CN116305798A - Superconducting magnetic resonance magnet passive shimming optimization design method and system - Google Patents
Superconducting magnetic resonance magnet passive shimming optimization design method and system Download PDFInfo
- Publication number
- CN116305798A CN116305798A CN202310078452.0A CN202310078452A CN116305798A CN 116305798 A CN116305798 A CN 116305798A CN 202310078452 A CN202310078452 A CN 202310078452A CN 116305798 A CN116305798 A CN 116305798A
- Authority
- CN
- China
- Prior art keywords
- shimming
- magnetic field
- bare
- epsilon
- superconducting
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Pending
Links
Images
Classifications
-
- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F30/00—Computer-aided design [CAD]
- G06F30/20—Design optimisation, verification or simulation
-
- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F2111/00—Details relating to CAD techniques
- G06F2111/10—Numerical modelling
-
- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F2119/00—Details relating to the type or aim of the analysis or the optimisation
- G06F2119/02—Reliability analysis or reliability optimisation; Failure analysis, e.g. worst case scenario performance, failure mode and effects analysis [FMEA]
Landscapes
- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Theoretical Computer Science (AREA)
- Computer Hardware Design (AREA)
- Evolutionary Computation (AREA)
- Geometry (AREA)
- General Engineering & Computer Science (AREA)
- General Physics & Mathematics (AREA)
- Magnetic Resonance Imaging Apparatus (AREA)
Abstract
The invention provides a passive shimming optimization design method and system of a superconducting magnetic resonance magnet, which belong to the technical field of superconducting magnet system optimization, and the method comprises the following steps: simulating the magnetic field drift condition caused by geometric parameters and electromagnetic parameter tolerance by Monte Carlo to obtain n groups of bare magnetic field groups to be shimmed; selecting the value ranges of four shimming factors according to the size requirement of the magnet, and constructing a DOE test set; carrying out shimming on a bare magnetic field with the largest non-uniformity by utilizing a bipartite improved OTMF algorithm to each test group and taking the minimum total thickness of a shimming piece as an objective function; selecting a plurality of shimming design methods according to shimming results; respectively carrying out shimming on n groups of bare magnetic field groups to be shimmed under a plurality of shimming design methods; and selecting a shimming design method with the best shimming effect as the best passive shimming optimization design. The invention completes the passive shimming optimization of the superconducting magnetic resonance magnet by designing four shimming factors.
Description
Technical Field
The invention belongs to the technical field of superconducting magnet system optimization, and particularly relates to a passive shimming optimization design method and system for a superconducting magnetic resonance magnet.
Background
Magnetic Resonance Imaging (MRI) systems are important medical imaging devices in modern clinical diagnosis. The magnetic field strength and uniformity of the central spherical imaging region (Diameter of spherical volume, DSV) determine the imaging quality. However, due to factors such as manufacturing, engineering installation, and coil material tolerances, the magnetic field strength of the central spherical imaging region is easily changed, resulting in difficulty in achieving high quality imaging requirements for the uniformity of the DSV region. Therefore, shimming techniques are often used to correct for the inhomogeneities in the DSV region. The shimming method of an MRI system mainly comprises two methods: active shimming methods and passive shimming methods. Passive shimming requires no additional power and coils to generate the corrective magnetic field, as opposed to active shimming methods that require shimming coils to be energized, which merely rely on shimming pieces placed in the magnet cavity. Compared with the active shimming technology, the passive shimming has the advantages of low cost, simple operation and the like.
The existing magnetic resonance passive shimming method takes the whole thickness of an iron sheet as an objective function, takes the non-uniformity of a magnetic field peak value as a constraint, or takes the root mean square value of magnetic field uniformity as a constraint, realizes a better shimming effect under a fixed shimming design parameter, and does not provide a method for optimally designing the Wen Kongshi MRI passive shimming parameter. There are also some patents which propose a two-step shimming strategy for fine shimming, i.e. a shimming operation method, and which do not mention how to optimally design Wen Kongshi MRI passive shimming parameters. Chinese patent publication No. CN 114970861A proposes a design method of open MRI passive shimming, and determines shimming design parameters such as coil thickness and coil width through genetic algorithm, but the object is open MRI, and no description is given of how to optimally design the Wen Kongshi MRI passive shimming parameters. Therefore, the passive shimming technology proposed in the prior invention is a novel technology in the aspects of novel structure, shimming operation or shimming algorithm, and does not mention a method for optimally designing Wen Kongshi MRI passive shimming parameters.
Therefore, it is needed to provide a passive shimming optimization design method for designing shimming parameters such as the number of circumferential shimming strips, the number of cavities of each shimming strip, the size of shimming pieces, and the spacing between shimming pieces.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a superconducting magnetic resonance magnet passive shimming optimization design method and system, which aim to solve the problem of how to design optimal parameters of four shimming factors, namely the number of circumferential shimming strips, the number of cavities of each shimming strip, the size of shimming pieces and the distance between shimming pieces, so as to optimize the passive shimming of the superconducting magnetic resonance magnet.
