JP5291583B2 - Magnetic field distribution measuring method, magnetic field distribution measuring jig, magnet apparatus, and magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic field distribution measuring method obtaining highly precise uniformity of the magnetic field space even when the magnetic field space is measured at few measuring points of M1-M4 on the surface. <P>SOLUTION: In a method for measuring a magnetic field distribution in a magnetic field space by a magnetic device, the magnetic device includes a magnetic field generating source for generating a magnetic field in a magnetic field space and a magnetic field uniformity adjusting device wherein a plurality of magnetic materials are nonuniformly mounted to improve the uniformity of the magnetic field in the magnetic field space, and the position where the magnetic materials are mounted in the magnetic field unifromity adjusting device is changed while the magnetic materials obtain the magnetic field distribution formed on the surface of the magnetic field space, and the function of half width at half maximum d&theta; of the magnetic field distribution to the peak position &theta; of the magnetic field distribution is installed, and a plurality of measuring points M1-M4 for measuring the magnetic field mounted on the surface of the magnetic field space have distances d&theta;a, d&theta;b, d&theta;c, and the sizes of distances d&theta;a, d&theta;b, d&theta;c are changed by the positions of measuring points M1-M4 in order to be below the half width of half maximum d&theta; introduced by the function from the peak position &theta; accorded with the positions of measuring points M1-M4. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a magnetic field distribution measuring method in a magnetic field space in a magnet device having a magnetic field generation source and a magnetic field homogeneity adjusting device, and further, a magnetic field distribution measuring jig used when the magnetic field distribution measuring method is performed. And a magnet device provided with a magnetic field uniformity adjusting device adjusted based on a measurement result of the magnetic field distribution measuring method, and a magnetic resonance imaging device equipped with the magnet device.

  A magnetic resonance imaging (MRI) apparatus uses a nuclear magnetic resonance phenomenon that occurs when an object placed in a magnetic field space of a uniform static magnetic field is irradiated with a high-frequency pulse, and the physical and chemical properties of the object. An image representing the physical properties can be obtained and is mainly used for medical purposes. In a magnetic resonance imaging apparatus, in order to generate a uniform static magnetic field space in an imaging region into which a subject is carried, not only a magnetic field generation source that generates a magnetic field in the magnetic field space but also a magnetic field in the magnetic field space. A magnet device equipped with a magnetic field uniformity adjusting device for improving uniformity is mounted.

  In the magnetic field uniformity adjusting device, the magnetic material is arranged around the magnetic field space, and the magnetic field uniformity is improved by controlling the movement of the magnetic flux. At this time, in order to obtain the measurement result of the uniformity of the magnetic field in the magnetic field space, it is necessary to obtain the maximum value and the minimum value of the magnetic field in the magnetic field space. By obtaining the maximum value and the minimum value, the magnetic field uniformity inside the magnetic field space can be calculated. Thereby, since it is only necessary to measure the magnetic field on the surface of the magnetic field space, the number of measurement points can be reduced and the measurement time can be shortened. However, the measurement of the magnetic field on the surface of the magnetic field space has a large number of measurement points, and the measurement takes a considerable amount of time. For example, in Non-Patent Document 1, in an rθφ coordinate system in which the angle between the center of a spherical magnetic field space and the direction of the static magnetic field as θ is θ, the interval in the θ direction is equal to about 7.3 degrees. It is disclosed to place measurement points.

MetroLab product manual "PROBE-ARRAY MFC-3048 / 24 Probes DATA SHEET"

  One of the requirements for determining the image quality of the magnetic resonance imaging apparatus is the uniformity of the static magnetic field in the imaging region. The uniformity of the static magnetic field in the imaging region can be defined by, for example, the ratio of the difference between the maximum magnetic field strength in the imaging region and the minimum magnetic field strength to the average magnetic field strength in the imaging region. Specifically, as the uniformity of the static magnetic field in the imaging region, the required value varies depending on the imaging target, and when obtaining a very general image, a uniformity of 30 ppm or less is required, or the fat portion is received. A uniformity of 3 ppm or less is required to remove the signal.

ここでｒは撮像領域（磁場空間）の幾何的な中心からの距離、ｌ、ｍは次数を表す整数、ａ、ｂは未知係数であり、θはｚ軸からの角度（仰角）、φはｚ軸まわりの回転角度（方位角）であり、Ｐはルジャンドル陪関数である。磁場空間の表面上の複数の計測点で計測された磁場分布Ｂをもとに、未知係数ａ、ｂを求めることができれば、磁場空間の内側の磁場分布Ｂはａ、ｂの求まった式１を用いた内挿演算により算出することができる。すなわち、磁場空間の表面上の磁場分布Ｂさえ測定すれば、その内側の磁場分布Ｂは内挿演算により算出することができ、その内側の磁場分布Ｂを直接測定することなしに、磁場空間における磁場の均一度を取得することができる。 Since the magnetic field distribution B of the static magnetic field in the imaging region (magnetic field space) satisfies the Laplace equation, it can be expressed as Equation 1 as a general solution of the Laplace equation.

Here, r is a distance from the geometric center of the imaging region (magnetic field space), l and m are integers representing orders, a and b are unknown coefficients, θ is an angle (elevation angle) from the z axis, and φ is It is a rotation angle (azimuth angle) around the z axis, and P is a Legendre power function. If the unknown coefficients a and b can be obtained on the basis of the magnetic field distribution B measured at a plurality of measurement points on the surface of the magnetic field space, the magnetic field distribution B inside the magnetic field space can be obtained by the formula 1 in which a and b are obtained. Can be calculated by an interpolation operation using. That is, if only the magnetic field distribution B on the surface of the magnetic field space is measured, the inner magnetic field distribution B can be calculated by interpolation, and the inner magnetic field distribution B can be calculated directly in the magnetic field space without directly measuring it. The uniformity of the magnetic field can be acquired.

  In order to calculate the uniformity of the magnetic field with high accuracy, it is necessary to improve the accuracy of the interpolation calculation. In order to improve the accuracy of the interpolation operation, it is necessary to consider as much as possible the orders l and m in Equation 1. Since the number of unknown coefficients a and b increases, it is necessary to increase the number of measurement points in order to determine these. is there. Conversely, when the number of measurement points is small and only a few orders l and m can be taken into account, the magnetic field distribution B with high orders l and m that actually exists is measured as a magnetic field distribution with low orders l and m. Therefore, aliasing in the so-called sampling theorem occurs, and high-precision interpolation cannot be performed.

  Accordingly, an object of the present invention is to provide a magnetic field distribution measuring method, a magnetic field distribution measuring jig, a magnet device, and a magnetic device that can obtain high-precision uniformity of the magnetic field space even if the surface of the magnetic field space is measured with a small number of measurement points. A resonance imaging apparatus is provided.

