet center region, max(b) is the maximum magnetic field intensity value, min(b) is the minimum magnetic field intensity value, and mean(b) is the average magnetic field intensity value.

[0024] Furthermore, for the fifth step, the optimization equations when the superconducting shimming coil is used alone are shown in formulas (5) and (6):

[0025] Minimize:

[0026] Constraint condition: -I max ≤ I k ≤ I max (6)

[0027] Here A k is the magnetic field transfer coefficient matrix of the k-th superconducting shimming coil, and the meanings of other parameters are the same as those in formulas (2) and (3).

[0028] Beneficial effects:

[0029] The superconducting shimming control method of the present invention has high stability for superconducting shimming operations, can well reduce the manufacturing error of the shimming coil, eliminate the assembly deviation of the superconducting shimming device, and reduce the coupling interference of the multi-coil interaction, thereby significantly improving the magnetic field uniformity of the superconducting magnet. Description of the Drawings

[0030] Figure 1 are the operation steps of the superconducting shimming control method of the present invention;

[0031] Figure 2 is the schematic diagram of the position of the superconducting shimming device and the superconducting magnet;

[0032] Figure 3 is the schematic diagram of the connection of the superconducting shimming coils;

[0033] Figure 4 is the magnetic field distribution law of the superconducting shimming coils;

[0034] Description of the reference numerals: 1 is the superconducting shimming device, 2 is the superconducting magnet, 3 is the superconducting shimming coil, 4 is the superconducting switch, 5 is the superconducting switch heating wire lead, and 6 is the superconducting shimming coil lead. Detailed Embodiments

[0035] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0036] As Figure 1 shown, the specific steps of the superconducting field shimming control method of the present invention are as follows:

[0037] In the first step, the superconducting field shimming coils are turned on one by one, and the induced current in the superconducting field shimming coils is eliminated by heating the superconducting switches. The criterion for determining whether there is residual induced current in the superconducting field shimming coils is: when heating the heating wire of the superconducting switch of the superconducting field shimming coil, whether there is voltage at both ends of the incoming and outgoing lines of the superconducting field shimming coil. If there is voltage, it means that the induced current has not been completely eliminated, and the heating wire of the superconducting switch needs to be continuously heated until the voltage at both ends of the incoming and outgoing lines of the superconducting field shimming coil completely disappears;

[0038] In the second step, the magnetic field distribution on the central axis of the central region of the magnet is measured, which is the basic magnetic field. Then, an axial superconducting field shimming coil is turned on. After applying a certain intensity of current, the axial superconducting field shimming coil is closed, and the magnetic field distribution on the central axis of the central region of the magnet is measured again. Then, the axial superconducting field shimming coil is turned on, and the current in the axial superconducting field shimming coil is removed. Such operations are performed for the measurement of other axial superconducting field shimming coils. Then, the magnetic field distribution on the spherical surface of the central region of the magnet is measured, which is the basic magnetic field. Then, a radial superconducting field shimming coil is turned on. After applying a certain intensity of current, the radial superconducting field shimming coil is closed, and the magnetic field distribution on the spherical surface of the central region of the magnet is measured again. Then, the radial superconducting field shimming coil is turned on, and the current in the radial superconducting field shimming coil is removed. Such operations are performed for the measurement of other radial superconducting field shimming coils. Before each current application to the superconducting field shimming coil in this step, the basic magnetic field needs to be measured. In addition, the number of sampling points for measuring the magnetic field distribution can be set according to actual needs. For the axial superconducting field shimming coil, since its magnetic field is circumferentially distributed, the required number of sampling points can determine the direction of the coil current; for the radial superconducting field shimming coil, since its magnetic field is non-circumferentially distributed, the required number of sampling points needs to be able to determine both the direction of the coil current and the spatial orientation of the coil in the circumferential direction;

