MRI device

Abstract

To provide an MRI apparatus equipped with a refrigerating machine which is filled with a cold storage material that has large specific heat and small magnetization in a cryogenic region and has good manufacturability, and has high efficiency and excellent cooling performance.SOLUTION: An MRI apparatus of an embodiment is equipped with a refrigerating machine which accommodates a cold storage material including particles composed of an intermetallic compound with the ThCr2Si2 type structure occupying 80 vol.% or more, the particles having a crystallite size of 70 nm or less. In the cold storage material, in the ThCr2Si2 type structure, a Th site is at least one element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y, the Cr site is at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni and Cu, and the Si site is at least one element selected from Si and Ge.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The embodiment relates to an MRI apparatus equipped with a refrigerator that uses a cold storage material used at extremely low temperatures. [Background technology]

[0002] Superconducting electromagnets used in magnetic resonance imaging systems (MRI) and heavy particle accelerators operate in cryogenic environments below several tens of K. This cryogenic environment is usually achieved by a regenerative refrigerator, such as the Gifford-McMahon (GM) refrigerator.

[0003] Refrigerators use several types of regenerator materials with large specific heat capacities for each operating temperature range. GM refrigerators, which are currently widely used, use a Cu mesh in the first regenerator, spherical particles of Pb and Bi alloys on the high-temperature side of the second regenerator, and particles of rare earth compounds such as Gd2O2S (GOS), HoCu2, and Er3Ni on the low-temperature side of the second regenerator. Among these regenerator materials, GOS has high specific heat capacities in the temperature range around 5K.

[0004] By the way, synthesizing oxide regenerator materials such as GOS requires a multi-step process, including synthesis of raw materials, granulation, sintering at high temperatures, and polishing to form a spherical shape.

[0005] Furthermore, many refrigerators that achieve extremely low temperatures are used to cool superconducting coils. Therefore, if the magnetization of the regenerator material is high, the magnetic field generated by the superconducting coil will exert a large force on the regenerator material, which may cause the shaft containing the regenerator material to break, reducing the reliability of the refrigerator. Furthermore, as mentioned above, superconducting coils are used in MRIs, etc., and if the regenerator material has a high magnetization, magnetic noise from the regenerator material may cause noise in the images. For this reason, the regenerator material must have a low magnetization.

[0006] In refrigerators such as GM refrigerators, pulse tube refrigerators, and Stirling refrigerators, high-pressure working gas flows back and forth through the gaps in the regenerator material. Furthermore, in GM refrigerators and Stirling refrigerators, the regenerator filled with regenerator material vibrates. Therefore, the regenerator material must have sufficient mechanical strength.

[0007] In contrast to oxides, which require a multi-step manufacturing process, including synthesis of raw materials, granulation, sintering at high temperatures, and polishing to form spherical shapes, intermetallic compounds can be manufactured through a simple process of melting and solidifying, making them preferable from the perspective of manufacturing regenerator materials. RCu2X2 (R = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, X = Si, Ge), a candidate intermetallic compound regenerator material, is known to have a large specific heat capacity at extremely low temperatures.

[0008] However, RCu2X2-based intermetallic compounds are produced by, for example, melting raw materials using an arc melting method, and then subjecting the resulting ingot to a high-temperature, long-term homogenization heat treatment in a vacuum (for example, at 800°C for one week). The need for a high-temperature, long-term heat treatment process after melting and solidification thus leads to increased costs when applied to industrial mass production. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 09-014774 [Patent Document 2] Japanese Patent Application Publication No. 06-101915 [Non-patent literature]

[0010] [Non-Patent Document 1] L. Gonedek, et. al., Acta Phys Pol A 122, 391 (2012). [Non-patent document 2] Y. Takeda, et. al., J. Phys. Soc. Jpn. 77, 104710 (2008). Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention provides an MRI system equipped with a refrigerator that is highly efficient and has excellent cooling performance by using a regenerator material that has a large specific heat capacity in the cryogenic temperature range, small magnetization, and good manufacturability.The present invention also provides an MRI system that can reduce the effects of magnetic noise originating from the regenerator material. [Means for solving the problem]