A first aspect of the present invention includes a magnetic field generation source that generates a magnetic field in a magnetic field space, and a magnetic field uniformity adjusting device that non-uniformly arranges a plurality of magnetic materials and improves the uniformity of the magnetic field in the magnetic field space. In the magnetic field distribution measuring method of the magnetic field space in the magnet device,
While changing the position of the magnetic material arranged in the magnetic field uniformity adjusting device, obtain the magnetic field distribution that the magnetic material creates on the surface of the magnetic field space,
Obtain a half-width function of the magnetic field distribution with respect to the peak position of the magnetic field distribution,
The interval between a plurality of measurement points that are arranged on the surface of the magnetic field space and measure the magnetic field is equal to or less than the half-value half width derived by the function from the peak position that coincides with the position where the measurement point is placed. The size of the interval is changed according to the position of the measurement point.

  The second aspect of the present invention provides a plurality of magnetic materials based on measurement results obtained by measuring a magnetic field generated by the magnetic field generation source at the measurement points arranged by the magnetic field distribution measurement method of the first aspect of the present invention. The magnetic field uniformity adjusting device in which the magnetic field is arranged to improve the uniformity of the magnetic field in the magnetic field space.

According to a third aspect of the present invention, there is provided a magnetic field generation source that generates a magnetic field in a magnetic field space, and a magnetic field uniformity adjusting device that non-uniformly arranges a plurality of magnetic materials to improve the uniformity of the magnetic field in the magnetic field space. In the magnetic field distribution measuring jig of the magnetic field space in the magnet device provided,
When the position of the magnetic material to be arranged on the magnetic field uniformity adjusting device is changed, the magnetic material distribution is obtained by using the magnetic field distribution created on the surface of the magnetic field space by the magnetic material. Using a half-value half-width function, the interval between a plurality of measurement points arranged on the surface of the magnetic field space and measuring the magnetic field is derived by the function from the peak position that coincides with the position where the measurement point is placed. A holder for fixing a magnetic field sensor to the measurement point where the size of the interval is changed according to the position of the measurement point so as to be equal to or less than a half value half width;
The holder has a seat configured to be able to rotate around a rotation axis parallel to the direction of the magnetic field in the magnetic field space.

  And 4th this invention is based on the measurement result which measured the magnetic field generated with the said magnetic field generation source in the said measurement point set with the jig | tool for magnetic field distribution measurement of 3rd this invention, and several magnetic It is a magnet device that improves the uniformity of the magnetic field in the magnetic field space by the magnetic field uniformity adjusting device in which a material is arranged.

  Finally, the fifth aspect of the present invention is a magnetic resonance imaging apparatus equipped with the magnet device of the second aspect of the present invention or the fourth aspect of the present invention.

  According to the present invention, a magnetic field distribution measuring method, a magnetic field distribution measuring jig, a magnet device, and a magnetic resonance imaging capable of acquiring high-precision uniformity of the magnetic field space even if the surface of the magnetic field space is measured with a small number of measurement points. Equipment can be provided.

It is a perspective view of the magnet apparatus which concerns on the 1st Embodiment of this invention. It is a longitudinal cross-sectional view of the magnet apparatus which concerns on the 1st Embodiment of this invention. 1 is a schematic configuration diagram of a magnetic resonance imaging apparatus according to a first embodiment of the present invention. In the magnet device according to the first embodiment of the present invention, when one magnetic material is separated from the central axis of the magnetic field uniformity adjusting device by _RA from the central axis, the magnetic materials are in a uniform magnetic field space. It is a graph of the magnetic field distribution made on the surface of (imaging space). 4 is a graph showing a relationship between a half-value half width of a magnetic field distribution and a magnetic field distribution peak position (direction in which a magnetic material is arranged) in the magnet device according to the first embodiment of the present invention. 5 is a graph for explaining an example of a magnetic field distribution measurement method for determining an interval between measurement points of a magnetic field on the surface of a uniform magnetic field space in the magnet device according to the first embodiment of the present invention. It is an arrangement | positioning figure which shows two-dimensionally an example of the arrangement | positioning of the measurement point of the magnetic field on the surface of the uniform magnetic field space determined by the magnetic field distribution measuring method which concerns on the 1st Embodiment of this invention. It is an arrangement | positioning figure which shows an example of arrangement | positioning of the measurement point of the magnetic field on the surface of the uniform magnetic field space determined by the magnetic field distribution measuring method which concerns on the 1st Embodiment of this invention in three dimensions. It is a graph explaining another example of the magnetic field distribution measuring method which determines the space | interval of the measurement point of the magnetic field on the surface of uniform magnetic field space in the magnet apparatus which concerns on the 1st Embodiment of this invention. It is a front view of the jig for magnetic field distribution measurement concerning a 1st embodiment of the present invention. It is a longitudinal cross-sectional view of the magnet apparatus which installed the jig | tool for magnetic field distribution measurement which concerns on the 1st Embodiment of this invention. It is a front view of the jig for magnetic field distribution measurement concerning a 2nd embodiment of the present invention. It is a perspective view of the magnet apparatus which concerns on the 3rd Embodiment of this invention. It is a longitudinal cross-sectional view of the magnet apparatus which installed the jig | tool for magnetic field distribution measurement which concerns on the 3rd Embodiment of this invention. It is a perspective view of the magnet apparatus which concerns on the 4th Embodiment of this invention. It is a longitudinal cross-sectional view of the magnet apparatus which concerns on the 4th Embodiment of this invention. It is a schematic block diagram of the magnetic resonance imaging device which concerns on the 4th Embodiment of this invention. In the magnet device according to the fourth embodiment of the present invention, in the case where one magnetic material is on the equator plane of the magnetic field uniformity adjusting device and the distance from the equator plane is R _A , those magnetic materials are in a uniform magnetic field space ( It is a graph of the magnetic field distribution made on the surface of (imaging space). It is a graph explaining an example of the magnetic field distribution measuring method which determines the space | interval of the measurement point of the magnetic field on the surface of uniform magnetic field space in the magnet apparatus which concerns on the 4th Embodiment of this invention. It is an arrangement | positioning figure which shows two-dimensionally an example of arrangement | positioning of the measurement point of the magnetic field on the surface of the uniform magnetic field space determined by the magnetic field distribution measuring method which concerns on the 4th Embodiment of this invention. It is an arrangement | positioning figure which shows an example of arrangement | positioning of the measurement point of the magnetic field on the surface of the uniform magnetic field space determined by the magnetic field distribution measuring method which concerns on the 4th Embodiment of this invention in three dimensions. It is a longitudinal cross-sectional view of the magnet apparatus which installed the jig | tool for magnetic field distribution measurement which concerns on the 4th Embodiment of this invention.

