JP5536429B2 - Magnetic resonance imaging apparatus and magnetic field homogeneity adjustment method of the apparatus - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) that images a desired portion of a subject using a magnetic resonance phenomenon, and more particularly to a technique for adjusting a static magnetic field having a desired uniformity in an imaging space.

  The MRI apparatus measures the NMR signal obtained from the magnetic resonance phenomenon (hereinafter referred to as “NMR phenomenon”) of the nuclear spin in the subject placed in the imaging space in a uniform static magnetic field, and The spin density distribution, relaxation time distribution, and the like are displayed as tomographic images. As a magnetic field generation source of the MRI apparatus, a static magnetic field generation apparatus that generates the static magnetic field and a gradient magnetic field generation apparatus that generates a gradient magnetic field for giving positional information to an NMR signal are used. At this time, if the uniformity of the static magnetic field generated by the static magnetic field generator fluctuates, the linearity of the gradient magnetic field superimposed on it deteriorates and the positional information shifts, resulting in distortion or loss on the image. This will impair the accuracy and clarity of the image, which is a major obstacle to diagnosis. For this reason, it is very important to maintain the uniformity of the static magnetic field.

  Since a static magnetic field generator is required to have a high uniformity in an imaging space, a long-term stable and high magnetic field, it is common to use a superconducting magnet. Furthermore, as a superconducting magnet, a cylindrical magnet that is highly efficient for generating a high magnetic field is widespread.

  Inside the cylindrical superconducting magnet, a plurality of superconducting coils are arranged inside a cryogenic vessel or a cryogenic vessel filled with liquid helium or other cryogenic cryogen. Furthermore, a radiation shield and a vacuum layer are provided outside the cryostat or the cryocontainer to insulate heat from entering from the outside. Usually, a refrigerator for maintaining a low temperature is connected to the cryostat or the cryocontainer.

  The superconducting magnet is required to have a highly uniform static magnetic field in which the difference between the maximum value and the minimum value of the magnetic field is 3 ppm or less, for example, in the imaging space near the center of the magnetic field. However, in practice, it is difficult to obtain the highly uniform static magnetic field due to manufacturing dimensional errors at the time of manufacturing the super-electric magnet. Furthermore, the uniform magnetic field is disturbed by the magnetic field of the building materials arranged around the superconducting magnet due to the leakage magnetic field from the superconducting magnet and the influence of the ferromagnetic material such as a magnetic shield provided to suppress the leakage magnetic field. (Hereinafter, the disturbance of the magnetic field is referred to as an irregular magnetic field). For this reason, the superconducting magnet for the MRI apparatus is provided with means for finely adjusting the magnetic field in the imaging space (hereinafter referred to as shimming).

  Shimming has a cylindrical hollow space (hereinafter referred to as a cylindrical bore) in the inner periphery of a cylindrical superconducting magnet, and is approximately 45-50 cm in diameter at a substantially central position inside the cylindrical bore. This is an operation to adjust the magnetic field uniformity of a uniform magnetic field space that is spherical. As one means for performing this shimming, Patent Document 1 discloses a passive shimming means that uses a ferromagnetic shim piece (hereinafter referred to as a magnetic shim) made of a magnetic material having a high magnetic permeability. . This means is that a magnetic shim of an appropriate amount is arranged at an appropriate position in the magnetic field generated by the superconducting magnet, and the magnetic field in the substantially spherical magnetic field space is disturbed by using the magnetic field generated by the magnetization of the magnetic shim. To make the magnetic field strength uniform.

  In the passive shimming, generally, a magnetic shim such as iron or an electromagnetic steel plate having a thin plate shape having a width of several centimeters, a length of several centimeters, and a thickness of several millimeters to several centimeters is used. A shim tray for storing a magnetic shim has a strip shape made of a non-magnetic material such as resin, and has a plurality of pockets for storing magnetic shims. A shim tray containing a magnetic shim in each shim pocket is inserted from the opening side of the cylindrical bore, and is fixed around the uniform magnetic field space inside the cylindrical bore.

  At this time, shim trays are arranged on the inner periphery of the columnar bore at substantially constant intervals for every certain angle. It is possible to adjust the magnetic field in the uniform magnetic field space by changing the thickness and number of magnetic shims arranged in each shim pocket.

  In the shimming, first, a superconducting magnet is excited in a state where a magnetic shim is not attached, and static magnetic fields at many points in a uniform magnetic field space are measured to evaluate the uniformity of the static magnetic field. In general, a magnetic field component indicating the uniformity of a static magnetic field is expressed using a Legendre function expansion term, and is referred to by each (n, m) term (m and n are 0 and a positive integer), respectively. At this time, the (0,0) term is a required uniform component, and the other terms are irregular magnetic field components indicating disturbance of the static magnetic field. Furthermore, among the irregular magnetic field components, the expansion term ((n, 0) term) where m = 0 (n ≠ 0) has a magnetic field component that is axisymmetric with respect to the central axis of the cylindrical bore, which is axisymmetric. It is called an ingredient. On the other hand, the expansion term (n, m) term in which m ≠ 0 (n ≠ 0) is referred to as a non-axisymmetric component.

  Next, in order to minimize the irregular magnetic field component of the developed term as much as possible, the arrangement position and the arrangement amount of the magnetic substance shim arranged in each pocket of each shim tray are obtained by optimization calculation. Based on the optimization calculation result, the magnetic material shim is placed in each pocket of the shim tray. At this time, since a large number of magnetic shims are usually arranged in each shim tray, when the magnet is in an excited state, an extremely strong electromagnetic attraction force works. When the shim tray that accommodates the shim amount obtained by the optimization calculation is disposed at a desired position, it is extremely difficult to ensure the safety of the operator who adjusts the magnetic field. For this reason, the super-electric magnet is once demagnetized, and a shim tray in which a large amount of magnetic material is arranged is arranged at a shim tray installation location provided in the superconducting magnet. Next, the superconducting magnet is excited again, the static magnetic field strength is measured, and the uniformity of the static magnetic field strength is evaluated. Usually, there is an influence of the calculation accuracy of the optimization calculation due to variations in the magnetization of the magnetic shim placed in the shim tray and manufacturing errors of the shim tray. Shimming is required. For this reason, the amount and position of the magnetic shim are optimized and calculated again, and the re-demagnetization of the superconducting magnet → the arrangement of the magnetic shim → re-excitation of the superconducting magnet → re-measurement of the magnetic field strength is made uniform. Repeat until you get sex. When the desired uniformity of the static magnetic field is obtained, each shim tray having the magnetic shim amount at this time is arranged at the location where the superconducting magnet is installed, and the shimming operation is completed.

