JP3990410B2 - Superconducting magnet and magnetic resonance imaging apparatus - Google Patents

Description

  The present invention relates to a superconducting magnet apparatus suitable for a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and in particular, to give a subject a sense of freedom by having a wide opening, and to an operator. The present invention relates to a superconducting magnet device that facilitates access to an examiner.

FIG. 7 shows an example of a conventional superconducting magnet device for an MRI apparatus.
The one shown in FIG. 7 is a horizontal magnetic field superconducting magnet device. This superconducting magnet is composed of main coils 13, 14, 15, 16, 17, 18 having a small diameter and shield coils 19, 20 having a large diameter, and generates a magnetic field in the horizontal direction (Z-axis direction). In this example, the main coils 13 to 18 create a magnetic field along the central axis 22 of the magnet, and the shield coils 19 and 20 are arranged to shield magnetic field leakage to the surroundings. By configuring the magnet in this way, a uniform magnetic field region 21 having a magnetic field uniformity of about 10 ppm or less is formed in the magnetic field space. Magnetic resonance imaging is performed in the uniform magnetic field region 21.

  Since these coils are usually made using superconducting wire, the temperature is a predetermined temperature (e.g., liquid helium temperature (4.2K) in the case of alloy-based superconductors or liquid nitrogen temperature (77K in the case of oxide superconductors). )) To cool down. For this reason, the coil is held in a cooling container including a vacuum container, a heat shield (not shown), a refrigerant container (containing liquid helium, etc.), and the like.

In the example of FIG. 7, the coils 13 to 20 are disposed in a refrigerant container 11 containing a superconducting refrigerant 12 such as liquid helium, supported by a support (not shown), and the refrigerant container 11 is a vacuum. It is held in the container 10.
Further, in order to keep the coil temperature low, a refrigerator (not shown) is used to keep the temperature of the heat shield constant or to reduce the evaporation amount of the superconducting refrigerant 12. Recently, the performance of the refrigerator has improved, and the refrigerant container 11 may not be used by directly cooling the superconductor coil with the refrigerator.

  In the case of the superconducting magnet device shown in FIG. 7, the measurement space in which the subject enters for imaging is narrow and the periphery is also surrounded, so that the subject has a feeling of blockage. For this reason, the subject sometimes refused to enter the apparatus. In addition, it is difficult for the surgeon to access the subject from the outside of the apparatus.

FIG. 8 shows another example (open type horizontal magnetic field method) of a conventional superconducting magnet device for an MRI apparatus. This conventional example is disclosed in (Patent Document 1), which improves the feeling of blockage of the device and the difficulty of access to the operator's subject, which were the disadvantages of the conventional example of FIG.
U.S. Pat.No. 5,410,287

FIG. 8 (a) shows a cross-sectional view, and FIG. 8 (b) shows an external view.
In FIG. 8 (a), three coils 23A, 24A, 25A and 23B, 24B, 25B are arranged coaxially with the central axis 22 with the uniform magnetic field region 21 in between. Each set of coils is supported by a support (not shown) and directly cooled by a refrigerator. The entire coil is surrounded by heat shields 9A and 9B, and these heat shields 9A and 9B are vacuum vessels 10A and 10B. Is held in.
The coils 23A, 23B, 24A, and 24B are main coils, and current in the same direction flows. The coils 25A and 25B are auxiliary coils, and current in the direction opposite to that of the main coils flows. In the magnet of this configuration, the main coils 23A, 23B, 24A, 24B create a magnetic field in the direction along the central axis 22, and the auxiliary coils 25A, 25B are arranged to improve the magnetic field uniformity of the uniform magnetic field region 21. Yes. In addition, this magnet does not use a shield coil, but provides a magnetic shield in the room where the apparatus is installed.

In FIG. 8 (a), the opposing vacuum vessels 10A and 10B have a donut shape, and are supported by two support rods 26 therebetween. For this reason, the space between the vacuum vessels 10A and 10B is an open space. The subject is inserted into the uniform magnetic field region 21 along the central axis 22 through the central hole of the vacuum vessel, and imaging is performed there.
According to this configuration, since the side surface of the imaging region (homogeneous magnetic field region 21) is released, the subject does not need to feel a blockage. In addition, the surgeon can easily access the subject from the side and can be used for a monitor during surgery.

