JP6392141B2 - Magnetic field uniformity adjustment method, magnetic field uniformity adjustment program, and magnetic field uniformity adjustment apparatus - Google Patents

Description

  The present invention relates to a method for adjusting the magnetic field uniformity of a static magnetic field generator for magnetic resonance imaging (hereinafter referred to as MRI).

  The MRI apparatus measures a magnetic resonance (hereinafter referred to as NMR signal) signal of a nuclear spin in a subject placed in an imaging space in a uniform static magnetic field, and calculates a nuclear spin density distribution and relaxation time distribution in the subject. Etc. are displayed as tomographic images. The MRI apparatus includes a static magnetic field generation apparatus that generates a static magnetic field and a gradient magnetic field generation apparatus that generates a gradient magnetic field for giving positional information to NMR signals.

  If the uniformity of the static magnetic field generated by the static magnetic field generator is disturbed, the linearity of the gradient magnetic field superimposed on it deteriorates, so that the positional information given to the NMR signal is displaced, and image distortion or Cause loss. Image distortion, loss, or the like impairs the accuracy and clarity of the image and is a major obstacle to diagnosis. Therefore, a very high degree of uniformity is required for the static magnetic field in the imaging space. On the other hand, since the intensity of the NMR signal is substantially proportional to the static magnetic field intensity, a static magnetic field generator having a large static magnetic field intensity is desired to obtain a high-quality MRI image. Thus, the static magnetic field generated by the MRI apparatus is required to have a high degree of uniformity and a large magnetic field strength (high magnetic field), and the static magnetic field generation apparatus is one of the extremely important components of the MRI apparatus. .

  It is known that an apparatus for generating a static magnetic field using a superconducting magnet can stably form a static magnetic field having a high degree of uniformity and a large intensity in an imaging space for a long time. Cylindrical superconducting magnets are known as shapes that can generate a high magnetic field with high efficiency. A cylindrical superconducting magnet has a structure in which a plurality of superconducting coils are arranged inside a cryogenic vessel or a cryogenic vessel filled with liquid helium or other cryogenic cryogen. The cryostat or cryocontainer is disposed inside the vacuum chamber, and a radiation shield for insulating heat from entering from the outside is disposed inside the vacuum chamber. Furthermore, a refrigerator is attached to the vacuum chamber, and a cooling unit of the refrigerator is connected to a low temperature chamber or a low temperature container and a radiation shield, and maintains a low temperature.

  In an MRI apparatus using a superconducting magnet as a static magnetic field generator, the volume and shape of the imaging space known as the field of view (FOV) vary depending on the imaging object required, but is defined by the peak-to-peak value of the magnetic field uniformity. The space is generally spherical. In recent years, in an MRI apparatus having a central magnetic field strength of 1.5 Tesla, an FOV with a diameter of about 45 to 50 cm and a peak-to-peak value of the uniformity of the static magnetic field is several tens of ppm (about 20 to 40 ppm) Is common.

  Since the uniformity of the static magnetic field of the superconducting magnet is mainly determined by the arrangement of the superconducting coils, the arrangement is designed to generate a uniform magnetic field required in a desired space. However, in reality, it is difficult to achieve the designed magnetic field uniformity due to the manufacturing dimensional error of the superconducting magnet. It is about several hundred ppm. Therefore, in order to correct the non-uniformity of the static magnetic field of the superconducting magnet, there is generally a method called a passive shim, in which a small magnetic piece is arranged around the imaging space to finely adjust the static magnetic field. Used.

  Patent Document 1 proposes a method of calculating the amount and position of a magnetic piece to be arranged for magnetic field adjustment in a static magnetic field generator by calculation. The adjustment method of Patent Document 1 measures the spatial distribution of a magnetic field in an imaging space, and calculates an eigen distribution function obtained by singular value decomposition to determine the amount and arrangement of magnetic pieces for correcting an error magnetic field with respect to a desired uniform magnetic field. Use to calculate.

Japanese Patent No. 4902787

  Since the static magnetic field generation device of the MRI apparatus needs to realize correction of the static magnetic field by the shim iron piece while securing a large imaging space for inserting the subject, a plurality of concave portions in which a plurality of shim iron pieces can be arranged are arranged. The shim tray provided is provided on the imaging space side of the static magnetic field generator. The size (depth) of the concave portion of the shim tray is limited to ensure the size of the imaging space, and there is an upper limit on the amount of magnetic pieces that can be placed in one concave portion. It is necessary to realize a static magnetic field having a desired uniformity or more with a magnetic piece having an upper limit value or less.

  Further, the arranged shim iron pieces generate a uniform static magnetic field component (B0 component) in addition to the magnetic field component for correcting the error magnetic field. The amount of B0 component produced at this time tends to be roughly proportional to the total amount of shim iron. On the other hand, the magnetization of the shim iron piece is affected by temperature due to heat generated by the gradient coil disposed adjacent to the room temperature or the static magnetic field space, and as a result, causes static magnetic field fluctuations. In particular, since the temperature dependence of the B0 component has the highest sensitivity, it is necessary to set a certain limit for the total amount of shim iron pieces, in addition to the arrangement amount for each recess described above.

  In addition, the magnetic pieces actually arranged in the recess of the shim tray have a predetermined size, and the amount of the magnetic pieces to be arranged in the recess is set depending on how many (how many) the magnetic pieces are arranged. The Therefore, the amount of the magnetic piece in the concave portion of the shim tray is calculated as a continuous arbitrary value, but in practice, it can take only a discrete value, and the magnetic field uniformity to be realized is calculated. An error occurs with respect to the magnetic field uniformity.

  According to Patent Document 1, when the target magnetic field is changed, the intensity of each eigen distribution included in the error magnetic field and the magnetic field intensity remaining (residual uniformity) remaining as a residual also change. It is disclosed that there is a need (paragraphs 0051, 0069). However, since the technique of Patent Document 1 does not consider the case where there is an upper limit on the amount of magnetic pieces that can be arranged, the relationship between the upper limit of the amount of magnetic pieces, the target magnetic field, and the degree of uniformity of arrival. Not disclosed. Further, Patent Document 1 does not consider that the amount of magnetic pieces actually arranged can take only discrete values, and the magnetic field uniformity error is caused accordingly.

  An object of the present invention is to achieve high magnetic field uniformity even when there is a limit to the amount of magnetic pieces that can be placed at each position of a shim tray.

  In order to achieve the above object, according to the present invention, a static magnetic field generation apparatus including shim trays for holding magnetic pieces for adjusting the uniformity of a generated static magnetic field at a plurality of predetermined positions, respectively. A method for adjusting the magnetic field uniformity is provided. That is, when the distribution of the static magnetic field generated by the static magnetic field generator is measured, the error magnetic field between the distribution of the static magnetic field and the target magnetic field is calculated, and the magnetic pieces are arranged at one or more of the plurality of positions of the shim tray The magnetic field uniformity that can be reached is calculated while changing the target magnetic field in a predetermined magnetic field range, and the amount of magnetic material pieces at each position of the shim tray is less than or equal to a predetermined upper limit value, and can be reached. It is assumed that a target magnetic field having a value equal to or less than a predetermined value is selected, and the arrangement and amount of magnetic pieces corresponding to the target magnetic field are magnetic pieces to be placed on the shim tray.

