JPH08215168A - Magnetic field uniformity adjustment method and its device - Google Patents

Abstract

PURPOSE: To materialize the magnetic field ununiformity adjustment method by a passive shimming method and its device, which reduces labor and time required by shimming operations. CONSTITUTION: As a magnetic body for adjusting a magnetic ununiformity to be disposed close to an adjusted magnetic field, each shim pack 412 combining the magnetic body with a temperature adjusting means is used, and its permeability is changed by adjusting the temperature of the magnetic body by the temperature adjusting means, so that the ununiformity of the adjusted magnetic field is thereby adjusted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic field homogeneity adjusting method and apparatus. More specifically, the present invention adjusts the homogeneity of the magnetic field to be adjusted by changing the magnetic permeability of the homogeneity adjusting magnetic body according to temperature when adjusting the homogeneity of the magnetic field by a passive shimming method. The method and apparatus for adjusting magnetic field homogeneity.

[0002]

2. Description of the Related Art For example, the magnetic field of a medical magnetic resonance imaging apparatus is required to have a magnetic field homogeneity of about several ppm in a spherical region having a diameter of 40 cm or a cylindrical region having a diameter of 40 cm and a length of 40 cm. Fine adjustment of magnetic field (shimming (shimm
ing)) is performed.

As one of the shimming methods, there is a passive shimming method which uses a shim plate made of a magnetic material having a high magnetic permeability. This method adjusts the homogeneity of the magnetic field by adjusting the volume and the arrangement of the shim plate, and in view of the advantage that the magnetic field can be adjusted by a passive element that does not generate the magnetic field itself. , Widely used.

A conventional magnetic field homogeneity adjusting operation by the passive shimming method will be described with reference to FIGS. FIG.
In 0, 10 is a cylindrical magnetic field generator using, for example, a superconducting magnet, 11 is a spherical region in which the magnetic field should be uniform, 2
Reference numeral 0 denotes a shim mechanism provided near the internal magnetic field space of the magnetic field generator 10.

The shim mechanism 20 includes a plurality of shim units (shi
m unit) 21 and multiple trays containing them
It is composed of 22. The plurality of shim units 21 inserted in the plurality of trays 22 have their longitudinal directions aligned with the magnetic field generator 1.
The internal magnetic field space of the magnetic field generator 10 is surrounded by a cylinder so as to be parallel to the central axis of 0.

The tray 22 is non-magnetic, and the shim unit 21 can be put in and taken out of the tray 22. As shown in FIGS. 11 and 12, the shim unit 21 includes a shim holder (shi
m holder) 211 and a shim plate 212 attached to a plurality of places on it. The shim plate 212 is composed of a rectangular plate made of a magnetic material having high magnetic permeability, and a plurality of types having different vertical and horizontal lengths and different thicknesses are prepared.

The shim plate 212 has a pair of holes 213 and a pair of pins 214 planted in the shim holder 211.
Can be installed using. A plurality of shim plates can be stacked and mounted at each mounting position, and they are fixed by an appropriate fastener (not shown). Pin 214
Both fasteners are non-magnetic. The thickness and number of the shim plates 212 to be attached determine the volume of the shim plates 212 at that position.

The volume of the shim plate 212 attached to each attachment position of the shim holder 211 is determined according to the correction amount of magnetic field nonuniformity. For that decision, FIG.
The following shimming device is used. In FIG. 13, 30
Is a probe for detecting a magnetic field, 31 is a probe positioning device, 32 is a magnetic field measuring device, 33 is a shim calculation device, and 34 is an indicating device.

The probe 30 is a probe positioning device 3
1, the magnetic field space is sequentially positioned at a plurality of predetermined measurement points, and the magnetic field at each measurement point is detected. As the measurement points, for example, as shown in FIG. 14, a spherical region 11 having a diameter of 40 cm, which is concentric with the magnetic field center 110, is equally spaced along the magnetic field axis direction.
Divide into 5 planes, 15 for each plane
The point defined by the angle step of ° is used.

Based on the detection signal of the probe 30, the magnetic field measuring device 32 determines the magnetic field at each measurement point. Magnetic field measuring device 3
The shim calculation device 33 calculates the shimming amount for correcting the magnetic field inhomogeneity based on the magnetic fields of the respective measurement points given from 2. The amount of shimming is determined by which part of the shim mechanism 20
How much volume of the magnetic body is arranged, that is, it is obtained as the number of shim plates attached at each attachment position in the shim mechanism 20. An algorithm for performing such calculation has been established as a passive shimming algorithm and is commonly used.

