Medical cyclotron isochronous magnetic field shimming method and system

Abstract

The invention relates to the technical field of magnetic field shimming, in particular to a medical cyclotron isochronous magnetic field shimming method and system, and the method comprises the steps: collecting the magnetic field intensity of each position in a magnetic field region of a medical cyclotron; determining local magnetic field non-uniformity at each position; equally dividing a preset number of magnetic pole steps on the magnetic pole surface of the magnetic field area, and determining shimming performance of each magnetic pole step; all the magnetic pole steps are divided into first-class magnetic pole steps and second-class magnetic pole steps based on the correlation between shimming properties of the magnetic pole steps; determining shimming complexity of each type of magnetic pole step; and determining a fitness function by combining the shimming complexity and the shimming difference before and after shimming of the first-class magnetic pole step, and obtaining the optimal height of each magnetic pole step during magnetic field shimming by using a particle swarm optimization algorithm. Therefore, the accuracy of shimming the isochronous magnetic field of the medical cyclotron is improved.

Description

Technical Field

[0001] The present application relates to the field of magnetic field shimming technology, and in particular to a method and system for isochronous magnetic field shimming of a medical cyclotron. Background Art

[0002] A medical cyclotron is a device used to generate high-energy particles, primarily used in radiotherapy and medical imaging. It accelerates charged particles (usually protons or ions) to extremely high speeds, then impacts them with a target, producing radioactive isotopes used in medical diagnosis and treatment.

[0003] In the design and construction of medical cyclotrons, accurate shimming of the actual magnetic field is necessary to eliminate magnetic field distortion caused by non-ideal magnetic properties of ferromagnetic materials, machining and assembly errors, and magnet structural deformation, thereby achieving the theoretically designed isochronous magnetic field. Traditional shimming techniques often focus on a single plane or dimension, whereas magnetic fields are typically three-dimensional. Accurately determining the shimming height in a three-dimensional field is a key challenge facing magnetic field shimming. Summary of the Invention

[0004] In order to solve the above technical problems, the purpose of this application is to provide a method and system for isochronous magnetic field shimming of a medical cyclotron. The technical solutions adopted are as follows: In a first aspect, an embodiment of the present application provides a method for shimming an isochronous magnetic field of a medical cyclotron, the method comprising the following steps: Collecting the magnetic field strength at each position within the magnetic field area of ​​a medical cyclotron; Analyzing the difference in magnetic field intensity at each position and its radially adjacent positions, and combining the spatial distance between each position and its radially adjacent positions, determining the local magnetic field inhomogeneity at each position; equally dividing a preset number of magnetic pole steps on the magnetic pole surface of the magnetic field region, analyzing the difference in magnetic field intensity at each position in the three-dimensional region corresponding to each magnetic pole step in the magnetic field and the target magnetic field intensity, and combining the local magnetic field inhomogeneity to determine the shimming property of each magnetic pole step; Based on the correlation between the shimming properties of the various magnetic pole steps, all magnetic pole steps are divided into Class I and Class II. For each Class I magnetic pole step, the distribution of local magnetic field inhomogeneity at all positions within the corresponding three-dimensional region in the magnetic field is analyzed, as well as the difference in local magnetic field inhomogeneity between each position and its axially adjacent position. Combined with the shimming properties, the shimming complexity of each Class I magnetic pole step is determined. Based on the difference of the pad before and after the pad of a class of magnetic pole step, combined with the pad complexity, and the difference of the pad before and after the pad of a class of magnetic pole step, the fitness function is determined, and the intelligent optimization algorithm is used to obtain the optimal height of each magnetic pole step for magnetic field pad.

[0005] In one embodiment, the determination of the local magnetic field inhomogeneity includes: The difference between the axial component of the magnetic field strength at each position and the axial component of the magnetic field strength at the adjacent position in the radial direction of the position is calculated, denoted as the first difference, and the ratio of the first difference to the spatial distance is calculated, and the local magnetic field inhomogeneity is the fusion result of the ratio at all adjacent positions of each position in the radial direction.