JP 2008-289703 A

  As described above, in the shimming process, a large number of magnetic shims are arranged on each shim tray, and when the superconducting magnet is in an excited state, a strong electromagnetic attractive force acts on each shim tray. For this reason, it is necessary to demagnetize the superconducting magnet so that an electromagnetic attractive force does not act on the magnetic material shim before the magnetic material shim is placed, that is, when each shim tray is removed and attached during shimming. However, demagnetization and excitation during the shimming process consume a large amount of cryogen such as liquid helium for quenching the superconducting magnet and keeping the superconducting magnet in the superconducting state.

  In general, the superconducting state becomes stable when the permanent current mode is entered, but during re-excitation, it becomes unstable when the current rises or when the permanent current switch is turned on and off, and the heat of the power lead that supplies current to the superconducting coil is generated. May lead to quenching. Further, during excitation and demagnetization, a large amount of helium is consumed for the following reason. That is, when power is supplied to the superconducting coil in the low temperature bath, a power lead connected from the normal temperature part to the cryogenic part is used. A conductive material such as copper is used for the power lead, but a conductive material such as copper having resistance generates Joule heat during energization. Since this Joule heat may cause quenching of the superconducting coil, the power lead is cooled to near cryogenic temperature and the resistivity of the conductive material such as copper used for the power lead is sufficiently lowered to reduce the Joule heat. It is necessary to suppress it.

  The power lead is generally cooled by using a cryogen gas such as liquid helium that cools the superconducting coil, and this is used as a cooling gas for the power lead. For this reason, the cryogen is gasified, that is, evaporated during excitation and demagnetization. The power lead is connected to the cryogenic temperature from the normal temperature part, but in order to energize a large current of several hundred amperes, it is necessary to increase the cross-sectional area of the energization part. However, when the cross-sectional area of the current-carrying part is increased, the structure is such that heat is easily transferred from the normal temperature part to the cryogenic part, and the cryogen in the cryogenic part evaporates during excitation and demagnetization.

  Thus, during excitation and demagnetization, a large amount of cryogen such as liquid helium in the cryogenic part is consumed, and therefore it is necessary to replenish a large amount of cryogen such as liquid helium after completion of excitation and demagnetization. Furthermore, when liquid injection after completion of excitation, if the high-temperature helium, etc., gasified by cryogen accidentally touches the superconducting coil, the superconducting coil is quenched, and replenishment of cryogen, cooling of the superconducting coil, and excitation are performed again. It needs to be repeated. Moreover, the quenching of the superconducting magnet causes an increase in the number of installation steps of the apparatus, the installation process is greatly delayed, and the customer cannot use the apparatus for a certain period. Furthermore, repeated excitation and demagnetization using a large amount of cryogen such as liquid helium also leads to an increase in installation costs.

  For the above reasons, it is desirable to minimize the number of times of demagnetization and excitation accompanying the placement work of the magnetic material shim in the shimming process. However, on the other hand, as described above, shimming is affected by variations in magnetization of the magnetic shim placed on the shim tray, manufacturing errors, calculation accuracy of optimization calculation, and the like. It was necessary to asymptotically once, and a plurality of shimming times were essential.

  Therefore, the object of the present invention is to reduce the number of times of shimming, thereby reducing the number of demagnetization and excitation of the superconducting magnet, preventing quenching of the superconducting magnet and reducing the consumption of cryogen such as liquid helium, and the shimming adjustment man-hours There are provided a magnetic resonance imaging apparatus and a method for adjusting the magnetic field uniformity of the apparatus.

  To achieve the above object, the present invention provides a first shim tray that houses a desired amount of magnetic shim, and a second shim tray that contains a magnetic shim amount that is smaller than the magnetic shim amount of the first shim tray. Using at least two shim trays and a shim tray, the first shim tray was installed in the inner circumferential direction of a cylindrical superconducting magnet that generates a static magnetic field, and the static magnetic field was coarsely adjusted to excite the superconducting magnet. The second shim tray is installed as it is and fine adjustment of the static magnetic field is performed. As a result, the number of times of excitation and demagnetization of the super-electric magnet can be minimized, and specifically, this can be achieved by the following means.

(1) having a cylindrical bore in which a subject is arranged, and adjusting a static magnetic field generation means using a cylindrical superconducting magnet that generates a uniform static magnetic field in the cylindrical bore, and the uniformity of the static magnetic field A plurality of types of shim trays that store magnetic material shims and have a shim tray disposed in the circumferential direction of the cylindrical bore, the magnetic field adjusting devices having different amounts or shapes of magnetic material shims to be stored. Have The shim tray includes a plurality of pocket groups divided into arbitrary widths, lengths, and depths for accommodating the magnetic shims.

  The shim tray includes a plurality of first shim trays that store a required amount of magnetic shim, a plurality of second shim trays that store a magnetic shim amount smaller than the magnetic shim amount of the first shim tray, and these shim trays. And a first shim tray disposing unit disposed at an arbitrary angular interval in the circumferential direction of the cylindrical superconducting magnet at a position having substantially the same diameter from the axial center of the columnar bore.

  The uniformity of the magnetic field configured in this way is to adjust the static magnetic field roughly by exciting the superconducting magnet by attaching the first shim tray containing the desired amount of magnetic shim to the inner circumference of the cylindrical bore. The second shim tray is installed and fine adjustment of the static magnetic field is performed with this excitation state maintained, that is, without demagnetization. In this case, the magnetic shim amount of the second shim tray is set to a value that provides an electromagnetic attractive force that does not cause a safety problem at the time of attachment.

  In the magnetic resonance imaging apparatus according to the present invention, the shim tray may be variously changed as follows.

  1) The width, length, depth, and / or the number of pockets of the pocket group of the second shim tray are configured differently from the pocket group of the first shim tray, and the second shim tray has the second shim tray. Stores less magnetic shim than 1 shim tray.

