Superconducting magnet apparatus having circulating path for coolant

Abstract: A superconducting coil (1) incorporated in the superconducting magnet apparatus of the invention is shaped annular or cylindrical, and thus surrounds a photographing field. The coil has an axis being substantially perpendicular to the line of the gravity, i.e., extending in the horizontal direction. The coil (1) is contained in a coolant vessel of a double structure filled with a coolant (e.g. liquid helium). Specifically, it is contained a first coolant vessel (2), together with a predetermined small amount of liquid helium (3). The vessel (2) has a receiving space (4) defined therein and shaped similar to the coil (1). The space (4) is large enough to receive the coil and the coolant with a predetermined gap. The coolant vessel (2) comprises a first cylindrical wall (5) contacting the inner periphery of the coil (1), a second cylindrical wall (6) being coaxial with the first cylindrical wall and having an inner diameter larger than the outer diameter of the coil (1), and a pair of annular end walls arranged at both opposite ends of the first and second cylindrical walls (5, 6), thereby closing, in liquidtight airtight manner, the gap formed by the walls (5, 6). That is, the coil (1) is received, in a manner coaxial with the first and second cylindrical walls, in a receiving space (4) defined by the cylindrical walls and annular end walls.

Description: This invention relates to a superconducting magnet apparatus for use in a magnetic resonance tomographic diagnosing apparatus (i.e. MRI, NMR), which has a heat insulating structure such as a coolant circulating path. Most magnetic resonance tomographic diagnosing apparatuses obtain a magnetic field of required high intensity by means of superconducting magnets. A magnet apparatus employed in such a magnetic resonance tomographic diagnosing apparatus as has a magnetic field whose axis is substantially at right angles to the line of the gravity (i.e. arranged in the horizontal direction), since a bed must be put into and out of the magnetic field, together with a patient lying thereon. The magnet apparatus is generally annular or cylindrical, and comprises a superconducting coil having an axis substantially perpendicular to the line of the gravity, a cylindrical vessel containing liquid helium for cooling the coil, and a space (for accommodating a patient and the like) which has a shape similar to the superconducting coil, and houses the coil and vessel. The vessel is generally made of a non-magnetic material such as stainless steel. The vessel and super heat insulating layers covering the entire inner and outer surfaces form a cryostat. To keep the superconducting coil in a stable superconducting state, it is desirable that the entire coil is always immersed in liquid helium. It is also desirable to resupply liquid helium after at least one month elapses since the last supply. To meet these, a great amount of liquid helium need be stored in a position higher than the upper surface of the superconducting coil. To store a great amount of liquid helium in such a position, conventional superconducting magnetic apparatuses have been designed such that the above-described receiving space has an outer diameter and an inner diameter much smaller than the former. This conventional apparatus, however, has the following problems: Since its liquid helium vessel is obtained by setting large the difference between the inner and outer diameters, a great amount of liquid helium necessarily exists also in a position lower than the upper surface of the coil. Little of the liquid helium stored in the lower position is used effectively, i.e., liquid helium of an amount more than required is stored in the vessel. If the superconducting coil is quenched to a normal conducting state, the stored liquid helium will be abruptly vaporized. There is a dangerous case of a conventional apparatus whose liquid helium vessel was damaged due to an increase in pressure within the vessel. Further, form an economical point of view, this is disadvantageous to that scanner for scanning the entire body of a patient which requires as much as 200 - 300 ℓ of expensive liquid helium. To eliminate the above disadvantage, it is considered to reduce the amount of liquid helium stored in a position lower than the upper surface of the superconducting coil, by arranging the vessel to have an axis extending in a plane higher than that in which the axis of the coil extends. In this case, however, since the vessel made of a conductive material is asymmetry with respect to the axis of a gradient coil, which is excited pulsively, an uniform eddy current will be caused along the vesssel, providing distorted tomograms. DE-A-1 814 783 discloses a superconducting magnet apparatus according to the preamble of claim 1. The coolant vessel of this known superconducting magnet apparatus has a double wall structure surrounding the superconducting coil with the exception of its upper part. By this double wall structure there is defined a buffer receiving space, which in case of a high pressure condition takes up coolant from a receiving space which surrounds the superconducting coil. A coolant communication pipe connecting both receiving spaces has its one end below the superconducting coil, extends around the superconducting coil and has its other end connected to the uppermost part of the buffer receiving space. By this design the gap between the superconducting coil and the inner wall of the double wall vessel is increased. The double wall structure surrounding the superconducting coil is not formed symmetrically. It is an object of the invention is to provide a superconducting magnet apparatus in which the amount of the coolant contained below the tip portion of the superconducting coil employed is reduced to a minimum necessary value without losing the symmetry to the gradient coil of the cryostat of MRI, thereby enhancing