In order to achieve the above purpose, the invention provides a passive shimming optimization design method of a superconducting magnetic resonance magnet, which comprises the following steps:
s1: giving geometric parameters and electromagnetic parameters of a superconducting magnet to be shimmed;
s2: simulating the magnetic field drift condition caused by geometric parameters and electromagnetic parameter tolerance by Monte Carlo to obtain n groups of bare magnetic field groups to be shimmed;
s3: selecting the number of circumferential shimming strips, the number of cavities of each shimming strip, the size of shimming pieces and the value range of four shimming factors of the interval between the shimming pieces according to the size requirement of the magnet, and constructing a DOE test set;
s4: carrying out shimming on the bare magnetic field with the largest non-uniformity by utilizing a bipartite improved OTMF algorithm and taking the minimum total thickness of the shimming piece as an objective function to obtain a non-integer solution of the DOE (Design of Experiment) test group;
s5: selecting a plurality of shimming design methods according to shimming results of the DOE test group;
s6: respectively carrying out shimming on n groups of bare magnetic field groups to be shimmed under a plurality of shimming design methods to obtain integer solutions of the n groups of bare magnetic fields;
s7: and selecting a shimming design method with the best integer shimming effect as the optimal passive shimming optimization design.
Further preferably, the geometric parameters include cylinder size of the shim bar placement, individual shim thickness, and shim maximum thickness; the electromagnetic parameters include the number of superconducting coils of the bare wire tube, the positional relationship of the superconducting coils of the bare wire tube and the material of the superconducting wire.
Further preferably, the OTMF model in the OTMF algorithm is:
Min:Lx′
wherein x' = [ x ] 1 ,x 2 ,…,x M ,B t ] T T is the maximum thickness of the cavity capable of accommodating the shim, kappa is the coefficient of the control target magnetic field, B avr Average magnetic field intensity of the bare magnetic field corresponding to the group with the largest bare magnetic field unevenness; l= [1, …,1]Is (M+1) x 1 identity matrix; the sensitivity coefficient matrices A 'and A' can be expressed as:
x＝[x 1 ,x 2 ,…,x M ] T ，x i representing the thickness of the shim in the ith cavity; b (B) m A bare magnetic field corresponding to a group having the greatest bare magnetic field unevenness; b (B) t Is the target magnetic field; a is that i,j A magnetic field in the z-axis direction generated by the shim of unit thickness in the jth cavity to the ith sampling point; epsilon is the maximum unevenness allowed.
Further preferably, the specific implementation method of the bipartite modified OTMF algorithm comprises the following steps:
a. initializing epsilon min ＝0，ε max ＝H 0 Wherein H is 0 Is the non-uniformity of the DSV region before shimming; epsilon min Minimum to allow maximum non-uniformity; epsilon max To a maximum value that allows maximum non-uniformity;
b. taking out
Substituting into an OTMF model for solving;
c. if there is no solution, it indicates the minimum value H of the unevenness of the DSV region min Greater than epsilon, let epsilon min =ε, go to step b;
d. if there is a solution, it indicates the maximum size H of the unevenness of the DSV region min Smaller than epsilon, let epsilon max =ε; if epsilon max -ε min ≤10 -7 The unevenness of the DSV area reaches a set precision value, the OTMF algorithm is ended, and otherwise, the step b is switched to;
wherein, the calculation formula of the unevenness of the DSV area is:
wherein A is a sensitivity coefficient matrix with the size of N multiplied by M; m is the total number of cavities; n is the number of sampling points; a is that i,j And generating a magnetic field in the z-axis direction for the ith sampling point for the shim of unit thickness in the jth cavity.
In another aspect, the present invention provides a superconducting magnetic resonance magnet passive shimming optimization design system, comprising:
the parameter setting module is used for giving geometric parameters and electromagnetic parameters of the superconducting magnet to be shimmed;
the bare magnetic field group determining module is used for simulating the magnetic field drift condition caused by geometric parameters and electromagnetic parameter tolerance through Monte Carlo to obtain n groups of bare magnetic field groups to be shimmed;
the DOE test group construction module is used for selecting the number of circumferential shimming strips, the number of cavities of each shimming strip, the size of the shimming pieces and the value ranges of four shimming factors of the interval between the shimming pieces according to the size requirement of the magnet to construct a DOE test group;
the non-integer solution shimming module is used for shimming the bare magnetic field with the largest non-uniformity by utilizing a binary improved OTMF algorithm for each DOE test group and taking the minimum total thickness of the shimming piece as an objective function to obtain a non-integer solution of the DOE test group;
the initial shimming design scheme screening module is used for selecting a plurality of shimming design methods according to shimming results of the DOE test group;
the integer solution shimming module is used for shimming n groups of bare magnetic field groups to be shimmed under a plurality of shimming design methods respectively to obtain integer solutions of n groups of bare magnetic fields;
and the optimal passive shimming optimal design screening module is used for selecting a shimming design method with the optimal integer shimming effect as the optimal passive shimming optimal design.
