WO2016136465A1 - 磁気共鳴イメージング装置、静磁場均一度調整方法、プログラム及び計算機 - Google Patents
磁気共鳴イメージング装置、静磁場均一度調整方法、プログラム及び計算機 Download PDFInfo
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- WO2016136465A1 WO2016136465A1 PCT/JP2016/053882 JP2016053882W WO2016136465A1 WO 2016136465 A1 WO2016136465 A1 WO 2016136465A1 JP 2016053882 W JP2016053882 W JP 2016053882W WO 2016136465 A1 WO2016136465 A1 WO 2016136465A1
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves
- A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/387—Compensation of inhomogeneities
- G01R33/3873—Compensation of inhomogeneities using ferromagnetic bodies ; Passive shimming
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/381—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets
- G01R33/3815—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets with superconducting coils, e.g. power supply therefor
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/385—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using gradient magnetic field coils
Definitions
- the present invention relates to a magnetic resonance imaging apparatus, a static magnetic field uniformity adjustment method, a program, and a computer.
- the magnetic field generator installed in the magnetic resonance imaging (MRI) device has high magnetic field uniformity (for example, the difference between the maximum value and the minimum value of the magnetic field in the imaging space (FOV: Field of View) near the center of the magnetic field. ppm or less) is required.
- This high magnetic field uniformity is disturbed by the influence of manufacturing dimensional errors in the manufacturing stage of the MRI apparatus and the influence of the surrounding magnetic body where the MRI apparatus is installed.
- shimming the uniformity of the magnetic field strength in the FOV is finely adjusted.
- One type of shimming is passive shimming in which a magnetic shim piece for correction (hereinafter referred to as a magnetic piece) is arranged around the FOV to finely adjust the static magnetic field distribution.
- Passive shimming is a technique for adjusting the static magnetic field distribution of FOV to a desired uniformity by arranging an appropriate amount of magnetic material pieces at appropriate positions by optimization calculation (see, for example, Patent Documents 1 and 2). ). That is, the homogeneity of the FOV static magnetic field distribution is adjusted by the magnetic field distribution generated by the magnetic moment of the magnetic pieces arranged in the magnetic field.
- Patent Documents 1 and 2 do not take into consideration that the magnetic field distribution of the magnetic moment generated in the FOV, particularly the polarity, changes according to the arrangement position of the magnetic piece. As a result, in the fine adjustment shimming performed when the total iron amount of the magnetic piece used for the first shimming is sufficiently smaller (for example, 1/10 or less) in the shimming performed a plurality of times, the magnetic piece is There is a problem that there is a case where the amount of arrangement of is increased.
- a magnetic piece arranged near the bore opening far from the FOV generates a magnetic field in the positive direction with respect to the static magnetic field of the FOV.
- Low magnetic field adjustment capability the magnetic piece in the region close to the FOV generates a magnetic field in the negative direction with respect to the FOV static magnetic field, but has a high ability to adjust the static magnetic field because the distance to the FOV is short. Therefore, if a plurality of magnetic pieces are arranged from the vicinity of the bore opening far from the FOV to the region close to the FOV, the magnetic field adjustment ability by these magnetic pieces may be offset, and the arrangement amount of the magnetic pieces may increase. is there.
- the discretization error increases, the shimming accuracy is lowered, and a desired magnetic field uniformity may not be achieved.
- the arrangement amount of the magnetic piece increases, an error in the arrangement position is likely to occur, and the workability of fine adjustment shimming may be reduced.
- a problem to be solved by the present invention is a method for adjusting the static magnetic field homogeneity of a magnetic resonance imaging apparatus capable of reducing the amount of arrangement of magnetic material pieces in adjusting the magnetic field homogeneity and achieving a desired magnetic field homogeneity with high accuracy. Is to provide.
- the present invention calculates the positions of a plurality of magnetic material pieces away from the imaging space by shimming calculation with respect to the static magnetic field in the imaging space generated by the magnetic field generator, A method of adjusting the static magnetic field uniformity in the imaging space by arranging the plurality of magnetic pieces at positions obtained by calculation, wherein the magnetic bodies are arranged at the positions during the shimming calculation.