the economy and safety during operation. To attain this object, there is provided a superconducting magnet apparatus according to claim 1. Advantageous embodiments of this superconducting magnet apparatus are characterized by the features of the subclaims. The superconducting magnet apparatus constructed as above can store a coolant, required for operating the apparatus for a long time, in the space defined in the second coolant vessel, so that the space defined in the first coolant vessel may have a minimum possible size capable of receiving the superconducting coil with a predetermined gap. Thus, the apparatus can operate for a long time with a minimum necessary amount of coolant. Further, the coil is coaxial with the first and second coolant vessels, which results in the axial symmetry of the apparatus. Therefore, if, for example, a magnetic resonance tomographic diagnosing apparatus (MRI) uses a superconducting magnet apparatus constructed as above, it will provide tomograms free from distortion. In addition, in a case where an abnormality occurs, and the superconductive state is shifted to a normal conductive state, the coolant contained in the first coolant vessel starts to vaporize at first, and then the coolant contained in the second coolant vessel starts to vaporize slowly, which prevents the coolant from abruptly increasing in pressure. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a transverse sectional view showing an essential part of a superconducting magnet apparatus according to an embodiment of the invention; Fig. 2 is a partially cutaway view of the apparatus of the invention; and Fig. 3 is a longitudinal sectional view of the apparatus of the invention. The superconducting magnet apparatus according to an embodiment of the invention and shown in Fig. 1 has indispensable components such as a power lead for energizing or deenergizing a superconducting coil, hereinafter referred to, a permanent current switch for enabling the coil to operate in permanent current mode, a line for supplying a coolant, a supporting mechanism supporting the superconducting coil and a coolant vessel, hereinafter referred to, a heat-shielding plate for shielding the vessel from radiation heat, and a vacuum vessel. However, the essence of the present invention can be explained without referring to those components, and therefore they are not shown in the figure and not explained here. As is shown in Fig. 1, a superconducting coil 1 is shaped annular or cylindrical, thereby defining a photographing field therein. The axis of the coil 1 is arranged substantially at right angles to the line of the gravity, i.e., arranged in the horizontal direction. The coil 1 is received in a double-structure vessel filled with liquid helium. Specifically, the coil is received in a first coolant vessel 2, together with a predetermined small amount of liquid helium 3′. The vessel 2 is made of a nonmagnetic material such as stainless steel, and has a receiving space 4 defined therein. The space 4 is shaped similar to the coil 1 and is large enough to receive the coil with a predetermined gap (capable of storing a predetermined amount of liquid helium). That is, the vessel 2 comprises a first cylindrical wall 5 being in contact with the inner peripheral surface of the coil 1, a second cylindrical wall 6 (which serves also as the inner wall of a second vessel, hereinafter referred to) arranged coaxial with the first cylindrical wall 5, and a pair of annular end walls (not shown) arranged at opposite ends of the walls 5 and 6. The annular end walls close, in liquidtight/airtight manner, the gap defined by the walls 5 and 6, thereby forming the receiving space 4. Thus, the coil 1 is arranged coaxial with the cylindrical walls 5 and 6. A second coolant vessel 7 is provided around and coaxial with the first coolant vessel 2, and is made of a non-magnetic material such as stainless steel. The vessel 7 comprises the second cylindrical wall 6, a third cylindrical wall 8 arranged coaxial with the wall 6, a pair of annular end walls (not shown). These cylindrical and end walls define a liquidtight annular space, and this space is divided by means of separating walls 10 and 11 into an upper space 9 for receiving liquid helium 3, and a lower space 15. The walls 10 and 11 extend radially, and are located above the axis of the vessel. A bottom region of the space 9 is communicated with a bottom region of the space 4 through a communication pipe 12. A vaporized gas collecting pipe 13 extends, between the space 4 and the outside, through the tip portion of the wall 6, also through the upper space 9, and through the tip portion of the wall 8. A hole 14 for guiding vapor (i.e., helium gas) generated in the space 9 into the pipe 13 is formed in that portion of the pipe 13 which is located in the space 9. In the vessel constructed as above, liquid helium 3 required for operating the apparatus for a long time can be stored in the space 9 formed above the second vessel 7 at all times in a recycling manner. Thus, the space 4 of the first vessel 2 may have a minimum possible size capable of receiving the superconducting coil 1 with a predetermined gap. Therefore, the apparatus can operate for a long time with a minimum necessary amount of liquid helium. That is, maintenance free operation can be performed for a long time without storing liquid helium in the vacant space 15 defined by the separating walls 10 and 11 and the cylindrical wall 6. Further, since the superconducting coil 1 is coaxial with the first and second vessels 2 and 7, the supercon ducting magnet apparatus is symmetric to the gradient magnetic field. Consequently, a magnetic resonance tomographic diagnosing apparatus incorporating the above-described superconducting magnet apparatus can provide a tomogram without distortion. Further, in the apparatus of the invention, when an abnormality occurs and the superconducting coil 1 is quenched to a normal conducting state, the