  2) The second shim tray further includes a third shim tray having a smaller amount of magnetic shim than the first shim tray, wherein any of the width, length, depth, or number of pockets of the pocket group of the second shim tray is different, A part or the whole of the second shim tray is replaced with the third shim tray at the position where the shim tray is arranged.

  3) The first shim tray further includes a fourth shim tray having a larger amount of magnetic shim than the second shim tray, which is different in any of the width, length, depth, or number of pockets of the pocket group of the first shim tray, A part or all of the first shim tray is replaced with the fourth shim tray at the position where the shim tray is arranged.

  In the magnetic resonance imaging apparatus of (1), the bore in which the subject is arranged is a cylindrical hollow space, but the present invention provides a magnetic device having an elliptical columnar bore with an excellent open feeling that the subject feels. The present invention can also be applied to a resonance imaging apparatus.

  (2) Static magnetic field generation means using a cylindrical superconducting magnet that generates a uniform static magnetic field in the elliptical columnar bore where the subject is arranged, and adjustment of the uniformity of the static magnetic field in the inner peripheral axis direction of the elliptical columnar bore A magnetic field adjusting means using a shim tray that stores a magnetic shim that is arranged, and the magnetic field adjusting means includes a plurality of the shim trays arranged along a circumferential direction of the elliptical columnar bore, and is housed in the shim tray. There are at least two kinds of shim trays having different magnetic shim amounts, and the shim tray is provided with a plurality of pocket groups divided into arbitrary widths, lengths and depths for accommodating the magnetic shims. is there.

  The shim tray has a plurality of fifth shim trays that store a required amount of magnetic shim in the plurality of pocket groups, and a plurality of sixth shim trays that store a magnetic shim amount smaller than the magnetic shim amount of the fifth shim tray. Shim trays and second shim tray arrangement means for arranging these shim trays at an arbitrary angular interval in the circumferential direction of the cylindrical superconducting magnet are provided.

  The uniformity of the magnetic field configured in this way is to adjust the static magnetic field roughly by exciting the superconducting magnet by attaching a first shim tray containing a desired amount of magnetic shim to the inner peripheral axis of the elliptical columnar bore. The second shim tray is installed and fine adjustment of the static magnetic field is performed with this excitation state maintained, that is, without demagnetization. In this case, the magnetic shim amount of the second shim tray is set to a value that provides an electromagnetic attractive force that does not cause a safety problem at the time of attachment.

  In the magnetic resonance imaging apparatus according to the present invention, the shim tray may be variously changed as follows.

  1) The width, length, depth, and / or the number of pockets of the pocket group of the sixth shim tray are different from those of the pocket group of the fifth shim tray so that the sixth shim tray Stores less magnetic shim than 5 shim trays.

  2) The sixth shim tray further includes a seventh shim tray that has a smaller amount of magnetic shim than the fifth shim tray, which is different in any of the width, length, depth, or number of pockets of the pocket group of the sixth shim tray, A part or all of the sixth shim tray is replaced with the seventh shim tray at the position where the shim tray is arranged.

  3) The fifth shim tray further comprises an eighth shim tray having a larger amount of magnetic shim than the sixth shim tray, wherein any of the width, length, depth, or number of pockets of the pocket group of the fifth shim tray is different, A part or all of the fifth shim tray is replaced with the eighth shim tray at the position where the shim tray is arranged.

  4) It further includes a ninth shim tray having a pocket group of any width, length, depth, or number of pockets for storing the magnetic shim, and any one of the fifth shim tray to the eighth shim tray is disposed. In addition, the ninth shim tray or the fifth shim tray is disposed in a vertical area of the elliptical columnar bore, which is a wide region of the elliptical columnar bore.

  5) further comprising a tenth shim tray having a pocket group of any width, length, depth, or number of pockets in accordance with the vertical spacing of the elliptical columnar bore, which is a wide area of the elliptical columnar bore, The tenth shim tray is replaced with the fifth shim tray in the vertical direction of the columnar bore, or the tenth shim tray is arranged so that the longitudinal direction of the shim tray is perpendicular to the substantially circumferential direction of the elliptical columnar bore. Arrange vertically.

  6) an eleventh shim tray having a pocket group of any width, length, depth, or number of pockets that matches the vertical spacing of the elliptical columnar bore, which is a wide area of the elliptical columnar bore, and the elliptical columnar bore A twelfth shim tray having a pocket group of any width, length, depth, or number of pockets that matches the space in the left-right direction of the elliptical columnar bore, which is a narrow area of the ellipsoidal columnar bore, and the eleventh shim tray The vertical direction of the elliptical columnar bore is arranged such that the longitudinal direction of the shim tray is perpendicular to the substantially circumferential direction of the elliptical columnar bore, and the twelfth shim tray is arranged in the vertical direction of the elliptical columnar bore. Or arrange | position so that the circumferential direction may be met.

Further, the adjustment of the static magnetic field uniformity of the magnetic resonance imaging apparatus according to the present invention is adjusted by the following (3) when the bore in which the subject is arranged is cylindrical, and (4) when the bore is elliptical cylindrical. Adjusted by
(3) Static magnetic field generation means using a cylindrical superconducting magnet that generates a uniform static magnetic field in a cylindrical bore in which the subject is arranged, and adjustment of the uniformity of the static magnetic field in the direction of the inner peripheral axis of the cylindrical bore A magnetic resonance imaging apparatus having a magnetic field adjusting means using a shim tray containing a magnetic shim for adjusting a magnetic field uniformity, wherein the shim tray has a substantially same diameter from an axial center of the cylindrical bore. A first shim tray that stores at least a required amount of magnetic shim and a second shim tray that stores a smaller amount of magnetic shim than the first shim tray at a position along the circumferential direction of the cylindrical bore. A magnetic field measurement step of exciting the superconducting magnet and measuring the static magnetic field strength so that the central magnetic field strength of the cylindrical bore becomes a desired magnetic field strength, and calculating an irregular magnetic field component based on the measured value And Magnetic field shim adjustment magnetic field shim calculation step for calculating the magnetic shim amount and shim placement position stored in the first shim tray for coarse adjustment of the irregular magnetic field, and temporarily demagnetizing the superconducting magnet, A first shim tray arrangement step of storing the magnetic shim amount calculated in the magnetic field rough adjustment magnetic material shim calculation step in the first shim tray and arranging the first shim tray in the cylindrical bore; and the superconducting magnet. Is re-excited to measure the static magnetic field strength, and based on this measurement value, the magnetic field fine adjustment for calculating the amount and arrangement position of the magnetic shim stored in the second shim tray for fine adjustment of the static magnetic field strength. The magnetic shim calculation step for magnetic field and the magnetic material shim amount calculated in the magnetic field rough adjustment magnetic material shim calculation step are stored in the second shim tray, and the second shim tray is arranged in the cylindrical bore. Fine adjustment of the static magnetic field Cormorants including a static magnetic field fine-tuning step.