  According to the present invention, even if there is an upper limit for the amount of magnetic material pieces that can be placed on the shim tray, it is possible to efficiently adjust the magnetic field uniformity to a high level.

(A) The perspective view of an example of a static magnetic field generator (state which pulled out the shim tray 71 partially), (b) The perspective view of the shim tray 71. The graph which shows an example of distribution of the measured magnetic field strength. (A) a graph showing the relationship between the target magnetic field strength, the reachable magnetic field uniformity and the total amount of necessary magnetic shims, (b) the target magnetic field strength and the reach when there is an upper limit in the capacity of the recess of the shim tray The graph which shows the relationship between possible magnetic field uniformity and the total amount of a magnetic body shim. The block diagram which shows the structure of the magnetic field uniformity adjustment apparatus of 1st Embodiment. The flowchart which shows operation | movement of the magnetic field uniformity adjustment program of 1st Embodiment. Explanatory drawing which shows the structure of the measurement part 74 of a magnetic field. Explanatory drawing which shows the screen which receives the setting of conditions from an operator in 1st Embodiment. The flowchart which shows the detail of step 17 of FIG. The graph which shows the value of the magnetic field intensity _| strength _Bei produced for every eigenmode. The table | surface which shows the relationship between the number of a shim tray and the recessed part 72 displayed at step 20 of FIG. (a)-(c) The table | surface which shows the calculation result of step 17 of FIG. 6 is a flowchart showing details of step 81 in FIG. 5. The graph which shows the magnetic field uniformity at the time of changing the target magnetic field Btg calculated at step 81 of FIG. The graph which shows the magnetic field uniformity at the time of changing the maximum value of the order of an eigenmode, making target magnetic field Btg constant. (a)-(c) The graph which shows the calculation result of Fig.11 (a)-(c). The flowchart which shows operation | movement of the magnetic field uniformity adjustment program of 2nd Embodiment. The perspective view which shows the static magnetic field generator by which the magnetic field uniformity adjustment apparatus 33 of 3rd Embodiment is arrange | positioned. The flowchart which shows operation | movement of the magnetic field uniformity adjustment program of 3rd Embodiment.

  An embodiment of the present invention will be described with reference to the drawings.

<< First Embodiment >>
A static magnetic field adjustment method according to the first embodiment of the present invention will be described. The static magnetic field adjustment method of the present embodiment is a method of adjusting the magnetic field uniformity of a static magnetic field generator 73 having a shim tray 71 as shown in FIG. The shim tray 71 holds the magnetic piece 9 for adjusting the uniformity of the static magnetic field generated in the uniform magnetic field space 77 by the static magnetic field generator 73 at a plurality of predetermined positions (numbers 1 to 20). In addition, a recess 72 is provided at each position (numbers 1 to 20). A plurality of shim trays 71 are arranged side by side on the uniform magnetic field space 77 side of the static magnetic field generator 73. Thereby, the plurality of positions P1 to P20 for arranging the magnetic pieces 9 of the plurality of shim trays 71 are arranged in a matrix.

  In this embodiment, the case where the static magnetic field generator 73 is a static magnetic field generator of an MRI apparatus will be described as an example. However, the static magnetic field adjustment of other apparatuses that require a uniform static magnetic field is not limited to the MRI apparatus. It is possible to apply the method of this embodiment to. In addition, FIG. 1 shows an example in which the static magnetic field generator (Z-axis) is horizontally arranged in the cylindrical static magnetic field generator, but a pair of flat magnets are arranged above and below at intervals. It is also possible to apply to a static magnetic field generator in which the static magnetic field direction is vertical. Moreover, although the case where a static magnetic field generator is a superconducting magnet is demonstrated below, a normal conducting magnet and a permanent magnet may be sufficient.

  In the magnetic field uniformity adjusting method of the present embodiment, in order to make the magnetic field uniformity of the uniform magnetic field space 77 equal to or less than a predetermined value, the magnetic piece to be arranged in the recesses 72 at the positions (numbers 1 to 20) of the plurality of shim trays 71 The position and amount of 9 are obtained by calculation using the singular value decomposition method. At this time, the size (depth) of the recess 72 of the shim tray 71 is predetermined in order to secure a large uniform magnetic field space 77. Therefore, the amount (capacity) of the magnetic piece 9 that can be disposed in the concave portion 72 at the position (numbers 1 to 20) has an upper limit. The inventors have found that when the amount of the magnetic piece 9 that can be arranged has an upper limit, changing the target magnetic field changes the minimum reachable magnetic field uniformity. Therefore, by changing the target magnetic field within an allowable range, a uniform magnetic field space 77 having a smaller magnetic field uniformity (small magnetic field distribution) is formed. As a method for calculating the arrangement and amount of the magnetic piece 9 using the singular value decomposition method, a well-known method is used (for example, Japanese Patent No. 4902787).

  The principle of changing the minimum magnetic field uniformity that can be reached by changing the target magnetic field will be described. If the magnetic field strengths measured at a plurality of measurement points in the uniform magnetic field space 77 are distributed as shown in FIG. 2, for example, a magnetic field for correcting an error magnetic field between the target magnetic field 31 and the measured magnetic field strength is generated. The magnetic piece 9 needs to be disposed on the shim tray 71. When an iron piece is arranged as the magnetic piece 9 on the shim tray 71, the iron piece is magnetized in the same direction as the magnetic field of the uniform magnetic field space 77 by the magnetic field generated by the static magnetic field generator 73 and has a magnetic moment (positive magnetic moment). To generate a magnetic field. When the iron piece having a positive magnetic moment is arranged in the vicinity of the uniform magnetic field space 77, the magnetic flux generated by the iron piece is the direction of the magnetic field in the uniform magnetic field space 77 when passing through the uniform magnetic field space 77. The direction is reversed, and the magnetic field in the uniform magnetic field space 77 is lowered. Thereby, the error magnetic field higher than the target magnetic field 31 in the distribution of the magnetic field strength in FIG. On the other hand, in order to increase the magnetic field lower than the target magnetic field 31 toward the target magnetic field 31, when the magnetic flux generated by the iron piece having a positive magnetic moment passes through the uniform magnetic field space 77, the uniform magnetic field space 77. A method is used in which iron pieces are arranged in the recesses 72 at positions away from the uniform magnetic field space 77 in the axial direction so as to be in the same direction as the magnetic field. Alternatively, a permanent magnet piece that has been previously magnetized in the concave portion 72 in the vicinity of the uniform magnetic field space 77 in the direction opposite to the direction of the uniform magnetic field space 77, that is, a negative magnetic moment is disposed as the magnetic piece 9. It can be adjusted by generating a magnetic field in the same direction as the static magnetic field in the uniform magnetic field space 77.

  As described above, the magnetic piece 9 for reducing the magnetic field strength of the uniform magnetic field space 77 and the magnetic piece 9 for increasing the magnetic field strength are arranged depending on what magnetic field strength is set as the target magnetic field 31. The position and amount change.

FIG. 3A shows the target magnetic field 31 and the predicted value of the magnetic field uniformity of the uniform magnetic field space 77 (the predicted arrival uniformity: left vertical axis) and the total amount of the magnetic pieces 9 to be arranged (total amount of arranged shim iron: right vertical). Axis).
For example, as the target magnetic field is decreased as shown in FIG. 3A, a larger number of magnetic pieces 9 are required, but the magnetic field in the uniform magnetic field space 77 can be made asymptotic to the target magnetic field. In particular, when the target magnetic field 32 smaller than the lower limit of the magnetic field strength distribution in the uniform magnetic field space 77 is set, there is no need to increase the magnetic field strength distribution toward the target magnetic field, and many magnetisms that lower the magnetic field strength in the uniform magnetic field space 77. By disposing the body piece 9, the magnetic field in the uniform magnetic field space 77 can be made asymptotic to the target magnetic field 32.

