Multi-target planning method for MRI passive shimming

Abstract

The invention relates to the technical field of MRI magnet systems, in particular to a multi-target planning method for MRI passive shimming. Acquiring magnetic field data of a target measuring point, and preprocessing the magnetic field data; calculating the magnetic field influence of the shimming sheet with unit thickness on the target measuring point; constructing a sensitivity array shiminfl (j, i, jj, ii); constructing a target function; generating a solving file according to the objective function and a preset constraint condition, and outputting a shimming sheet distribution scheme through a solver; and performing shimming operation according to the shimming sheet distribution scheme. By presetting the weight coefficients K1, K2 and K3, the relationship among the peak-to-peak value (P-P), the root-mean-square error (RMSE) and the shimming piece use amount can be flexibly adjusted, the requirements of different application scenes are met, and the shimming flexibility and applicability are remarkably improved; by constructing the objective function and the constraint condition, the magnetic field uniformity is accurately controlled, the peak-to-peak value and the root-mean-square error are effectively reduced, and therefore the quality of the MRI image is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of MRI magnet system, in particular to a multi-objective planning method for MRI passive shimming. BACKGROUND

[0002] In a magnetic resonance imaging (MRI) system, the key to obtaining high-quality images lies in the high uniformity of the main magnetic field (B0 field). Passive shimming technology (PS) compensates for magnetic field inhomogeneity by using shimming iron sheets, which has the advantages of high stability, no power consumption and low cost. However, the traditional passive shimming method has the following technical problems: first, it is difficult to dynamically adjust the relationship between the peak-to-peak value (P-P), the root mean square error (RMSE) and the shimming sheet usage during the shimming process, resulting in limited shimming effect; second, the existing method cannot flexibly adjust the shimming parameters according to different application scenarios, making it difficult to optimize the usage of shimming sheets while meeting the demand for magnetic field uniformity; finally, the traditional method is time-consuming and laborious, and the precision and efficiency of the shimming result are difficult to balance. Therefore, there is an urgent need for a passive shimming method that can dynamically adjust the shimming parameters, adapt to different scene requirements and quickly output results to solve the above technical problems. SUMMARY

[0003] The purpose of the present application is to provide a multi-objective planning method for MRI passive shimming to improve the problem that it is difficult to dynamically adjust the relationship between the peak-to-peak value (P-P), the root mean square error (RMSE) and the shimming sheet usage during the shimming process, resulting in limited shimming effect; second, the existing method cannot flexibly adjust the shimming parameters according to different application scenarios, making it difficult to optimize the usage of shimming sheets while meeting the demand for magnetic field uniformity and the traditional method is time-consuming and laborious, and the precision and efficiency of the shimming result are difficult to balance.

[0004] To achieve the above purpose, the embodiments of the present application provide the following technical solutions: The embodiments of the present application provide a multi-objective planning method for MRI passive shimming, comprising the following steps: Step one: obtaining the magnetic field data of the target measurement point and preprocessing the magnetic field data; Step two: calculating the magnetic field influence of the unit thickness shimming sheet on the target measurement point, the calculation formula is: ; In the formula, B is the magnetic field intensity generated by the magnetic dipole in space; is the vacuum permeability; is the polar angle of the measurement point relative to the shimming sheet; r is the distance from the shimming sheet to the measurement point; M is the magnetization intensity of the shimming sheet; Step three: constructing a sensitivity array shiminfl(j, i, jj, ii) representing the influence of the i-th shim strip in the j-th shim strip on the ii-th target point on the jj-th dsv arc line; Step four: constructing a target function: ; In the formula, K i is a weight coefficient; SP9999 represents a peak-to-peak value PPM; SP9997 represents positive uniformity; SM9998 represents negative uniformity; m and n respectively represent the number of shim strips and the number of single shim strip grids; represents the number of shim pieces to be added in the j-th shim grid; represents the number of shim pieces to be reduced in the j-th shim grid; Step five: generating a solving file according to the target function and a preset constraint condition, and outputting a shim piece distribution scheme by a solver; Step six: performing a shim operation according to the shim piece distribution scheme.