  (4) Static magnetic field generation means using a cylindrical superconducting magnet that generates a uniform static magnetic field in the elliptical columnar bore where the subject is arranged, and adjustment of the uniformity of the static magnetic field in the inner circumferential axis direction of the elliptical columnar bore The magnetic resonance imaging apparatus includes a magnetic field adjustment unit that adjusts the magnetic field uniformity of the magnetic resonance shim. The shim tray includes at least a circumferential direction of the elliptical columnar bore, A first shim tray that stores a required amount of magnetic shim, and a second shim tray that stores a magnetic shim amount smaller than the first shim tray, and the magnetic field strength of the cylindrical bore is a desired magnetic field. A magnetic field measurement step of exciting the superconducting magnet to measure the strength and measuring the static magnetic field strength, and calculating the irregular magnetic field component based on the measured value, and the first shim for coarsely adjusting the irregular magnetic field Magnetic field shim adjustment magnetic material shim calculation step for calculating the amount of magnetic material shim stored in the tray and shim arrangement position, and demagnetizing the superconducting magnet temporarily, and calculating by the magnetic material rough adjustment magnetic material shim calculation step The first shim tray placement step of placing the magnetic shim amount in the first shim tray and placing the first shim tray in the elliptical columnar bore, and re-exciting the superconducting magnet to measure the static magnetic field strength, Magnetic substance shim calculation step for fine adjustment of magnetic field for calculating the amount and arrangement position of the magnetic substance shim stored in the second shim tray for fine adjustment of the static magnetic field intensity based on the measured value, and for this magnetic field rough adjustment A static magnetic field fine adjustment step in which the magnetic shim amount calculated in the magnetic material shim calculation step is stored in the second shim tray, and the second shim tray is disposed in the elliptical columnar bore to finely adjust the static magnetic field; ,including.

  (5) The magnetic field fine adjustment magnetic material shim calculation step and the static magnetic field fine adjustment step are performed until a desired static magnetic field uniformity is obtained.

  The present invention provides at least two shim trays: a first shim tray that stores a required amount of magnetic shim, and a second shim tray that stores a magnetic shim amount smaller than the magnetic shim amount of the first shim tray. Is used to coarsely adjust the static magnetic field with the first shim tray arranged in the direction of the inner circumferential axis of the cylindrical superconducting magnet, and the second shim tray is arranged while the superconducting magnet is excited, thereby reducing the static magnetic field finely. Make adjustments. Thereby, since the number of times of excitation and demagnetization of the super-electric magnet can be minimized, it is possible to prevent quenching of the superconducting magnet, reduce consumption of cryogen such as liquid helium, and reduce shimming adjustment man-hours.

The schematic block diagram of the cylindrical superconducting magnet provided with the shimming mechanism in case the magnetic field space of the magnetic resonance imaging apparatus by this invention is a cylindrical bore. The block diagram of the shim tray of the shimming mechanism used for this invention. FIG. 6 is a diagram showing another embodiment of a shimming mechanism when the magnetic field space of the magnetic resonance imaging apparatus according to the present invention is a cylindrical bore. FIG. 4 is a process chart of magnetic field intensity adjustment in a uniform magnetic field space of the magnetic resonance imaging apparatus according to the present invention. FIG. 5 is a diagram showing an embodiment of a shimming mechanism when the magnetic field space of the magnetic resonance imaging apparatus according to the present invention is an elliptical columnar bore. FIG. 6 is a diagram showing another embodiment of a shimming mechanism when the magnetic field space of the magnetic resonance imaging apparatus according to the present invention is an elliptical columnar bore. FIG. 6 is a diagram showing another embodiment of a shimming mechanism when the magnetic field space of the magnetic resonance imaging apparatus according to the present invention is an elliptical columnar bore. FIG. 6 is a diagram showing another embodiment of a shimming mechanism when the magnetic field space of the magnetic resonance imaging apparatus according to the present invention is an elliptical columnar bore.

  Embodiments of a magnetic resonance imaging apparatus and a magnetic field uniformity adjustment method of the apparatus according to the present invention will be described below with reference to the drawings.

First Embodiment
A magnetic resonance imaging apparatus according to a first embodiment and a magnetic field uniformity adjustment method of the apparatus will be described with reference to FIGS. The superconducting magnet of this magnetic resonance imaging apparatus is a cylindrical superconducting magnet that is highly efficient for generating a high magnetic field. FIG. 1 shows an external perspective view as a schematic configuration diagram of this cylindrical superconducting magnet.

  In FIG. 1, a cylindrical superconducting magnet 3 contains a superconducting coil (not shown), which is a main coil, in a cryogenic bath (not shown) that stores a cryogen such as liquid helium contained in a vacuum vessel 4. In the internal space of the cylindrical bore 6 of the cylindrical superconducting magnet 3 along the axial direction (Z-axis) 5, it has a substantially spherical magnetic field space 7 that is a substantially uniform imaging space for imaging the subject. . On the inner surface of the vacuum vessel 4 along the cylindrical bore 6, holes (not shown) provided at substantially equal intervals in the circumferential direction of the cylindrical superconducting magnet 3 have a plurality of strip-shaped magnetic shim amounts. A plurality of shim tray groups 1 having different first shim trays 1a and second shim trays 1b are inserted and fixed so as to be detachable (first arrangement means). The shim tray group 1 may be disposed inside the gradient magnetic field coil 8 accommodated in the cylindrical bore 6 of the vacuum vessel 4.