  However, since the capacity of the recess 72 of the shim tray 71 has an upper limit value, if the magnetic piece 9 that needs to be arranged exceeds the upper limit value, the error magnetic field cannot be corrected and the target magnetic field 32 can be made asymptotic. Can not. Here, FIG. 3B shows a relationship between the amount of the magnetic piece 9 and the magnetic field uniformity of the uniform magnetic field space 77 when the capacity of the recess 72 of the shim tray 71 has an upper limit. As described above, when the target magnetic field is set low, the amount of the magnetic piece 9 increases to correct the error magnetic field, and the magnetic field uniformity tends to decrease (improve). However, since the capacity of the concave portion 72 has an upper limit, the magnetic field uniformity has an extreme value because not all of the necessary magnetic material pieces 9 can be arranged in a place required for correcting the error magnetic field.

  As described above, when the amount of the magnetic piece 9 to be arranged at a certain position obtained by calculation is larger than the upper limit value of the capacity of the recess 72 of the shim tray 71, the amount of the magnetic piece 9 is actually placed in the shim tray 71. Since the error magnetic field cannot be completely corrected, it cannot be matched with the target magnetic field. Therefore, when there is an upper limit value for the amount of the magnetic piece 9 that can be arranged at each position (numbers 1 to 20) of the shim tray 71, the minimum reachable magnetic field uniformity varies depending on the value of the target magnetic field 31. become.

  Further, since it is not easy to precisely process the permanent magnet into a minute size, it is desirable to adjust the magnetic field uniformity without using a permanent magnet having a negative magnetic moment as much as possible as the magnetic piece 9.

  Therefore, in the present embodiment, the target magnetic field is changed within a predetermined range, and the reachable magnetic field uniformity is calculated, thereby achieving the predetermined magnetic field uniformity or less with the amount of the magnetic piece 9 below the upper limit value. Select a target magnetic field. Further, what is arranged in the recess 72 of the actual shim tray 71 is a magnetic piece 9 of a predetermined size, and the amount of the magnetic piece 9 is adjusted depending on how many (how many) pieces are arranged. The Therefore, the amount of the magnetic piece 9 can take only a discrete value, and an error occurs with respect to the amount of the magnetic piece 9 obtained by calculation. Therefore, the reachable magnetic field uniformity is also the calculated magnetic field uniformity. An error will occur. In view of this, a target magnetic field that can achieve a predetermined magnetic field uniformity or less is selected. This will be specifically described below.

  In the present embodiment, as shown in FIG. 4, a uniform magnetic field adjustment device 33 including a CPU 34 and a memory 35 is used. A predetermined magnetic field uniformity adjustment program is stored in the memory 35, and the CPU 34 reads and executes the program in the memory 35 to operate as shown in the flow of FIG. Realize the adjustment method. FIG. 4 shows an example in which the uniform magnetic field adjusting device 33 is connected to the measuring unit 74 and the excitation power source 2 of the superconducting coil 4a. However, in the first embodiment, even if it is not connected. I do not care.

First, the operator measures the distribution of the magnetic field strength in the uniform magnetic field space 77. For example, as shown in FIG. 6, the measurement unit 74 in which the magnetic field measurement elements 78 are arranged in a predetermined pattern (for example, 24 planes) corresponding to the shape of the uniform magnetic field space 6 is rotated about the axial direction 75 of the static magnetic field. Accordingly, measuring the magnetic field strength _{B m} of uniform magnetic field space 77.

CPU34 at step 12 in FIG. 5, captures the magnetic field strength _{B m} is the measurement result.

In steps 13 to 16, the CPU 34 determines the position information of the recesses 72 at the positions (numbers 1 to 20) of the plurality of shim trays 71 and the upper limit value of the amount of the magnetic body piece 9 (shim iron amount) arranged for each recess 72. The upper limit value of the total capacity of the magnetic bodies arranged in all the recesses 72 in the shim tray 71, the required magnetic field uniformity, the range of the target magnetic field B _tg , and the maximum value of the eigenmode used for calculation are received. For example, the CPU 34 displays a reception screen as shown in FIG. 7 on the connected display device 220, and in the input fields 61 to 63 of the reception screen, the central value of the target magnetic field B _tg (target central magnetic field B _tgc ), Input of the step width ΔB and the number of steps n (n is an integer) can be received from the operator. In this case, the range of the target magnetic field B _tg is represented by (B _tgc −ΔB · n / 2) to (B _tgc + ΔB · n / 2). In addition, the maximum value of the eigenmode used for the calculation in the input field 64 is the upper limit of the total amount of the magnetic piece 9 (positive magnetic moment, negative magnetic moment) permitted to be arranged in the entire shim tray 71 in the input fields 65 and 66. In the input fields 65a and 66a, the upper limit value of the amount of the magnetic piece 9 (positive magnetic moment, negative magnetic moment) permitted to be placed in one recess 72 is entered in the input field 68a and 66a. Each can be accepted once.

Next, in step 17, the CPU 34 determines the minimum magnetic field uniformity that can be reached when the magnetic piece 9 is arranged at one or more of the positions (numbers 1 to 20) of the plurality of shim trays 71 as the target magnetic field B. Each _tg is calculated while changing within the accepted magnetic field range.

The calculation method of step 17 will be described in detail with reference to the flow of FIG. First, CPU 34, in step 51, calculates the error magnetic field _{B e} based on the target magnetic field _{B tg} accepted in measurement field _{B m} and step 16 taken in step 12.

The error magnetic field _Be can be described by a linear equation of the following equation (2).

Here, if the response matrix A of the magnetic field is regular, an inverse matrix A ⁻¹ exists, and the matrix I of the current potential representing the position and amount of the magnetic piece 9 can be obtained by Expression (3).

The response matrix A is obtained in advance as follows using a vector r indicating the magnetic moment of iron and the distance r and the direction from the recess 72 of the shim tray 71 of the MRI apparatus to the FOV (imaging field of view). Keep it. The point-arranged magnetic moment m can be defined as a magnetic dipole in any direction of m = (m _X , m _Y , m _Z ). In general, the magnetic field B (vector) created by the magnetic moment m at a position separated from the position vector r (= (X, Y, Z,)) can be expressed by the equation (4). In (5) and (6), bold r represents a position vector, and thin r represents a distance.

Here, when the magnetic moment per unit volume is F, the magnetic moment m of the iron amount I is as shown in Equation (5). When the magnetic moment (iron) is used for magnetic field correction, the response matrix A is It is represented by Formula (6).
The

When number j is assigned to the recess 72 of the shim tray 71, the magnetic field at the k-th measurement point is expressed by Equation (7) with respect to the iron amount I _j (m ³ ) of each recess 72.

However, since the response matrix A expressed by Equation (6) is not regular and there is no inverse matrix, the singular value decomposition method (SVD) is performed, and the response matrix A of the magnetic field is obtained by Equation (8).