  In the magnetic resonance imaging apparatus according to the present invention, the size of the imaging region (magnetic field space), the position and size of the magnetic field uniformity adjusting device (shim tray), the arrangement method of the magnetic material (shim), and the interval are different for each apparatus. In carrying out the present invention, the optimum measurement point interval is determined for each apparatus in accordance with the magnetic field distribution measurement method of the present invention, and a magnetic field distribution measurement jig corresponding to this is manufactured. The magnetic resonance imaging apparatus of the present invention includes a vertical magnetic field type that generates a magnetic field space in which the direction of the static magnetic field is vertical and the magnetic field strength is uniform in the imaging region between the magnetic poles that are vertically opposed to each other, and the center that is oriented in the horizontal direction. A horizontal magnetic field type that generates a magnetic field space in which the direction of the static magnetic field is horizontal and the magnetic field intensity is uniform in the imaging region inside the solenoidal coil group having an axis is roughly classified into those types. The magnet device of the present invention mounted on the magnetic resonance imaging apparatus of the present invention is also roughly classified into a vertical magnetic field type, a horizontal magnetic field type, and their types according to the type of the magnetic resonance imaging apparatus of the present invention. The According to the magnetic field distribution measurement method and the magnetic field distribution measurement jig of the present invention, it is possible to determine the optimum interval between measurement points for any of these types.

  Hereinafter, embodiments of the present invention will be described in correspondence with the types of magnet devices (magnetic resonance imaging devices).

(First embodiment)
In FIG. 1, the perspective view of the magnet apparatus 8 which concerns on the 1st Embodiment of this invention is shown. The magnet device 8 of the first embodiment is a vertical magnetic field type magnet device. In the magnet device 8, a pair of magnetic poles 1 and 2 are separated from each other by two connecting pillars 5 and are arranged facing each other in the vertical direction (z-axis direction) so as to sandwich the (uniform) magnetic field space 3. Yes. Each of the magnetic poles 1 and 2 incorporates permanent magnets 6 and 7 as magnetic field generation sources (magnetomotive force sources) and shim trays as magnetic field uniformity adjusting devices 15 and 16. The permanent magnets 6 and 7, the magnetic field uniformity adjusting devices 15 and 16, the connecting column 5, and the gap 11 between the magnetic field uniformity adjusting devices 15 and 16 constitute a magnetic circuit 10. The uniform magnetic field space 3 is formed in the gap 11. The outer shape of the uniform magnetic field space 3 may be any shape, but in many cases, it is a spherical or flat spheroid, and FIG. 1 illustrates the case of a spherical shape. The magnetic field direction 4 in the uniform magnetic field space 3 coincides with the vertical direction (z-axis direction).

  In FIG. 2, the longitudinal cross-sectional view of the magnet apparatus 8 which concerns on the 1st Embodiment of this invention is shown. The z axis is defined so as to coincide with the central axis 13 of the magnet device 8. An equatorial plane 14 is defined as a plane of symmetry between the magnetic poles 1 and 2 having a plane symmetry relationship. The equator plane 14 is orthogonal to the central axis 13 (z-axis), and the intersection associated with this orthogonality is the magnet center (magnet symmetry point) 12. The magnet center 12 substantially coincides with the spherical center of the outer shape of the uniform magnetic field space 3.

  Considering the case where the intensity of the magnetic field is measured at a sweep measurement point M0 at an arbitrary point on the outer surface of the uniform magnetic field space 3, the position of the sweep measurement point M0 is the spherical radius r of the outer shape of the uniform magnetic field space 3 And the elevation angle θ formed by the direction from the magnet center 12 to the sweep measurement point M0 and the central axis 13 (z axis) and the angle (azimuth angle) φ around the central axis 13 (z axis). it can.

  The magnetic field uniformity adjusting devices (shim trays) 15 and 16 are arranged so as to sandwich the uniform magnetic field space 3 up and down. The shape of the magnetic field uniformity adjusting devices (shim trays) 15 and 16 is arbitrary, and the shape does not affect the essence of the present invention, but in many cases, as shown in FIG. And it has a pair of disk shape which makes the central axis 13 the common central axis. FIG. 2 shows a state in which the magnetic material (shim) 17 is arranged on the magnetic field uniformity adjusting device (shim tray) 15. By arranging the magnetic material (shim) 17 on the magnetic field uniformity adjusting devices (shim trays) 15 and 16, control for moving the magnetic flux in the uniform magnetic field space 3 (so-called shimming operation) is performed, and the magnetic field equalization in the uniform magnetic field space 3 is performed. You can improve once. As the magnetic material (shim) 17, a permanent magnet, iron, or the like can be used.

The position of the magnetic material (shim) 17 on the disks of the magnetic field uniformity adjusting devices (shim trays) 15 and 16 is the R coordinate in the radial direction of the disk with the shim tray centers 24 and 25 on the center axis 13 (z axis) as the origin. And an angle (azimuth angle) φ around the z-axis. FIG. 2 shows a case where the magnetic material (shim) 17 is arranged at a position of R = 0 and a position of R = _RA on the magnetic field uniformity adjusting device (shim tray) 15.

  FIG. 3 shows a schematic diagram of the magnetic resonance imaging apparatus 9 according to the first embodiment of the present invention. The magnetic resonance imaging apparatus 9 is equipped with the magnet device 8, further includes gradient magnetic field coils 18 and 19 on the equator plane 14 side of the magnetic field uniformity adjusting devices (shim trays) 15 and 16, and further includes the gradient magnetic field coil 18, RF transmitting / receiving coils 20 and 21 are arranged on the equator plane 14 side of 19. Outside the magnet device 8, there are provided a driving device 22 and a control device 23 for driving the gradient magnetic field coils 18 and 19 and the RF transmitting / receiving coils 20 and 21.

In FIG. 4, in the magnet device 8, one magnetic material (shim) 17 is separated from _RA on the shim tray center 24 (R = 0) and the shim tray center 24 of the magnetic field uniformity adjusting device (shim tray) 15 (R = R _A ) shows magnetic field distributions 26 and 27 which are formed on the surface of the uniform magnetic field space (imaging space) 3 by the magnetic material 17 when arranged in each of (R _A ). These magnetic field distributions 26 and 27 can be measured by moving the sweep measurement point M0 in the θ direction while measuring the magnetic field intensity at the sweep measurement point M0 (see FIG. 2). A magnetic field sensor is disposed at the moving sweep measurement point M0. Instead of this measurement, the magnetic field distributions 26 and 27 may be calculated by simulation. Further, due to the symmetry around the central axis (z axis) 13 of the magnet device 8, the tendency of the magnetic field distributions 26 and 27 in the θ direction is established at an arbitrary azimuth angle φ.

As shown in FIG. 4, the magnetic field distribution 26 when R = 0, that is, when the shim 17 is placed at the shim tray center 24, has a peak intensity at the peak position θ ₀ that is higher than the magnetic field distribution 27 when R = R _A. The magnetic field distribution has a sharp peak that is larger than the peak intensity at the position θ _A and the half-value half-width dθ ₀ is narrower than the half-value half-width dθ _A.

According to the teaching of the sampling theorem, in order to know these magnetic field distributions 26 and 27 accurately, the magnetic field distributions 26 and 27 must be measured at measurement point intervals equal to or less than the half-value half widths dθ ₀ and dθ _A. The half-value half widths dθ ₀ and dθ _A have different values depending on the peak positions θ ₀ and θ _A , that is, the positions (R) where the shims 17 are placed on the shim trays 15 and 16. That is, the half widths dθ ₀ and dθ _A indicate the maximum values of the measurement point intervals at the peak positions θ ₀ and θ _A and the shim 17 positions R = 0 and R _A. Since the peak positions θ ₀ and θ _A and the positions R = 0 and R _{A of} the shim 17 correspond one-to-one as a physical phenomenon, the peak positions θ ₀ and θ _A represent the position of the shim 17. It can also be used as a substitute for the R value.