The shim plate 2 obtained by the shim calculation device 33
The arrangement of the 12 volumes is instructed by the instruction device 34. The indicating device 34 indicates the number of shim plates 212 to be attached together with the plate thickness for each shim plate attaching position of the shim holder 211 for all the shim units 21.

According to this instruction, the worker attaches the shim plate 212 to each shim unit 21.
The operation is to take out the plurality of shim units 21 one by one from the tray 22, attach shim plates 212 of the plate thickness and the number of sheets as instructed to each attachment position of the shim holder 211, and return them to the tray 22. In the magnetic field generator of the medical magnetic resonance imaging apparatus, there are more than ten shim units 21 and several tens of shim plate attachment positions in each unit, so that the installation work reaches several hundreds in total.

After this work is completed, the magnetic field is measured in the spherical region 11 and the degree of achievement of the magnetic field homogeneity is inspected.
At this time, if the required uniformity has not been reached yet, the shim calculation device 33 calculates an additional shimming amount, and the instruction device 34 is instructed to do so. Perform plate mounting work. The worker repeats such work until the required magnetic field homogeneity is achieved.

[0014]

In such a shimming method, several hundred shim plate mounting operations are required for one adjusting operation, and this is repeated until the required uniformity is obtained. However, it requires a lot of work and takes a lot of time. In particular, the magnetic field generator of the medical magnetic resonance imaging apparatus has a large frame and a large and heavy shim unit, which increases the work effort.

The present invention has been made to solve the above problems, and an object of the present invention is to realize a magnetic field homogeneity adjusting method and apparatus by the passive shimming method which reduces the labor and time of the shimming work.

[0016]

The first means for solving the above-mentioned problems is a method of adjusting the homogeneity of the magnetic field by a magnetic body arranged in the vicinity of the magnetic field to be adjusted. Is a method for adjusting magnetic field homogeneity, which is characterized by changing the temperature according to the temperature.

A second means for solving the above-mentioned problems is an apparatus for adjusting the homogeneity of the magnetic field by a magnetic body arranged in the vicinity of the magnetic field to be adjusted, and means for changing the magnetic permeability of the magnetic body according to the temperature. It is a magnetic field homogeneity adjusting device characterized by having.

In the second means for solving the above-mentioned problems, the means for changing the magnetic permeability of the magnetic body depending on temperature is to keep the magnetic body at a temperature at which the desired magnetic permeability is exhibited. It is preferable in that the shimming is performed.

In the second means for solving the above-mentioned problems, the means for changing the magnetic permeability of the magnetic body according to the temperature gives a temperature history to the magnetic body to generate a temperature history in the magnetic permeability. However, it is preferable in that shimming is performed with low power consumption.

[0020]

In the first means for solving the problem, the magnetic permeability of the magnetic material is changed according to the temperature to adjust the magnetic field homogeneity.

In the second means for solving the problem, the means for changing the temperature of the magnetic body adjusts the magnetic field homogeneity by changing the magnetic permeability of the magnetic body according to the temperature.

[0022]

Embodiments of the present invention will be described below in detail with reference to the drawings. 1 to 3 show the configuration of the main part of the apparatus according to the embodiment of the present invention. 1 is a plan view, FIG. 2 is a cross-sectional view, and FIG. 3 is a block diagram of an electrical configuration. FIG. 4 shows a conceptual configuration of a magnetic field generator incorporating the device of the present invention. FIG. 5 shows a conceptual configuration of the magnetic field homogeneity adjusting device. The method of the embodiment of the present invention is shown by the operation of the apparatus of the embodiment of the present invention.

First, referring to FIG. 4, a magnetic field generator incorporating the apparatus of the embodiment of the present invention will be described. In FIG. 4, reference numeral 10 is a cylindrical magnetic field generating device using, for example, a superconducting magnet, 11 is a spherical region where a magnetic field should be uniform, and 40 is an embodiment of the present invention provided near the internal magnetic field space of the magnetic field generating device 10. It is a shim mechanism.

The shim mechanism 40 includes a plurality of shim units 41.
And a plurality of trays 42 for accommodating them.
Plural shim units 41 inserted in plural trays 42
Surrounds the internal magnetic field space of the magnetic field generator 10 in a cylindrical shape with its longitudinal direction parallel to the central axis of the magnetic field generator 10. The tray 42 is non-magnetic.