  The shim tray group 1 preferably has a structure that can be moved smoothly in the Z-axis direction in order to easily attach and remove the shim tray during magnetic field adjustment at the inner surface position of the vacuum vessel 4. It has a plurality of divided pockets that accommodate magnetic shims with different amounts of magnetic shims arranged in the Z-axis direction. For example, as shown in FIG. 2, the shim tray group 1 is larger in the width, length, depth, or number of pockets of the pocket group 11a in the first shim tray 1a than in the pocket group 11b in the second shim tray 1b. The magnetic shim amount of the first shim tray 1a is larger than the magnetic shim amount of the second shim tray 1b.

  Although not shown, the shim tray having a smaller amount of magnetic material shim than the first shim tray 1a in FIG. 1 is not limited to the second shim tray 1b, and the condition that the amount of magnetic material shim is smaller than that of the first shim tray 1a. A configuration including a plurality of second shim trays may be used as long as it satisfies the requirements. For example, a shim tray in which at least one of the width, length, depth, and the number of pockets of the pockets 11b group of the second shim tray 1b is a third shim tray, and the third shim tray is replaced with the second shim tray 1b. A configuration in which the third shim tray is disposed may be employed, or a third shim tray may be disposed in any position of the second shim tray 1b. With this configuration, more detailed magnetic field adjustment is possible based on the magnetic field optimization calculation, which contributes to improvement in the uniformity of the magnetic field strength and reduction in the number of magnetic field adjustment operations.

  Similarly, as long as the condition that the magnetic shim amount of the first shim tray 1a is larger than the magnetic shim amount of the second shim tray 1b is satisfied, a configuration including a plurality of first shim trays may be used. For example, a shim tray in which at least one of the width, length, depth, or number of pockets of the pockets 11a group of the first shim tray 1a is a fourth shim tray, and the fourth shim tray is replaced with the first shim tray 1a. The arrangement may be such that the fourth shim tray is arranged at an arbitrary position in the first shim tray 1a. With this configuration, more detailed magnetic field adjustment is possible based on the magnetic field optimization calculation, which contributes to improvement in the uniformity of the magnetic field strength and reduction in the number of magnetic field adjustment operations.

  As described above, the first embodiment of the present invention performs the coarse adjustment of the spatial magnetic field intensity using the first shim tray 1a having a large amount of magnetic material shim, and in addition to the first shim tray 1a that has performed this coarse adjustment, A second shim tray 1b having a smaller amount of magnetic material shim than the first shim tray 1a is disposed, and the magnetic material shim amount is increased as compared with the coarse adjustment to finely adjust the spatial magnetic field strength.

The amount of the magnetic shim accommodated in each pocket of the second shim tray 1b configured as described above is finely adjusted to adjust the static magnetic field uniformity of the magnetic field space 7 to a target value.
Thereby, at the time of fine adjustment, since a shim tray with a small amount of magnetic shim is used, the electromagnetic attractive force acting on the shim tray can be reduced. Accordingly, since the shim tray can be removed and attached without demagnetizing the superconducting magnet, that is, with the magnet being excited, the magnetic field intensity can be adjusted without generating a quench.

Note that FIG. 1 shows an example in which 16 first shim trays 1a and 16 second shim trays 1b are configured as shim trays, but these combinations are not limited to the same number, and the first shim tray 1a If the condition that the amount of the magnetic shim is larger than the amount of the magnetic shim of the second shim tray 1b is satisfied, the number of the second shim tray 1b may be reduced and arranged at substantially equal intervals in the circumferential direction. . Conversely, the number of the second shim trays 1b may be increased, and the second shim trays 1b may be arranged at substantially equal intervals in the circumferential direction. FIG. 3 is an example of the case where the first shim tray 1a and the 12 second shim trays 1b are configured in the embodiment.

  Next, the shimming process by the passive shim mechanism of the magnetic resonance imaging apparatus according to the present invention will be described with reference to the flowchart of FIG.

  (1) First, the superconducting magnet 3 is excited so that the central magnetic field strength of the uniform magnetic field space 7 becomes a desired magnetic field strength without arranging the first shim tray 1a and the second shim tray 1b. (Step S21).

  (2) Next, the magnetic field strength in the uniform magnetic field space 7 shown in FIG. 1 is measured (step S22). In general, the superconducting magnet 3 has an error in manufacturing the superconducting magnet 3 and an error magnetic field generated in design, that is, an irregular magnetic field (nonuniform) component. At this time, for example, on a spherical surface with a diameter of 30 cm, if the difference between the maximum and minimum peak values of the magnetic field strength is several hundred ppm, for example, in order to accurately measure the irregular magnetic field component, The magnetic field is measured at a total of 586 evaluation points, 24 points in the arc direction and 24 points in the circumferential direction perpendicular thereto. Based on this measurement result, for example, an irregular magnetic field component is accurately calculated using a Legendre function expansion term.

  (3) Based on the calculation result, the amount and arrangement position of the magnetic material shim are accurately calculated (step S23). At this time, the arrangement of the magnetic shim is calculated by setting a constraint that the magnetic shim is arranged only in the pocket group 11a of the first shim tray 1a. Therefore, the second shim tray 1b is not arranged.

  (4) Next, the superconducting magnet 3 is temporarily demagnetized (step S24).

  (5) A magnetic shim is placed in the pocket group 11a of the first shim tray 1a (step S step 25).

  (6) Next, the superconducting magnet 3 is excited again (step S26).

  (7) Similarly to step S22, the magnetic field strength of the uniform magnetic field space 7 is measured (step S27). By the process using the first shim tray 1a so far, that is, by rough adjustment of the magnetic field strength, for example, on a spherical surface with a diameter of 30 cm, the difference between the maximum and minimum peak values of the magnetic field strength is reduced from several hundred ppm to several tens ppm. The static magnetic field uniformity is improved.

  (8) Next, based on the result measured in step S27, the amount and arrangement position of the magnetic shim stored in the second shim tray 1b are accurately calculated (step S28).

  At this time, the arrangement of the magnetic shim is calculated by setting a constraint that the magnetic shim is arranged only in the pocket group 11b of the second shim tray 1b. Therefore, the first shim tray 1a may not be arranged at the time of calculation, or may be arranged.