  FIG. 5 shows a simulation result of the relationship between the peak position θ of the magnetic field distribution and the half-value half width dθ of the magnetic field distribution on the surface of the uniform magnetic field space 3 at the peak position θ. The peak position θ is displayed from 0 degree to 90 degrees because the shim 17 is arranged on the upper shim tray 15. Since the first embodiment exemplifies a vertically symmetric system having the equator plane 14 as a symmetry plane, when considering an example in which the shim 17 is arranged on the lower shim tray 16, the peak position θ is 90. The range is from 180 degrees to 180 degrees. It is clear that the half-value half-width dθ when the peak position θ is 180 degrees substantially matches the half-value half-width dθ when the peak position θ is 0 degrees.

According to FIG. 5, with respect to half width at half maximum dθmin (= dθ ₀₎ peak positions theta is at 0 degrees near, and peak positions theta becomes large toward the half width at half maximum d [theta] _A of the theta _A, further 90 degrees near It can be seen that the half-value half-width dθmax is larger. The half-value half width dθmax near 90 degrees is approximately three times as large as half or more the half-value half width dθmin near 0 degrees. In other words, it can be seen that the peak position θ is between 0 ° and 90 °, and the half-value half-width dθ increases as the peak position θ increases. The measurement point interval is set to be less than or equal to the half-value half-width dθ as the maximum measurement point interval according to the sampling theorem. Therefore, changing the size of the half-value half-width dθ according to the peak position θ The size can be changed by the peak position θ. Then, by increasing the measurement point interval within an allowable range of the half value half width dθ according to the peak position θ, the number of measurement points can be reduced while maintaining the measurement accuracy of the magnetic field distribution or the accuracy of the interpolation calculation with high accuracy. it can. That is, if the size of the measurement point interval is changed according to the peak position θ (corresponding to the measurement point) so that the measurement point interval is equal to or less than the half-value half width dθ derived according to the curve of the graph of FIG. The magnetic field distribution can be measured with high accuracy by the optimum and minimum measurement point arrangement.

FIG. 6 shows an example of a magnetic field distribution measurement method for determining the measurement point intervals dθa to dθc of the measurement points M1 to M4 of the magnetic field on the surface of the uniform magnetic field space 3. First, the measurement point M1 is arranged at a position where the peak position θ is 0 degrees (θ = 0 degrees). Next, the half-value half width dθa (= dθmin = dθ ₀ ) at the position where the peak position θ is 0 degree (θ = 0 degree) is obtained by reading from the graphs of FIGS. 5 and 6 (or may be obtained by simulation). Hereinafter, this half value half width dθa is defined as a measurement point interval (dθa) between the measurement point M1 and the measurement point M2. Specifically, the measurement point M2 is arranged at a position away from the measurement point M1 by the measurement point interval (dθa). In FIG. 6, the scales of the vertical axis and the horizontal axis are different.

  Next, the half-value half-width dθb at the peak position θ at the measurement point M2 (θ = dθa) is obtained by reading from the graphs of FIGS. 5 and 6, and this half-value half-width dθb is obtained between the measurement point M2 and the measurement point M3. The measurement point interval (dθb) is used. The half-value half-width dθb is different from the half-value half-width dθa because the peak position θ at the time of reading is different (dθa ≠ 0). Specifically, the measurement point M3 is arranged at a position (θ = dθa + dθb) that is separated from the measurement point M2 by the measurement point interval (dθb).

  Similarly, the half-value half-width dθc at the position where the peak position θ is the measurement point M3 (θ = dθa + dθb) is obtained by reading from the graphs of FIGS. 5 and 6, and this half-value half-width dθc is obtained between the measurement point M3 and the measurement point M4. The measurement point interval (dθc) is used. The half-value half-width dθc has a different value (dθc ≠ dθa, dθc ≠ dθb) and the half-value half-width dθa because the peak position θ when read is different from the half-value half-widths dθa and dθb (dθa + dθb ≠ 0, dθa + dθb ≠ dθa). , Dθb (dθc> dθb> dθa). Specifically, the measurement point M4 is arranged at a position (θ = dθa + dθb + dθc) that is separated from the measurement point M3 by the measurement point interval (dθc). As described above, the difference between the positions where the measurement points M1 to M4 are arranged is dθa to dθc (dθc> dθb> dθa), so that there is no equal difference relationship. Since the half-value half width dθmax near 90 degrees is more than twice the half-value half width dθmin near 0 degrees and is almost three times, the measurement point interval is the minimum value and the maximum value, and the maximum value is minimized. It can be set to about 3 times more than twice the value.

  FIG. 7A shows two-dimensionally an example in which the measurement points M1 to M4 are arranged on the surface of the uniform magnetic field space 3. The operation of arranging the measurement points M1 to M4 is first repeated within a range in which the positions of the measurement points M1 to M4 do not exceed 90 degrees (θ = 90 degrees). The range where the peak position θ is 90 to 180 degrees can be set as shown in FIG. 7A from the symmetry with respect to the equator plane 14 (see FIG. 2) of the magnet device 8. That is, the measurement point M1 is arranged at a position where the peak position θ is 180 degrees (θ = 180 degrees). Next, the measurement point M2 is arranged at a position (θ = 180−dθa) away from the measurement point M1 by the measurement point interval (dθa). Next, the measurement point M3 is arranged at a position (θ = 180−dθa−dθb) away from the measurement point M2 by the measurement point interval (dθb). Finally, the measurement point M4 is arranged at a position (θ = 180−dθa−dθb−dθc) that is separated from the measurement point M3 by the measurement point interval (dθc).

FIG. 7B shows an example in which measurement points M1 to M4 are arranged on the surface of the uniform magnetic field space 3 in a three-dimensional manner. The measurement point interval dφ around the z-axis is equal to the half-value half width dθ ₉₀ (= dθmax, see FIG. 6) when the peak position θ is 90 degrees (θ = 90 degrees) based on the principle such as the sampling theorem described above. (Dφ = dθ ₉₀ ). According to this, the measurement point interval between the adjacent measurement points M3 and M3 can be made longer than the measurement point interval between the adjacent measurement points M2 and M2. Similarly, the measurement point interval between the adjacent measurement points M4 and M4 can be made longer than the measurement point interval between the adjacent measurement points M3 and M3. The measurement points M1 to M4 can be optimally arranged, and the number of measurement points M1 to M4 can be minimized.