The shim unit 41, as shown in FIG.
A shim holder 411 composed of an elongated plate made of a non-magnetic material
And a shim pack 412, each attached to a plurality of attachment points thereon.

The shim pack 412 is shown in FIG.
As shown in, the shim bar 413 and the heater
(heater) 414, heat insulating cover 415, and temperature detector 4
16 and. The shim bar 413 is made of a magnetic material having a high magnetic permeability, and is configured as a rectangular parallelepiped having a constant size. A heater 414 is in contact with the shim bar 413, and they are covered with a heat insulating cover 415.

FIG. 3 shows a heater 414 and a temperature detector 41.
6 shows electrical connections for 6. In FIG. 3, 51 is a temperature measuring device, 52 is a heater power supply, 53 is a temperature control device, and 62 is a temperature setting device.

Temperature detector 41 for all shim packs 412
6 is connected to the temperature measuring device 51, and the temperature measuring device 51 measures the temperature of the shim bar 413 of each shim pack 412. The heaters 414 of all the shim packs 412 are connected to the heater power supply 52, and the heater power supply 52 supplies an electric current to each heater 414.

The temperature control device 53 controls each heater power source 52 based on the difference between the temperature of each shim bar 413 measured by the temperature measuring device 51 and the temperature set value set by the temperature setting device 62, thereby controlling each heater power source 52. The temperature of the shim bar 413 is made to match the temperature set value. The temperature setting device 62 will be described later again.

When the relationship between the current flowing through the heater 414 and the temperature of the shim bar 413 is known in advance, the temperature measurement is unnecessary and the temperature detector 416 and the temperature measuring device 51 can be omitted.

Such a shim pack 412 utilizes the phenomenon that the magnetic permeability of the magnetic material changes with temperature, and the heater 414 changes the temperature of the shim bar 413 to change its magnetic permeability and change the volume of the shim bar 413. It has the same effect as. The relationship between the temperature and the magnetic permeability can be obtained from the temperature characteristic of the magnetic permeability of the magnetic material used as the shim bar 413.

When a magnetic material whose magnetic permeability decreases with increasing temperature is used, the shim bar 413 is dimensioned so as to have a volume that gives the necessary maximum value of the shimming amount at room temperature, and the magnetic permeability decreases due to heating. Also, the effective volume is reduced.

FIG. 5 is a block diagram of an example of an apparatus for adjusting the magnetic field homogeneity of the magnetic field generator 10 using the shim mechanism 40. In FIG. 5, 30 is a probe for magnetic field detection,
31 is a probe positioning device, 32 is a magnetic field measuring device, 6
Reference numeral 1 is a shim calculation device, and 62 is a temperature setting device.

The probe 30 is a probe positioning device 3
1, the magnetic field is sequentially positioned at a plurality of measurement points in the magnetic field space, and the magnetic field at each measurement point is detected. As the measurement points, for example, as shown in FIG. 14, a spherical region 11 having a diameter of 40 cm and concentric with the center of the magnetic field is divided into 15 planes at equal intervals in the magnetic field axis direction, and each plane has an angle of 15 °. Use the points defined in the step.

The magnetic field measuring device 32 obtains the magnetic field at each measurement point based on the detection signal of the probe 30. Known devices are used as the probe 30, the probe positioning device 31, and the magnetic field measuring device 32.

The shim calculation device 61 calculates a shimming amount for correcting the magnetic field inhomogeneity based on the magnetic field at each measurement point given from the magnetic field measuring device 32, and makes the shimming amount into a magnetic permeability corresponding to the shimming amount. After conversion, the temperature corresponding to the magnetic permeability is obtained from the temperature characteristic of the magnetic material that is known in advance.

The shimming amount calculation by the shim calculation device 61 is performed by an algorithm similar to the conventional shimming algorithm, and the volume and arrangement of the shim bars 413 are obtained. Then, the conversion from the shimming amount to the magnetic permeability is performed by calculating the magnetic permeability of the shim bar 413 for matching the effective volume with the calculated volume.

The temperature setting device 62 supplies the temperature value obtained by the shim calculation device 61 to the temperature control device 53 as a set value for temperature control of each shim pack 412. The shim calculation device 61 and the temperature setting device 62 are composed of, for example, a computer and its input / output device.

The operation of the thus constructed apparatus of the embodiment of the present invention will be described below with reference to the flowchart of FIG. The operation is started from step 70, and in step 71, the magnetic field is measured by the magnetic field measuring device 32 while the probe positioning device 31 sequentially positions the probe 30 at a predetermined measurement point, and the homogeneity of the magnetic field is obtained.