- An adjustment step of adjusting the static magnetic field homogeneity is imposed by imposing a constraint that a polarity of the magnetic field distribution generated by the piece in the imaging space is either positive or negative.
- the present invention it is possible to reduce the total amount of magnetic substance pieces arranged in the magnetic field uniformity adjustment, and it is possible to achieve a desired magnetic field uniformity with high accuracy.
- the total amount of magnetic pieces used could be reduced by about 65% compared to the conventional method.
- by reducing the total amount of the magnetic pieces it is possible to reduce the influence of discretization error and to reduce the work time in the fine adjustment process in shimming.
- the present invention it is possible to provide a method for adjusting the static magnetic field uniformity of a magnetic resonance imaging apparatus that can reduce the amount of magnetic material pieces arranged in adjusting the magnetic field uniformity and achieve a desired magnetic field uniformity with high accuracy. Can do.
- FIG. 1 is a schematic configuration diagram of a cylindrical superconducting magnet provided with a shimming mechanism of the magnetic resonance imaging apparatus according to the first embodiment of the present invention.
- Configuration diagram of shim tray of cylindrical superconducting magnet of first embodiment Flow chart of shimming according to the first embodiment Positive and negative magnetic field distribution diagram generated by the magnetic moment of the magnetic piece according to the first embodiment Positive and negative magnetic field distribution map generated in FOV by the magnetic moment of the magnetic piece according to the first embodiment Arrangement of magnetic pieces in coarse adjustment shimming according to the first embodiment Arrangement of magnetic pieces in fine adjustment shimming according to the first embodiment
- region in the fine adjustment shimming which concerns on 1st Embodiment The figure which showed the magnetic field strength on the FOV surface before and after fine adjustment shimming concerning a 1st embodiment
- the figure which showed the modification of the shim tray used for the fine adjustment shimming which concerns on 1st Embodiment Schematic configuration diagram of an open super
- FIG. 1 is a schematic diagram of an external perspective view of a cylindrical superconducting magnet 2 of the magnetic resonance imaging apparatus according to the first embodiment.
- the superconducting magnet 2 can generate a high magnetic field, the present invention may be applied to a normal conducting magnet.
- a superconducting magnet 2 is formed in a vacuum vessel 3 by containing a refrigerant together with a superconducting coil which is a main coil (not shown).
- the superconducting magnet 2 has an imaging space (FOV) 6 for imaging a subject in an internal space of a cylindrical bore 5 having an axial direction (Z axis) 4 parallel to the horizontal direction as a central axis.
- the superconducting magnet 2 generates a static magnetic field having a substantially spherical shape, a uniform magnetic field intensity, and a constant magnetic field direction in the FOV 6.
- a gradient magnetic field coil 7 is accommodated on the inner surface of the cylindrical bore 5 on the FOV 6 side, and a plurality of holes 7a are provided at equal angular intervals in the circumferential direction inside the gradient magnetic field coil 7, and the shim tray 1 is attached to and detached from this hole 7a. Inserted as possible.
- the shim tray 1 is used for passive shimming, and each shim tray 1 is given an arbitrary number #, and shimming can be performed by individually selecting each based on the number.
- the shim tray 1 has a length corresponding to the axial length of the cylindrical bore 5, and is formed with a plurality of shim pockets 8 dispersed at predetermined positions in the length direction. . In the shim pocket 8, a plurality of magnetic pieces 9 are formed so as to be arranged.
- the shim tray 1 for storing the magnetic piece 9 is formed of a non-magnetic material such as resin.
- the shim tray containing the magnetic piece 9 is fixed at a predetermined position of the superconducting magnet 2.
- the disturbance of the FOV magnetic field is adjusted.
- the accuracy is affected by the variation in the magnetization of the magnetic piece 9 arranged on the shim tray 1 and the error in the shape of the magnetic piece 9 due to the minimum dimension of the magnetic piece 9 (hereinafter referred to as discretization error). May not achieve the desired magnetic field uniformity. Multiple times of shimming are required to obtain the desired magnetic field uniformity.