  To adjust the uniformity of the magnetic field strength, the first shim tray 1a is arranged, the superconducting magnet 3 remains in the excited state, and the difference between the maximum and minimum peak values of the spatial magnetic field strength is the target value, that is, the magnetic field Since the installation and removal of the second shim tray 1b are repeated until the strength becomes uniform, the magnetic shim of the second shim tray 1b is sufficiently reduced so that the electromagnetic attraction force is sufficiently small so that this operation can be performed safely. The amount is designed in advance to a sufficiently small amount. Furthermore, the amount of magnetic shim of the second shim tray 1b can be corrected, for example, on a spherical surface with a diameter of 30 cm, so that the difference between the maximum and minimum peak values of the magnetic field strength can be corrected from several tens ppm to a target value of several ppm. It is designed to have a sufficient amount of magnetic shim. That is, the magnetic shim amount of the second shim tray 1b is designed so that the electromagnetic attractive force and the magnetic field correctable amount are optimal.

  In this way, by arranging the second shim tray 1b in the circumferential direction of the superconducting magnet 3 at approximately equal intervals between the first shim trays 1a of the shim tray group 1, the correction amount of the magnetic field uniformity is better in the second shim tray 1b. Although there are few, the irregular magnetic field components that can be adjusted by the first shim tray 1a group and the second shim tray 1b group are almost the same type. Therefore, by designing the amount of the irregular magnetic field component remaining due to the coarse adjustment of the magnetic field strength by the first shim tray 1a group to be equal to or less than the correctable amount of the second shim tray 1b group, the target that is a standard value in principle. It becomes possible to achieve the static magnetic field uniformity.

  That is, the coarse magnetic field adjustment is performed by the first shim tray 1a group, and the remaining shim magnetic field component is finely adjusted by adding the second shim tray 1b group thereto.

  (9) The magnetic shim is stored in the second shim tray 1b based on the amount and arrangement position of the magnetic shim calculated in step S28 (step S29).

(10) Similar to Step S22 and Step S27, the magnetic field strength of the uniform magnetic field space is measured (Step S30).

  By fine adjustment of the magnetic field using the second shim tray 1b so far, for example, on a spherical surface with a diameter of 30 cm, the difference between the maximum and minimum peak values of the magnetic field strength is several tens ppm to several ppm, and the static magnetic field uniformity is improved. doing.

  (11) Then, for example, steps S28, S29, and S30 are repeated until the spatial magnetic field uniformity is within a standard value (eg, 3 ppm) (steps S31 and S32).

  As described above, the target uniformity of the magnetic field strength can be achieved. That is, as in the past, in order to remove the first shim tray 1a and attach a shim tray having a different shim amount than the first shim tray 1a, the superconducting magnet 3 is temporarily demagnetized, and a shim tray having a different shim amount from the first shim tray 1a is provided. The superconducting magnet 3 must be excited again after installation. For this reason, quenching occurs and replenishment of a cryogen such as liquid helium is necessary. Therefore, as described above, the present invention does not remove the first shim tray 1a in order to adjust the magnetic field intensity in the excited state, and the second shim tray 1b having a smaller amount of magnetic material shim than the first shim tray 1a. to add.

  As a result, the total amount of shim can be increased while the electromagnetic attractive force remains small, and the magnetic field uniformity can be finely adjusted.

In this way, by repeating the fine adjustment many times without demagnetizing and exciting the superconducting magnet, which involves the risk of quenching and waste of cryogen such as liquid helium during magnetic field adjustment, It becomes possible to achieve static magnetic field uniformity. Furthermore, it is possible to minimize the influence of the calculation accuracy of the optimization calculation due to the variation in the magnetization of the magnetic shim placed on the shim tray and the manufacturing error by multiple adjustments. A uniform static magnetic field space can be obtained, and the obtained MRI image can have high image quality.

  Further, in the step S23 for performing the rough adjustment, not only the first shim tray 1a but also the second shim tray 1b is used, and the accommodation shim amount of the second shim tray 1b is, for example, half the total accommodation amount that can be accommodated in the second shim tray 1b. The magnetic shim amount is calculated, the position and amount of the magnetic shim amount to be arranged in the second shim tray 1b are calculated, and the magnetic shim amount obtained by this calculation is accommodated in the second shim tray 1b and the first shim tray 1b receives the magnetic shim amount. Arranged together with the shim tray 1a for rough adjustment of the magnetic field strength. Then, the arrangement position and shim amount of the remaining magnetic material shim amount to be accommodated in the second shim tray 1b are calculated (step S28), and the arrangement position and shim amount of the magnetic material shim amount based on the calculation result are calculated as the second value. Is stored in the shim tray 1b (step S29), and the magnetic field strength is finely adjusted. Thereby, since the fine adjustment work is shortened, the convergence of the shimming work can be further improved.

  When calculating the magnetic shim amount of the second shim tray 1b in step S28, the magnetic shim amounts arranged up to the previous shimming are integrated, and the second shim tray is arranged at the next magnetic shim placement. If the amount exceeds the possible position of 1b, that is, if it becomes a safety problem to attach and detach the second shim tray in the excited state, demagnetize it once and return to the coarse adjustment of the magnetic field strength by the first shim tray 1a. Good. As a result, it is possible to prevent accidents due to electromagnetic attraction and improve safety in shimming work.

  Furthermore, the second shim tray 1b can be safely attached and detached in the excited state by using a non-magnetic jig for attaching and detaching.

  In addition, since a large electromagnetic attraction force acts on the first shim tray 1a, the superconducting magnet 3 is structured so that the first shim tray 1a cannot be attached or detached when the superconducting magnet 3 is excited. This makes it possible to further improve the safety of shimming work.

  In addition, after the shimming is performed once, the static magnetic field uniformity changes after the magnetic field environment around the superconducting magnet has changed, such as the addition of magnetic shields or the addition or replacement of the superconducting magnet peripheral unit with magnetic material. The standard value may be exceeded. At this time, in the shimming process by the passive shim mechanism, when fine adjustment of the static magnetic field uniformity is performed, the magnetic field strength can be easily finely adjusted without demagnetization and excitation by starting shimming from step S27. It becomes possible and contributes to reduction of the man-hours for magnetic field adjustment.