Further preferably, the geometric parameters include cylinder size of the shim bar placement, individual shim thickness, and shim maximum thickness; the electromagnetic parameters include the number of superconducting coils of the bare wire tube, the positional relationship of the superconducting coils of the bare wire tube and the material of the superconducting wire.
Further preferably, the OTMF model in the OTMF algorithm is:
Min:Lx′
wherein x' = [ x ] 1 ,x 2 ,…,x M ,B t ] T T is the maximum thickness of the cavity capable of accommodating the shim, kappa is the coefficient of the control target magnetic field, B avr Average magnetic field intensity of the bare magnetic field corresponding to the group with the largest bare magnetic field unevenness; l= [1, …,1]I.e., (m+1) ×1 identity matrix; x= [ x ] 1 ,x 2 ,…,x M ] T ，x i Representing the thickness of the shim in the ith cavity; b (B) m A bare magnetic field corresponding to a group having the greatest bare magnetic field unevenness; b (B) t Is the target magnetic field; a is that i,j A magnetic field in the z-axis direction generated by the shim of unit thickness in the jth cavity to the ith sampling point; epsilon is the maximum allowable unevenness; the sensitivity coefficient matrices A 'and A' can be expressed as:
further preferably, the specific implementation method of the bipartite modified OTMF algorithm comprises the following steps:
a. initializing epsilon min ＝0，ε max ＝H 0 The method comprises the steps of carrying out a first treatment on the surface of the Wherein H is 0 Is the non-uniformity of the DSV region before shimming; epsilon min Minimum to allow maximum non-uniformity; epsilon max To a maximum value that allows maximum non-uniformity;
b. taking out
Substituting into an OTMF model for solving;
c. if there is no solution, it indicates the minimum value H of the unevenness of the DSV region min Greater than epsilon, let epsilon min =ε, go to step b;
d. if there is a solution, it indicates the maximum size H of the unevenness of the DSV region min Smaller than epsilon, let epsilon max =ε; if epsilon max -ε min ≤10 -7 The unevenness of the DSV area reaches a set precision value, the OTMF algorithm is ended, and otherwise, the step b is switched to;
wherein, the calculation formula of the unevenness of the DSV area is:
wherein A is a sensitivity coefficient matrix with the size of N multiplied by M; m is the total number of cavities; n is the number of sampling points; a is that i,j And generating a magnetic field in the z-axis direction for the ith sampling point for the shim of unit thickness in the jth cavity.
In general, the above technical solutions conceived by the present invention have the following compared with the prior art
The beneficial effects are that:
the invention provides a passive shimming optimization design method of a superconducting magnetic resonance magnet, wherein the number of circumferential shimming strips, the number of cavities of each shimming strip, the size of shimming pieces and the value ranges of four shimming factors of the interval between the shimming pieces are selected according to the size requirement of the magnet, and a DOE test set is constructed; based on the method, the optimal passive shimming optimization design is obtained in two steps, and in the first step, a binary improved OTMF algorithm is utilized for each test group of the DOE, so that the bare magnetic field with the largest non-uniformity is shimmed by using the minimum total thickness of the shimming piece as an objective function, and a non-integer solution is obtained; secondly, shimming is carried out on n groups of bare magnetic field groups to be shimmed respectively under a plurality of shimming design methods, and a shimming design method with the best integer shimming effect is screened out to be used as the optimal passive shimming optimization design. A superconducting magnet of 1.5T designed with optimal passive shimming achieves excellent magnetic field uniformity, enabling nearly 0.5ppm uniformity to be produced.
According to the passive shimming optimization design method for the superconducting magnetic resonance magnet, provided by the invention, the split improved OTMF algorithm is utilized for each test group of the DOE, so that the bare magnetic field with the largest non-uniformity is shimmed by using the minimum total thickness of the shimming piece as an objective function, and the simplicity in installation and the lowest cost are realized.
Drawings
FIG. 1 is a flow chart of a passive shimming optimization design of a superconducting magnetic resonance magnet provided by an embodiment of the invention;
FIG. 2 is a schematic diagram of Monte Carlo simulation tolerances provided by an embodiment of the present invention;
FIG. 3 is a chart of 1000 Monte Carlo simulated field drift statistics provided by an embodiment of the present invention;
FIG. 4 is a schematic diagram of a magnetic field coordinate system provided by an embodiment of the present invention;
FIG. 5 is a block G with maximum non-uniformity of bare magnetic field according to an embodiment of the present invention max Is a magnetic field profile of (2);
FIG. 6 is a flowchart of a bipartite modified OTMF algorithm provided by an embodiment of the present invention;
FIG. 7 is a graph of DOE test results provided by an embodiment of the present invention;
fig. 8 is a diagram of 1000 sets of shim effect statistics for three schemes provided by an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
One or more aspects of the present invention are now summarized to facilitate a basic understanding of the invention, wherein the summary is not an extensive overview of the invention, and is intended neither to identify certain elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form before the more detailed description is presented below.