[0006] In one embodiment, the local magnetic field inhomogeneity is the cumulative sum of the ratio at all adjacent positions of each position in the radial direction.

[0007] In one embodiment, the target magnetic field strength is the magnetic field strength required to maintain the isochronous magnetic field at each position in the magnetic field region.

[0008] In one embodiment, the determination of the pad includes: The difference between the axial component of the magnetic field strength at each position and the target magnetic field strength at the position is denoted as the second difference, and the product of the local magnetic field inhomogeneity at each position and the second difference is calculated, and the pad is the fusion result of the product at all positions in the three-dimensional region corresponding to each magnetic pole step in the magnetic field.

[0009] In one embodiment, the division of all magnetic pole steps into a class of magnetic pole steps and a class of magnetic pole steps includes: The clustering algorithm is used to cluster the pad of all magnetic pole steps, and all magnetic pole steps are divided into two clustering clusters, the mean value of the pad of all magnetic pole steps in each clustering cluster is calculated, the magnetic pole step in the clustering cluster corresponding to the maximum mean value is denoted as a class of magnetic pole steps, and the magnetic pole step in the remaining clustering cluster is denoted as a class of magnetic pole steps.

[0010] In one embodiment, the determination of the pad complexity includes: The difference between the local magnetic field inhomogeneity at each position in the three-dimensional region corresponding to each class of magnetic pole step in the magnetic field and the axial adjacent position of the position is calculated, denoted as the third difference, the fusion value of the third difference at all positions in the three-dimensional region corresponding to each class of magnetic pole step in the magnetic field is calculated, the chaos degree of the local magnetic field inhomogeneity at all positions in the three-dimensional region corresponding to each class of magnetic pole step in the magnetic field is calculated, and the pad complexity of each class of magnetic pole step is determined based on the chaos degree, the fusion value and the pad.

[0011] In one embodiment, the shimming complexity is the product of the disorder degree, the fusion value, and the shimming property of each type of magnetic pole step.

[0012] In one embodiment, the fitness function is expressed as: ; Where S represents the fitness function, C1 is the number of the first type of magnetic pole steps, C2 is the number of the second type of magnetic pole steps, is the shimming complexity of the i-th type-one magnetic pole step, is the shimming property of the i-th type-one magnetic pole step before shimming, is the shimming property of the ith type I magnetic pole step after shimming, is the shimming property of the jth second-class magnetic pole step before shimming, is the shimming property of the jth second-class magnetic pole step after shimming, The default value is greater than 0.

[0013] In a second aspect, an embodiment of the present application also provides a medical cyclotron isochronous magnetic field shimming system, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, wherein the processor implements the steps of any one of the above methods when executing the computer program.

[0014] This application has at least the following beneficial effects: The present application collects the magnetic field strength at each position in the magnetic field area of ​​a medical cyclotron; analyzes the difference in magnetic field strength at each position and its adjacent positions in the radial direction, and determines the local magnetic field inhomogeneity at each position in combination with the spatial distance between each position and its adjacent positions in the radial direction; the local magnetic field inhomogeneity reflects the degree of inconsistency of magnetic field changes at each position, reflects the necessity of magnetic field shimming at each position to maintain an isochronous magnetic field, and improves the reliability of magnetic field shimming analysis; divides a preset number of magnetic pole steps equally on the magnetic pole surface of the magnetic field area, analyzes the difference in magnetic field strength at each position in the three-dimensional area corresponding to each magnetic pole step in the magnetic field and the target magnetic field strength, and determines the shimming property of each magnetic pole step in combination with the local magnetic field inhomogeneity; the shimming property reflects the urgency of the need for magnetic field shimming in the three-dimensional area corresponding to each magnetic pole step, and improves the accuracy of determining the magnetic field shimming position; based on the correlation between the shimming properties of each magnetic pole step, all magnetic pole steps are divided into a class of magnetic pole steps and a class of magnetic pole steps. Two types of magnetic pole steps; by dividing the magnetic pole steps into Class I and Class II, the magnetic pole steps that significantly need to be padded are screened out, thereby improving the accuracy of subsequent magnetic pole step height determination; for each Class I magnetic pole step, the distribution of local magnetic field inhomogeneity at all positions in the three-dimensional region corresponding to the magnetic field, as well as the difference in local magnetic field inhomogeneity between each position and the axially adjacent position are analyzed, and combined with the padded property, the padded complexity of each Class I magnetic pole step is determined; the padded complexity reflects the difficulty of the padded process of each Class I magnetic pole step, and this is used as a weight to improve the rationality of the subsequent fitness function determination; based on the difference in the padded property before and after the padded of the Class I magnetic pole step, combined with the padded complexity, and the difference in the padded property before and after the padded of the Class II magnetic pole step, the fitness function is determined, thereby improving the optimization ability of the particle swarm optimization algorithm, and using the particle swarm optimization algorithm to obtain the optimal height of each magnetic pole step when the magnetic field is padded, ultimately improving the accuracy of the height determination of each magnetic pole step when the magnetic field is padded. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions and advantages of the embodiments of the present application or the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0016] Figure 1 A flowchart of a method for shimming the isochronous magnetic field of a medical cyclotron provided in one embodiment of the present application; Figure 2 is a schematic diagram of the magnetic field response curve; Figure 3 Determine a flow chart for the fitness function. DETAILED DESCRIPTION