Next, at step 72, the shim calculation device 6
1 calculates the shimming amount for correcting the magnetic field inhomogeneity, calculates the control temperature of each shim pack 412 corresponding to the shimming amount, and the temperature setting device 62 causes the temperature controller 53 to set the temperature set value of each shim pack 412. Give as.

Next, in step 73, the temperature of each shim pack 412 is controlled in accordance with the temperature set value, so that each shim bar 413 is kept at the set temperature and has a magnetic permeability corresponding to that.

Next, in step 74, the probe 3
0 and the magnetic field measuring device 32 measure the magnetic field to determine the homogeneity of the magnetic field. Next, in step 75, it is determined whether the magnetic field homogeneity is within a predetermined specification, and if it is not within the specification, the process returns to step 72, and the operations of steps 72 to 75 are repeated until the specification is achieved.

When the magnetic field homogeneity is within the specifications, step 7
The procedure ends in 6 and the operation is completed. In this way, the shimming is performed by the automatic control, and the shim unit 40 does not require any attachment / detachment work of the shim unit or the shim bar. Therefore, there is an effect that labor and time are significantly reduced as compared with the conventional method.

The construction of the shim unit of another embodiment of the present invention is shown in FIGS. 7 is a plan view and FIG. 8 is a sectional view. The shim unit 41 of this embodiment has substantially the same configuration as that of the embodiment of FIG.
Is different in that it does not have an insulating cover.

Further, as the shim bar 413, a magnetic material whose magnetic permeability has a hysteresis characteristic with respect to temperature is used. Due to the temperature history characteristic of permeability, the shim bar 413
Since the magnetic permeability at room temperature (hereinafter referred to as "residual magnetic permeability") changes according to the temperature reached during heating, shimming is performed using this.

Further, the shim calculation device 61 obtains the magnetic permeability corresponding to the shimming amount in accordance with the characteristics of the shim bar 413, and then reaches the heating for matching the "residual magnetic permeability" to it. A function to find the temperature has been added.
The reached temperature of such heating is given to the temperature control device 53 through the temperature setting device 62 as the temperature set value of each shim pack 412.

The shimming operation according to this embodiment will be described with reference to the flowchart of FIG. The operation is started from step 80, and in step 81, the magnetic field is measured by the magnetic field measuring device 32 while the probe positioning device 31 sequentially positions the probe 30 at a predetermined measurement point, and the homogeneity of the magnetic field is obtained.

Next, at step 82, the shim calculation device 6
1 calculates the shimming amount for correcting the magnetic field inhomogeneity, calculates the control temperature of each shim pack 412 corresponding to the shimming amount, that is, the reached temperature of heating, and the temperature setting device 6
2 to the temperature control device 53 as a temperature set value of each shim pack 412.

Next, at step 83, the temperature controller 5
3 controls the temperature of each shim pack 412 according to the set value, so that the temperature of each shim bar 413 matches the set temperature.

Next, at step 84, each shim pack 4
The temperature control of the temperature detector 414 of No. 12 is stopped, the temperature of the shim bar 413 is returned to normal temperature, and the magnetic permeability of the shim bar 413 is made into "residual magnetic permeability".

Next, in step 85, the probe 3 is again used.
0 and the magnetic field measuring device 32 measure the magnetic field to determine the homogeneity of the magnetic field. Next, at step 86, it is judged whether the magnetic field homogeneity is within a predetermined specification, and if it is not within the specification, the process returns to step 82, and the operations of steps 82 to 86 are repeated until the specification is satisfied.

When the magnetic field homogeneity is within the specifications, step 8
The procedure ends in 7 and the operation ends. In this way, shimming is performed by automatic control. According to this embodiment, in addition to the effects of the above-described embodiment, it is possible to obtain the effect that it is not necessary to constantly energize the heater 414 of the shim pack 412.

In the above embodiments, as a concrete example of the means for changing the magnetic permeability of the magnetic material according to the temperature,
I explained an example of heating a magnetic material with a heater,
The means for changing the magnetic permeability of the magnetic material depending on the temperature may be one that lowers the temperature by a cooler, or one that performs both heating and cooling by combining a heater and a cooler.

Further, the shim mechanism is not limited to the one shown in FIG. 4, and, in short, as long as it adjusts the homogeneity of the magnetic field by a magnetic body arranged in the vicinity of the magnetic field to be adjusted, The invention can be applied to perform shimming.