From equations (3) and (8), the matrix I of the current potential representing the position and amount of the magnetic piece can be obtained by equation (9).

From the equation (9), it can be seen that the magnetic field strength B _ei generated in the eigenmode of the order i is represented by n _p ^1/2 P _i u _i . The magnetic field strength B _ei for each eigenmode is distributed as shown in FIG. 9, and the smaller the order i, the larger the magnetic field strength B _ei . _Therefore, the value of ⁿ p _1/2 P _{i u} i, and adds the order i = 1 up to order i = _{M D} of the maximum value of the eigenmodes accepted in step 15, further measuring the magnetic field _{B m} and the following formula (6 ), The estimated magnetic field distribution after adjustment: B _Predicted can be calculated.

The matrix B _ep representing the distribution of the non-uniform magnetic field from the target magnetic field can be calculated by obtaining the difference between the adjusted magnetic field distribution estimated value B _Predicted obtained by Expression (10) and the target magnetic field B _tg .

In step 52, the CPU 34 calculates the expressions (8) and (9) to obtain the position and amount of the magnetic piece 9 for each eigenmode. Then, store the location and amount of the magnetic material piece 9 for eigenmodes below the maximum value M _D set in the memory 35 in correspondence to the target magnetic field B _tg. When the magnetic piece 9 is arranged in the concave portion 72 at the same position in a plurality of natural modes, the total amount of the magnetic piece 9 in the plurality of natural modes is calculated, and the magnetic piece 9 after the summation for each concave portion 72 is calculated. Are stored in the memory 35.

The calculated arrangement and amount of the magnetic piece 9 to be arranged in the recess 72 of the shim tray 71 are represented by a matrix I _J. Matrix elements Ij of the matrix I _J represents the amount of the magnetic material pieces 9 of the j-th recess 72. For example, if there are 16 shim trays 71 and 20 recesses 72 in the shim tray 71, the number of matrix elements I _j of the matrix I _J is 320 (j = 1 to 320).

In the above step 52, when the calculated amount of the magnetic piece 9 exceeds the upper limit of the amount of the magnetic piece 9 permitted to be disposed in the recess 72 received in step 13, the calculated piece of the magnetic piece 9 the amount of the (matrix elements Ij of the matrix I _J), reduced by equation (12) to the upper limit of the amount of the magnetic material piece 9 to allow the placement. However, in Expression (12), I _max indicates the upper limit of the amount of the magnetic piece 9 that is allowed to be disposed in the recess 72.

The matrix I _J after the application of the equation (12) is obtained by applying the upper limit for allowing the concave portion 72 to be arranged to the amount of the magnetic piece 9 for correcting the error magnetic field calculated in step 52. Represents quantity and placement. For example, as shown in FIG. 10, the amount of the magnetic piece 9 is stored in the memory 35 in the form of a table for specifying the position of the recess 72 by the number of the recess 72.

Since the magnetic field generated by the amount and arrangement of the magnetic piece 9 represented by the matrix I _J is represented by AI _J from Equation (2), the magnetic field B _Predicted after the arrangement of the magnetic piece 9 and the magnetic substance The matrix B _ep representing the inhomogeneous magnetic field distribution from the target magnetic field after the piece 9 is arranged can be obtained from the equations (13) and (14) based on the equations (10) and (11).

The CPU 34 obtains a matrix B _ep representing the non-uniform magnetic field distribution by using the matrix I _J representing the amount and distribution of the magnetic piece 9 obtained in step 52 and the equations (13) and (14). However, equation (8) is used as response matrix A in equation (13).

Further, in step 53, the CPU 34 calculates the reachable magnetic field uniformity (reached expected magnetic field uniformity) from the matrix elements of the matrix B _ep representing the inhomogeneous magnetic field. For example, the inhomogeneous magnetic field is obtained by calculating the relative value of the difference between the maximum value and the minimum value of the error magnetic field represented by the matrix B _ep (PkPk value: peak-to-peak value) with respect to the average magnetic field intensity from the equation (15). . The calculated magnetic field uniformity is stored in the memory 35 in correspondence with the target magnetic field B _tg .

The calculation in steps 51 to 53 is performed by changing the target magnetic field B _tg from the minimum value (B _tgc -ΔB · n / 2) of the set magnetic field range to the maximum value (B _tgc + ΔB · n / 2) by ΔB. repeat. Thereby, the reachable magnetic field uniformity can be calculated for each of n target magnetic fields in increments of ΔB within the set magnetic field range.

FIGS. 11A to 11C and FIGS. 15A to 15C show examples of calculation results as tables and graphs. An example of the calculation result will be described with reference to FIG. In the example of FIG. 11A, ΔB = 5 × 10 ⁻⁴ Tesla, n = 6, and the magnetic field uniformity that can be reached for six target magnetic fields Btg in the range of 1.498848 Tesla to 1.501348 Tesla. Calculated. The result of FIG. 11A is shown as a graph in FIG. FIGS. 11A and 15A show the total capacity of the magnetic piece 9 to be arranged for each of the magnetic piece 9 having a positive magnetic moment and the magnetic piece 9 having a negative magnetic moment. .

Here, with respect to the total capacity of the magnetic piece 9 that can be arranged with a required magnetic field uniformity of 10 ppm or less, the total amount of positive magnetic moment is 1.5 × 10 ⁻³ [m ³ ] (the magnetic piece is iron or electromagnetic In the case of a steel plate, an embodiment in which the total amount of negative magnetic moment (lower limit value) is set to 0 will be described below with reference to FIGS. 11 (a) and 15 (a).

As can be seen from FIG. 15A, the reachable magnetic field uniformity also changes as the target magnetic field Btg changes. Based on the reachable magnetic field uniformity calculated in step 53, the CPU 34 determines that the amount of the magnetic piece 9 and the total capacity of each recess 72 at each position of the shim tray 71 are equal to or less than a predetermined upper limit value. Then, it is determined whether or not there is a target magnetic field Btg whose reachable magnetic field uniformity is not more than a predetermined value (step 18), and if there is a target magnetic field Btg that satisfies such conditions, it is selected (step 19). For example, in the examples of FIGS. 11A and 15A, the reachable magnetic field uniformity has a target magnetic field Btg indicating 9.3 ppm as a minimum value, but the total amount of the magnetic piece 9 at that time is The total positive magnetic moment satisfies the above constraint (1.5 × 10 ⁻³ [m ³ ] or less), but the negative total is −5.6 × 10 ⁻⁴ [m ³ ], and the above constraint Does not meet. Therefore, a target magnetic field Btg (1.499848 Tesla, magnetic field uniformity: 9.9 ppm) that satisfies the above-described constraints and satisfies the required magnetic field uniformity of 10 ppm or less is selected. In the example of FIG. 15A, the magnetic piece 9 needs only the magnetic piece 9 having a positive magnetic moment, and the amount of the magnetic piece 9 having a negative magnetic moment is 0. It is not necessary to use and is preferable.

  If there is no target magnetic field Btg that satisfies the condition in step 18, the process returns to step 13, the operator sets steps 13 to 17 under different conditions, and performs the calculation again.