Example 1
As shown in fig. 1, the embodiment of the invention provides a passive shimming optimization design method of a superconducting magnetic resonance magnet, which comprises the following steps:
s1: obtaining geometric parameters of a 1.5T brain function imaging superconducting magnet system, as shown in table 1; wherein, the shimming strips are required to be placed on the inner surface of a cylinder with the radius of 24.99cm and the length of 120cm; the thickness of a single shim plate is 0.05mm, and the maximum thickness of the shim plate is 6mm;
and acquiring electromagnetic design parameters of the 1.5T brain function imaging superconducting magnet system, as shown in Table 2; wherein, the 1.5T brain function imaging superconducting magnet system consists of 10 bare wire tube superconducting coils; B+/B-, L+/L-, M+/M-, S+/S-, and C+/C-are symmetrical about the z-axis (wherein the xyz coordinate system is a Cartesian coordinate system), form a coil pair, and the superconducting wire is an NbTi wire;
TABLE 1
| Magnetic field strength | 1.5T |
| DSV diameter | 20cm |
| Cylindrical surface radius for placing shimming strips | 24.99cm |
| Cylindrical surface length for placing shimming bar | 120cm |
| Thickness of single shim | 0.05mm |
| Maximum thickness of shim | 6mm |
TABLE 2
S2: the magnetic field drift condition caused by parameter tolerance is simulated through Monte Carlo to obtain n groups of bare magnetic field groups G to be shimmed n And record the group with the largest unevenness as G max ；
FIG. 2 is a schematic diagram of tolerance generation, wherein the tolerance distribution is a uniform distribution U (-0.015,0.015) with a mean value equal to 0.015mm, because the tolerance in NbTi wire production is wire tolerance; since the tolerance of the wire installation position is the position tolerance, the tolerance distribution is a normal distribution N0 with a mean value of 0mm and a variance of 0.254mm, (0.254/3) 2 ]The method comprises the steps of carrying out a first treatment on the surface of the 1000 magnetic field drift test groups G due to tolerance were simulated using Monte Carlo n As shown in fig. 3; the magnetic field peak-to-peak non-uniformity due to tolerance is concentrated at 50pBetween pm and 750ppm, the average peak unevenness was 310.3094ppm; group G in which bare magnetic field unevenness is greatest max The uniformity of (2) is 1199.2373ppm and the magnetic field distribution is shown in FIG. 4;
s3: the number of circumferential shimming strips, the number of cavities of each shimming strip, the size of the shimming pieces and the value ranges of four shimming main factors of the interval between the shimming pieces are shown in table 3 according to the geometric constraint of the magnet; the size of the shim is three specifications (the length in the z direction is multiplied by the length in the phi direction is multiplied by the thickness of a single slice); meanwhile, as the distance between the shim plates is a discrete quantity, the distance is also 3.5cm, 7cm and 10 cm; the number of the cavities of each shimming strip is increased from 2 to explore, and the total length of the cavities of each shimming strip is required to be smaller than 120cm; the number of the circumferential shimming strips is 10, 16, 24 and 32, so that corresponding DOE test level combinations can be obtained;
TABLE 3 Table 3
S4: group G with maximum non-uniformity to bare magnetic field max Bare magnetic field B of (2) m Collecting through a Hall probe, and setting the number of DSV collecting points as N, then B m Can be represented as [ B ] 1 ,B 2 ,B 3 ,…,B N ]The method comprises the steps of carrying out a first treatment on the surface of the The magnetic field contribution of the unit volume shim to the sampling point in the DSV region can be obtained through measurement or calculation at any point in space; taking the sphere center of the DSV area as an origin, establishing a coordinate system as shown in fig. 5, wherein a point Q represents the center point of any shim, and a point P represents any sampling point of the DSV area; then
Magnetic field dB in z-axis direction of unit volume shim pair P (r, theta, phi) point at point z Can be calculated from formula (1):
wherein mu 0 For vacuum permeability, M z For the magnetization of shim ε m As a result of the Neuman factor,
representing a continuous band Legend function; dV is a unit volume element; n is the order of the Legend function; m is Legend function series; r is the radius of the DSV region;
the sensitivity coefficient matrix A with the size of N multiplied by M can be calculated by the formulas (1) and (2); wherein M is the total number of cavities; n is the number of sampling points; a is that i,j A magnetic field in the z-axis direction generated by the shim of unit thickness in the jth cavity to the ith sampling point; introducing a shim total thickness decision vector x with length M and a target magnetic field B t Obtaining a linear optimization model of passive shimming; wherein x= [ x ] 1 ,x 2 ,…,x M ] T ，x i Representing the thickness of the shim in the ith cavity, the non-uniformity H can be expressed as:
where ε is the maximum allowable non-uniformity;
will B t Introducing the decision variable x, an OTMF model is obtained, which can be expressed as:
Min:Lx′
wherein x' = [ x ] 1 ,x 2 ,…,x M ,B t ] T T is the maximum thickness of the cavity capable of accommodating the shim, kappa is the coefficient of the control target magnetic field, B avr Is bareAverage magnetic field strength of the bare magnetic field corresponding to the group with the largest magnetic field unevenness; l= [1, …,1]I.e., (m+1) ×1 identity matrix; the sensitivity coefficient matrices A 'and A' can be expressed as:
the bipartite modified OTMF algorithm flow is as follows in fig. 6:
a. initializing epsilon min ＝0，ε max ＝H 0 Wherein H is 0 Is the non-uniformity of the DSV region before shimming;
b. taking out
Substituting into an OTMF model for solving;
c. if there is no solution, it indicates the minimum value H of the unevenness of the DSV region min Greater than ε, i.e. H min Between epsilon and epsilon max Between, thus make ε min =ε, go to step b;
d. if there is a solution, it indicates the minimum value H of the unevenness of the DSV region min Smaller than epsilon, i.e. H min Between 0 and epsilon, thus making epsilon max =ε; if epsilon at this time max -ε min ≤10 -7 I.e. the set precision value is reached, the algorithm is ended; if the set precision value is not reached, turning to the step b;
s5: preliminarily selecting 3 shimming design methods according to shimming results of the DOE test group; more specifically, the following are:
the shimming results for the DOE trial group are shown in fig. 7, where: (1) Considering the convergence rate and the total volume of the shim consumed when the convergence length is just reached, the number of circumferential shim bars is preferably selected to be 24; (2) The number of cavities should be selected according to shim size and spacing; the number is too large, and the total volume of the consumed shim pieces is increased, so that the convergence length is preferable just before; based on the shimming results of the DOE test group, three sets of shimming design methods are initially selected, as shown in table 4;
TABLE 4 Table 4
S6: the 1000 groups of magnetic field drift test groups G are respectively subjected to a binary improved OTMF algorithm by three sets of shim design methods which are preliminarily selected n Carrying out shimming; and the result is subjected to integer processing, so that shimming effects of three sets of methods under integer solution are obtained as shown in figure 8; under the method of No. 1 shimming design, the average peak-to-peak non-uniformity after shimming of 1000 test groups is 0.4346ppm, the peak-to-peak non-uniformity after shimming of 99% of test groups is less than 0.5ppm, and the shimming effect is optimal;
s7: selecting a No. 1 shimming design method as the optimal passive shimming design, wherein parameters are shown in a table 5;
TABLE 5
Example 2
The embodiment of the invention provides a superconducting magnetic resonance magnet passive shimming optimization design system, which comprises the following components:
the parameter setting module is used for giving geometric parameters and electromagnetic parameters of the superconducting magnet to be shimmed;
the bare magnetic field group determining module is used for simulating the magnetic field drift condition caused by geometric parameters and electromagnetic parameter tolerance through Monte Carlo to obtain n groups of bare magnetic field groups to be shimmed;
the DOE test group construction module is used for selecting the number of circumferential shimming strips, the number of cavities of each shimming strip, the size of the shimming pieces and the value ranges of four shimming factors of the interval between the shimming pieces according to the size requirement of the magnet to construct a DOE test group;
the non-integer solution shimming module is used for shimming the bare magnetic field with the largest non-uniformity by utilizing a binary improved OTMF algorithm for each DOE test group and taking the minimum total thickness of the shimming piece as an objective function to obtain a non-integer solution of the DOE test group;
the initial shimming design scheme screening module is used for selecting a plurality of shimming design methods according to shimming results of the DOE test group;
the integer solution shimming module is used for shimming n groups of bare magnetic field groups to be shimmed under a plurality of shimming design methods respectively to obtain integer solutions of n groups of bare magnetic fields;
and the optimal passive shimming optimal design screening module is used for selecting a shimming design method with the optimal integer shimming effect as the optimal passive shimming optimal design.
Further preferably, the geometric parameters include cylinder size of the shim bar placement, individual shim thickness, and shim maximum thickness; the electromagnetic parameters include the number of superconducting coils of the bare wire tube, the positional relationship of the superconducting coils of the bare wire tube and the material of the superconducting wire.