- the magnetic piece 9 is made of a magnetic material having a high magnetic permeability.
- a thin plate-like magnetic material such as an iron plate, preferably a silicon steel plate can be used.
- the magnetic field created at the point where the magnetization direction (magnetic moment direction) 12 of the magnetic piece 9 of the magnetic piece 9 is away from the position r is expressed by the following equation (1).
- the axial magnetic field Bz component handled by passive shimming is expressed as shown in Equation 2.
- B (r) is the magnetic field [T] created by the magnetic moment at the point r away M is the magnetic moment [Am 2 ]
- ⁇ is a coefficient
- Bz is the magnetic field in the Z-axis direction [T] created by the magnetic moment at the position r away
- Mx is the magnetic moment in the X-axis direction [Am 2 ]
- My is the magnetic moment in the Y-axis direction [Am 2 ]
- Mz is the magnetic moment in the Z-axis direction [Am 2 ]
- Bz shown in the following expression 3 is an amount that can be adjusted by shimming.
- FIG. 4 shows a distribution diagram of positive and negative magnetic fields generated in the FOV 6 by the magnetic moment of the magnetic piece 9.
- the Bz component created by the magnetic moment of the magnetic piece 9 has a positive region 10 and a negative region 11 with the following expression 4 as a boundary.
- FIG. 5 shows the relationship between the Bz component magnetization region created by the magnetic moment of the magnetic piece 9 used for shimming, the axial direction (Z-axis) 4, and FOV6.
- the arrow of the magnetic moment indicates the direction of magnetization of the magnetic piece 9
- the two-dot chain line indicates the arrangement direction of the magnetic piece 9.
- the negative region 11 that generates only the negative magnetic field distribution is selected as the arrangement region of the magnetic piece.
- the region that generates only one of the polar magnetic field distributions is selected as the arrangement area of the magnetic piece 9 (this book In the embodiment, negative polarity is selected).
- the region is limited to a region where the negative Bz region and the FOV region overlap.
- steps S301 to S311 indicate rough adjustment shimming
- steps S312 to S315 indicate fine adjustment shimming.
- the shimming method of the present invention is characterized by fine adjustment shimming.
- Fine adjustment shimming is shimming performed when the total amount of the magnetic piece 9 used for shimming is sufficiently smaller than the total amount of the magnetic piece 9 used for the first shimming (for example, 1/10 or less).
- non-uniformity of the low-frequency component of the magnetic field distribution of FOV6 for example, when the series expansion is performed with a spherical harmonic function, the non-uniformity of the magnetic field mainly corresponding to the low-order term magnetic field component
- step S301 the superconducting magnet 2 is excited to generate a static magnetic field in the FOV 6.
- step S302 although not shown, the magnetic field strength in FOV 6 is measured by a magnetic field measuring device.
- step S303 a known optimization calculation is performed using the magnetic field intensity measured in step S302, and shim pockets P (( For example, the magnetic piece amount (A) of the magnetic piece 9 stored in P-1 to P-24) is determined.
- a layout for example, shown in FIG. 6) of the determined magnetic piece 9 is output and displayed.
- step S304 the superconducting magnet 2 is demagnetized.
- step S305 the magnetic piece 9 is stored in the shim pocket P of the shim tray 1 according to the result obtained by the optimization calculation.
- step S306 the superconducting magnet 2 is excited to generate a static magnetic field in the FOV 6 again.
- step S307 the magnetic field strength in FOV6 is measured by the magnetic field measuring instrument.
- step S308 an optimization calculation is performed from the magnetic field intensity measured in step S307, and the magnetic piece amount (B) of the magnetic piece 9 stored in the shim pocket P of the shim tray 1 that cancels the magnetic field inhomogeneity is calculated. decide.
- the magnetic piece 9 of the present embodiment is prepared with two types A and B (for example, B is an iron plate thinner than A) in which the thickness of the square iron plate is changed, and the type accommodated in one shim pocket 8
- the number of the magnetic material pieces is adjusted to the amount obtained by the optimization calculation.
- the size and thickness can be selected as necessary.