Next, the process proceeds to step 81. Considering that the magnetic piece 9 has a predetermined size, the amount of the magnetic piece 9 can take only discrete values, and the target magnetic field Btg is selected more precisely. To do. This will be described in detail using the flow of FIG. First, in step 181, a predetermined discretization error calculation magnetic field range including the target magnetic field Btg selected in step 19 and smaller than the magnetic field range accepted in step 16 is set. Further, a step size ΔBd for calculating the discretization error smaller than ΔB received in step 16 is set. For example, as ΔBd, four types of 1 × 10 ⁻³ Tesla, 1 × 10 ⁻⁴ Tesla, 1 × 10 ⁻⁵ Tesla, and 1 × 10 ⁻⁶ Tesla are prepared in advance. What is smaller than the width ΔB and closest to ΔB may be selected as ΔBd. For example, if ΔB in step 16 is 1 × 10 ⁻³ Tesla, 1 × 10 ⁻⁴ Tesla is selected as ΔBd. If ΔB in step 16 is 1 × 10 ⁻⁵ Tesla, 1 × 10 ⁻⁶ Tesla is selected as ΔBd. The discretization error calculation magnetic field range can be set to the target magnetic field Btg selected in step 19 as the center value Btgc of the target magnetic field and n times the step width ΔBd (n is an integer).

Then, in steps 182 to 187, the reachable magnetic field uniformity is recalculated while changing the target magnetic field Btg by the increment ΔBd within the discretization error calculation magnetic field range, as in steps 50 to 55 of FIG. . However, in step 185, considering that the amount of the magnetic piece 9 set is a multiple of the capacity of the predetermined magnetic piece 9, the position of the magnetic piece 9 when the target magnetic field B _tg is set. From the above and the quantity, the magnetic moment _Id is expressed by Equation (16).

In addition, between the magnetic moment I of the magnetic piece 9 calculated with an arbitrary value which is calculated continuously and the magnetic moment I _d considering the discretization when the target magnetic field B _tg is used, the equation (17) There is a relationship like

Further, a matrix B _ep representing the distribution of the inhomogeneous magnetic field that can be reached by the target magnetic field Btg is calculated using the equation (18). A in Equation (18) is a magnetic field response matrix, and the matrix obtained by Equation (8) is used. The reachable magnetic field homogeneity is calculated from the inhomogeneous magnetic field B _ep of the equation (18) by the equation (15). This magnetic field uniformity is a magnetic field uniformity that takes into account the influence of discretization errors.

In step 188, based on the calculation result of step 185, a target magnetic field Btg that minimizes the magnetic field uniformity is selected. An example of the calculation result of step 185 is shown in FIG. In the example of FIG. 13, ΔBb = 1 × 10 ⁻⁶ Tesla, n = 10, and 10 target magnetic fields Btg centered on 1.499848 Tesla are set by a multiple of the predetermined number of magnetic material pieces 9. The magnetic field uniformity that can be reached by the amount of the magnetic piece 9 is calculated. The amount of one magnetic piece 9 is (for example, 0.06 cc). Since the magnetic field range is narrow, the magnetic field uniformity (ideal solution) is constant at 9.9 ppm in an ideal case where the amount of the magnetic piece 9 can take an arbitrary continuous value. However, in the case of the discretized amount of the magnetic piece 9, it varies as shown in FIG. Therefore, in step 188, the target magnetic field Btg (third ● from the largest) is selected to minimize the magnetic field uniformity. The magnetic field uniformity at that time is about 10 ppm.

  Then, the CPU 34 determines that the arrangement and the amount of the magnetic piece 9 for each recess 72 of the shim tray 71 stored in step 184 for the target magnetic field Btg selected in step 19 are the magnetic piece 9 to be arranged in the shim tray 71. It is displayed on the display device (step 20). For example, as shown in FIG. 10, a table showing the amount of the magnetic piece 9 for each recess 72 is displayed on the display device.

  The operator confirms the amount and position of the magnetic piece 9 displayed on the display device (the number of the concave portion 72 of the shim tray 71), and inserts the magnetic piece 9 of the amount displayed in each concave portion 72 of the shim tray 71. To do. Thereby, the adjustment of the static magnetic field uniformity is completed.

  When the static magnetic field generator is a static magnetic field generator of the MRI apparatus, the CPU 34 calculates the frequency of the high frequency magnetic field for exciting the nuclear magnetism of the subject arranged in the target magnetic field Btg selected in step 19; It is displayed on the display device (step 21). The operator sets the high frequency magnetic field generation unit of the MRI apparatus so that the frequency of the displayed high frequency magnetic field is irradiated. Thereby, the MRI apparatus can image the subject with high accuracy by the adjusted target magnetic field Btg.

Further, in the present embodiment, by using the order of the low order to the order M _D set from the order i = 1 as eigenmodes available large singular values λi lower order values, large with a small magnetic body weight A correction magnetic field can be generated. Therefore, magnetic field adjustment can be performed efficiently while suppressing the amount of magnetic material.

  According to the present embodiment, by changing the target magnetic field Btg, the required magnetic field uniformity can be achieved in consideration of the upper limit for the amount of the magnetic piece 9 that can be disposed in the recess 72 of the shim tray 71. . In addition, considering that the amount of the magnetic piece 9 is discrete, the required magnetic field uniformity can be achieved.

In this embodiment, in step 81, the target magnetic field Btg is selected by further changing the target magnetic field Btg in consideration of the discretization error of the amount of the magnetic piece 9, and the magnetic field uniformity is minimized. It was, but the more eigenmodes while changing the target magnetic field Btg the maximum value M _D orders varied in a predetermined range, the target magnetic field Btg and eigenmodes magnetic field uniformity becomes the smallest maximum value M _D orders You may make it the structure which searches a combination. As shown in FIG. 14, and the target magnetic field Btg constant, the case of changing the maximum value M _D orders eigenmodes, attainable magnetic field uniformity changes the (M _{D =} 90 in the example of FIG. 14 Change to the center). The change in the magnetic field uniformity varies depending on the target magnetic field Btg. Therefore, by searching a combination of the maximum value M _D orders of target magnetic field Btg and eigenmodes magnetic field uniformity is minimized in consideration of an error of the magnetic field homogeneity by discretization of the amount of the magnetic material piece 9, Minimal magnetic field uniformity can be obtained. Incidentally, without changing the target magnetic field Btg, a constant, only the maximum value M _D orders eigenmodes may be changed as shown in FIG. 14.

  On the screen for accepting the setting of conditions from the operator, as shown in FIG. 7, the CPU 34 displays the input field 67 for accepting the size of the required uniform magnetic field and the central magnetic field and the uniformity of the measured magnetic field captured in step 12 as reference information. It is also possible to provide columns 69a, 69b and the like for calculation and display.

<< Second Embodiment >>
A magnetic field uniformity adjustment method according to the second embodiment of the present invention will be described. The second embodiment reaches the required magnetic field uniformity (for example, 10 ppm or less) while changing the width ΔB for changing the target magnetic field even when the distribution of the measured magnetic field is large (for example, 100 to 1000 ppm). By repeating the calculation of possible magnetic field uniformity, the arrangement and amount of the magnetic pieces 9 that efficiently achieve the required magnetic field uniformity are obtained. This will be described with reference to the flow of FIG.