Further preferably, the OTMF model in the OTMF algorithm is:
Min:Lx′
wherein x' = [ x ] 1 ,x 2 ,…,x M ,B t ] T T is the maximum thickness of the cavity capable of accommodating the shim, kappa is the coefficient of the control target magnetic field, B avr Average magnetic field intensity of the bare magnetic field corresponding to the group with the largest bare magnetic field unevenness; l= [1, …,1]I.e., (m+1) ×1 identity matrix; x= [ x ] 1 ,x 2 ,…,x M ] T ，x i Representing the thickness of the shim in the ith cavity; b (B) m A bare magnetic field corresponding to a group having the greatest bare magnetic field unevenness; b (B) t Is the target magnetic field; a is that i,j A magnetic field in the z-axis direction generated by the shim of unit thickness in the jth cavity to the ith sampling point; epsilon is the maximum allowable unevenness; the sensitivity coefficient matrices A 'and A' can be expressed as:
further preferably, the specific implementation method of the bipartite modified OTMF algorithm comprises the following steps:
a. initializing epsilon min ＝0，ε max ＝H 0 The method comprises the steps of carrying out a first treatment on the surface of the Wherein H is 0 Is the non-uniformity of the DSV region before shimming; epsilon min Minimum to allow maximum non-uniformity; epsilon max To a maximum value that allows maximum non-uniformity;
b. taking out
Substituting into an OTMF model for solving;
c. if there is no solution, it indicates the minimum value H of the unevenness of the DSV region min Greater than epsilon, let epsilon min =ε, go to step b;
d. if there is a solution, it indicates the maximum size H of the unevenness of the DSV region min Smaller than epsilon, let epsilon max =ε; if epsilon max -ε min ≤10 -7 The unevenness of the DSV area reaches a set precision value, the OTMF algorithm is ended, and otherwise, the step b is switched to;
wherein, the calculation formula of the unevenness of the DSV area is:
wherein A is a sensitivity coefficient matrix with the size of N multiplied by M; m is the total number of cavities; n is the number of sampling points; a is that i,j And generating a magnetic field in the z-axis direction for the ith sampling point for the shim of unit thickness in the jth cavity.
In summary, compared with the prior art, the invention has the following advantages:
the invention provides a passive shimming optimization design method of a superconducting magnetic resonance magnet, wherein the number of circumferential shimming strips, the number of cavities of each shimming strip, the size of shimming pieces and the value ranges of four shimming factors of the interval between the shimming pieces are selected according to the size requirement of the magnet, and a DOE test set is constructed; based on the method, the optimal passive shimming optimization design is obtained in two steps, and in the first step, a binary improved OTMF algorithm is utilized for each test group of the DOE, so that the bare magnetic field with the largest non-uniformity is shimmed by using the minimum total thickness of the shimming piece as an objective function, and a non-integer solution is obtained; secondly, shimming is carried out on n groups of bare magnetic field groups to be shimmed respectively under a plurality of shimming design methods, and a shimming design method with the best integer shimming effect is screened out to be used as the optimal passive shimming optimization design. A superconducting magnet of 1.5T designed with optimal passive shimming achieves excellent magnetic field uniformity, enabling nearly 0.5ppm uniformity to be produced.
According to the passive shimming optimization design method for the superconducting magnetic resonance magnet, provided by the invention, the split improved OTMF algorithm is utilized for each test group of the DOE, so that the bare magnetic field with the largest non-uniformity is shimmed by using the minimum total thickness of the shimming piece as an objective function, and the simplicity in installation and the lowest cost are realized.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.
Claims (8)
1. The passive shimming optimization design method of the superconducting magnetic resonance magnet is characterized by comprising the following steps of:
s1: giving geometric parameters and electromagnetic parameters of a superconducting magnet to be shimmed;
s2: simulating the magnetic field drift condition caused by geometric parameters and electromagnetic parameter tolerance by Monte Carlo to obtain n groups of bare magnetic field groups to be shimmed;
s3: selecting the number of circumferential shimming strips, the number of cavities of each shimming strip, the size of shimming pieces and the value range of four shimming factors of the interval between the shimming pieces according to the size requirement of the magnet, and constructing a DOE test set;
s4: carrying out shimming on a bare magnetic field with the largest non-uniformity by utilizing a binary improved OTMF algorithm on each DOE test group and taking the minimum total thickness of a shimming piece as an objective function to obtain a non-integer solution of the DOE test group;
s5: selecting a plurality of shimming design methods according to shimming results of the DOE test group;
s6: respectively carrying out shimming on n groups of bare magnetic field groups to be shimmed under a plurality of shimming design methods to obtain integer solutions of the n groups of bare magnetic fields;
s7: and selecting a shimming design method with the best integer shimming effect as the optimal passive shimming optimization design.
2. The method of claim 1, wherein the geometric parameters include cylinder size of shim bar placement, thickness of individual shim and maximum thickness of shim; the electromagnetic parameters comprise the number of the superconducting coils of the bare wire tube, the position relation of the superconducting coils of the bare wire tube and the material of the superconducting wire.