  However, in this configuration, since a set of donut-shaped superconducting coils is used to insert the subject, an operation for improving the magnetic field uniformity in the space of the central portion of the donut-shaped vacuum vessel is performed. Cannot be used as an area Therefore, it is difficult to obtain good magnetic field uniformity over a wide space. Further, since the magnetic flux generated by the superconducting coil returns through the external space of the apparatus, the leakage magnetic field becomes wide. For this reason, it is necessary to install a device with a large installation area or to provide a thick magnetic field shield.

FIG. 9 shows a third example (vertical magnetic field method) of a conventional superconducting magnet device for an MRI apparatus. This conventional example is disclosed in (Patent Document 2). This magnet generates a magnetic field by two sets of superconducting coils 31 arranged facing each other in the vertical direction, and an iron shimming means 32 is provided inside the superconducting coil 31 to obtain a good magnetic field uniformity. The magnetic field uniformity in the region 21 is improved. Further, the upper and lower magnetic field generation sources are mechanically supported by the iron yoke 33 that also serves as a return path of the magnetic field generated by the upper and lower superconducting coils 31.
U.S. Patent No. 5,194,810

  In this conventional example, since it is opened in all directions, the subject does not need to feel a blockage, and the operator can easily access the subject. Further, since the return path of the magnetic flux is constituted by the iron yoke 33 and the iron plate 34, magnetic field leakage can be reduced. However, since the iron yoke 33 and the iron plate 34 are used, the magnet weight increases, and there is a problem that the floor needs to be strengthened when the apparatus is installed. Moreover, since the saturation magnetic flux density of iron is approximately 2 Tesla, there is a limitation that the magnetic field strength cannot be increased so much. Furthermore, since iron has hysteresis characteristics with respect to a magnetic field, the magnetic field generated by the gradient magnetic field coil affects the magnetic field distribution, which may hinder high-precision signal measurement.

As described in the above description of the prior art, in a superconducting magnet device having a wide opening that gives a sense of release to the subject, there is little magnetic field leakage and the weight of the device can be reduced without using iron. There was nothing planned. In addition, in the prior art, it has been difficult to achieve a wide uniform magnetic field region with high magnetic field strength.
Accordingly, an object of the present invention is to provide a superconducting magnet device that solves the above-described problems, has a wide opening, has little magnetic field leakage, is lightweight, and can achieve a wide uniform magnetic field region at a high magnetic field strength.

The object of the present invention is achieved by the following means. The superconducting magnet device according to the present invention is directed vertically opposite to a magnetic field generation source including a superconducting coil group accommodated in a refrigerant container filled with a superconducting refrigerant and having a central axis aligned and a vacuum container containing the refrigerant container. The pair of magnetic field generation sources is supported by a support means spaced apart by a predetermined distance, and a current is passed through the superconducting coils of the pair of magnetic field generation sources so as to be sandwiched between the pair of magnetic field generation sources. The superconducting coil group is configured to generate a uniform magnetic field in the facing direction, and the superconducting coil group is disposed at a position where the facing distance is short between the facing magnetic field generation sources, and the first coil that generates the uniform magnetic field in the space, and the first coil A second coil disposed at a position where the facing distance is larger and having a diameter larger than that of the first coil and a current flowing in a direction opposite to that of the first coil, and the vacuum vessel has an outer side the outer periphery is substantially circular And a, and an outer periphery between the vicinity of said the vicinity of the first coil the second coil, characterized in that the diameter with increasing distance from said space is formed to be larger.
Preferably, the vacuum vessel has a frustoconical shape, and a diameter of an outer periphery is made larger at a position where the second coil is located than at a position where the first coil is located. To do.
The magnetic resonance imaging apparatus of the present invention is characterized by using the superconducting magnet device.

  According to the present invention, by providing a wide opening in the superconducting magnet device, the surgeon can easily access the subject without giving the subject a feeling of blockage. In addition, it is possible to provide a superconducting magnet device that has a small magnetic field leakage, is lightweight, and can realize a wide uniform magnetic field region even at a high magnetic field strength. Therefore, the installation conditions of the superconducting magnet device are relaxed, and a good image can be taken.