[0005] Preferably, the preset constraint condition further comprises: ; In the formula, fave represents the average field strength of the current measurement point; a sensitivity matrix of the measurement point to the shim piece; fieldmax represents the maximum field strength of the current measurement point; fieldold represents the corresponding field strength of the current measurement point; tar represents the target uniformity of the measurement point; and ppm target represents a global peak-to-peak value PPM.

[0006] Preferably, the preset constraint condition further comprises: ; In the formula, represents the maximum number of single grids allowed by the shim piece; represents the number of shim pieces originally in the shim grid; and shimtotal represents the number of all shim pieces allowed.

[0007] Preferably, the weight coefficients K1, K 2 、 K3 are dynamically adjusted according to the shim requirements to adapt to different application scenarios.

[0008] Preferably, the shim piece distribution scheme comprises the number of shim pieces to be added, the number of shim pieces to be reduced, and the position information of the shim pieces in the shim grid.

[0009] Preferably, the magnetic field data of the target measurement point are obtained by a magnetic field camera measurement, and the magnetic field camera probe can be of any number and can be of any spherical or ellipsoidal shape in diameter.

[0010] Preferably, the shim strips and the shim grid can be of any number, and the shim strips are inserted into the gradient coil to perform the shim operation.

[0011] The beneficial effects of the present application are: By presetting the weight coefficients K1, K2 and K3, the relationship between the peak-to-peak value (P-P), the root mean square error (RMSE) and the amount of shim sheets can be flexibly adjusted to adapt to the needs of different application scenarios, significantly improving the flexibility and applicability of the shimming; by constructing the objective function and the constraint condition, the accuracy of the magnetic field uniformity is realized, the peak-to-peak value and the root mean square error are effectively reduced, and the quality of the MRI image is improved; the sensitivity array and the solver are used to generate the shim sheet distribution scheme, which greatly reduces the manual intervention and the calculation time, making the shimming process more efficient; by optimizing the amount of shim sheets, the waste of resources caused by excessive use of shim sheets is avoided, and the shimming cost is reduced; the method can be applied to different types of MRI systems, and has high universality and scalability.

[0012] Through the above technical solutions, the present application successfully solves the technical problems of the traditional passive shimming method, such as difficulty in dynamically adjusting parameters, limited shimming effect and low efficiency, and provides an efficient, flexible and economical solution for the magnetic field shimming of the MRI system.

[0013] Other features and advantages of the present application will be set forth in the following description, and in part will become apparent to those skilled in the art from the description, or can be learned by practice of the present application as described in the written description and claims. The objects and other advantages of the present application will be realized and attained by the structure particularly pointed out in the written description and claims. BRIEF DESCRIPTION OF DRAWINGS

[0014] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope, and for those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0015] Figure 1 is a flow diagram of a multi-objective planning method for passive shimming of MRI described in the embodiments of the present application; Figure 2 is a gradient coil diagram of a multi-objective planning method for passive shimming of MRI described in the embodiments of the present application; Figure 3 is a shim strip diagram of a multi-objective planning method for passive shimming of MRI described in the embodiments of the present application; Figure 4A schematic diagram of the shimming slice distribution scheme for a multi-objective planning method for passive shimming in MRI, as described in this embodiment of the invention. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0017] It should be noted that similar reference numerals or letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. Furthermore, in the description of this invention, terms such as "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0018] Example 1:

[0019] like Figure 1 As shown, this embodiment provides a multi-objective planning method for passive shimming in MRI, the method including steps S100, S200, S300, S400, S500 and S600.