3. The method for designing a passive shimming optimization of a superconducting magnetic resonance magnet according to claim 1 or 2, wherein the OTMF model in the OTMF algorithm is:
Min:Lx′
S.t:
wherein x' = [ x ] 1 ,x 2 ,…,x M ,B t ] T T is the maximum thickness of the cavity capable of accommodating the shim, kappa is the coefficient of the control target magnetic field, B avr Average magnetic field intensity of the bare magnetic field corresponding to the group with the largest bare magnetic field unevenness; l= [1, …,1]Is an identity matrix; x= [ x ] 1 ,x 2 ,…,x M ] T ，x i Indicating that the ith cavity is uniformThe thickness of the field plate; b (B) m A bare magnetic field corresponding to a group having the greatest bare magnetic field unevenness; b (B) t Is the target magnetic field; a is that i,j A magnetic field in the z-axis direction generated by the shim of unit thickness in the jth cavity to the ith sampling point; epsilon is the maximum allowable unevenness; the sensitivity coefficient matrices A 'and A' can be expressed as:
4. the method for passive shimming optimization design of a superconducting magnetic resonance magnet according to claim 3, wherein the specific implementation method of the bipartite modified OTMF algorithm comprises the following steps:
a. initializing epsilon min ＝0，ε max ＝H 0 The method comprises the steps of carrying out a first treatment on the surface of the Wherein H is 0 Is the non-uniformity of the DSV region before shimming; epsilon min Minimum to allow maximum non-uniformity; epsilon max To a maximum value that allows maximum non-uniformity;
b. taking out
Substituting into an OTMF model for solving;
c. if there is no solution, it indicates the minimum value H of the unevenness of the DSV region min Greater than epsilon, let epsilon min =ε, go to step b;
d. if there is a solution, it indicates the maximum size H of the unevenness of the DSV region min Smaller than epsilon, let epsilon max =ε; if epsilon max -ε min ≤10 -7 The unevenness of the DSV area reaches a set precision value, the OTMF algorithm is ended, and otherwise, the step b is switched to;
wherein, the calculation formula of the unevenness of the DSV area is:
wherein A is a sensitivity coefficient matrix with the size of N multiplied by M; m is the total number of cavities; n is the number of sampling points; a is that i,j A magnetic field in the z-axis direction generated by the shim of unit thickness in the jth cavity to the ith sampling point; the method comprises the steps of carrying out a first treatment on the surface of the The xyz axis is the cadier coordinate system.
5. A superconducting magnetic resonance magnet passive shimming optimization design system, comprising:
the parameter setting module is used for giving geometric parameters and electromagnetic parameters of the superconducting magnet to be shimmed;
the bare magnetic field group determining module is used for simulating the magnetic field drift condition caused by geometric parameters and electromagnetic parameter tolerance through Monte Carlo to obtain n groups of bare magnetic field groups to be shimmed;
the DOE test group construction module is used for selecting the number of circumferential shimming strips, the number of cavities of each shimming strip, the size of the shimming pieces and the value ranges of four shimming factors of the interval between the shimming pieces according to the size requirement of the magnet to construct a DOE test group;
the non-integer solution shimming module is used for shimming the bare magnetic field with the largest non-uniformity by utilizing a binary improved OTMF algorithm for each DOE test group and taking the minimum total thickness of the shimming piece as an objective function to obtain a non-integer solution of the DOE test group;
the initial shimming design scheme screening module is used for selecting a plurality of shimming design methods according to shimming results of the DOE test group;
the integer solution shimming module is used for shimming n groups of bare magnetic field groups to be shimmed under a plurality of shimming design methods respectively to obtain integer solutions of n groups of bare magnetic fields;
and the optimal passive shimming optimal design screening module is used for selecting a shimming design method with the optimal integer shimming effect as the optimal passive shimming optimal design.
6. The passive shimming optimization design system of superconducting magnetic resonance magnet in accordance with claim 5, wherein the geometric parameters include cylinder size of shim bar placement, individual shim thickness, and shim maximum thickness; the electromagnetic parameters comprise the number of the superconducting coils of the bare wire tube, the position relation of the superconducting coils of the bare wire tube and the material of the superconducting wire.