[0017] To further illustrate the technical means and effectiveness of this application's implementation of the intended invention, the following, in conjunction with the accompanying drawings and preferred embodiments, details the specific implementation, structure, features, and effectiveness of a medical cyclotron isochronous magnetic field shimming method and system proposed in this application. In the following description, references to different "one embodiment" or "another embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics of one or more embodiments may be combined in any suitable manner.

[0018] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0019] The specific scheme of the medical cyclotron isochronous magnetic field shimming method and system provided by the present application is described in detail below with reference to the accompanying drawings.

[0020] See also Figure 1 , which shows a flowchart of a method for isochronous magnetic field shimming of a medical cyclotron provided by one embodiment of the present application, the method comprising the following steps: S1, collecting the magnetic field strength at each position in the magnetic field area of ​​the medical cyclotron.

[0021] In a medical cyclotron, the region corresponding to the particle motion trajectory in the magnetic field is recorded as a characteristic magnetic field region. This embodiment uses the finite cloud method to simulate the three-dimensional magnetic field between the two magnetic poles of the medical cyclotron, i.e., the characteristic magnetic field region. This embodiment takes N preset positions in the characteristic magnetic field region, which are recorded as N measurement points. The acquisition process of N measurement points is as follows: uniformly obtain U horizontal magnetic pole surfaces in the characteristic magnetic field region, and uniformly select n measurement points on each horizontal magnetic pole surface, then N = n × U. In this embodiment, N = 1000 and U = 10. The implementer can set them according to actual conditions, and this embodiment does not impose any restrictions here.

[0022] It should be noted that the finite cloud method is an existing publicly known technology, and implementers can select other existing feasible simulation algorithms according to actual conditions, and this embodiment does not impose any limitation thereto.

[0023] This embodiment uses a Hall Gauss meter to measure the magnetic field intensity at each measurement point in the characteristic magnetic field area.

[0024] S2, analyzing the difference in magnetic field strength at each position and its adjacent positions in the radial direction, and determining the local magnetic field inhomogeneity at each position in combination with the spatial distance between each position and its adjacent positions in the radial direction; evenly dividing a preset number of magnetic pole steps on the magnetic pole surface of the magnetic field region, analyzing the difference in magnetic field strength at each position in the three-dimensional region corresponding to each magnetic pole step in the magnetic field and the target magnetic field strength, and determining the shimming property of each magnetic pole step in combination with the local magnetic field inhomogeneity.

[0025] In medical cyclotrons, an isochronous magnetic field typically refers to a magnetic field whose strength and direction do not change over time, creating a constant magnetic field. The magnetic field within a medical cyclotron is typically isochronous, meaning that during the acceleration process, the strength and direction of the magnetic field do not change, thereby keeping the particles on a fixed trajectory. During this process, the isochronous magnetic field ensures that the particles maintain a stable trajectory, unaffected by fluctuations in the magnetic field. Therefore, the design of medical cyclotrons typically relies on a uniform and constant magnetic field.