First, the CPU 34 performs steps 12 to 19 in FIG. 5 of the first embodiment, and selects a target magnetic field Btg that satisfies a condition. In step 121, a second magnetic field range that includes the target magnetic field Btg selected in step 19 and that is smaller than the magnetic field range that has been accepted in step 16 is set. Further, a step size ΔB2 smaller than ΔB accepted in step 16 is set. For example, as the step width ΔB2, three types of 5 × 10 ⁻⁴ Tesla, 2 × 10 ⁻⁴ Tesla, and 1 × 10 ⁻⁵ Tesla are prepared in advance, and the step width ΔB is equal to or smaller than the step width ΔB used in the previous step 18. In this case, a value close to ΔB may be selected. For example, if ΔB in step 16 is 5 × 10 ⁻⁴ Tesla, 2 × 10 ⁻⁴ Tesla is selected as ΔB2. With respect to the second magnetic field range, the target magnetic field Btg selected in step 19 can be set to the center value Btgc of the target magnetic field, and can be set by n times (n is an integer) the step size ΔB2.

Then, the process proceeds to step 122, and similarly to step 18, the reachable magnetic field homogeneity is calculated again while changing the target magnetic field Btg within the second magnetic field range by the increment ΔB2 by the flow of FIG. In step 123, based on the calculation result of step 122, a target magnetic field Btg in which the amount of the magnetic piece 9 in each recess 72 of the shim tray 71 is equal to or less than a predetermined upper limit and the magnetic field uniformity is minimum is selected. An example of the calculation result of step 122 is shown as a table in FIG. In the example of FIG. 11 (b), the center Btgc of the target magnetic field Btg is 1.499848 Tesla, ΔB2 = 2 × 10 ⁻⁴ Tesla, n = 6 selected in Step 19, 1.499248 Tesla or more and 1.500248. The reachable magnetic field uniformity is calculated for the six target magnetic fields Btg in the range of Tesla or lower. A target magnetic field Btg (= 1.500408 Tesla) is selected in which the amount of the magnetic piece 9 in each recess 72 of the shim tray 71 is not more than a predetermined upper limit value and the magnetic field uniformity is minimum. The magnetic field uniformity at that time is 8.7 ppm.

Next, the routine proceeds to step 124, where it is determined whether or not the set step size ΔB2 is a predetermined minimum value. For example, as the step size ΔB2, when three types of 5 × 10 ⁻⁴ Tesla, 2 × 10 ⁻⁴ Tesla, and 1 × 10 ⁻⁵ Tesla are prepared in advance, ΔB2 is still a minimum value of 1 × 10 ^−5. Since it is not Tesla, it returns to step 121.

In step 121, a second magnetic field range including the target magnetic field Btg selected in step 123 and smaller than the previous time is set. The step size ΔB2 is also set to a value smaller than the previous time. For example, as the step size ΔB2, 1 × 10 ⁻⁵ Tesla is set, and for the second magnetic field range, the target magnetic field Btg selected in step 123 is set as the center value Btgc of the target magnetic field, and n times the step size ΔB2 ( n is an integer).

Then, the process proceeds to step 122, and the reachable magnetic field uniformity is calculated again while changing the target magnetic field Btg within the second magnetic field range in increments of ΔB2 according to the flow of FIG. An example of the calculation result of step 122 is shown as a table in FIG. In the example of FIG. 11C, the center Btgc of the target magnetic field Btg is 1.500048 Tesla, ΔB2 = 1 × 10 ⁻⁵ Tesla, n = 6 selected in Step 123, and is 1.5000018 Tesla or more 1.500068 The reachable magnetic field uniformity is calculated for the six target magnetic fields Btg in the range of Tesla or lower. The target magnetic field Btg = 1.0005958 Tesla, in which the amount of the magnetic piece 9 in each recess 72 of the shim tray 71 is equal to or less than a predetermined upper limit, the amount of the negative magnetic piece 9 is zero, and the magnetic field uniformity is minimum. 8.6 ppm. FIG. 11C is a graph and shown in FIG. As shown in FIG. 15 (c), the required magnetic field uniformity is achieved in all six target magnetic fields with the target magnetic field Btg from 1.5000018 Tesla to 1.500068 Tesla, and the minimum magnetic field uniformity. One time is 8.5 ppm in the case of Btg = 1.0005608 Tesla, but the amount of the negative magnetic piece 9 is −4.9 × 10 ⁻¹⁰ , and does not satisfy the constraint condition of the arrangement amount of the magnetic piece 9, It cannot be selected. Therefore, 1.000558 Tesla, which is the minimum magnetic field uniformity within the range of the constraint condition, is selected as the target magnetic field.

Here, as a constraint condition, the negative lower limit value is set to zero. However, depending on the unit amount of the magnetic piece 9, the amount −4.9 × 10 ⁻¹⁰ of the negative magnetic piece 9 can be ignored. In some cases, the magnetic field can be a large size, and even better magnetic field uniformity can be achieved. Thus, the constraint condition may be set in consideration of the unit amount of the magnetic piece 9 and the existing arrangement amount of the magnetic piece 9.

  Then, the process proceeds to step 81, and as described in the first embodiment, the target magnetic field Btg having the smallest uniformity is selected in consideration of the discretization error of the amount of the magnetic piece 9. Proceeding to step 20, as in the first embodiment, it is assumed that the arrangement and amount of the magnetic material corresponding to the target magnetic field Btg selected in step 123 is the magnetic piece 9 to be arranged in the recess 72 of the shim tray 71. Display on the display device. For example, the recess 72 of the shim tray 71 in FIG. Furthermore, it progresses to step 21 and calculates and displays the frequency of the high frequency magnetic field corresponding to the selected target magnetic field Btg.

  In the second embodiment, even when the measured magnetic field uniformity is large (for example, 100 to 1000 ppm) with respect to the required magnetic field uniformity (for example, 10 ppm or less), the target magnetic field range and step ΔB are gradually increased. By reducing the size and calculating the magnetic field uniformity that can be repeatedly reached, it is possible to efficiently obtain the arrangement and amount of magnetic bodies that achieve the required magnetic field uniformity.

  In the second embodiment, the configuration and operation other than those described above are the same as those in the first embodiment, and thus the description thereof is omitted.

<< Third Embodiment >>
In the third embodiment, a case where magnetic field measurement and excitation of a superconducting magnet of a static magnetic field generator are automatically performed will be described. In the present embodiment, as shown in FIGS. 4 and 17, the uniform magnetic field adjustment device 33 is built in the measurement unit 74 and the static magnetic field generation device 73 arranged in the uniform magnetic field space 77 of the static magnetic field generation device. It is connected to the exciting power source 2 of the superconducting coil 4a.

  As shown in the flow of FIG. 18, the CPU 34 of the uniform magnetic field adjusting device 33 first controls the excitation power source 2 to supply a predetermined superconducting current to the superconducting coil 4a to excite it (step 11). In step 12, the CPU 34 controls the operation of the measuring device 74, rotates the measuring device 74 in the uniform magnetic field space 77, and measures the magnetic field. And conditions are set in steps 13-16. In the present embodiment, in steps 13 and 14, the upper limit value of the position and amount of the magnetic piece 9 and the required magnetic field uniformity are received from the operator as in the first embodiment. A predetermined value is used as the maximum value MD of the 15 eigenmodes.

In Step 16, the range of the target magnetic field Btg is not accepted from the operator, and a value between the maximum value and the minimum value of the magnetic field distribution measured in Step 12 is calculated by a predetermined mathematical formula (for example, an average value), The center value Btgc of the target magnetic field Btg is set, and (B _tgc -ΔB · n / 2) to (B _tgc + ΔB · n / 2) are set by calculation as a magnetic field range based on ΔB and n determined in advance.