[0020] Step S100: Obtain the magnetic field data of the target measuring point and preprocess the magnetic field data; Step S200: Calculate the influence of the unit thickness uniform field sheet on the magnetic field of the target measuring point. The calculation formula is as follows: In the formula, B is the magnetic field strength generated by the magnetic dipole in space; Permeability of free space; θ is the polar angle of the measuring point relative to the shim plate; r is the distance from the shim plate to the measuring point; M is the magnetization intensity of the shim plate. Step S300: Construct a sensitivity array shiminfl(j,i,jj,ii), which represents the influence of the i-th uniform grid in the j-th uniform strip on the ii-th target point on the jj-th dsv arc; Step S400: Construct the objective function: ; In the formula, K i are weight coefficients; SP9999 represents a peak-to-peak value PPM; SP9997 represents positive uniformity; SM9998 represents negative uniformity; m and n respectively represent the number of shim bars and the number of shim grids; represents the number of shim pieces to be added in the jth shim grid; represents the number of shim pieces to be reduced in the jth shim grid; Step S500, generating a solving file according to the objective function and a preset constraint condition, and outputting a shim piece distribution scheme through a solver, as shown in the following table: Figure 4 which is a schematic diagram of a 32*24 shim piece distribution scheme, wherein the column items represent shim bars, the horizontal items represent the number of shim pieces in the corresponding cells in each shim bar, and the numbers in the second column at the right end represent the total number of shim pieces in a shim bar; Step S600, performing a shim operation according to the shim piece distribution scheme.

[0021] By presetting the weight coefficients K1, K2 and K3, the relationship between the peak-to-peak value (P-P), the root mean square error (RMSE) and the amount of shim pieces can be flexibly adjusted to adapt to the needs of different application scenarios, and the flexibility and applicability of the shim are significantly improved; by constructing the objective function and the constraint condition, the magnetic field uniformity is accurately controlled, the peak-to-peak value and the root mean square error are effectively reduced, and the quality of the MRI image is improved; by using the sensitivity array and the solver to generate the shim piece distribution scheme, the manual intervention and the calculation time are greatly reduced, and the shim process is more efficient; by optimizing the amount of shim pieces, the waste of resources caused by excessive use of shim pieces is avoided, and the shim cost is reduced; the method can be applied to different types of MRI systems and has high universality and scalability.

[0022] Through the above technical solutions, the technical problems such as difficulty in dynamically adjusting parameters, limited shim effect and low efficiency in the traditional passive shim method are successfully solved, and an efficient, flexible and economical solution for the magnetic field shim of the MRI system is provided.

[0023] In this embodiment, data acquisition and preprocessing are performed first: target measurement point magnetic field data are read and standardized processing is performed; shim piece magnetic field influence modeling is used: the magnetic field contribution of a unit thickness shim piece to the target measurement point is calculated; a sensitivity matrix is constructed: a magnetic field influence relationship matrix between the shim piece and the target measurement point is established; an objective function is optimized: a multi-objective optimization model is constructed in combination with the weight coefficient, and the optimal shim piece distribution is solved; constraint conditions are set: the number of shim pieces, the maximum amount of single grid and the like are limited to ensure the feasibility of the scheme; finally, solving and execution are performed: a shim scheme is output through an optimization algorithm, and a shim operation is performed.

[0024] Data acquisition and preprocessing are performed in step S100. First, magnetic field data acquisition is performed. A magnetic field camera (such as MFC9046) with a 32x32 probe is used to measure the magnetic field distribution of an imaging area with a diameter of 500 mm. The specific operation mode of the 32x32 probe magnetic field camera is briefly described as follows. Thirty-two probes are arranged on an arc segment, and then a circle is divided into 32 equal parts and quantitatively rotated and measured (i.e., the magnetic field strength is measured and recorded after the arc segment is rotated by 11.25°, and then the magnetic field distribution data in the target area is obtained). It should be noted that the number of probes and the angle of quantitative rotation are not limited in the present embodiment, and can be changed by those skilled in the art according to actual needs. p The average field strength f ave , the maximum field strength f max , and the minimum field strength f min are calculated.

[0025] The original magnetic field data is converted to ppm units: The processed magnetic field data matrix fieldold is output, which is used for subsequent calculation, thereby ensuring uniform data format and facilitating subsequent modeling and optimization.

[0026] A magnetic dipole model is established. The shim can be regarded as a magnetic dipole, and the magnetic field strength B generated by the magnetic dipole in space is calculated by the following formula: ; The magnetic field contribution of each shim grid (such as 24x24 grid) to all target measurement points is calculated to generate a basic influence matrix, thereby quantifying the correction ability of the shim to the magnetic field and laying a foundation for the construction of the sensitivity matrix.