[0026] However, actual magnetic fields often exhibit non-uniformity due to manufacturing errors, current fluctuations, and core saturation. This can cause particle trajectories to deviate and become unstable, thus affecting acceleration efficiency. Step shimming is primarily used to correct for magnetic field inhomogeneities. By placing specific compensating magnetic fields within the accelerator, this discontinuity is eliminated or reduced, ensuring stable particle trajectories.

[0027] In this embodiment, the entire magnetic pole surface of the bottom magnet is evenly divided into M steps along the radial direction, and the width of each step is kept uniform. In this embodiment, M=5, and the implementer can set it according to actual conditions. This embodiment does not impose any limitation here.

[0028] When shimming the isochronous magnetic field of a medical cyclotron, the magnetic field distribution within the characteristic magnetic field region of the cyclotron must first be analyzed to identify any inhomogeneous areas of the magnetic field, which can then be shimmed. Specifically, for each measurement point, R adjacent measurement points are selected in the radial direction. In this embodiment, R = 10. This value can be set by the user based on actual conditions and is not a limitation of this embodiment.

[0029] The local magnetic field inhomogeneity of each measuring point is calculated by the difference in magnetic field intensity between local measuring points in the radial direction of the measuring point. Specifically, the difference between the axial component of the magnetic field intensity at each measuring point and its adjacent measuring points in the radial direction is calculated, recorded as the first difference, and the ratio of the first difference to the spatial distance is calculated. The local magnetic field inhomogeneity is the fusion result of the ratios between each measuring point and all its adjacent measuring points in the radial direction.

[0030] It should be noted that the difference represents the degree of difference between two variables, which can be calculated specifically by using the absolute value of the difference, the square of the difference, etc. This embodiment uses the absolute value of the difference as the calculation method for the difference. Fusion represents combining multiple variables, which can be calculated specifically by using multiplication, addition, or a mixture of addition and multiplication, etc. This embodiment uses addition as the calculation method for fusion. In addition, for calculating the difference in the axial component of the magnetic field strength, the axial component is the component in the vertical direction. Since in the cyclotron, the particles move along a circular orbit in the horizontal plane, the magnetic field formed between the two magnetic poles is perpendicular to the horizontal plane. The magnetic field strength in the vertical direction, that is, the axial component of the magnetic field strength, has the most significant effect on the particle motion. Therefore, the difference in the axial component of the magnetic field strength between the measurement points is analyzed.

[0031] In this embodiment, the local magnetic field inhomogeneity of each measuring point is calculated as follows: Where, is the local magnetic field inhomogeneity at the qth measurement point, is the magnetic field intensity amplitude of the axial component of the magnetic field intensity at the qth measurement point, is the magnetic field intensity amplitude on the axial component of the magnetic field intensity at the zth measuring point adjacent to the qth measuring point in the radial direction, is the spatial distance between the qth measurement point and its adjacent zth measurement point in the radial direction. Recorded as the first difference.

[0032] It should be noted that the spatial distance is calculated using Euclidean distance, and the radial direction represents the measurement point located along the same diameter as the qth measurement point. The closer the spatial distance between each measurement point and its radially adjacent measurement point, and the greater the difference in magnetic field intensity amplitude, the more nonuniform the magnetic field is in the local area of ​​that measurement point, and the greater the need for shimming processing. Consequently, the local magnetic field nonuniformity increases.

[0033] When shimming step blocks are used to shim the measuring points with uneven magnetic fields, the magnetic field emitted by the steps mainly affects the area above the steps and indirectly affects the entire magnetic field area. Therefore, when judging whether to shim the measuring points only by the magnetic field unevenness, the following phenomenon may occur. Since the characteristic magnetic field area is a three-dimensional area, the unevenness in the radial direction of the measuring point does not guarantee that the measuring points in the axial direction of the measuring point will also have unevenness. Therefore, when shimming is performed directly, the measuring points above or below may be affected by the magnetic field of the shim step blocks, causing the magnetic field unevenness to occur again.