  Steps 17 to 19, 81, 20, and 21 are performed in the same manner as in the first embodiment.

  In step 22, the total amount of the magnetic material pieces 9 displayed in step 20 is withdrawn from the static magnetic field generator 73 while the superconducting coil 4 a is excited, and the magnetic material pieces 9 are placed in the recesses 72. It is determined whether or not the total amount of 71 can be inserted. That is, if the total amount of the magnetic piece 9 is small, the attractive force received by the static magnetic field is small, so that the magnetic piece 9 can be arranged while being excited. However, if the amount of the magnetic piece 9 is large, a large attractive force acts. Since it cannot be arranged with being excited, the process proceeds to step 23, and the superconducting magnet 4a is once demagnetized. Since re-excitation after demagnetization takes time and the cost of the refrigerant, in step 22, the total amount of the magnetic piece 9 that has been obtained by calculation in advance as to whether it can be placed in the excited state, and the magnetic substance displayed in step 20. Determination is made by comparing the total amount of the pieces 9. If demagnetization is not necessary, the process proceeds to step 24 as it is. If demagnetization is necessary, the process proceeds to step 23, where the CPU 34 controls the excitation power source 2 to reduce the excitation current to zero or a predetermined value, and then proceeds to step 24.

  In step 24, a display prompting the operator to arrange the magnetic piece 9 in the recesses 72 of the plurality of shim trays 71 as shown in step 20 is performed.

  If the operator arranges the magnetic piece 9, the operation of the measuring unit 74 is controlled in step 25, and the magnetic field is measured again. When excitation is performed in step 23, an excitation current is supplied from the excitation power source 2 to the superconducting coil 4a before measurement, and re-excitation is performed.

  In step 26, the uniformity of the magnetic field measured in step 25 is calculated, and it is determined whether it is equal to or less than the required magnetic field uniformity received in step 14. If the measured magnetic field uniformity is equal to or less than the required magnetic field uniformity, the adjustment of the magnetic field uniformity is completed. If it is greater than the required magnetic field uniformity, the process returns to step 13 and repeats step 13 and subsequent steps.

At this time, when setting the restricting condition of the amount of magnetic substance in step 13, since the magnetic piece 9 is already arranged in step 24, the upper limit value is set by allowing it. For example, when 2.0 × 10 ⁻⁶ m ³ of the magnetic piece 9 can be arranged for each recess 72, the upper limit value of the amount of the magnetic piece 9 is set to 1.5 × 10 ⁻⁶ m ³ at the first time. In this case, in the second step 13, the upper limit value is set to a value smaller than 0.5 × 10 ⁻⁶ m ³ .

  In this embodiment, since the CPU 34 can automatically perform excitation of the superconducting coil 4a, demagnetization as necessary, and measurement of the magnetic field without bothering the operator, the burden on the operator can be reduced. it can. In addition, since it is automatically determined whether or not demagnetization is necessary, the number of times of demagnetization can be minimized.

  Further, after demagnetizing in step 23 and re-exciting in step 24, the magnetic field strength after re-excitation may differ from the original magnetic field strength due to an error in the excitation current value depending on the accuracy of the excitation power source 2. For this reason, the target magnetic field Btg used for calculating the arrangement and amount of the magnetic piece 9 after re-excitation needs to be changed according to the excitation current value (that is, the magnetic field strength after re-excitation). In step 16, the target magnetic field is set by calculation based on the magnetic field distribution measured in step 12 or 25. Therefore, the arrangement of the required magnetic field homogeneity corresponding to the error of the excitation current value after re-excitation is set. Can be sought.

  The third embodiment is the same as the first embodiment except for the configuration described above, and a description thereof will be omitted. Not only the first embodiment but also step 11 and steps 22 to 26 of the third embodiment may be performed before step 12 and after step 21 of the flow of FIG. 16 of the second embodiment. Of course it is possible.

Further, in the first to third embodiments, in step 15 receives the maximum value M _D eigenmodes used for the calculation, the position and amount of the magnetic material piece 9 by changing only the target magnetic field Btg In step 17, as well as Although the magnetic field homogeneity is calculated, the calculation in step 17 may be performed by changing both the target magnetic field Btg and the maximum value MD of the natural mode. In this case, the combination of the target magnetic field Btg that can further reduce the magnetic field uniformity and the maximum value MD of the optimum natural mode, and the position and amount of the magnetic piece 9 at that time can be obtained.

  Further, in the first to third embodiments, the eigenmode is selected in the low-order predetermined range (the range of the order 1 to the maximum value MD), but the present embodiment is not limited to this, and the high mode It is possible to select the next eigenmode range, and it is also possible to select an arbitrary plurality of orders in succession.

  DESCRIPTION OF SYMBOLS 2 ... Excitation power supply, 4a ... Superconducting coil, 9 ... Magnetic body piece, 33 ... Magnetic field uniformity adjustment apparatus, 34 ... CPU, 35 ... Memory, 71 ... Shim tray, 72 ... Recessed part, 73 ... Static magnetic field generator, 74 ... Measurement Part, 77 ... uniform magnetic field space

Claims (15)