[0039] In the third step, subtract the measured magnetic field of the superconducting shimming coil from the corresponding basic magnetic field to obtain the net magnetic field of the superconducting shimming coil 3. The superconducting shimming coil includes at least first-order shimming coils, namely the axial Z1 superconducting shimming coil, the radial X superconducting shimming coil, and the radial Y superconducting shimming coil. Determine the direction of the Z-axis and the current direction of the Z1 superconducting shimming coil according to the magnetic field direction of the Z1 superconducting shimming coil, determine the direction of the X-axis and the current direction of the X superconducting shimming coil according to the magnetic field direction of the X superconducting shimming coil, and determine the direction of the Y-axis and the current direction of the Y superconducting shimming coil according to the magnetic field direction of the Y superconducting shimming coil. Then, based on the Cartesian coordinate system composed of the Z-axis, X-axis, and Y-axis, determine the corresponding current directions according to the net magnetic field directions of the remaining superconducting shimming coils. In this step, it is necessary to compare the measured net magnetic field distribution of the superconducting shimming coil with the theoretically calculated magnetic field distribution of the superconducting shimming coil to find the phase difference between the two, and accordingly determine the current direction and spatial orientation of the superconducting shimming coil;

[0040] In the fourth step, open the superconducting shimming coils one by one again, eliminate the induced current in the superconducting shimming coils by heating the superconducting switches, and measure the magnetic field distribution on the spherical surface in the central region of the magnet, which is the initial magnetic field. Then, according to the current direction and spatial orientation of the superconducting shimming coil determined in the third step, use the magnetic field distribution generated by a unit current of the superconducting shimming coil multiplied by the corresponding current intensity to be solved to perform a fitting operation on the initial magnetic field. Pass the calculated current intensity into all superconducting shimming coils and close the superconducting shimming coils, and then measure the magnetic field distribution on the spherical surface in the central region of the magnet and evaluate the magnetic field uniformity. The magnetic field of the coil in the imaging region can be calculated according to formula (1):

[0041]

[0042] where μ0 is the vacuum permeability, M is the total number of turns of the coil, N is the number of units per turn of the coil, l ij is the position coordinate of the j-th unit of the i-th turn of the coil, dl ij is the direction vector of the line segment l ij , r is the position coordinate of the magnetic field point, I k is the current intensity of the superconducting shimming coil, and B(r) is the magnetic field intensity generated by the superconducting shimming coil at the magnetic field point r.

[0043] The solution of the superconducting shimming coil current uses the least squares method, that is, to solve the optimal current combination to minimize the magnetic field non-uniformity in the central region. The corresponding optimization equations are shown in formulas (2) and (3):

[0044] Minimize:

[0045] Constraint condition: -I max ≤I≤Imax (3)

[0046] Among them, b n is the magnetic field intensity at the nth sampling point on the spherical surface of the central region of the magnet. There are a total of S sampling points. A is the magnetic field transfer coefficient matrix, that is, the calculated value after removing the current I in formula (1) k I is the current vector of the superconducting shimming coil, x is the central magnetic field intensity to be solved, and I max is the maximum current that can be passed through the superconducting shimming coil.

[0047] The calculation of the magnetic field uniformity is shown in formula (4).

[0048] (max(b)-min(b)) / mean(b) (4)

[0049] Among them, b is the magnetic field intensity vector on the spherical surface of the central region of the magnet, max(b) is the maximum magnetic field intensity value, min(b) is the minimum magnetic field intensity value, and mean(b) is the average magnetic field intensity value;

[0050] Step 5: Take the magnetic field distribution on the spherical surface of the central region of the magnet after shimming in the fourth step as the initial magnetic field, and further use a single superconducting shimming coil for shimming. Calculate the expected magnetic field uniformity when each superconducting shimming coil is used alone, and compare the expected magnetic field uniformities when all superconducting shimming coils are used alone. Select the superconducting shimming coil with the optimal expected magnetic field uniformity for magnetic field regulation. The optimization equations when the superconducting shimming coil is used alone are shown in formulas (5) and (6):

[0051] Minimize:

[0052] Constraint condition: -I max ≤I k ≤I max (6)

[0053] Here A k is the magnetic field transfer coefficient matrix of the kth superconducting shimming coil, and the meanings of other parameters are the same as those in formulas (2) and (3).

[0054] Pass the calculated current intensity into the superconducting shimming coil and close the superconducting shimming coil, and then measure the magnetic field distribution on the spherical surface of the central region of the magnet. Perform the magnetic field regulation calculation and test of the remaining superconducting shimming coils one by one in this way until the magnetic field uniformity is improved to the highest level.