[0034] Therefore, in this embodiment, the area between each step surface vertically upward to the magnetic pole surface of the top magnet is recorded as each three-dimensional step area. Each measurement point in the three-dimensional step area has a corresponding local magnetic field inhomogeneity. The necessity of shimming the three-dimensional step area is further determined by combining the difference between the magnetic field intensity of each measurement point and the target magnetic field intensity. Specifically, the difference between the axial component of the magnetic field intensity of each measurement point and its target magnetic field intensity is recorded as the second difference, and the product of the local magnetic field inhomogeneity of each measurement point and the second difference is calculated. The shimming property of each magnetic pole step is the fusion result of the above products of all measurement points in the three-dimensional step area corresponding to each magnetic pole step in the magnetic field.

[0035] It should be noted that the target magnetic field strength is the magnetic field strength that needs to be achieved at each position in order to maintain isochronism when the particles move along a circular orbit in the magnetic field.

[0036] In this embodiment, the shimming performance of each magnetic pole step is calculated as follows: Where, is the shimming property of the k-th magnetic pole step, is the number of measurement points in the three-dimensional step area corresponding to the k-th magnetic pole step, is the local magnetic field inhomogeneity of the vth measurement point in the three-dimensional step area corresponding to the kth magnetic pole step, is the magnetic field intensity amplitude of the axial component of the magnetic field intensity at the vth measurement point in the three-dimensional step area corresponding to the kth magnetic pole step, is the target magnetic field intensity amplitude at the vth measurement point in the three-dimensional step area corresponding to the kth magnetic pole step. Recorded as the second difference.

[0037] It should be understood that the greater the difference between the magnetic field strength of each measuring point in the three-dimensional step area and the target magnetic field strength, and the greater the local magnetic field inhomogeneity of the measuring point, the more the three-dimensional step area needs to be padded, and therefore, the greater the padded nature of the corresponding magnetic pole step.

[0038] S3. Based on the correlation between the padding properties of each pole step, all pole steps are divided into Class I pole steps and Class II pole steps; for each Class I pole step, the distribution of local magnetic field inhomogeneity at all positions in the three-dimensional region corresponding to the magnetic field, as well as the difference in local magnetic field inhomogeneity at each position and the axially adjacent position are analyzed, and combined with the padding properties, the padding complexity of each Class I pole step is determined.

[0039] The padding properties of all magnetic pole steps are clustered using the K-means clustering algorithm, with the clustering parameter K=2. The distance metric is the absolute value of the difference in the padding properties of the magnetic pole steps. The K-means clustering algorithm is an existing well-known technology. The implementer can choose other existing feasible clustering algorithms according to actual conditions. This embodiment does not limit this.

[0040] Based on the above clustering results, all magnetic pole steps are divided into two clusters, and the mean of the padding of all magnetic pole steps in each cluster is calculated. All magnetic pole steps in the cluster with the maximum mean are recorded as Class I magnetic pole steps, and all magnetic pole steps in the remaining clusters are recorded as Class II magnetic pole steps. Among them, Class I magnetic pole steps are magnetic pole steps that obviously need to be padded, and the magnetic field inhomogeneity in the corresponding three-dimensional step area is more obvious. Therefore, Class I magnetic pole steps are used as magnetic pole steps to be padded. By distinguishing the magnetic pole steps, they are divided into Class I magnetic pole steps and Class II magnetic pole steps, so that the magnetic pole steps can be padded more accurately.

[0041] When padding a three-dimensional step area, the padding magnetic field strength at a certain position is the superposition of the magnetic field strengths generated by multiple different magnetic pole steps, and the padding magnetic field strength values ​​of magnetic pole steps at different positions and different heights at the same position are often different. That is, the more uneven the magnetic field strength distribution in the three-dimensional step area is, the more magnetic pole steps are required, and the more complex the height setting of the magnetic pole steps is.