A magnetic field homogeneity adjusting method for a static magnetic field generating apparatus provided with shim trays for holding magnetic pieces for adjusting the uniformity of a generated static magnetic field at a plurality of predetermined positions, respectively,
A first step of measuring a distribution of a static magnetic field generated by the static magnetic field generator and calculating an error magnetic field between the distribution of the static magnetic field and a target magnetic field;
A second step of calculating a magnetic field uniformity that can be reached when the magnetic piece is arranged at one or more of the plurality of positions of the shim tray while changing the target magnetic field in a predetermined magnetic field range;
Based on the reachable magnetic field uniformity calculated in the second step, the amount of the magnetic piece at each position of the shim tray is less than or equal to a predetermined upper limit value, and the reachable magnetic field uniformity is a predetermined value. A third step of selecting the target magnetic field that is:
A discretization error calculation magnetic field range including the target magnetic field selected in the third step and smaller than the predetermined magnetic field range is set, and a value that can be taken as the amount of the magnetic piece is a predetermined discrete value In some cases, the reachable magnetic field uniformity is calculated again while changing the target magnetic field within the discretization error calculation magnetic field range, and the target magnetic field with the minimum reachable magnetic field uniformity is re-calculated. A fourth step to select,
A magnetic field comprising: a fifth step in which the arrangement of the magnetic material corresponding to the target magnetic field reselected in the fourth step and the amount thereof are magnetic material pieces to be arranged in the shim tray. Uniformity adjustment method.   The magnetic field homogeneity adjusting method according to claim 1, wherein the fourth step includes an arithmetic method for calculating the arrangement and the amount of the magnetic pieces using a plurality of eigenmodes calculated by the singular value decomposition method. A method for adjusting the magnetic field uniformity, wherein the reachable magnetic field uniformity is calculated by changing a plurality of orders of eigenmodes used in the calculation within a predetermined range while changing the target magnetic field. 2. The magnetic field uniformity adjustment method according to claim 1, wherein the target magnetic field selected in the third step is included after the third step and before the fourth step, and is more than the predetermined magnetic field range. A sixth step of setting a small second magnetic field range;
A seventh step of recalculating the reachable magnetic field uniformity while changing the target magnetic field within the second magnetic field range;
Based on the calculation result of the seventh step, the target magnetic field in which the amount of the magnetic piece at each position of the shim tray is not more than a predetermined upper limit and the reachable magnetic field uniformity is the smallest is selected. 8 steps,
In the fourth step, the discretization error calculation magnetic field range is set to a range including the target magnetic field selected in the seventh step.   2. The magnetic field homogeneity adjusting method according to claim 1, wherein when the static magnetic field generation device is a static magnetic field generation device of a magnetic resonance imaging apparatus, a high-frequency magnetic field corresponding to the target magnetic field obtained in the fourth step. The magnetic field uniformity adjusting method further comprising a ninth step of calculating the frequency of the frequency and setting the frequency in the high-frequency magnetic field generator of the magnetic resonance imaging apparatus.   2. The magnetic field uniformity adjustment method according to claim 1, wherein the second step sets a center value of the target magnetic field based on the distribution of the static magnetic field measured in the first step, and A magnetic field uniformity adjusting method, wherein the predetermined magnetic field range is set around a center value.   2. The magnetic field uniformity adjustment method according to claim 1, wherein the second step is a calculation method for calculating the arrangement and the amount of the magnetic pieces using a plurality of eigenmodes calculated by a singular value decomposition method. And a magnetic field uniformity adjusting method, wherein the arrangement and the amount of the magnetic piece are calculated by selectively using only a predetermined plurality of orders among the plurality of eigenmodes.   The magnetic field homogeneity adjustment method according to claim 6, wherein the second step determines not only the target magnetic field but also the reachable magnetic field homogeneity while changing a plurality of orders of eigenmodes used for the calculation. A magnetic field uniformity adjustment method characterized by calculating. In order to adjust the magnetic field uniformity of a static magnetic field generator having a shim tray for holding magnetic pieces for adjusting the uniformity of the generated static magnetic field at a plurality of predetermined positions,
A first step of taking a measurement result of a static magnetic field generated by the static magnetic field generation device and calculating an error magnetic field between the static magnetic field and a target magnetic field;
A second step of calculating a magnetic field uniformity that can be reached when the magnetic pieces are arranged at one or more of the plurality of positions of the shim tray while changing the target magnetic field in a predetermined magnetic field range;
Based on the calculation result of the reachable magnetic field uniformity calculated in the second step, the amount of the magnetic material piece at each position of the shim tray is equal to or less than a predetermined upper limit value, and the reachable magnetic field uniformity is A third step of selecting the target magnetic field for which is less than or equal to a predetermined value;
A discretization error calculation magnetic field range including the target magnetic field selected in the third step and smaller than the predetermined magnetic field range is set, and a value that can be taken as the amount of the magnetic piece is a predetermined discrete value In some cases, the reachable magnetic field uniformity is calculated again while changing the target magnetic field within the discretization error calculation magnetic field range, and the target magnetic field with the minimum reachable magnetic field uniformity is re-calculated. A fourth step to select, and
The fifth step of causing the display device to display the arrangement and the amount of the magnetic piece corresponding to the target magnetic field reselected in the fourth step as the magnetic piece to be arranged on the shim tray is performed. Magnetic field uniformity adjustment program.   9. The magnetic field uniformity adjustment program according to claim 8, wherein the fourth step includes an arithmetic method for calculating the arrangement and the amount of the magnetic pieces using a plurality of eigenmodes calculated by the singular value decomposition method. A magnetic field uniformity adjustment program for calculating the reachable magnetic field uniformity by changing a plurality of orders of eigenmodes used for the calculation in a predetermined range while changing the target magnetic field. The magnetic field uniformity adjustment program according to claim 8,
A sixth step of setting a second magnetic field range that includes the target magnetic field selected in the third step and is smaller than the predetermined magnetic field range;
A seventh step of calculating again the reachable magnetic field uniformity while changing the target magnetic field within the second magnetic field range; and
Based on the calculation result of the seventh step, the target magnetic field in which the amount of the magnetic piece at each position of the shim tray is not more than a predetermined upper limit and the reachable magnetic field uniformity is the smallest is selected. Let the computer perform the 8 steps further,
The fourth step sets the discretization error calculation magnetic field range to a range including the target magnetic field selected in the eighth step.   9. The magnetic field uniformity adjustment program according to claim 8, wherein when the static magnetic field generator is a static magnetic field generator of a magnetic resonance imaging apparatus, a high frequency magnetic field corresponding to the target magnetic field obtained in the fourth step The computer further executes a ninth step of displaying the frequency of the high-frequency magnetic field on the display device as a frequency to be set in the high-frequency magnetic field generation unit of the magnetic resonance imaging apparatus. Once adjusted program.   9. The magnetic field uniformity adjustment program according to claim 8, wherein the second step includes a center of the target magnetic field in a distribution width of the static magnetic field based on the distribution of the static magnetic field captured in the first step. A magnetic field uniformity adjusting program characterized by setting a value and setting the predetermined magnetic field range around the center value of the target magnetic field.   9. The magnetic field uniformity adjustment program according to claim 8, wherein the second step is a calculation method for calculating the arrangement and the amount of the magnetic pieces using a plurality of eigenmodes calculated by a singular value decomposition method. A magnetic field uniformity adjustment program that uses the plurality of eigenmodes to selectively calculate only the plurality of orders received from an operator and to calculate the arrangement and the amount of the magnetic material pieces.   14. The magnetic field uniformity adjustment program according to claim 13, wherein the second step calculates the reachable magnetic field uniformity while changing not only the target magnetic field but also the order of the eigenmode. Magnetic field uniformity adjustment program. Magnetic field uniformity adjusting apparatus for adjusting the magnetic field uniformity of a static magnetic field generating apparatus having a shim tray for holding magnetic pieces for adjusting the uniformity of a generated static magnetic field at a plurality of predetermined positions, respectively Because
A receiving unit that receives the measurement result of the static magnetic field generated by the static magnetic field generation device, the upper limit value of the amount of the magnetic piece at the plurality of positions, and the range of the target magnetic field;
A calculation unit that calculates the amount of the magnetic piece to be arranged on the shim tray using the condition received by the reception unit;
A first step of measuring the distribution of the static magnetic field generated by the static magnetic field generator and calculating an error magnetic field between the distribution of the static magnetic field and a target magnetic field;
A second step of calculating a magnetic field uniformity that can be reached when the magnetic piece is arranged at one or more of the plurality of positions of the shim tray while changing the target magnetic field in a predetermined magnetic field range;
Based on the reachable magnetic field uniformity calculated in the second step, the amount of the magnetic piece at each position of the shim tray is less than or equal to a predetermined upper limit value, and the reachable magnetic field uniformity is a predetermined value. A third step of selecting the target magnetic field that is:
A discretization error calculation magnetic field range including the target magnetic field selected in the third step and smaller than the predetermined magnetic field range is set, and a value that can be taken as the amount of the magnetic piece is a predetermined discrete value In some cases, the reachable magnetic field uniformity is calculated again while changing the target magnetic field within the discretization error calculation magnetic field range, and the target magnetic field with the minimum reachable magnetic field uniformity is re-calculated. A fourth step to select,
Magnetic field characterized in that in the fourth step, a fifth step is performed in which the magnetic material corresponding to the re-selected target magnetic field and the amount thereof are the magnetic material pieces to be placed on the shim tray. Uniformity adjustment device.

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