Embodiments of the present invention will be specifically described below with reference to the drawings. 1, 2 and 6 (a) show a first embodiment of the superconducting magnet apparatus of the present invention.
1 is a cross-sectional view showing the overall configuration of the apparatus, FIG. 2 is a view showing the external appearance of the apparatus, and FIG. 6 (a) is a reprint of the upper right portion of the cross-sectional view of FIG. It is.

In FIG. 1, cylindrical vacuum vessels 10A and 10B containing superconducting coils are arranged facing each other in the vertical direction, and the upper vacuum vessel 10A is supported by a support column 26 at a predetermined interval from the lower vacuum vessel 10B. Has been. Refrigerant containers 11A, 11B filled with superconducting refrigerant 12 are held inside vacuum containers 10A, 10B, and superconducting coils 41A, 42A, 43A, 44A, 45A and 41B, 42B, 43B, 44B and 45B are supported and arranged on a support (not shown).
For the superconducting coil, a commonly used NbTi wire is used. Liquid helium is used as the superconducting refrigerant. Vacuum containers 10A and 10B are disposed outside the refrigerant containers 11A and 11B to prevent heat convection, and a heat shield (not shown) and the like are disposed to prevent heat radiation.

  The column 26 serves to mechanically support the upper vacuum vessel 10A, but may have a function to thermally connect the upper and lower refrigerant vessels 11A and 11B if necessary. By doing so, it is not necessary to provide one refrigerator (not shown) for each of the upper and lower refrigerant containers 11A, 11B, and the entire apparatus can be made in time with one refrigerator. In addition, the number of columns 26 need not be limited to the two illustrated, and can be increased to three or four, and in order to obtain a feeling of release from the subject, as a single cantilever column Also good.

  In FIG. 1, five sets of superconducting coils are arranged coaxially with the central axis 22A to form a magnetic field with high magnetic field uniformity in the uniform magnetic field region 21. The functions of these superconducting coils can be broadly classified into three. First, the superconducting coils 41A and 41B (corresponding to the main coil) are for generating a magnetic field having a high magnetic field strength and a magnetic field uniformity of a predetermined level or more in the uniform magnetic field region 21. Regarding the main coil, generally, when the diameter of the coil is increased while the distance between the opposing coils is kept constant, the uniformity of the magnetic field generated between the two coils tends to be improved. Therefore, in order to obtain good magnetic field uniformity, it is better to increase the diameter of the coil as much as possible. On the other hand, since the magnetic field strength decreases as the diameter of the main coil increases, the magnetomotive force of the main coil necessary to obtain a magnetic field with a constant strength increases, leading to an increase in the price of the apparatus. In order to reduce the feeling of pressure on the subject, it is desirable that the outer diameter of the apparatus be as small as possible. From the balance between the two, the diameters of the superconducting coils 41A and 41B, which are the main coils, are determined, and the outer diameters of the vacuum vessels 10A and 10B are substantially determined by the diameters of the coils.

Next, in this embodiment, a canceling coil for canceling the magnetic field to the outside generated by the main coils 41A and 41B is arranged in order to suppress leakage of the magnetic field to the outside of the apparatus. Superconducting coils 42A, 42B in the coil cancel each other, the coil 42A, 42 B are main coils 41A, is disposed 41B coaxially. In addition, a current in a direction opposite to that of the main coil is supplied to generate a magnetic field in a direction opposite to that generated by the main coils 41A and 41B outside the apparatus, thereby canceling the magnetic field outside the apparatus. In order to effectively reduce the magnetic field outside the apparatus, it is effective to make the diameter of the cancellation coil as large as possible. On the other hand, in order not to lower the magnetic field strength of the uniform magnetic field region 21 by the canceling coils 42A and 42B, it is important to keep the canceling coils 42A and 42B as far as possible from the main coils 41A and 41B. As a result, as shown in FIG. 1 or FIG. 6 (a), the canceling coils 42A and 42B have substantially the same diameter as the main coils 41A and 41B, and the distance between the two canceling coils 42A and 42B is reduced. It is appropriate to make the distance larger than the distance between the two main coils 41A and 41B.

 Further, in this embodiment, in order to improve the uniformity of the magnetic field generated in the uniform magnetic field region 21, the uniformity correction coils 43A, 44A, 45A, 43B, 44B, and 45B are arranged. This uniformity correction coil is a coil provided for correcting a non-uniform component of the magnetic field in the uniform magnetic field region 21 formed by the main coil and the cancellation coil. In general, if the main coils 41A and 41B have a sufficiently large diameter, the above-mentioned magnetic field inhomogeneity component is not so large, so the uniformity correction coil is as large as the main coils 41A and 41B and the cancellation coils 42A and 42B. Magnetomotive force is not required. The direction of the current may be determined for each coil according to the non-uniform component of the magnetic field generated by the main coils 41A and 41B, and need not be limited to a certain direction.