  In the above description, the example in which the second shim tray 1b is arranged in the excited state and the magnetic field is finely adjusted has been described. However, the positions of the first shim tray 1 and the second shim tray 1b are obtained by calculation without being excited. Excitation may be performed after the arrangement.

<< Second Embodiment >>
The superconducting magnet 3 in FIG. 1 is a case where the uniform magnetic field space in which the subject is arranged is a cylindrical hollow space, but the present invention is not limited to this, and is disclosed in JP 2001-327478 A The present invention can also be applied to an MRI apparatus using a superconducting magnet 44 in which a uniform magnetic field space has an elliptic columnar cavity space. As shown in FIG. 5, the gradient magnetic field coil 88 is obtained by expanding a space in the left and right direction inside the main coil in which the subject is arranged. Can improve the feeling of opening.

  Hereinafter, an example using an elliptic cylindrical gradient magnetic field coil having an elliptic columnar internal space will be described for the shim tray and its arrangement. 5 to 8, the gradient magnetic field coil 88 disposed in the columnar bore 66 is disposed radially inward and has an elliptical cylindrical main coil (not shown), and radially disposed outside in the cylindrical shape. This is an example in which a magnetic shim member is accommodated in a shim tray between a main coil and a shield coil when combined with a shield coil (not shown).

  In such a configuration, the interval between the main coil and the shield coil is wider in the vertical direction than in the horizontal direction. Conversely, the gradient magnetic field coil may be configured by combining an elliptical cylindrical main coil and a cylindrical shield coil so that the horizontal direction is wider than the vertical direction.

  FIG. 5 shows a plurality of strip-shaped fifth shim trays 101a and sixth shim trays 101b having different magnetic shim amounts in holes (not shown) provided at substantially equal angular intervals in the circumferential direction of the cylindrical superconducting magnet 33. 3 is a case corresponding to the case where the uniform magnetic field space has a cylindrical shape shown in FIG. 3 and includes a passive shim mechanism in which a plurality of shim tray groups having a plurality of shim trays are inserted and fixed detachably (second arrangement means). That is, in the example of the case where it is composed of 20 fifth shim trays 101a and 12 sixth shim trays 101b, the magnetic field strength is roughly adjusted using the fifth shim tray 101a having a large amount of magnetic shim, and the superconducting magnet The sixth shim tray 101b having a magnetic material shim amount smaller than that of the fifth shim tray 101a is arranged with the magnet 33 being excited to finely adjust the magnetic field strength. Thereby, even when the internal space of the superconducting magnet suitable for improving the feeling of openness felt by the subject has an elliptical columnar shape, it is possible to obtain the same effect as when the internal space of the superconducting magnet has a cylindrical shape. . In the example of FIG. 5, the number of shim trays is larger in the fifth shim tray 101a, but the present invention is not limited to this example, and the magnetic shim amount of the fifth shim tray 101a is the sixth shim tray 101a. Any number of combinations of the fifth shim tray 101a and the sixth shim tray 101b may be used as long as the condition that the amount of the shim tray 101b is larger than the magnetic shim amount is satisfied.

  Although there are various modifications of FIG. 5, the following (1) to (3) are given as typical examples.

  (1) The width, length, depth, and / or the number of pockets of the pocket group of the sixth shim tray is different from the pocket group of the fifth shim tray, and the sixth shim tray has the Stores a smaller amount of magnetic shim than the fifth shim tray.

  (2) The seventh shim tray further comprises a seventh shim tray having a smaller amount of magnetic shim than the fifth shim tray, wherein any of the width, length, depth, or number of pockets of the pocket group of the sixth shim tray is different, A part or all of the sixth shim tray is replaced with the seventh shim tray at the position where the six shim tray is disposed.

  (3) The fifth shim tray further includes an eighth shim tray having a larger amount of magnetic shim than the sixth shim tray, wherein any of the width, length, depth, or number of pockets of the pocket group of the fifth shim tray is different, A part or all of the fifth shim tray is replaced with the eighth shim tray at the position where the fifth shim tray is disposed.

  In the embodiment of FIG. 5, even if the fifth shim tray 101a and the sixth shim tray 101b are arranged between the main coil and the shield coil in the vertical direction, a space is generated in the vertical direction. For this reason, it can be considered that the uniformity of the magnetic field strength in the vertical direction is lower than the uniformity of the magnetic field strength in the horizontal direction. Therefore, as shown in FIG. 6, by arranging, for example, four fifth shim trays 101a in the empty area in the vertical direction, the magnetic field uniformity can be made substantially the same as that in the horizontal direction. Thereby, shortening of magnetic field adjustment time can be aimed at. The embodiment of FIG. 6 is an example in which only the fifth shim tray 101a is arranged in two layers, but only the sixth shim tray 101b or the fifth shim tray 101a and the sixth shim tray 101b are combined into two layers. You may arrange. For example, although not shown, the sixth shim tray 101b may be disposed between the four fifth shim trays 101a disposed in the empty space in the vertical direction.

  Alternatively, it further includes a ninth shim tray having a pocket group of any width, length, depth, or number of pockets for storing the magnetic shim, and any one of the fifth shim tray to the eighth shim tray is disposed. In addition, the ninth shim tray is arranged in an empty area in the vertical direction of the elliptical columnar bore, which is a wide region of the elliptical columnar bore.

  FIG. 7 shows an example in which the tenth shim tray 104a having a larger amount of magnetic material shim than the fifth shim tray 101a is arranged in accordance with the interval between the main coil and the shield coil. In this case, the amount of the magnetic shim disposed between the main coil and the shield coil in the vertical direction is larger than the amount of the magnetic shim disposed between the main coil and the shield coil in the left-right direction. Thereby, since it is not necessary to arrange in two layers, the passive shim mechanism can be made simpler than the arrangement example of FIG.