FIG. 8 shows a magnetic field distribution measurement method for determining the measurement point intervals dθa and dθe (dθe ≠ dθa, dθe> dθa) of the magnetic field measurement points M1 to M5 on the surface of the uniform magnetic field space 3 and the method shown in FIG. Shows another example. First, the measurement point M1 is arranged at a position where the peak position θ is 0 degrees (θ = 0 degrees). Next, the half-value half-width dθa (= dθmin = dθ ₀ ) at the position where the peak position θ is 0 degree (θ = 0 degree) is obtained by reading from the graphs of FIGS. 5 and 6, and this half-value half-width dθa is obtained as a predetermined peak. The measurement point intervals of the measurement points M1 to M3 that do not reach the position θ (45 degrees in FIG. 8) are set. Specifically, the measurement point M2 is arranged at a position (θ = dθa) that is separated from the measurement point M1 by the measurement point interval (dθa), and similarly, the position (θ that is separated from the measurement point M2 by the measurement point interval (dθa)). = 2 × dθa), the measurement point M3 is arranged. In FIG. 8, the scales of the vertical axis and the horizontal axis are different.

  Next, the half-value half-width dθe when the peak position θ is 45 degrees is obtained by reading from the graphs of FIGS. 5 and 6, and at least one of the measurement points M4 and M5 exceeds 45 degrees of the peak position θ. An interval between measurement points M3 to M5 is set. Specifically, the measurement point M4 is arranged at a position (θ = 2 × dθa + dθe) away from the measurement point interval (dθe) from the measurement point M3, and similarly, a position away from the measurement point M4 by the measurement point interval (dθe). The measurement point M5 is arranged at (θ = 2 × dθa + 2 × dθe). The half-value half-width dθe is not equal to the half-value half-width dθa (dθe ≠ dθa) and is larger than the half-value half-width dθa (dθe> dθa). Also in this example, the measurement point interval does not exceed the half-value half-width dθ at any peak position θ of 0 to 90 degrees. For this reason, the accuracy of calculation such as interpolation is not reduced. Since equal measurement intervals are used for repeated calculations, the calculation can be simplified. However, the number of measurement points is larger than in the case of FIG. Which one should be selected may be determined in consideration of the measurement time. In any case, the measurement point interval near 90 degrees (θ = 90 degrees) can be made larger than the measurement point interval near peak position θ of 0 degrees (θ = 0 degrees). The number of measurement points M1 to M5 can be reduced rather than arranging them at equal intervals.

  FIG. 9A shows a front view of the magnetic field distribution measurement jig 32 according to the first embodiment of the present invention. The magnetic field distribution measuring jig 32 includes a non-magnetic, flat plate shaped holder 28 with a hole 31, a seat 29 that supports the holder 28 so as to rotate about the central axis 13 (z axis), And a magnetic field sensor 30 that is fitted and held in the hole 31. Here, the magnetic field sensor 30 may be anything as long as it can measure the magnetic field intensity. However, since the magnetic resonance imaging apparatus 9 requires highly accurate measurement, for example, an NMR (Nuclear Magnetic Resonance) sensor or the like is used.

  The holder 28 is provided with holes 31 at positions of the measurement points M1 to M4 along an arc having the same size as the arc on the surface of the uniform magnetic field space 3. The measurement point intervals of the measurement points M1 to M4 are set to be the same as the measurement point intervals dθa to dθc and dθe described in the magnetic field distribution measurement method. For this reason, the interval between the adjacent holes 31 of the holder 28 is wider in the vicinity of the equator plane 14 (θ = 90 degrees) away from the z axis than in the vicinity of the z axis (θ = 0 degrees and 180 degrees). Has appearance characteristics. By using such a magnetic field distribution measurement jig 32, it is possible to measure the magnetic field distribution at the measurement point intervals dθa to dθc and dθe that vary depending on the position θ.

  FIG. 9B shows a state in which the magnetic field distribution measurement jig 32 according to the first embodiment of the present invention is installed in the magnet device 8 during magnetic field distribution measurement. At the time of magnetic field distribution measurement, the magnetic field distribution measurement jig 32 is set in the magnet device 8 so that all of the plurality of holes 31 are arranged on the surface of the uniform magnetic field space 3. The magnetic material (shim) 17 is removed from the shim trays 15 and 16. Then, the magnetic field sensor 30 is sequentially fitted into the plurality of holes 31, and the magnetic field is generated every interval dφ (see FIG. 7B) of the azimuth angle φ while rotating the holder 28 with the seat 29 fixed to the magnet device 8. Measure strength. Thereby, it can measure with respect to all the measurement points M1-M4, and can acquire a highly accurate magnetic field distribution and also a highly accurate uniformity. This magnetic field distribution is a magnetic field distribution of the magnetic field generated by the magnetic field generation sources (magnetic poles) 1 and 2 that do not depend on the shim 17. Based on the measurement result, one or a plurality of shims 17 are arranged on the shim trays 15 and 16, and high-precision uniformity is obtained again as described above. By repeating the arrangement of the shim 17 and the acquisition of high accuracy uniformity, the uniformity of the magnetic field in the magnetic field space 3 can be improved.

  In the first embodiment, the example in which the permanent magnets 6 and 7 (see FIG. 2) are adopted as the magnetic field generation sources (magnetic poles) 1 and 2 as the magnet device 8 is shown. Instead of the permanent magnets 6 and 7, for example, a superconducting magnet may be adopted.

(Second Embodiment)
FIG. 10 shows a magnetic field distribution measurement jig 32 according to the second embodiment of the present invention. The magnetic field distribution measurement jig 32 of the second embodiment is different from the magnetic field distribution measurement jig 32 of the first embodiment in that the magnetic field sensor 30 is fitted in each of the holes 31. There is no need to replace the magnetic field sensor 30 from the hole 31 to the hole 31, and when measuring the magnetic field distribution, it is only necessary to make one rotation around the z axis, and there is no need to rotate every time it is replaced. Thereby, the time required for magnetic field distribution measurement can be shortened.

  Further, the second embodiment is different from the rectangular holder 28 of the first embodiment in that the holder 33 has a half-moon shape. A hole 31 is provided along the outer periphery of the half-moon shaped holder 33, and the outer circumference of the half-moon shaped holder 33 and the outer shape of the uniform magnetic field space 3 are set in parallel. For this reason, there is an advantage that the person who performs the setting can easily grasp the positional relationship between the magnet device 8 and the magnetic field distribution measurement jig 32. In the second embodiment, the case of a semicircular (half-moon) shape is illustrated. However, the present invention is not limited thereto, and the holder 33 may be a circular plate whose outer periphery is parallel to the outer shape of the uniform magnetic field space 3 or a sphere. Obviously it may be.

(Third embodiment)
FIG. 11A shows a perspective view of a magnet device 8 according to a third embodiment of the present invention. The magnet device 8 of the third embodiment is a type of the vertical magnetic field type magnet device 8 of the first embodiment, and seems to have tilted the vertical magnetic field type magnet device 8 of the first embodiment horizontally by 90 degrees. It has a simple structure. Along with this, the z-axis is also tilted 90 degrees and changed in the horizontal direction. For this reason, also in the magnet apparatus 8 of 3rd Embodiment, the discussion similar to the magnet apparatus 8 of 1st Embodiment can be performed, and the same effect can be acquired.