- the amount of shim iron used means the amount of magnetic piece, and the unit is represented by volume [cm 3 ], but is not limited to this.
- the size of shim iron A is a square iron plate of 20 [mm] x 20 [mm]
- the thickness is 0.1 [mm]
- the size of shim iron B is 20 [mm] x 20 [mm] ]
- these specific values can be set arbitrarily.
- the numerical values of A and B shown in each shim pocket are the number of sheets.
- step S309 when the magnetic piece amount (B) is sufficiently smaller than the magnetic piece amount (A) (for example, 1/10 or less), the process proceeds to step S312 to shift to fine adjustment shimming. If not, the process proceeds to step S310, the superconducting magnet 2 is demagnetized, and in step S311, the magnetic piece 9 of the magnetic piece amount (B) obtained by the optimization calculation in step S308 is placed in the shim pocket 8. Store. Then, the process returns to step S306 and the process is repeated.
- Steps after step S312 correspond to fine adjustment shimming according to the feature of the present invention. That is, in step S312, the shim tray 1 on which the magnetic piece 9 is arranged is selected as will be described later, and how the uniformity is generated within the selected position arrangement is calculated by shimming calculation. Obtained and determined as the amount of the magnetic piece (B-2).
- the layout of the determined magnetic piece 9 is output and displayed. An example of this layout is shown in FIG. In FIG. 7, the regions of shim pockets (P-1) to (P-5) and (P-20) to (P-24) filled with shading are restricted regions where the magnetic piece 9 is not arranged. Is selected. In other words, the magnetic moment of the magnetic piece 9 arranged in these regions has a positive polarity of the magnetic distribution generated in the FOV 6 (see FIG. 4). Area.
- step S313 the magnetic piece 9 of the calculation result (B-2) is placed in the shim pocket P of some shim trays 1 (for example, even numbered shim trays).
- step S314 the magnetic field strength at FOV 6 is measured by the magnetic field measuring instrument. If the magnetic field intensity measured in step S314 satisfies the desired value of the magnetic field uniformity in FOV 6, the shimming is terminated. Otherwise, the process returns to step S312, and fine adjustment shimming is repeated.
- FIG. 8 is a cross-sectional view of the superconducting magnet 2 cut along a plane that passes through the axial direction (Z-axis) 4.
- a region (negative region 11) where only the negative magnetic field distribution is generated is selected as the arrangement region of the magnetic piece 9. To do. That is, as shown in FIG.
- the range of the position on the shim tray 1 of the magnetic material piece 9 that generates a negative magnetic field distribution in the FOV 6 is set as the adjustment region 13, and only the shim pocket 8 in the adjustment region 13 Is selected, and a fine adjustment shimming magnetic piece 9 is arranged. That is, as described above, only the shim pockets (P-6) to (P-19) are selected.
- a highly uniform static magnetic field can be generated as shown in FIG. That is, this figure compares and shows the magnetic field strength on the surface of FOV 6 before and after fine adjustment shimming, the vertical axis is the static magnetic field strength, and the horizontal axis is the axial direction (Z axis).
- the solid line indicates the magnetic field strength before fine adjustment shimming, and the broken line indicates the magnetic field strength after shimming.
- the magnetic field of FOV6 is adjusted in the direction of decreasing, and the uniformity of the magnetic field is improved because the magnetic material piece 9 is selected to generate a negative magnetic field distribution in the FOV6 region in fine adjustment shimming.
- the magnetic field uniformity is higher as the difference between the maximum value and the minimum value of the solid line or the broken line is smaller.
- the total usage amount of the magnetic piece 9 was able to be reduced by about 65% compared with the past.
- the positions of the plurality of magnetic pieces separated from the FOV 6 are calculated by shimming calculation, and the shimming
- a method of adjusting the static magnetic field uniformity in the imaging space by arranging the plurality of magnetic pieces at positions obtained by calculation, wherein the magnetic bodies are arranged at the positions during the shimming calculation.