[0027] A sensitivity array shiminfl(j,i,jj,ii) is defined, j: shim bar number (such as 24 bars); i: shim grid number (such as 24 grids per bar); jj: DSV (imaging area) arc line number; ii: measurement point number on the arc line. Based on the magnetic dipole model of step S200, the magnetic field influence value of each shim grid (j, i) on each measurement point (jj, ii) is calculated, the shiminfl matrix is filled, and the quantitative relationship between the shim and the magnetic field measurement point is established to provide input data for the optimization problem, wherein the objective function is: ; K1 、K2、 k3 is a weight coefficient (the weight coefficient can be adjusted, such as K1 =0.5, K2=0.3, k3=0.2); SP9999 is the peak-to-peak value PPM; SP9997 represents positive uniformity; SM9998 represents negative uniformity; represents the number of shims increased in the jth shim grid. represents the number of shims reduced in the jth shim bin, the optimization objective, the minimization of TH, i.e. balancing P-P, RMSE and the amount of shims; by adjusting the weight coefficients, P-P can be prioritized (e.g. K1 is adjusted higher) or the amount of shims can be reduced (e.g. K3 is adjusted higher).

[0028] Secondly, the preset constraint condition in step S500 further includes: In the formula, fave represents the average field strength of the current measurement point; fieldmax represents the maximum field strength in the current measurement point; fieldold represents the corresponding field strength of the current measurement point; tar represents the target uniformity of the measurement point; and ppmtarget represents the global peak-to-peak value PPM.

[0029] Secondly, the preset constraint condition in step S500 further includes: ; In the formula, represents the maximum number of shims allowed in a single bin; represents the number of shims originally in the shim bin; and shimtotal represents the total number of shims allowed.

[0030] In step S400 of the embodiment, the weight coefficients K1, K 2 、 K3 is dynamically adjusted according to the shim requirement to adapt to different application scenarios.

[0031] The magnetic field uniformity constraint and the shim number constraint are performed to ensure that the shim scheme is physically feasible and to avoid excessive use of shims.

[0032] The objective function and the constraint condition are input into an optimization solver (such as a linear programming tool); and a shim scheme is output: the solver returns the SP j and SM j values of each shim bin, and a shim distribution map (such as Figure 4 ) is generated; shim bars (such as 24 shim bars) as shown in Figure 3 are inserted into the gradient coil (such as Figure 2 ), and the shims are placed according to the scheme; and an optimal shim scheme is quickly obtained, which significantly improves the magnetic field uniformity.

[0033] Embodiment 2:

[0034] This embodiment is based on embodiment 1 and is used to present two groups of specific optimization effects based on the method described in embodiment 1: Case 1: Before shim: P-P = 550 ppm, RMSE = 2.16e-4.

[0035] After optimization: Calculated: P-P = 15.08 ppm, RMSE = 7.35e-6; Measured: P-P = 15.06 ppm, RMSE = 7.35e-6.

[0036] Case 2: Before shimming: P-P = 434 ppm, RMSE = 1.78e-4.

[0037] After optimization: Calculated: P-P = 14.8 ppm, RMSE = 7.39e-6; Measured: P-P = 15.3 ppm, RMSE = 7.34e-6.

[0038] As can be seen from the specific embodiments, by adjusting the weight coefficient, different MRI devices and imaging requirements can be adapted; P-P and RMSE are significantly reduced; the calculation time is shortened from several hours of traditional method to minutes; the amount of shimming sheet is reduced by 10%~20%, reducing the material cost.

[0039] In step S500 of the embodiment, the shimming sheet distribution scheme includes the increase in the number of shimming sheets, the decrease in the number of shimming sheets, and the position information of the shimming sheets in the shimming grid.

[0040] In step S100 of the embodiment, the magnetic field data of the target measurement point is obtained by measuring with a magnetic field camera, the number of probes of the magnetic field camera is 32x32, and the measurement diameter is 500mm.

[0041] In step S300 of the embodiment, the number of shimming strips is 24, the number of shimming grids in each shimming grid is 24, and the shimming strips are inserted into the gradient coil to perform the shimming operation.