  FIG. 11B shows a state in which the magnetic field distribution measurement jig 32 according to the third embodiment of the present invention is installed in the magnet device 8 during the magnetic field distribution measurement. As the z-axis becomes horizontal, the holder 28 needs to be rotated around the z-axis, that is, around the horizontal axis in order to obtain the same effect as that of the first embodiment. Therefore, the seat 29 is changed so as to be rotatable about the z axis which is a horizontal axis. Also in the third embodiment, as in the first embodiment, a highly accurate magnetic field distribution can be measured with the optimum and minimum number of measurement points, and a highly accurate uniformity can be calculated.

(Fourth embodiment)
FIG. 12A shows a perspective view of a magnet device 8 according to a fourth embodiment of the present invention. The magnet device 8 of the fourth embodiment is a horizontal magnetic field type magnet device, and the z-axis faces the horizontal direction. The magnet device 8 includes a cylindrical vacuum vessel 41 having a central axis that coincides with the z-axis, and a cylindrical magnetic field uniformity adjusting device that is disposed along the inner wall of the vacuum vessel 41 and that has a central axis that coincides with the z-axis ( Shim tray) 45 and a refrigerator 46 disposed on the upper portion of the vacuum vessel 41.

  FIG. 12B shows a longitudinal sectional view of the magnet device 8 according to the fourth embodiment of the present invention. In the magnet device 8, a double cylindrical vacuum vessel 41 arranged so as to surround the uniform magnetic field space 3, a radiation shield 42 arranged inside the vacuum vessel 41, and further arranged inside the radiation shield 42. In order to reduce the leakage magnetic field to the outside of the magnet device 8 and the annular main coil 51 made of a superconducting wire, which is arranged inside the refrigerant vessel 43, is disposed inside the refrigerant vessel 43, and generates a main magnetic field. In addition, an annular shield coil 52, which is energized in the opposite direction to the main coil 51 and is made of a superconducting wire, is arranged coaxially so that the respective central axes 13 coincide with the z-axis. The refrigerant container 43 stores a cryogenic refrigerant, for example, liquid helium 44, for keeping the main coil 51 and the shielding coil 52 in superconductivity. The cryogenic refrigerant can maintain a cryogenic temperature by the refrigerator 46 (see FIG. 12A).

  The uniform magnetic field space 3 may have any shape, but in most cases, it is a spherical or flat spheroid. In the fourth embodiment, a spherical space having a radius r is illustrated as the uniform magnetic field space 3. The positive direction of the z-axis is made to coincide with the magnetic field direction 4 in the magnetic field space 3. A plane in which the magnet center 12 that is a point-symmetrical symmetry point of the plurality of main coils 51 is set on the z-axis and includes the magnet center 12 and has the z-axis as a normal line is defined as an equatorial plane 14. The magnet center 12 substantially coincides with the spherical center of the outer shape of the uniform magnetic field space 3.

  Considering the case where the intensity of the magnetic field is measured at a sweep measurement point M0 at an arbitrary point on the outer surface of the uniform magnetic field space 3, the position of the sweep measurement point M0 is the spherical radius r of the outer shape of the uniform magnetic field space 3 And the elevation angle θ formed by the direction from the magnet center 12 to the sweep measurement point M0 and the central axis 13 (z axis) and the angle (azimuth angle) φ around the central axis 13 (z axis). it can.

  The magnetic field uniformity adjusting device (shim tray) 45 is disposed along the inner peripheral surface of the double cylindrical vacuum vessel 41 and has a cylindrical shape. A magnetic material (shim) 17 is embedded and held in the shim tray 45 by screwing or the like, and can be disposed at any place on the shim tray 45. By disposing the shim 17 on the shim tray 45, control (so-called shimming work) for moving the magnetic flux in the uniform magnetic field space 3 is performed, and the uniformity of the magnetic field in the uniform magnetic field space 3 can be improved. For the shim 17, a permanent magnet, iron, or the like can be used.

The position of the shim 17 on the cylinder of the shim tray 45 is determined by the R coordinate in the length direction of the cylinder with the origin (zero) of the shim tray center 24 on the equatorial plane 14 and the angle (azimuth angle) φ around the z axis. It can be determined uniquely. FIG. 12B shows a case where the shim 17 is arranged at the position of R = 0 (zero) and the position of R = _RA on the shim tray 45.

  FIG. 13 shows a schematic diagram of a magnetic resonance imaging apparatus 9 according to the fourth embodiment of the present invention. The magnetic resonance imaging apparatus 9 is equipped with the magnet device 8 of the fourth embodiment, further has a gradient magnetic field coil 48 disposed on the z-axis (center axis 13) side of the magnetic field uniformity adjusting device (shim tray) 45, and An RF transmitting / receiving coil 50 is arranged on the z-axis (center axis 13) side of the gradient magnetic field coil 48. A driving device 22 and a control device 23 for driving the gradient magnetic field coil 48 and the RF transmitting / receiving coil 50 are provided outside the magnet device 8. The gradient magnetic field coil 48 is disposed along the inner peripheral surface of the magnetic field uniformity adjusting device (shim tray) 45, and the RF transmitting / receiving coil 50 is disposed along the inner peripheral surface of the gradient magnetic field coil 48. . There are examples in which the shim tray 45 is disposed in the gap between the vacuum vessel 41 and the gradient magnetic field coil 48 and examples in which the shim tray 45 is disposed in the structural gap of the gradient magnetic field coil 48. Although the essence is not affected, in the fourth embodiment (FIG. 13), an example in which the vacuum vessel 41 and the gradient magnetic field coil 48 are arranged in the gap region is illustrated.

  Since the uniform magnetic field space 3 and the shim tray 45 have the shortest distance at each position on the equator plane 14, the shim 17 is at a position where the shim tray 45 and the equator plane 14 intersect (R = 0), that is, the shim tray center 53 ( 24), the influence of the shim 17 on the magnetic field distribution of the uniform magnetic field space (imaging space) 3 is maximized. Conversely, when the shim 17 is disposed at the shim tray end 54, the influence on the magnetic field distribution of the uniform magnetic field space 3 is minimized. Further, when the shim 17 is disposed between the shim tray center 53 (24) and the shim tray end 54, the influence on the magnetic field distribution of the uniform magnetic field space 3 is the case where the shim tray center 53 (24) is disposed. , And a value between the case where it is arranged at the shim tray end portion 54.

In FIG. 14, in the magnet device 8 of the fourth embodiment, one shim 17 is placed on the shim tray center 24 (R = 0) of the shim tray 45 and at a position R _A away from the top of the shim tray center 24 (R = R _A ). The magnetic field distributions 56 and 57 that the shims 17 form on the surface of the uniform magnetic field space 3 when they are arranged respectively are shown. These magnetic field distributions 56 and 57 can be measured by moving the sweep measurement point M0 in the θ direction while measuring the magnetic field intensity at the sweep measurement point M0 (see FIG. 12B). A magnetic field sensor is disposed at the moving sweep measurement point M0. Note that the magnetic field distributions 56 and 57 may be calculated by simulation instead of this measurement. Further, due to the symmetry around the central axis (z axis) 13 of the magnet device 8, the tendency of the magnetic field distributions 56 and 57 in the θ direction is established at an arbitrary azimuth angle φ.