- a static magnetic field of a magnetic resonance imaging apparatus comprising a step of adjusting a static magnetic field homogeneity by imposing a constraint that a polarity of a magnetic field distribution generated by a piece in the FOV 6 is either positive or negative This is a uniformity adjustment method.
- the positions of the plurality of magnetic body pieces away from the imaging space are calculated by shimming calculation, and the static magnetic field in the imaging space is calculated.
- a program for adjusting the degree of uniformity, wherein the polarity of the magnetic field distribution generated in the imaging space by the magnetic piece arranged at the position when the shimming calculation is performed is either positive or negative You may use the program characterized by having the function to impose the restrictions.
- the positions of the plurality of magnetic body pieces away from the imaging space are calculated by shimming calculation, and the static magnetic field in the imaging space is calculated.
- a computer for adjusting the uniformity, wherein the polarity of the magnetic field distribution generated in the imaging space by the magnetic piece arranged at the position when performing the shimming calculation is either positive or negative You may use the computer characterized by having the function to impose the restrictions.
- this corresponds to the case where the direction 12 of the magnetic moment of the magnetic piece 9 is parallel to the direction of the static magnetic field of FOV6.
- the position of the magnetic piece 9 where the magnetic piece 9 generates a negative magnetic field distribution in the FOV 6 is selected.
- the boundary of the position of the magnetic piece 9 to be selected is a boundary where the negative magnetic field distribution generated in the FOV 6 by the magnetic piece 9 satisfies Equation 4.
- the negative region 11 is selected so as not to arrange the magnetic piece 9 in the shim pocket 8 one by one from the bore opening of the superconducting magnet 2 in fine adjustment shimming, in addition to obtaining by solving Equation 4.
- the negative region 11 may be estimated by evaluating the expected uniformity of the magnetic field after completion of shimming and the increase / decrease in the amount of magnetic material piece (B-2) used for shimming.
- the shim pocket 8 used for fine adjustment shimming of the present embodiment is only an area close to FOV6. For example, it is an area up to about 35% of the total length of the shim tray 1 on one side from the center position of the FOV 6. Therefore, a shim tray 1 used for fine adjustment shimming of the present embodiment can be determined in advance and formed as shown in FIG. As in the shim tray 1-a shown in FIG. 5A, it is sufficient to arrange the shim pocket 8 in an area of about 80% with respect to the center of the FOV 6 with respect to the entire length of the shim tray 1. Further, as in the shim tray 1-B, one of the end portions in the axial direction of the shim tray 1-a may be deleted and shortened.
- the superconducting magnet 2 that is a magnetic field generator forms a FOV (imaging space) in a cylindrical internal space by a cylindrical electromagnet, and is dispersed in the inner peripheral surface of the electromagnet in the axial direction.
- a plurality of shim trays 1 that are straight magnetic body holding members that hold the magnetic body pieces 9 are arranged along the shim tray 1, and shim pockets 8 that are a plurality of recesses that accommodate the magnetic body pieces 9 are arranged in the longitudinal direction of the shim tray 1 Has been. Therefore, when the shim tray of FIG. 10 is used, it is necessary to use the shim tray separately for coarse adjustment shimming and fine adjustment shimming. In this case, for example, the rough adjustment is performed using an odd-numbered shim tray, and the fine adjustment is performed using an even-numbered # shim tray.
- a magnetic field generator that forms an imaging space in the cylindrical internal space with a cylindrical electromagnet, and is distributed along the inner circumferential surface of the electromagnet and disposed along the axial direction, and holds a magnetic piece for adjusting the magnetic field.
- a magnetic resonance imaging apparatus in which a plurality of straight magnetic body holding members and a plurality of recesses that accommodate magnetic pieces are arranged in the longitudinal direction of the magnetic body holding member, the magnetic body of the plurality of magnetic body holding members A part of the magnetic body holding member in which the recesses for accommodating the magnetic body pieces are not arranged from the end of the body holding member to the set length is included.