[0055] The superconducting shimming device 1 involved in the present invention is installed in the superconducting magnet 2, as shown in Figure 2As shown, it is assumed that the horizontal direction of the superconducting magnet 2 is the x-axis, the vertical direction is the y-axis, and the axial direction along the superconducting magnet 2 is the z-axis. However, the spatial orientation of the superconducting field shimming device 1 after being assembled into the superconducting magnet 2 is not determined. The actual spatial orientation of the superconducting field shimming coil 3 determined according to the operations in the second and third steps of the present invention is represented as the x'-axis, y'-axis, and z'-axis in the figure.

[0056] Figure 3 The connection schematic diagram of the superconducting field shimming coil is shown. The superconducting field shimming coil 3 and the corresponding superconducting switch 4 form a closed loop. Each superconducting switch 4 is wound with a heating wire, and the corresponding superconducting switch heating wire lead 5 is connected. All the superconducting field shimming coils 3 are connected in series, and current is applied through a pair of superconducting field shimming coil leads 6.

[0057] Figure 4 This is the theoretical magnetic field distribution of the superconducting field shimming coil. The curves shown by the Z1 shimming coil and the Z2 shimming coil are the magnetic field distributions on the axis of the superconducting field shimming device 1 from -l to l. The curves shown by the X shimming coil, Y shimming coil, X2-Y2 shimming coil, and XY shimming coil are the magnetic field distributions in the circumferential direction from 0 to 2π at the position of the central symmetry plane of the superconducting field shimming device. The curves shown by the ZX shimming coil and ZY shimming coil are the magnetic field distributions in the circumferential direction from 0 to 2π at the plane position l / 2 in the positive z-axis direction at the position of the central symmetry plane of the superconducting field shimming device. The above theoretical magnetic field distribution is used to compare with the net magnetic field distribution of the superconducting field shimming coil obtained by actual measurement, find the phase difference between the two, and thereby determine the current direction and spatial orientation of the superconducting field shimming coil.

[0058] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included within the protection scope of the present invention.

Claims

1. A superconducting shimming control method, characterized in that, It includes the following steps: In the first step, open the superconducting shim coils one by one, and eliminate the induced current in the superconducting shim coils by heating the superconducting switches; In the second step, measure the magnetic field distribution on the central axis in the central region of the magnet as the basic magnetic field. Then, open an axial superconducting shim coil, after applying a certain intensity of current, close the axial superconducting shim coil, and measure the magnetic field distribution on the central axis in the central region of the magnet again. Then, open the axial superconducting shim coil and remove the current in the axial superconducting shim coil. Perform such operations to measure other axial superconducting shim coils. Then, measure the magnetic field distribution on the spherical surface in the central region of the magnet as the basic magnetic field. Then, open a radial superconducting shim coil, after applying a certain intensity of current, close the radial superconducting shim coil, and measure the magnetic field distribution on the spherical surface in the central region of the magnet again. Then, open the radial superconducting shim coil and remove the current in the radial superconducting shim coil. Perform such operations to measure other radial superconducting shim coils; In the third step, subtract the measured magnetic field of the superconducting shim coil from the corresponding basic magnetic field to obtain the net magnetic field of the superconducting shim coil. The superconducting shim coil at least includes first-order shim coils, namely, the axial Z1 superconducting shim coil, the radial X superconducting shim coil, and the radial Y superconducting shim coil. Determine the direction of the Z coordinate axis and the current direction of the Z1 superconducting shim coil according to the magnetic field direction of the Z1 superconducting shim coil, determine the direction of the X coordinate axis and the current direction of the X superconducting shim coil according to the magnetic field direction of the X superconducting shim coil, determine the direction of the Y coordinate axis and the current direction of the Y superconducting shim coil according to the magnetic field direction of the Y superconducting shim coil. Then, taking the Cartesian coordinate system composed of the Z coordinate axis, the X coordinate axis, and the Y coordinate axis as the reference, determine the corresponding current directions according to the net magnetic field directions of the remaining superconducting shim coils; In the fourth step, open the superconducting shim coils one by one again, and eliminate the induced current in the superconducting shim coils by heating the superconducting switches. Measure the magnetic field distribution on the spherical surface in the central region of the magnet as the initial magnetic field. Then, according to the current directions and spatial orientations of the superconducting shim coils determined in the third step, perform a fitting operation on the initial magnetic field by multiplying the magnetic field distribution generated by the unit current of the superconducting shim coil by the corresponding current intensity to be solved. Pass the calculated current intensity into all superconducting shim coils and close the superconducting shim coils. Then, measure the magnetic field distribution on the spherical surface in the central region of the magnet and evaluate the magnetic field uniformity; In the fifth step, the magnetic field distribution on the spherical surface in the central region of the magnet after field shimming in the fourth step is used as the initial magnetic field. Further, a single superconducting field shimming coil is used for field shimming. The expected magnetic field uniformity when each superconducting field shimming coil is used alone is calculated, and the expected magnetic field uniformities when all superconducting field shimming coils are used alone are compared. The superconducting field shimming coil with the optimal expected magnetic field uniformity is selected for magnetic field regulation. The calculated current intensity is passed into this superconducting field shimming coil and the superconducting field shimming coil is closed. Then, the magnetic field distribution on the spherical surface in the central region of the magnet is measured. Such operations are carried out one by one for the magnetic field regulation calculation and test of the remaining superconducting field shimming coils until the magnetic field uniformity is increased to the highest level.