  In FIG. 1, three sets of coils are arranged, but this number can be determined according to the inhomogeneous component of the magnetic field. In general, the larger the diameters of the main coils 41A and 41B, the smaller the magnetic field inhomogeneity component, so the number of uniformity correction coils can be reduced.

As a specific example of this embodiment, when the outer diameter of the main coil is 1,600 mm to 1,800 mm and the size and magnetic field strength of the uniform magnetic field region are 450 mmφ and 1 Tesla, respectively, a magnetic field uniformity of 5 ppm or less is obtained in the uniform magnetic field region. Has been achieved.
By configuring the superconducting magnet device as described in this embodiment, it is possible to realize a device having a wide opening and less magnetic field leakage. Moreover, since iron is not used to suppress magnetic field leakage, the weight of the apparatus can be reduced. Furthermore, magnetic flux saturation, which is a problem when iron is used, does not occur. Therefore, even when the magnetic field strength is increased, good magnetic field uniformity can be achieved over a wide uniform magnetic field region.

3 and 4 show a second embodiment of the superconducting magnet device of the present invention.
FIG. 3 is a sectional view showing the overall configuration of the apparatus, and FIG. 4 is a view showing the appearance of the apparatus.
In FIG. 3, vacuum containers 10A and 10B and refrigerant containers 11A and 11B arranged above and below have a donut shape, and central portions 51A and 51B are hollow. By adopting such a configuration, it is possible to install gradient magnetic field or static magnetic field shimming means in the central portions 51A and 51B. Therefore, it is not necessary to provide a space for installing them between the opposing surfaces of the vacuum vessels 10A and 10B, so that the opening as a device can be made wider.

FIG. 5 shows a third embodiment of the superconducting magnet apparatus of the present invention.
In FIG. 5, the facing distance between the main coils 41A and 41B is made smaller than the facing distance between the uniformity correcting coils 43A, 44A, 45A and 43B, 44B, 45B. With such a configuration, the strength of the magnetic field generated in the uniform magnetic field region can be increased without increasing the magnetomotive force of the main coils 41A and 41B. In addition, the recesses 52A and 52B are provided in the central portions on the opposite sides of the vacuum vessels 10A and 10B, and the recesses 52A and 52B contain gradient magnetic field coils, high frequency coils, magnetic field shimming means, etc. The effective opening can be widened.

  Next, another embodiment of the first embodiment of the present invention is shown in FIGS. 6 (b), (c), and (d). These embodiments are different from the first embodiment shown in FIG. 6 (a) in the arrangement of the main coil or the cancellation coil.

  In the embodiment shown in FIG. 6 (b), the diameters of the canceling coils 42A and 42B are made larger than the diameters of the main coils 41A and 41B, and the outer peripheral shape of the refrigerant container 11A is on the side where the main coils 41A and 41B are located. The size of the refrigerant containers 11A and 11B as a whole is a truncated cone shape, which is small and large on the side where the cancellation coils 42A and 42B are located. By adopting such a configuration, the efficiency of the cancellation coils 42A and 42B can be increased, so that magnetic field leakage to the outside of the apparatus can be more effectively reduced. In this case, even if the diameter of the cancellation coils 42A and 42B is increased, the refrigerant containers 11A and 11B are formed in a truncated cone shape with a small diameter on the side of the uniform magnetic field region 21 serving as a measurement space. When entering the apparatus, the substantial field of view is not impaired, and the subject feels only a feeling of pressure similar to that in the case of FIG. 6 (a).