- the range of the position on the shim tray 1 of the magnetic piece 9 that generates a negative magnetic field distribution in the FOV 6 is referred to as an adjustment region 13, and the shim in the adjustment region 13
- the magnetic piece 9 may be disposed in the shim pocket 8 in the negative region 11 that is out of the adjustment region 13. In this case, the weights of the negative region 11 deviated from the adjustment region 13 and the magnetic piece 9 arranged in the adjustment region 13 are changed. For example, a small amount of the magnetic piece 9 is disposed in the negative region 11 that is out of the adjustment region 13.
- FIG. 11 is a schematic diagram of an external perspective view of the open superconducting magnet 2 of the magnetic resonance imaging apparatus according to the second embodiment.
- the difference from the first embodiment is an example in which the fine adjustment shimming of the present invention is applied to the open superconducting magnet 2 instead of the cylindrical type.
- the direction of the magnetic field of the generated FOV 6 is vertical.
- a method for adjusting the static magnetic field uniformity of the open superconducting magnet 2 will be described with reference to FIGS.
- a superconducting magnet 2 having an open magnetic field space is formed by accommodating a superconducting coil in a pair of vacuum vessels 3 provided vertically above and below.
- an FOV 6 is formed with an axial direction (Z axis) 4 parallel to the vertical direction as a central axis.
- a pair of disc-shaped shim trays 21 having the Z axis 4 as a central axis are provided on the opposing surfaces of the upper and lower superconducting magnets 3.
- a plurality of rectangular pocket shim pockets 22 are provided at equal intervals in a lattice shape on the respective FOV 6 side surfaces of the pair of shim trays 21.
- FIG. 12 shows a flowchart of a shimming processing procedure that is a static magnetic field adjustment method according to the present embodiment. Shimming of the open-type superconducting magnet 2 is performed only once and shimming is continuously performed without demagnetization. Therefore, steps S304, S306, and S310 in FIG. 3 of the first embodiment can be omitted. In other words, in the case of the cylindrical superconducting magnet 2, the strong magnetic force of the superconducting magnet 2 acts on the shim tray 21, so the shim tray 21 cannot be extracted to accommodate the magnetic piece 9 unless demagnetized. .
- FIGS. 13 and 14 show arrangement views of the magnetic piece 9 used for shimming according to the second embodiment.
- the disc-shaped shim tray 21 has a plurality of shim tray numbers (# 1 to # 12), and each shim tray number (# 1 to # 12) corresponds to a shim pocket number (A to L).
- a plurality of shim pockets 22 are formed at the lattice position.
- Each shim pocket 22 is formed in a rectangular hole that can accommodate the magnetic piece 9 as in the first embodiment.
- the shim pocket 22 of only the region that generates the positive region 10 is used in the positive and negative magnetic field distribution generated by the magnetic moment of the magnetic piece 9 in the FOV 6. That is, the shaded shim pocket 22 corresponds to the negative region 11 and is not used.
- FIG. 15 shows a distribution diagram of positive and negative magnetic fields generated in the FOV 6 by the magnetic moment of the magnetic piece 9 of the present embodiment.
- the main magnetic field generated by the superconducting magnet 2 in the region of FOV 6 is vertically upward.
- the fine adjustment shimming of the present invention can be applied even when the direction of the main magnetic field generated in the FOV 6 is vertically downward.
- FIG. 16 shows an adjustment area 13 in fine adjustment shimming.
- fine adjustment shimming the region of the shim pocket 22 where the magnetic moment of the magnetic piece 9 generates a positive magnetic field in the FOV 6 is selected, and the region where the magnetic piece 9 is arranged is selected as the arrangement region 14.
- Fig. 17 shows the magnetic field strength on the surface of FOV6 before and after fine adjustment shimming.
- the vertical axis represents the static magnetic field strength
- the horizontal axis represents the axial direction (Z axis) 4
- the solid line represents the magnetic field strength before fine adjustment shimming
- the broken line represents the magnetic field strength after fine adjustment shimming.
- the uniformity is adjusted using the magnetic piece 9 that generates the magnetic field distribution in the positive region 10 of the FOV6 region, so that the magnetic field strength changes in the direction of increasing, Adjustments are made.
- the second embodiment corresponds to the case where the magnetic moment direction 12 of the magnetic piece 9 is orthogonal to the direction of the static magnetic field of the FOV 6.