2. A superconducting field shimming regulation method according to claim 1, characterized in that: In the first step, the determination criterion for whether there is residual induced current in the superconducting field shimming coil is: when heating the heating wire on the superconducting switch of the superconducting field shimming coil, measure whether there is voltage at both ends of the incoming and outgoing lines of the superconducting field shimming coil. If there is voltage, it means that the induced current has not been completely eliminated. Continue to heat the heating wire of the superconducting switch until the voltage at both ends of the incoming and outgoing lines of the superconducting field shimming coil completely disappears.

3. A superconducting field shimming regulation method according to claim 1, characterized in that: In the second step, before each current loading on the superconducting field shimming coil, the basic magnetic field is measured, and the number of sampling points for measuring the magnetic field distribution is set according to actual needs; for the axial superconducting field shimming coil, since the magnetic field is circularly distributed, the required number of sampling points only needs to meet the criterion for determining the coil current direction; for the radial superconducting field shimming coil, since the magnetic field is non-circularly distributed, the required number of sampling points needs to meet both the criterion for determining the coil current direction and the criterion for determining the spatial orientation of the coil in the circumferential direction.

4. A superconducting field shimming regulation method according to claim 1, characterized in that: In the third step, the measured net magnetic field distribution of the superconducting field shimming coil is compared with the magnetic field distribution of the superconducting field shimming coil obtained by theoretical calculation, and the phase difference between the two is found, and based on this, the current direction and spatial orientation of the superconducting field shimming coil are determined.

5. A superconducting field shimming regulation method according to claim 1, characterized in that: In the fourth step, the magnetic field of the coil in the imaging region is calculated according to formula (1): （1） where μ0 is the magnetic permeability of vacuum, M is the total number of turns of the coil, N is the number of units per turn of the coil, l ij is the position coordinate of the j-th unit of the i-th turn of the coil, dl ij is the direction vector of the line segment l ij r is the position coordinate of the magnetic field point, I k is the current intensity of the superconducting shim coil, and B(r) is the magnetic field intensity generated by the superconducting shim coil at the magnetic field point r; The solution of the current of the superconducting field shimming coil adopts the least squares method, that is, the optimal current combination is solved to minimize the magnetic field non-uniformity in the central region. The corresponding optimization equations are shown in formulas (2) and (3): Minimization: (2) Constraints: (3) Among them, b n is the magnetic field strength at the nth sampling point on the spherical surface of the central region of the magnet. There are a total of S sampling points. A is the magnetic field transfer coefficient matrix, that is, the calculated value after removing the current I in formula (1). k I is the superconducting shimming coil current vector, x is the central magnetic field strength to be solved, and I max is the maximum current that can be passed through the superconducting shimming coil; The calculation of the magnetic field uniformity is shown in formula (4): （4） where b is the magnetic field intensity vector on the spherical surface in the central region of the magnet, max(b) is the maximum magnetic field intensity value, min(b) is the minimum magnetic field intensity value, and mean(b) is the average magnetic field intensity value.

6. A superconducting field shimming regulation method according to claim 5, characterized in that: In the fifth step, the optimization equations when the superconducting field shimming coil is used alone are shown in formulas (5) and (6): Minimize: (5) Constraints: (6) Here A k is the magnetic field transfer coefficient matrix of the k-th superconducting shim coil, and the meanings of other parameters are the same as those in formulas (2) and (3).

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