From FIG. 14, when R = 0, that is, when the shim 17 is placed at the shim tray center 24, the magnetic field distribution 56 has a peak intensity at the peak position θ ₀ , which is higher than that of the magnetic field distribution 57 when R = R _A. The magnetic field distribution has a sharp peak that is larger than the peak intensity at the position θ _A and the half-value half-width dθ ₀ is narrower than the half-value half-width dθ _A. The graph of FIG. 14 corresponds to a graph in which θ is shifted (tilted) by 90 degrees with respect to the graph of FIG. 14 of the first embodiment. That is, similarly to the first embodiment, in order to accurately know the magnetic field distributions 56 and 57 by the sampling theorem, the magnetic field distributions 56 and 57 must be measured at measurement point intervals equal to or less than the half-value half widths dθ ₀ and dθ _A. I must. The half-value half widths dθ ₀ and dθ _A take different values depending on the peak positions θ ₀ and θ _A , that is, the position (R) where the shim 17 is placed on the shim tray 45. That is, the half widths dθ ₀ and dθ _A indicate the maximum values of the measurement point intervals at the peak positions θ ₀ and θ _A and the shim 17 positions R = 0 and R _A. Since the peak positions θ ₀ and θ _A and the positions R = 0 and R _{A of} the shim 17 correspond one-to-one as a physical phenomenon, the peak positions θ ₀ and θ _A represent the position of the shim 17. It can also be used as a substitute for the R value.

  FIG. 15 shows a simulation result of the relationship between the peak position θ of the magnetic field distribution and the half-value half width dθ of the magnetic field distribution on the surface of the uniform magnetic field space 3 at the peak position θ. Furthermore, an example of a magnetic field distribution measurement method for determining the measurement point intervals dθa to dθc of the measurement points M1 to M4 of the magnetic field on the surface of the uniform magnetic field space 3 is shown. The graph of FIG. 15 displays 0 to 90 degrees as the peak position θ, but the values from 90 degrees to 180 degrees are symmetrical with respect to 90 degrees (equatorial plane 14). Then, the graph of FIG. 15 is also obtained by shifting the graph of FIG. 6 of the first embodiment by 90 degrees. In FIG. 15, the scales of the vertical axis and the horizontal axis are different.

According to FIG. 15, the half-value half-width dθ _{A with} the peak position θ being θ _A is larger than the half-value half-width dθmin (dθa = dθ ₀ ) when the peak position θ is around 90 degrees, and further, 0 degree It can be seen that the half width at half maximum dθmax in the vicinity is larger. The half-value half-width dθmax near 0 degrees is approximately three times as large as twice or more the half-value half-width dθmin near 90 degrees. That is, it can be seen that the half-value half-width dθ is a decreasing function that decreases as the peak position θ is between 0 and 90 degrees and the peak position θ increases. The measurement point interval is set to be less than or equal to the half-value half-width dθ as the maximum measurement point interval according to the sampling theorem. Therefore, changing the size of the half-value half-width dθ according to the peak position θ The size can be changed by the peak position θ. Then, by increasing the measurement point interval within an allowable range of the half value half width dθ according to the peak position θ, the number of measurement points can be reduced while maintaining the measurement accuracy of the magnetic field distribution or the accuracy of the interpolation calculation with high accuracy. it can. That is, if the size of the measurement point interval is changed according to the peak position θ (corresponding to the measurement point) so that the measurement point interval becomes equal to or less than the half-value half width dθ derived according to the curve of the graph of FIG. The magnetic field distribution can be measured with high accuracy by the optimum and minimum measurement point arrangement.

A method for determining the measurement point intervals dθa to dθc of the magnetic field measurement points M1 to M4 on the surface of the uniform magnetic field space 3 as a magnetic field distribution measurement method will be described. First, the measurement point M1 is arranged at a position where the peak position θ is 90 degrees (θ = 90 degrees). Next, the half-value half width dθa (= dθmin = dθ ₀ ) at the position where the peak position θ is 90 degrees (θ = 90 degrees) is obtained by reading from the graph of FIG. 15 (or may be obtained by simulation. Similarly, the half value half width dθa is defined as a measurement point interval (dθa) between the measurement point M1 and the measurement point M2. Specifically, the measurement point M2 is arranged at a position away from the measurement point M1 by the measurement point interval (dθa).

  Next, the half-value half-width dθb at the position where the peak position θ is the measurement point M2 (θ = 90−dθa) is obtained by reading from the graph of FIG. 15, and this half-value half-width dθb is measured between the measurement point M2 and the measurement point M3. The point interval (dθb) is used. The half-value half-width dθb is different from the half-value half-width dθa because the peak position θ at the time of reading is different (90−dθa ≠ 90). . Specifically, the measurement point M3 is arranged at a position (θ = 90− (dθa + dθb)) away from the measurement point M2 by the measurement point interval (dθb).

  Similarly, the half-value half width dθc at the peak position θ at the measurement point M3 (θ = 90− (dθa + dθb)) is obtained by reading from the graph of FIG. 15, and this half-value half-width dθc is obtained between the measurement point M3 and the measurement point M4. The measurement point interval (dθc). The half-value half-width dθc is different from the half-value half-widths dθa and dθb because the peak positions θ when read are different (90− (dθa + dθb) ≠ 90, 90− (dθa + dθb) ≠ 90−dθa). dθa, dθc ≠ dθb), and half width at half maximum dθa, dθb (dθc> dθb> dθa). Specifically, the measurement point M4 is arranged at a position (θ = 90− (dθa + dθb + dθc)) away from the measurement point M3 by the measurement point interval (dθc). Since the half-value half-width dθmax near 0 degrees is twice or more than the half-value half-width dθmin near 90 degrees and is almost tripled, the measurement point interval is the minimum value and the maximum value, and the maximum value is minimized. It can be set to about 3 times more than twice the value.

  FIG. 16A shows two-dimensionally an example in which the measurement points M1 to M4 are arranged on the surface of the uniform magnetic field space 3. The operation of arranging the measurement points M1 to M4 is first repeated in the range of the positions of the measurement points M1 to M4 from 90 degrees (θ = 90 degrees) to 0 degrees (θ = 0 degrees). The range where the peak position θ is 90 to 180 degrees can be set as shown in FIG. 16A from the symmetry with respect to the equator plane 14 (see FIG. 12B) of the magnet device 8. That is, since the measurement point M1 is already arranged at the position where the peak position θ is 90 degrees (θ = 90 degrees), the arrangement of the further measurement point M1 is omitted, and then the measurement is performed from the measurement point M1. The measurement point M2 is arranged at a position (θ = 90 + dθa) that is separated from the point interval (dθa). Next, the measurement point M3 is arranged at a position (θ = 90 + dθa + dθb) away from the measurement point M2 by the measurement point interval (dθb). Finally, the measurement point M4 is arranged at a position (θ = 90 + dθa + dθb + dθc) that is separated from the measurement point M3 by the measurement point interval (dθc).