- the magnetic material piece 9 selects the position of the magnetic material piece 9 that generates a positive magnetic field distribution in the FOV 6.
- the region of the shim pocket 22 where the magnetic moment of the magnetic piece 9 generates a positive magnetic field in the FOV 6 is selected, in other words, the positive region 10
- the fine adjustment shimming magnetic piece 9 is arranged in the adjustment region 14 where the FOV 6 region and the FOV 6 region overlap is shown.
- the magnetic piece 9 may be disposed in the positive region 10 that is out of the adjustment region 14.
- the weights of the positive region 10 that is out of the adjustment region 14 and the magnetic piece 9 disposed in the shim pocket 22 of the adjustment region 14 are changed.
- a small amount of the magnetic piece 9 may be disposed in the positive region 10 that is out of the adjustment region 14.
- a magnetic resonance imaging apparatus to which the static magnetic field homogeneity adjustment method of the first embodiment or the second embodiment is applied, a magnetic field generator including an electromagnet that forms a static magnetic field in an imaging space, and a surface portion of the electromagnet on the imaging space side And a magnetic material holding member in which a plurality of recesses for accommodating magnetic material pieces for magnetic field adjustment are arranged, and the density of the amount of the magnetic material pieces accommodated in a portion near the imaging space of the magnetic material holding member is The density of the amount of the magnetic piece in the portion far from the imaging space is higher.
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Abstract
Description
図1に、第1実施形態に係る磁気共鳴イメージング装置の円筒型の超電導磁石2の外観斜視図の概略図を示す。超電導磁石2は、高磁場を発生させることができるが、常電導電磁石に本発明を適用してもよい。
Mxは、X軸方向の磁気モーメント[Am2]
Myは、Y軸方向の磁気モーメント[Am2]
Mzは、Z軸方向の磁気モーメント[Am2]
さらに、磁性体片9を磁化させる磁場は、主にZ軸方向を向いているのでMzのみに着目して式3となる。つまり、下式の式3に示されるBzがシミングにより調整することができる量である。
Zは、Z軸方向の位置
図4に、磁性体片9の磁気モーメントがFOV6に生成する正負の磁場分布図を示す。
磁性体片9の磁気モーメントがつくるBz成分は、下式の式4を境界として、正の領域10と、負の領域11がある。
図5に、シミングで用いる磁性体片9の磁気モーメントがつくるBz成分の磁化の領域と、軸方向(Z軸)4と、FOV6の関係を示す。磁気モーメントの矢印は磁性体片9の磁化の向きを示しており、二点鎖線は磁性体片9の配置方向を示している。本実施形態では、磁性体片9の磁気モーメントがFOV6に生成する正負の磁場分布のうち、負の磁場分布のみ生成する負の領域11を磁性体片の配置領域として選択する。