[0042] Therefore, this embodiment calculates the shimming complexity of each type of magnetic pole step, specifically: calculates the difference in local magnetic field inhomogeneity between each measurement point in the three-dimensional step area corresponding to each type of magnetic pole step in the magnetic field and its axially adjacent measurement point, recorded as the third difference, calculates the fusion value of the third difference of all measurement points in the three-dimensional step area corresponding to each type of magnetic pole step in the magnetic field, calculates the chaos degree of local magnetic field inhomogeneity of all measurement points in the three-dimensional step area corresponding to each type of magnetic pole step in the magnetic field, and determines the shimming complexity of each type of magnetic pole step based on the chaos degree, the fusion value, and the shimming property.

[0043] It should be noted that the calculation method of the chaos degree in this embodiment adopts information entropy for calculation. The implementer can choose other existing feasible algorithms for calculation on the basis of being able to measure the distribution chaos degree of local magnetic field inhomogeneity of all measurement points. This embodiment does not impose any restrictions on this.

[0044] In this embodiment, the shimming complexity of each type of magnetic pole step is calculated as follows: Where, is the shimming complexity of the x-th first-class magnetic pole step, is the shimming property of the x-th first-class magnetic pole step, is the information entropy of the local magnetic field inhomogeneity of all measurement points in the three-dimensional step area corresponding to the x-th type of magnetic pole step, is the local magnetic field inhomogeneity at the vth measurement point in the three-dimensional step region corresponding to the xth first-class magnetic pole step, is the local magnetic field inhomogeneity of the nearest measuring point axially above the vth measuring point in the three-dimensional step region corresponding to the xth first-class magnetic pole step, is the number of all measurement points in the three-dimensional step area corresponding to the x-th type of magnetic pole step. Recorded as the third difference, Denote the fused value of the third difference.

[0045] It should be noted that in the process of calculating the filling complexity of each type of magnetic pole step, if there is no nearest measurement point axially above the measurement point in the three-dimensional step area corresponding to each type of magnetic pole step, the nearest measurement point axially below the measurement point is selected.

[0046] The third difference reflects the uneven distribution of axial magnetic field intensity at the measuring points in the three-dimensional step area. The larger the third difference is, the more uneven the distribution of axial magnetic field intensity is, and the more difficult it is to fill the magnetic pole steps. In addition, if the distribution of local magnetic field inhomogeneity of all measuring points in the three-dimensional step area is more chaotic, the greater the calculated information entropy is, which also indicates that the magnetic field in the area is more uneven, and correspondingly, the greater the complexity of the filling is.

[0047] S4, based on the difference in the shimming properties before and after the shimming of the first type of magnetic pole step, combined with the shimming complexity, and the difference in the shimming properties before and after the shimming of the second type of magnetic pole step, determine the fitness function, and use the particle swarm optimization algorithm to obtain the optimal height of each magnetic pole step when performing magnetic field shimming.

[0048] After placing a pole step of arbitrary height at any position on the pole surface of the magnet, since the magnetic field strength generated by the pole step will be linearly superimposed with the magnetic field strength of the original magnet, the magnetic field strength of each measurement point is repeatedly obtained by the Hall Gaussmeter, and the new three-dimensional magnetic field generated by the characteristic magnetic field region between the two magnetic poles of the medical cyclotron is simulated by the finite cloud method, and the magnetic field response curve of the pole step shim block of arbitrary height at the arbitrary position can be obtained. Similarly, the magnetic field response curves of the pole step shim blocks of different heights at different positions can be obtained. The magnetic field response curve is usually approximately Gaussian distributed, and the closer to the center of the pole step, the larger the amplitude. Conversely, the farther the position is, the amplitude shows a rapid decreasing trend. That is, the magnetic field strength value of the shim block directly above the shim block is the largest, and gradually decreases to both sides. The magnetic field strength of each shim block at different positions and heights can be obtained through the magnetic field response curve. The schematic diagram of the magnetic field response curve is shown as follows. Figure 2 As shown, Figure 2 The horizontal axis is distance, indicating that the farther away from the magnetic pole step, the faster the amplitude decreases. Figure 2 The vertical axis on the left is the magnetic field intensity, and the vertical axis on the right is the height, which indicates the height of the magnetic pole step.