7. The superconducting magnetic resonance magnet passive shimming optimization design system according to claim 5 or 6, wherein the OTMF model in the OTMF algorithm is:
Min:Lx′
S.t:
wherein x' = [ x ] 1 ,x 2 ,…,x M ,B t ] T T is the maximum thickness of the cavity capable of accommodating the shim, kappa is the coefficient of the control target magnetic field, B avr Average magnetic field intensity of the bare magnetic field corresponding to the group with the largest bare magnetic field unevenness; l= [1, …,1]Is (M+1) x 1 identity matrix; x= [ x ] 1 ,x 2 ,…,x M ] T ，x i Representing the thickness of the shim in the ith cavity; b (B) m A bare magnetic field corresponding to a group having the greatest bare magnetic field unevenness; b (B) t Is the target magnetic field; a is that i,j A magnetic field in the z-axis direction generated by the shim of unit thickness in the jth cavity to the ith sampling point; epsilon is the maximum allowable unevenness; the sensitivity coefficient matrices A 'and A' can be expressed as:
8. the superconducting magnetic resonance magnet passive shimming optimization design system according to claim 7, wherein the specific implementation method of the bipartite modified OTMF algorithm comprises the following steps:
a. initializing epsilon min ＝0，ε max ＝H 0 The method comprises the steps of carrying out a first treatment on the surface of the Wherein H is 0 Is the non-uniformity of the DSV region before shimming; epsilon min Minimum to allow maximum non-uniformity; epsilon max To a maximum value that allows maximum non-uniformity;
b. taking out
Substituting into an OTMF model for solving;
c. if there is no solution, it indicates the minimum value H of the unevenness of the DSV region min Greater than epsilon, let epsilon min =ε, go to step b;
d. if there is a solution, it indicates the maximum size H of the unevenness of the DSV region min Smaller than epsilon, let epsilon max =ε; if epsilon max -ε min ≤10 -7 The unevenness of the DSV area reaches a set precision value, the OTMF algorithm is ended, and otherwise, the step b is switched to;
wherein, the calculation formula of the unevenness of the DSV area is:
wherein A is a sensitivity coefficient matrix with the size of N multiplied by M; m is the total number of cavities; n is the number of sampling points; a is that i,j A magnetic field in the z-axis direction generated by the shim of unit thickness in the jth cavity to the ith sampling point; the xyz axis is the cadier coordinate system.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202310078452.0A CN116305798A (en) | 2023-01-12 | 2023-01-12 | Superconducting magnetic resonance magnet passive shimming optimization design method and system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202310078452.0A CN116305798A (en) | 2023-01-12 | 2023-01-12 | Superconducting magnetic resonance magnet passive shimming optimization design method and system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CN116305798A true CN116305798A (en) | 2023-06-23 |
Family
ID=86780561
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CN202310078452.0A Pending CN116305798A (en) | 2023-01-12 | 2023-01-12 | Superconducting magnetic resonance magnet passive shimming optimization design method and system |
Country Status (1)
| Country | Link |
|---|---|
| CN (1) | CN116305798A (en) |
-
2023
- 2023-01-12 CN CN202310078452.0A patent/CN116305798A/en active Pending
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CN106990373B (en) | A kind of decoupling axial direction shim coil design method of magnetic resonance system | |
| Vaughan et al. | 9.4 T human MRI: preliminary results | |
| IL96422A (en) | Method of optimizing passive shim placement in magnetic resonance magnets | |
| CN102667517B (en) | Open-bore magnet for use in magnetic resonance imaging | |
| JPS63226009A (en) | Uniform magnetic field generator | |
| WO2013166810A1 (en) | Magnetic resonance imaging superconducting magnet system and method and device for acquiring structural parameter thereof | |
| Liu et al. | A hybrid field-harmonics approach for passive shimming design in MRI | |
| EP0714521B1 (en) | Method and apparatus for compensation of field distortion in a magnetic structure using spatial filter | |
| CN113033028B (en) | Design method of magnetic resonance Halbach type shimming coil | |
| CN116305798A (en) | Superconducting magnetic resonance magnet passive shimming optimization design method and system | |
| CN108182329A (en) | A kind of radio frequency coil designs method and loop construction using particle cluster algorithm | |
| CN114970861A (en) | Design method of open MRI passive shimming | |
| CN108882498B (en) | High-intensity magnetic field synchrocyclotron and magnetic field shimming method thereof | |
| US7323875B2 (en) | Shimming structure and method for a magnetic resonance imaging apparatus | |
| CN113971349B (en) | Method for acquiring number and initial positions of solenoid-shaped superconducting magnet coils | |
| US5428333A (en) | Method and apparatus for compensation of field distortion in a magnetic structure | |
| CN100536769C (en) | Magnetic resonance imaging system | |
| CN114239266A (en) | Passive shimming optimization method for magnetic resonance imaging superconducting magnet | |
| Zhu et al. | A novel design method of passive shimming for 0.7-T biplanar superconducting MRI magnet | |
| CN110501663B (en) | Ultralow-field biplane nuclear magnetic resonance normally-conducting main magnet and construction method thereof | |
| GB2348960A (en) | Iterative method for determining shim positioning in NMR apparatus | |
| CN113030811A (en) | Design method of cylindrical shimming coil | |
| Ren et al. | Study on shimming method for open permanent magnet of MRI | |
| US7394255B2 (en) | Single-sided magnetic resonance magnet assembly | |
| Zhang et al. | Design and Optimization of High Uniformity Magnetic Field Gradient Coil |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PB01 | Publication | ||
| PB01 | Publication | ||
| SE01 | Entry into force of request for substantive examination | ||
| SE01 | Entry into force of request for substantive examination |