[0042] When shimming, the shimming strips need to be inserted into the gradient coil, as shown in Figure 2 , the shimming strips are as shown in Figure 3 , specifically, in the optimization process, the shimming strips are arranged in a plurality of mosaic strip holes in the gradient coil in a ring matrix distribution, and the magnetic field distribution parameters in the gradient coil are adjusted by arranging different numbers of shimming sheets in the unit cells of the shimming strips. The gradient coil adopts MFC22, and the magnetic field camera adopts MFC9046. The gradient coil has 32x24 shimming grids, and the magnetic field camera has 32x32 probes, with a measurement diameter of 500mm. Now, shimming is performed on two different magnetic resonance systems respectively:

[0043] The specific shimming process of the above case 1 and case 2 is as follows: set the initial peak-to-peak value / root mean square error before shimming, optimize and calculate the peak-to-peak value / root mean square error (theoretical value) by the method described in embodiment 1, then set the number of shim strips distributed in each cell in the plurality of shim strips in the gradient coil according to the optimized shim strip distribution scheme, then measure the peak-to-peak value / root mean square error to obtain the measured peak-to-peak value / root mean square error, input the measured peak-to-peak value / root mean square error into the algorithm described in embodiment 1 again, and optimize it to obtain a second optimized shim strip distribution scheme, repeat the above steps until the magnetic field distribution in the gradient coil meets the preset requirements.

[0044] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Those skilled in the art can make various modifications and changes to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A multi-objective planning method for MRI passive shimming, characterized in that, The method comprises: acquiring magnetic field data of target measurement points and pre-processing the magnetic field data; calculating the magnetic field influence of a unit-thickness shim on the target measurement points, the magnetic field influence being the magnetic field intensity generated by a magnetic dipole in space: constructing a sensitivity array shiminfl(j,i,jj,ii), wherein each element represents the influence of the i-th shim cell in the j-th shim bar on the ii-th target point on the jj-th dsv arc line; constructing an objective function, generating a solving file according to the objective function and a preset constraint condition, and outputting a shim distribution scheme by a solver; performing a shim operation according to the shim distribution scheme.

2. The multi-objective planning method for MRI passive shimming according to claim 1, wherein, The specific way of calculating the magnetic field influence of a unit-thickness shim on the target measurement points is: ； where B is the magnetic field strength produced by the magnetic dipole in space; is the vacuum permeability; is the polar angle of the measurement point relative to the shim; r is the distance from the shim to the measurement point; and M is the magnetization of the shim.

3. The multi-objective planning method for MRI passive shimming according to claim 1, wherein, The objective function is: ； In the formula, K1 is a weight coefficient; SP9999 represents a peak-to-peak value PPM; SP9997 represents positive uniformity; SM9998 represents negative uniformity; m and n respectively represent the number of shim strips and the number of single shim strip grids; represents the number of increased shim strips in the jth shim grid; represents the number of decreased shim strips in the jth shim grid.

4. The multi-objective planning method for MRI passive shimming according to claim 1, wherein, The preset constraint condition further comprises: ； In the formula, fave represents the average field strength of the current measurement point; is the sensitivity matrix of the measurement point to the shim sheet; fieldmax represents the maximum field strength in the current measurement point; fieldold represents the corresponding field strength of the current measurement point; tar represents the target homogeneity of the measurement point; and ppmtarget represents the global peak-to-peak value PPM.

5. The multi-objective planning method for MRI passive shimming according to claim 1, wherein, The preset constraint condition further comprises: ； In the formula, represents the maximum number of allowed single-shim slices; represents the number of shim slices originally within the shim bin; shimtotal represents the allowed number of all shim slices.

6. The multi-objective planning method for MRI passive shimming according to claim 3, wherein, The weight coefficients K1, K 2 、 K3 is dynamically adjusted according to the requirement of the shimming to adapt to different application scenarios.

7. The multi-objective planning method for MRI passive shimming according to claim 1, wherein, The shim distribution scheme comprises the increase number, the decrease number and the position information of the shim in the shim cell.

8. The multi-objective planning method for MRI passive shimming according to claim 1, wherein, The magnetic field data of the target measurement points is obtained by a magnetic field camera, the magnetic field camera probe can be of any number, and the measurement diameter can be of any spherical or ellipsoidal shape.

9. The multi-objective planning method for MRI passive shimming according to claim 1, wherein, The shim bar and the shim cell can be of any number, and the shim bar is inserted into a gradient coil to perform a shim operation.

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