  The embodiment shown in FIG. 6 (c) shows the case where the cancellation coils 42A and 42B of the embodiment shown in FIG. 6 (b) are divided into two cancellation coils 42AA, 42AB and 42BA, 42BB. It is. In general, each coil is arranged in a magnetic field generated by each coil, and an electric current flows through each coil, so that an electromagnetic force acts. Although this electromagnetic force depends on the magnetic field strength of the uniform magnetic field region 21, it reaches the order of 100 tons. Therefore, the idea of reducing this electromagnetic force is an important issue in manufacturing a superconducting magnet device. Therefore, by dividing the canceling coils 42A and 42B to which a particularly large electromagnetic force is applied into two coils as shown in FIG. 6 (c), the electromagnetic force acting on each coil can be reduced, and the superconducting magnet The manufacturing conditions of the apparatus can be relaxed. Moreover, although the case where it divided | segmented into two coils was shown in a present Example, it cannot be overemphasized that this method is not limited to this and may divide | segment into three or more coils.

  The embodiment shown in FIG. 6 (d) shows a case where the main coils 41A and 41B of the embodiment of FIG. 6 (b) are divided into two main coils 41AA, 41AB and 41BA, 41BB. . Also in this case, by dividing the main coil, the electromagnetic force applied to the divided main coil can be reduced. Also in the present embodiment, the number of divided coils is not limited to two, and it goes without saying that it may be three or more.

  In the above description of the embodiments of the present invention, the case of using a superconducting refrigerant such as liquid helium as the cooling means of the superconducting material has been described. However, when an oxide superconductor or the like is used as the superconducting material, In some cases, it is not necessary to use the refrigerant container by cooling with nitrogen or by directly cooling with a refrigerator.

1 is a cross-sectional view showing a first embodiment of a superconducting magnet device of the present invention. 1 is an external view of a first embodiment of a superconducting magnet device according to the present invention. FIG. Sectional drawing which shows the 2nd Example of the superconducting magnet apparatus of this invention. FIG. 6 is an external view of a second embodiment of the superconducting magnet device of the present invention. Sectional drawing which shows the 3rd Example of the superconducting magnet apparatus of this invention. The fragmentary sectional view which shows the other Example of the superconducting magnet apparatus of this invention. The figure which shows an example (horizontal magnetic field system) of the conventional superconducting magnet apparatus for MRI apparatuses. The figure which shows the other example (open type horizontal magnetic field system) of the conventional superconducting magnet apparatus for MRI apparatuses. The figure which shows the 3rd example (vertical magnetic field system) of the conventional superconducting magnet apparatus for MRI apparatuses.

Explanation of symbols

  10, 10A, 10B Vacuum container, 11, 11A, 11B Refrigerant container, 12 Superconducting refrigerant, 13, 14, 15, 16, 17, 18 Main coil, 19, 20 Shield coil, 21 Uniform magnetic field region, 22, 22A magnetic field Central shaft, 23A, 23B, 24A, 24B Main coil, 25A, 25B Auxiliary coil, 26 posts, 31 main coil, 32 shimming means, 33 Iron yoke, 34 Iron plate, 41A, 41B, 41AA, 41AB, 41BA, 41BB Main coil 42A, 42B, 42AA, 42AB, 42BA, 42BB Canceling coil, 43A, 43B, 44A, 44B, 45A, 45B Auxiliary coil, 51A, 51B Central part (hollow), 52A, 52B Concave

Claims (3)

A pair of magnetic field generation sources each including a superconducting coil group accommodated in a refrigerant container filled with a superconducting refrigerant so as to coincide with the central axis and a vacuum container accommodating the refrigerant container are vertically opposed to each other. The magnetic field generation source is supported at a predetermined distance by a support means, and a current is passed through the superconducting coils of the pair of magnetic field generation sources to generate a uniform magnetic field in the opposite direction in a space between the pair of magnetic field generation sources. In the superconducting magnet device to be
The superconducting coil group is disposed at a position where the facing distance is close between the opposing magnetic field generation sources and is disposed at a position where the facing distance is larger than that of the first coil, and a first coil that generates a uniform magnetic field in the space. And a second coil having a diameter larger than that of the first coil and a current flowing in a direction opposite to the first coil.
The vacuum vessel is formed so that the outer periphery is substantially circular, and the outer periphery between the vicinity of the first coil and the vicinity of the second coil increases in diameter as the distance from the space increases. A superconducting magnet device, characterized in that In the superconducting magnet device according to claim 1,
The superconducting magnet characterized in that the vacuum vessel has a truncated cone shape and the outer peripheral diameter is larger at a position where the second coil is located than at a position where the first coil is located. apparatus.   A magnetic resonance imaging apparatus using the superconducting magnet apparatus according to claim 1.

Images