Further, the measurement of the homogeneity of the magnetic field is performed by using a phantom instead of the probe as shown in FIG.
And a pulse sequence for magnetic field homogeneity measurement may be used.

[0056]

As described in detail above, according to the present invention, the magnetic permeability of the magnetic material is changed according to the temperature and the magnetic field homogeneity is adjusted accordingly, so that automatic shimming can be performed. The effect that the magnetic field homogeneity can be adjusted by the passive shimming method, which greatly reduces the labor and time of work, is obtained.

[Brief description of drawings]

FIG. 1 is a plan view showing a configuration of a main part of an apparatus according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a configuration of a main part of an apparatus according to an embodiment of the present invention.

FIG. 3 is a block diagram showing an electrical configuration of a main part of an apparatus according to an embodiment of the present invention.

FIG. 4 is a conceptual configuration diagram of a magnetic field generator incorporating the device of the embodiment of the present invention.

FIG. 5 is a block diagram of a magnetic field homogeneity adjusting apparatus using the apparatus of the embodiment of the present invention.

FIG. 6 is a flowchart of a magnetic field homogeneity adjusting operation according to the method of the embodiment of the present invention.

FIG. 7 is a plan view showing a configuration of a main part of an apparatus according to another embodiment of the present invention.

FIG. 8 is a cross-sectional view showing a configuration of a main part of an apparatus according to another embodiment of the present invention.

FIG. 9 is a flowchart of a magnetic field homogeneity adjusting operation according to a method of another embodiment of the present invention.

FIG. 10 is a conceptual configuration diagram of a magnetic field generator incorporating a conventional shim mechanism.

FIG. 11 is a plan view showing a configuration of a main part of a conventional shim mechanism.

FIG. 12 is an elevational view showing a configuration of a main part of a conventional shim mechanism.

FIG. 13 is a block diagram of a magnetic field homogeneity adjusting device using a conventional shim mechanism.

FIG. 14 is a conceptual diagram of a spherical region in which magnetic field measurement for uniformity adjustment is performed.

[Explanation of symbols]

 10 magnetic field generator 40 shim mechanism 41 shim unit 42 tray 411 shim holder 412 shim pack 413 shim bar 414 heater 415 heat insulating cover 416 temperature detector 30 probe for magnetic field detection 31 probe positioning device 32 magnetic field measuring device 51 temperature measuring device 52 heater power supply 53 temperature Control device 61 Shim calculation device 62 Temperature setting device

Front Page Continuation (72) Inventor Hideaki Uno 127 Gee Yokogawa Medical Systems Co., Ltd. 4-7 Asahigaoka, Hino City, Tokyo (72) Inventor Fumihiro Yoshizawa 127 Gee Yoko 4 Asahigaoka, Hino City, Tokyo Kawa Medical System Co., Ltd. (72) Inventor Kazuhiro Kono 127, 4-7 Asahigaoka, Hino City, Tokyo GE Yokogawa Medical System Co., Ltd. (72) Inventor Kenji Sato, 4-7 Asahigaoka, Hino City, Tokyo 127 GE Yokogawa Medical System Co., Ltd. (72) Inventor Yusuke Ito 4-7 Asahigaoka, Hino City, Tokyo 127 GE Yokogawa Medical System Co., Ltd. No. 127 GE Yokogawa Medical System Co., Ltd. (72) Inventor Koji Kato 127 GE Yokogawa Medical Systems, 4-7 Asahigaoka, Hino City, Tokyo Inside Tem Co., Ltd.

Claims (4)

[Claims] 1. A method for adjusting the homogeneity of the magnetic field by a magnetic body arranged near the magnetic field to be adjusted, wherein the magnetic permeability of the magnetic body is changed according to the temperature. 2. An apparatus for adjusting the homogeneity of the magnetic field by a magnetic body arranged in the vicinity of the magnetic field to be adjusted, comprising means for changing the magnetic permeability of the magnetic body according to temperature. Adjustment device. 3. The magnetic field homogeneity adjusting device according to claim 2, wherein the means for changing the magnetic permeability of the magnetic body according to temperature keeps the magnetic body at a temperature at which a desired magnetic permeability is exhibited. 4. The magnetic field homogeneity adjusting device according to claim 2, wherein the means for changing the magnetic permeability of the magnetic body according to temperature gives a temperature history to the magnetic body to generate a temperature history in the magnetic permeability.