liquid helium 3′ contained in the first vessel 2 starts to vaporize, and the liquid helium 3 contained in the space 9 of the second vessel 7 slowly starts to vaporize long after the start of vaporization of the liquid helium 3′. Thus, an abrupt increase in pressure in the vessels is prevented, which is advantageous in view of safety. Fig. 2 is a partial cutaway view of the superconducting magnet apparatus of the invention, showing a detailed cross section thereof. The actual superconducting magnet apparatus includes components other than those shown in Fig. 1, such as thermal shielding layers 25 which cover the coil-receiving space and the coil, have various thicknesses, and are made of various materials, also complex layers defining spaces (a nitrogen tank 30, helium tank 40, etc.) for receiving coolants such as liquid helium and nitrogen, and coolant-circulation paths. The shielding layers 25, tank 30, and tank 40 and the other components, which are located in the area in which the magnetic force is exerted, are thin walls made of non-magnetic materials such as aluminum and high manganese steel. These thin walls form a predetermined structure. As can be understood from Fig. 2, the apparatus further incorporates a load supporting member 35, an inlet 45 for a coolant and the like, etc. Fig. 3 shows a longitudinal cross section of the superconducting magnet apparatus with a patient P lying therein. The layers (the shielding layers, nitrogen tank 30, and helium tank 40) forming the structure for cooling and thermal insulation surround a plurality of superconducting magnets (coils 1). The liquid helium 3 is contained in the portion located above the separating wall 11. This liquid helium 3 can flow into the space defined by the cylindrical wall 6 and filled with the liquid helium 3′, through the communication pipe 12 (shown in Fig. 1). The liquid helium 3′ heated by the superconducting coils 1 becomes helium gas, and exhausted from the gas collect-ing pipe 13. Thus, the liquid helium 3 and 3′ are gradually decreased in amount. The superconducting coils 1 are therefore cooled to a desired temperature. The space located below the separating wall 11 is maintained in a vacuum state. As is evident from Fig. 3, the patient P is positioned in the magnetic axis of the apparatus, indicated by the one-dot chain line. The apparatus is shaped symmetrical so that a substantially circular magnetic field will be formed around the magnetic axis. Further, the apparatus may be constructed such that each magnetic coil is surrounded by multi layers. In this case, the apparatus is more complex in structure than that of the embodiment, but has a cooling efficiency higher than the latter. The invention is applicable also to an apparatus incorporating a superconducting coil to be cooled by liquid nitrogen or oxygen, in place of the superconducting coil to be cooled by liquid helium. As is explained above, the superconducting magnet apparatus of the invention can operate for a long time in a reliable manner with a small amount of coolant, and also can provide an undistorted tomogram.

Claims: A superconducting magnet apparatus having a cylindrical superconducting coil (1), the coil having an axis substantially perpendicular to the line of a direction in which the gravity acts, the apparatus comprising: a first coolant vessel (2) having a receiving space (4) defined therein and substantially the same shape as the superconducting coil, the space receiving the coil (1) and a coolant (3) of extremely low temperature for cooling the coil; a second coolant vessel (7) arranged concentrically around the first coolant vessel such that the axis of the second coolant vessel coincides with the axis of the first coolant vessel, and having a receiving space (9) defined therein for receiving the coolant; a coolant communication pipe (12) communicating the receiving space (9) of the second coolant vessel (7) with a bottom region of the receiving space (4) of the first coolant vessel (2), and gas exhaust means (13) for exhausting that steam of the coolant which generates in the receiving space (4) of the first coolant vessel (2), characterized in that, the second coolant vessel (7) comprises two radially extending separating walls (10, 11), which are arranged above or at the level of the axis of the coil (1) so that the receiving space (9) of the second coolant vessel (7) is located above the axis of the coil (1), and the coolant stored in the receiving space (9) of the second coolant vessel (7) is guided to the receiving space (4) of the first coolant vessel (2) through the coolant communication pipe (12), communicating a bottom region of the receiving space (9) of the second coolant vessel (7) with the bottom region of the receiving space (4) of the first coolant vessel (2). The superconducting magnet apparatus according to claim 1, characterized in that the exhausting means comprising a gas collecting pipe (13), an end portion of the gas collecting pipe (13) communicates with the receiving space (4) of the first coolant vessel (2), and the gas collecting pipe extends and pierces the second coolant vessel (7). The superconducting magnet apparatus according to claim 1 or 2, characterized in that the coolant communication pipe (12) pierces at least one of the separating walls (10, 11), extends in the circumferential direction of the first coolant vessel (2) and pierces a lowermost portion of the first coolant vessel or a portion located in the vicinity of the lowermost portion. The superconducting magnet apparatus according to claim 1, characterized in that the gas exhaust means comprises at least one gas collecting pipe (13), which extends upwards and pierces both an uppermost portion of the fist coolant vessel (2) and an uppermost portion of the second coolant vessel (7).

Inventors: SATO AKIO C O INTELLECTUAL PRO, SATO, AKIO, C/O INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H01F6
IPC codes: F17C13, H01L39, G01R33, F25D3