  Further, FIG. 8 shows an example in which shim trays having the same shape and size are arranged in accordance with the interval between the main coil and the shield coil. This example includes an eleventh shim tray having a pocket group having an arbitrary width, length, depth, or number of pockets according to the vertical interval of the elliptical columnar bore, which is a wide area of the elliptical columnar bore, and the elliptical columnar bore. A twelfth shim tray having a pocket group of any width, length, depth, or number of pockets that matches the space in the left-right direction of the elliptical columnar bore, which is a narrow region of the columnar bore, and the eleventh shim tray Are arranged in the vertical direction of the elliptical columnar bore so that the longitudinal direction of the shim tray is perpendicular to the substantially circumferential direction of the elliptical columnar bore, and the twelfth shim tray is arranged substantially in the longitudinal direction of the elliptical columnar bore. It arrange | positions along an up-down direction or the circumferential direction. By arranging the shim tray in this way, the shim tray can be efficiently arranged between the main coil and the shield coil, so that it is possible to improve the uniformity of the magnetic field strength and shorten the magnetic field adjustment time.

  As described above, in the second embodiment of the present invention, the elliptic cylindrical gradient magnetic field coil whose inner space is an elliptic cylinder can widen the space in the left-right direction inside the main coil in which the subject is arranged. The feeling of opening that the subject feels can be improved. If a magnetic shim member is arranged between the main coil and the shield coil of such a gradient magnetic field coil as shown in FIGS. 5 to 8, the uniformity of the static magnetic field can be improved. In particular, by disposing a large number of shim members corresponding to the space between the main coil and the shield coil, the space in the left-right direction is expanded, and the static magnetic field uniformity of the expanded space is increased. It becomes possible to improve.

  1 Shim tray group, 1a 1st shim tray, 1b 2nd shim tray, 3, 33 superconducting magnet, 6 cylindrical bore, 66 elliptical columnar bore, 7 uniform magnetic field space, 11a 1st shim tray pocket group, 11b 2nd shim tray pocket group 101a 5th shim tray, 101b 6th shim tray

Claims (10)

A static magnetic field generating means using a cylindrical superconducting magnet having a cylindrical bore in which a subject is arranged and generating a uniform static magnetic field in the cylindrical bore;
Magnetic field adjusting means having a plurality of pockets for storing magnetic shims for adjusting the uniformity of the static magnetic field and having shim trays arranged in the circumferential direction of the cylindrical bore;
With
The magnetic field adjusting means comprises a plurality of first shim trays and a plurality of second shim trays, wherein at least one of the width, length, depth and number of pockets of the pockets is different.
One or more of the first shim trays and one or more of the second shim trays are alternately arranged in the circumferential direction . In the first shim tray and the second shim tray, the width, length, depth, and / or the number of pockets of the pockets are different from each other, and the second shim tray has a different configuration than the first shim tray. 2. The magnetic resonance imaging apparatus according to claim 1, wherein a small amount of magnetic shim can be stored . The second shim tray has a pocket group that is different in width, length, depth, or number of pockets, and has a smaller amount of magnetic shim that can be stored than the first shim tray. Further equipped with 3 shim trays,
Magnetic resonance imaging apparatus according to claim 1 or 2, characterized in that part or all of the second shim trays disposed replaced with the third shim tray in a position to place said second shim tray. The first shim tray has a pocket group in which any one of the width, length, depth, and number of pockets of the pocket group is different, and has a larger amount of magnetic shim that can be stored than the second shim tray. 4 shim trays
Magnetic resonance imaging apparatus according to any one of claims 1 to 3, characterized in that arranged by replacing a part or all of the first shim tray in a position to place said first shim tray to the fourth shim tray . A gradient magnetic field generating means comprising a main coil having an elliptical cross section and a shield coil having a circular cross section;
5. The magnetic resonance imaging apparatus according to claim 1 , wherein the shim tray is disposed between the main coil and the shield coil . 6. 6. The magnetic resonance imaging apparatus according to claim 5 , wherein shim trays having different magnetic shim amounts that can be accommodated are arranged in accordance with an interval between the main coil and the shield coil . The shim tray is described from the shaft center of the cylindrical bore, the position substantially the same diameter, in any one of claims 1 to 6, characterized in that arranged along the circumferential direction of the cylindrical bore Magnetic resonance imaging equipment. The second shim tray is magnetic resonance imaging apparatus according to any one of claims 1 to 7, characterized in that it is arranged at approximately equal intervals in the circumferential direction. The magnetic resonance imaging apparatus according to claim 1 , wherein the plurality of shim pockets included in one shim tray have the same shape . A static magnetic field generating means using a cylindrical superconducting magnet having a cylindrical bore in which a subject is arranged and generating a uniform static magnetic field in the cylindrical bore;
Magnetic field adjusting means having a plurality of pockets for storing magnetic shims for adjusting the uniformity of the static magnetic field and having shim trays arranged in the circumferential direction of the cylindrical bore;
With
The magnetic field adjusting means comprises a plurality of first shim trays and a plurality of second shim trays, wherein at least one of the width, length, depth and number of pockets of the pockets is different.
In the magnetic resonance imaging apparatus in which one or more first shim trays and one or more second shim trays are alternately arranged in the circumferential direction ,
A magnetic field measurement step of measuring the static magnetic field strength by exciting the superconducting magnet so that the central magnetic field strength of the cylindrical bore becomes a desired magnetic field strength;
A rough magnetic field component for calculating an irregular magnetic field component based on a measurement value of the magnetic field measuring step and calculating a magnetic material shim amount and a shim arrangement position stored in the first shim tray for rough adjustment of the irregular magnetic field. A magnetic material shim calculation step;
The superconducting magnet is temporarily demagnetized, and the magnetic shim amount calculated in the magnetic field rough adjustment magnetic shim calculation step is stored in the first shim tray, and the first shim tray is disposed in the cylindrical bore. A first shim tray placement step;
The superconducting magnet is re-excited to measure the static magnetic field strength, and based on the measured value, the amount and arrangement position of the magnetic shim stored in the second shim tray for fine adjustment of the static magnetic field strength are calculated. A magnetic material shim calculation step for fine adjustment of the magnetic field;
The magnetic shim amount calculated in the magnetic field rough adjustment magnetic material shim calculation step is stored in the second shim tray, and the second shim tray is arranged in the cylindrical bore to perform fine adjustment of the static magnetic field. Magnetic field fine tuning process;
Magnetic field homogeneity adjustment method for magnetic resonance imaging apparatus.

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