[0049] Before and after the magnetic pole steps are padded, the padded property calculation method can be used to obtain the padded property of the magnetic pole steps before and after the padded property. In this embodiment, the particle swarm optimization algorithm is used to obtain the optimal height of each magnetic pole step when padded, wherein the dimension of the optimal solution of the particle swarm optimization algorithm is M, corresponding to the optimal height of M magnetic pole steps. In this embodiment, the number of particles of the particle swarm optimization algorithm is 200, the maximum number of iterations is 500, the inertia weight is 0.8, the individual learning factor and the social learning factor are both 1.5, and the implementer can set them according to the actual situation. This embodiment does not impose any restrictions here. The fitness function of the particle swarm optimization algorithm is ; Where S represents the fitness function, C1 is the number of the first type of magnetic pole steps, C2 is the number of the second type of magnetic pole steps, is the shimming complexity of the i-th type-one magnetic pole step, is the shimming property of the i-th type-one magnetic pole step before shimming, is the shimming property of the ith type I magnetic pole step after shimming, is the shimming property of the jth second-class magnetic pole step before shimming, is the shimming property of the jth second-class magnetic pole step after shimming, To preset a value greater than 0 to avoid the denominator being 0, in this embodiment , the implementer can set it according to the actual situation, and this embodiment does not limit it. The fitness function determination flow chart is as follows Figure 3 shown.

[0050] The optimization process of the particle swarm optimization algorithm in this embodiment is to maximize the fitness function. For a type of magnetic pole step, the more non-uniform the magnetic field in the corresponding three-dimensional step area is, the greater the difference in the shimming properties before and after shimming is, and the smaller the shimming properties after shimming are. For a type of magnetic pole step, compared with a type of magnetic pole step, the magnetic field in the corresponding three-dimensional step area is more uniform. Therefore, the difference in the shimming properties before and after shimming is smaller.

[0051] Finally, the optimal solution corresponding to the maximization of the fitness function, that is, the padding height corresponding to the padding property after padding, is used as the optimal height of each magnetic pole step. The optimization process of the particle swarm optimization algorithm is an existing well-known technology and will not be described in detail in this embodiment.

[0052] Based on the same inventive concept as the above-mentioned method, an embodiment of the present application also provides a medical cyclotron isochronous magnetic field shimming system, comprising a memory, a processor, and a computer program stored in the memory and running on the processor. When the processor executes the computer program, it implements the steps of any one of the above-mentioned medical cyclotron isochronous magnetic field shimming methods.

[0053] It should be noted that the order in which the embodiments of the present application are presented is for illustrative purposes only and does not necessarily represent the superiority or inferiority of the embodiments. Furthermore, the foregoing descriptions of specific embodiments of this specification are provided. Furthermore, the processes depicted in the accompanying drawings do not necessarily require the specific order or sequential sequence shown to achieve the desired results. In certain embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0054] The various embodiments in this specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on the differences from other embodiments.

[0055] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A method for shimming the isochronous magnetic field of a medical cyclotron, characterized in that: The method comprises the following steps: Collecting the magnetic field strength at each position within the magnetic field area of ​​a medical cyclotron; Analyzing the difference in magnetic field intensity at each position and its radially adjacent positions, and combining the spatial distance between each position and its radially adjacent positions, determining the local magnetic field inhomogeneity at each position; equally dividing a preset number of magnetic pole steps on the magnetic pole surface of the magnetic field region, analyzing the difference in magnetic field intensity at each position in the three-dimensional region corresponding to each magnetic pole step in the magnetic field and the target magnetic field intensity, and combining the local magnetic field inhomogeneity to determine the shimming property of each magnetic pole step; Based on the correlation between the shimming properties of the various magnetic pole steps, all magnetic pole steps are divided into Class I and Class II. For each Class I magnetic pole step, the distribution of local magnetic field inhomogeneity at all positions within the corresponding three-dimensional region in the magnetic field, as well as the difference in local magnetic field inhomogeneity between each position and its axially adjacent position, are analyzed. Combined with the shimming properties, the shimming complexity of each Class I magnetic pole step is determined. Based on the difference in shimming properties before and after shimming of the first type of magnetic pole step, combined with the shimming complexity, and the difference in shimming properties before and after shimming of the second type of magnetic pole step, the fitness function is determined, and the intelligent optimization algorithm is used to obtain the optimal height of each magnetic pole step when performing magnetic field shimming.