ステップS303で、ステップS302で計測した磁場強度を用いて公知の最適化計算を行い、磁場の不均一性を打ち消すような一部のシムトレイ1(例えば、奇数番目のシムトレイ1)のシムポケットP(例えば、P-1~P-24)に収納する磁性体片9の磁性体片量(A)を決定する。決定した磁性体片9の配置図(例えば、図6に示す。)を出力表示する。
ステップS304で、超電導磁石2を消磁する。
すなわち、ステップS312で、磁性体片9を配置するシムトレイ1を、後述するように選択し、選択された位置配置内でどのように配置すれば高い均一度が生成されるかを、シミング計算により求め、磁性体片量(B-2)として決定する。決定した磁性体片9の配置図を出力表示する。この配置図の一例を図7に示す。図7において、網掛けにより塗りつぶされたシムポケット(P-1)~(P-5)、(P-20)~(P-24)の領域は、磁性体片9を配置しない制約領域として、選択されている。つまり、これらの領域に配置される磁性体片9の磁気モーメントが、FOV6に生成する磁気分布の極性が正であるから(図4参照)、磁性体片9を配置しないシムポケットとして制約を加えた領域である。
図11に、第2実施形態に係る磁気共鳴イメージング装置の開放型の超電導磁石2の外観斜視図の概略図を示す。第1実施形態と異なる点は、円筒型に代えて、開放型の超電導磁石2に本発明の微調整シミングを適用する例である。開放型の超電導磁石2の場合は、生成されるFOV6の磁場の向きが垂直方向になる。以下、異なる箇所のみ説明し、同じ箇所の説明は省略する。その開放型の超電導磁石2の静磁場均一度調整方法を図12~図17に基づき説明する。
Claims (10)
- 磁場発生装置により発生された撮像空間における静磁場に対して、複数の磁性体片の前記撮像空間から離れた位置をシミング計算により計算して、該シミング計算により得られた位置に前記複数の磁性体片を配置して、前記撮像空間における静磁場均一度を調整する方法であって、
前記シミング計算の際に、前記位置に配置された前記磁性体片が前記撮像空間に生成する磁場分布の極性が正負のいずれか一方とする制約を課し、前記静磁場均一度を調整する調整ステップを含んでなることを特徴とする磁気共鳴イメージング装置の静磁場均一度調整方法。 - 前記磁性体片の磁気モーメントの方向が前記静磁場の方向に平行な場合は、前記磁性体片が前記撮像空間に負の磁場分布を生成する前記磁性体片の位置を選択することを特徴とする請求項1に記載の磁気共鳴イメージング装置の静磁場均一度調整方法。
- 前記磁性体片の磁気モーメントの方向が前記静磁場の方向に直交する場合は、前記磁性体片が前記撮像空間に正の磁場分布を生成する前記磁性体片の位置を選択することを特徴とする請求項1に記載の磁気共鳴イメージング装置の静磁場均一度調整方法。
- 前記磁性体片を配置する調整ステップにおいて、磁場発生装置のボア開口部から1箇所ずつ磁性体片を配置しないように選択し、前記調整ステップ完了後の予想される磁場均一度と前記調整ステップに用いる磁性体片量の増減を評価することで負の領域を推定することを特徴とする請求項1に記載の磁気共鳴イメージング装置の静磁場均一度調整方法。
- 撮像空間に静磁場を形成する電磁石を備えた磁場発生装置と、前記電磁石の前記撮像空間側の面部に磁場調整用の磁性体片を収容する複数の凹所が配列された磁性体保持部材とを備えてなる磁気共鳴イメージング装置において、
前記磁性体保持部材の前記撮像空間に近い部分に収容された磁性体片量の密度が、前記撮像空間から遠い部分の前記磁性体片量の密度よりも高いことを特徴とする磁気共鳴イメージング装置。 - 前記磁場発生装置は、円筒の内部空間に前記撮像空間を形成する円筒状の電磁石を備え、
前記磁性体保持部材は、前記電磁石の内周面に分散させて、かつ軸方向に沿って複数配置され、長手方向に前記磁性体片を収容する複数の凹所が配列されてなり、
一部の前記磁性体保持部材は、前記凹所が両端部から設定長さにわたって形成されていないことを特徴とする請求項6に記載の磁気共鳴イメージング装置。 - 前記一部の前記磁性体保持部材は、少なくとも前記凹所が形成されていない一端側の長さが他端側よりも短く形成されていることを特徴とする請求項7に記載の磁気共鳴イメージング装置。
- 磁場発生装置により発生された撮像空間における静磁場に対して、複数の磁性体片の前記撮像空間から離れた位置をシミング計算により計算して、前記撮像空間における静磁場均一度を調整するプログラムであって、前記シミング計算をする際に、該位置に配置された前記磁性体片が前記撮像空間に生成する磁場分布の極性が正負のいずれか一方であるものであるという制約を課する機能を備えていることを特徴とするプログラム。
- 磁場発生装置により発生された撮像空間における静磁場に対して、複数の磁性体片の前記撮像空間から離れた位置をシミング計算により計算して、前記撮像空間における静磁場均一度を調整する計算機であって、前記シミング計算をする際に、該位置に配置された前記磁性体片が前記撮像空間に生成する磁場分布の極性が正負のいずれか一方であるという制約を課する機能を備えていることを特徴とする計算機。
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