FIG. 16B three-dimensionally shows an example in which the measurement points M1 to M4 are arranged on the surface of the uniform magnetic field space 3. The measurement point interval dφ around the z-axis is equal to the half-value half width dθ ₉₀ (= dθmin, see FIG. 15) when the peak position θ is 90 degrees (θ = 90 degrees) based on the principle such as the sampling theorem described above. (Dφ = dθ ₉₀ ). According to this, the measurement point interval between the adjacent measurement points M3 and M3 can be made longer than the measurement point interval between the adjacent measurement points M4 and M4. Similarly, the measurement point interval between the adjacent measurement points M2 and M2 can be made longer than the measurement point interval between the adjacent measurement points M3 and M3. Moreover, the measurement point interval between the adjacent measurement points M1 and M1 can be made longer than the measurement point interval between the adjacent measurement points M2 and M2. The measurement points M1 to M4 can be optimally arranged, and the number of measurement points M1 to M4 can be minimized.

  FIG. 17 shows a state in which the magnetic field distribution measurement jig 32 according to the fourth embodiment of the present invention is installed in the magnet device 8 during the magnetic field distribution measurement. In the fourth embodiment, the same magnetic field distribution measurement jig 32 as in the first embodiment can be used. However, in the fourth embodiment, the z-axis is in the horizontal direction as in the third embodiment. Since it faces, the seat 29 is changed so as to be rotatable around the z axis that is the horizontal axis.

  Also in the fourth embodiment, the holder 28 is provided with the holes 31 at the positions of the measurement points M1 to M4 along an arc having the same size as the arc on the surface of the uniform magnetic field space 3. The measurement point intervals of the measurement points M1 to M4 are set to be the same as the measurement point intervals dθa to dθc described in the magnetic field distribution measurement method of the fourth embodiment. For this reason, the interval between the adjacent holes 31 of the holder 28 is narrower in the vicinity of the equator plane 14 (θ = 90 degrees) away from the z axis than in the vicinity of the z axis (θ = 0 degrees and 180 degrees). Has appearance characteristics. By using such a magnetic field distribution measurement jig 32, it is possible to measure the magnetic field distribution at the measurement point intervals dθa to dθc that vary depending on the position θ.

1, 2 Magnetic pole 3 (Uniform) Magnetic field space (imaging area)
4 Direction of magnetic field in magnetic field space 8 Magnet device 9 Magnetic resonance imaging device 12 Magnet center (magnet symmetry point)
13 Central axis 14 Equatorial plane (magnet symmetry plane)
15, 16 Magnetic field uniformity adjustment device (Shim tray)
17 Magnetic material (Shim)
24, 25 Shim tray center (R = 0)
28 Holder 29 Seat 30 Magnetic field sensor 31 Hole provided in holder 32 Magnetic field distribution measurement jig 33 Meniscus holder 44 Liquid helium 45 Magnetic field uniformity adjusting device (shim tray)
53 Center of shim tray 54 End of shim tray

Claims (11)

The magnetic field space in a magnet device comprising: a magnetic field generation source that generates a magnetic field in the magnetic field space; and a magnetic field uniformity adjusting device that non-uniformly arranges a plurality of magnetic materials to improve the uniformity of the magnetic field in the magnetic field space. In the magnetic field distribution measurement method of
While changing the position of the magnetic material arranged in the magnetic field uniformity adjusting device, obtain the magnetic field distribution that the magnetic material creates on the surface of the magnetic field space,
Set a half-width function of the magnetic field distribution with respect to the peak position of the magnetic field distribution,
The interval between a plurality of measurement points that are arranged on the surface of the magnetic field space and measure the magnetic field is equal to or less than the half-value half width derived by the function from the peak position that coincides with the position where the measurement point is placed. A magnetic field distribution measuring method, wherein the distance is changed according to the position of the measurement point.   A plurality of angles formed by the direction of the magnetic field generated in the magnetic field space and the respective directions from the center of the magnetic field space to the plurality of measurement points are not equal to each other. The magnetic field distribution measurement method according to claim 1. The magnetic field homogeneity adjusting device has a pair of disks arranged to face the direction of the magnetic field so as to sandwich the magnetic field space, and supports a plurality of the magnetic materials,
3. The magnetic field distribution measurement method according to claim 2, wherein the interval when the angle is approximately 90 degrees is wider than the interval when the angle is approximately 0 degrees and approximately 180 degrees.   4. The magnetic field distribution measurement method according to claim 3, wherein the interval when the angle is approximately 90 degrees is at least twice the interval when the angle is approximately 0 degrees and approximately 180 degrees. The magnetic field uniformity adjusting device has a cylinder that supports the plurality of magnetic materials, a central axis is parallel to the direction of the magnetic field, and surrounds the magnetic field space,
The magnetic field distribution measurement method according to claim 2, wherein the interval at the angle of approximately 0 degrees and approximately 180 degrees is wider than the interval at the angle of approximately 90 degrees.   6. The magnetic field distribution measurement method according to claim 5, wherein the interval when the angle is approximately 0 degrees and approximately 180 degrees is at least twice the interval when the angle is approximately 90 degrees.   A plurality of magnetic materials are formed based on a measurement result obtained by measuring a magnetic field generated by the magnetic field generation source at the measurement point arranged by the magnetic field distribution measurement method according to any one of claims 1 to 6. The magnet apparatus characterized by improving the uniformity of the magnetic field in the magnetic field space by the arranged magnetic field uniformity adjusting device. The magnetic field space in a magnet device comprising: a magnetic field generation source that generates a magnetic field in the magnetic field space; and a magnetic field uniformity adjusting device that non-uniformly arranges a plurality of magnetic materials to improve the uniformity of the magnetic field in the magnetic field space. In the magnetic field distribution measurement jig of
When the position of the magnetic material to be arranged in the magnetic field uniformity adjusting device is changed, the magnetic field distribution with respect to the peak position of the magnetic field distribution set by using the magnetic field distribution that the magnetic material creates on the surface of the magnetic field space. Using a half-value half-width function, the interval between a plurality of measurement points arranged on the surface of the magnetic field space and measuring the magnetic field is derived by the function from the peak position that coincides with the position where the measurement point is placed. A holder for fixing a magnetic field sensor to the measurement point where the size of the interval is changed according to the position of the measurement point so as to be equal to or less than a half value half width;
A jig for measuring a magnetic field distribution, comprising: a seat configured to be able to rotate the holder around a rotation axis parallel to a direction of a magnetic field in the magnetic field space.   9. The magnetic field distribution measuring jig according to claim 8, wherein the magnetic field sensor is arranged at each of the plurality of measurement points on the holder.   A plurality of magnetic materials are arranged based on a measurement result obtained by measuring a magnetic field generated by the magnetic field generation source at the measurement point set by the magnetic field distribution measurement jig according to claim 8 or 9. A magnet apparatus characterized in that the homogeneity of a magnetic field in the magnetic field space is improved by a magnetic field uniformity adjusting device.   A magnetic resonance imaging apparatus equipped with the magnet device according to claim 7 or 10.

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