2. The method for shimming the isochronous magnetic field of a medical cyclotron according to claim 1, wherein: The determination of the local magnetic field inhomogeneity comprises: The difference between the axial component of the magnetic field intensity at each position and its adjacent positions in the radial direction is calculated, recorded as the first difference, and the ratio of the first difference to the spatial distance is calculated. The local magnetic field inhomogeneity is the fusion result of the ratios at each position and all adjacent positions in the radial direction.

3. The method for shimming the isochronous magnetic field of a medical cyclotron according to claim 2, wherein: The local magnetic field inhomogeneity is the cumulative sum of the ratios at each position to all adjacent positions in the radial direction.

4. The method for shimming the isochronous magnetic field of a medical cyclotron according to claim 1, wherein: The target magnetic field strength is the magnetic field strength required to maintain an isochronous magnetic field at each position in the magnetic field region.

5. The method for shimming the isochronous magnetic field of a medical cyclotron according to claim 1, wherein: The determination of the shimming property includes: The difference between the axial component of the magnetic field strength at each position and its target magnetic field strength is recorded as the second difference, and the product of the local magnetic field inhomogeneity at each position and the second difference is calculated. The shimming property is the fusion result of the product of all positions in the three-dimensional area corresponding to each magnetic pole step in the magnetic field.

6. The method for shimming the isochronous magnetic field of a medical cyclotron according to claim 1, wherein: The method of dividing all magnetic pole steps into first-class magnetic pole steps and second-class magnetic pole steps includes: The clustering algorithm is used to cluster the padding properties of all magnetic pole steps, and all magnetic pole steps are divided into two clusters. The mean of the padding properties of all magnetic pole steps in each cluster is calculated, and the magnetic pole steps in the cluster corresponding to the maximum mean value are recorded as Class I magnetic pole steps, and the magnetic pole steps in the remaining clusters are recorded as Class II magnetic pole steps.

7. The method for shimming the isochronous magnetic field of a medical cyclotron according to claim 1, wherein: The determination of shimming complexity includes: The difference in local magnetic field inhomogeneity at each position in the three-dimensional region corresponding to each type of magnetic pole step in the magnetic field and its axially adjacent position is calculated and recorded as the third difference. The fusion value of the third difference at all positions in the three-dimensional region corresponding to each type of magnetic pole step in the magnetic field is calculated. The degree of disorder of the local magnetic field inhomogeneity at all positions in the three-dimensional region corresponding to each type of magnetic pole step in the magnetic field is calculated. Based on the degree of disorder, the fusion value and the shimming property, the shimming complexity of each type of magnetic pole step is determined.

8. The method for shimming the isochronous magnetic field of a medical cyclotron according to claim 7, wherein: The shimming complexity is the product of the degree of disorder, the fusion value, and the shimming property of each type of magnetic pole step.

9. The method for shimming the isochronous magnetic field of a medical cyclotron according to claim 1, wherein: The expression of the fitness function is: ; Where S represents the fitness function, C1 is the number of the first type of magnetic pole steps, C2 is the number of the second type of magnetic pole steps, is the shimming complexity of the i-th type-one magnetic pole step, is the shimming property of the i-th type-one magnetic pole step before shimming, is the shimming property of the ith type I magnetic pole step after shimming, is the shimming property of the jth second-class magnetic pole step before shimming, is the shimming property of the jth second-class magnetic pole step after shimming, The default value is greater than 0.

10. A medical cyclotron isochronous magnetic field shimming system, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that: When the processor executes the computer program, the steps of the method according to any one of claims 1 to 9 are implemented.

Images