[0012] The MRI apparatus of the embodiment is an MRI apparatus equipped with a refrigerator that houses a cold storage material including particles made of an intermetallic compound in which the ThCr2Si2 type structure accounts for 80% by volume or more and the crystallite size is 70 nm or less, and the cold storage material is characterized in that in the ThCr2Si2 type structure, the Th site is at least one element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y, the Cr site is at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni and Cu, and the Si site is at least one element selected from Si and Ge. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a schematic diagram of a ThCr2Si2 type structure showing the crystal structure of a regenerator material according to a first embodiment. [Figure 2] FIG. 2 is an explanatory diagram of the particle shape of the cold storage material according to the first embodiment. [Figure 3] FIG. 10 is a cross-sectional view of a two-stage expansion GM refrigerator exemplified as a refrigerator according to a second embodiment. [Figure 4] FIG. 11 is a cross-sectional view of an MRI apparatus exemplified as a superconducting coil built-in device according to a third embodiment. [Figure 5]1 is a graph showing the measurement results of Example 1 (top) and Comparative Example 1 (bottom) by powder X-ray diffraction. [Figure 6] 1 is a graph showing the specific heat characteristics in the cryogenic temperature range of Example 1 and Comparative Example 1. [Figure 7] 1 is a graph showing the measurement results of Example 1 (top) and Comparative Example 2 (bottom) by powder X-ray diffraction. [Figure 8] 1 is a table showing the crystallite size, volume percentage of ThCr2Si2 type structure, proportion of finely pulverized sample, peak temperature of specific heat, and peak value of specific heat of DyCu2Ge2, DyCu2Si2, GdCu2Si2, PrCu2Si2, and TbCu2Si2 intermetallic compounds in Examples 1 to 7 and Comparative Examples 1 to 14. [Figure 9] 3 is a graph showing magnetization characteristics in the cryogenic temperature range in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0014] (First embodiment) The embodiments will be described in detail below. Fig. 1 is a schematic diagram of a ThCr2Si2 structure 11 showing the crystal structure of the regenerator material according to the first embodiment. The regenerator material according to the first embodiment is a granular material made of an intermetallic compound in which the ThCr2Si2 structure 11 accounts for 80% by volume or more, and the crystallite size is 70 nm or less.

[0015] In this ThCr2Si2 type structure 11, the Th site 12 is at least one element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y, the Cr site 13 is at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir, and Pt, and the Si site 14 is at least one element selected from Si and Ge.

[0016] In refrigerators such as the GM refrigerator described below, a working gas such as He gas flows back and forth through the gaps in the regenerator material, and the heat generated by the gas compression-expansion cycle is stored in the regenerator material, cooling it from room temperature to extremely low temperatures. Therefore, the regenerator material used in refrigerators is required to have a large specific heat capacity within the operating temperature range.

[0017] When the ThCr2Si2 structure 11 accounts for 80% by volume or more of this intermetallic compound, a regenerator material with high specific heat properties in the cryogenic temperature range can be obtained. If the ThCr2Si2 structure 11 accounts for less than 80% by volume of the intermetallic compound, the specific heat properties may be inferior to those of general substances used as regenerator materials in the cryogenic temperature range. The volume percentage of the ThCr2Si2 structure can be calculated by Rietveld analysis of powder X-ray diffraction or by evaluating the phase ratio in multiple fields of view using a scanning electron microscope.

[0018] In addition, in GM refrigerators and Stirling refrigerators, the regenerator filled with regenerator material vibrates, so the regenerator material must have sufficient mechanical strength. Therefore, by keeping the crystallite size of the regenerator material fine, at 70 nm or less, the regenerator material's excellent mechanical strength is ensured. The crystallite size L is calculated using Scherrer's formula (Equation (1)) by evaluating the peak width (half-width) β in the X-ray diffraction pattern. If the crystallite size is small, the half-width of the X-ray diffraction pattern will be large. L = Kλ / (βcosθ) (1) (where K is the Scherrer constant and λ is the wavelength of the X-rays used) The mechanical strength can be evaluated by a vibration test.

[0019] If the crystallite size of the regenerator material is larger than 70 nm, the mechanical strength will be poor, and the particles will fatigue and break down into fine particles over the course of use, making it impossible to maintain the desired performance of the refrigerator. On the other hand, the crystallite size is preferably 1 nm or more, and more preferably 10 nm or more.

[0020] 2 is an explanatory diagram of the granular shape of the cold storage material according to the first embodiment. In terms of the particle size of the cold storage material granules, if the length in the longest direction of the powder is φmax and the length of the longest part in the direction perpendicular to the longest direction is φmin, then φmax and φmin fall within the range of 0.01 mm to 1 mm, and more preferably fall within the range of 0.05 mm to 0.5 mm. If the area of ​​a projected image 15 of this cold storage material is A and the area of ​​the smallest circumscribing circle 16 circumscribing this projected image 15 is M, then the shape factor expressed as M / A falls within the range of 1.0 to 5.0 in all projection directions.

[0021] By setting the particle size of the regenerator material to a range of 0.01 mm to 1 mm, the flow of the working gas (He gas) reciprocating in the regenerator filled with the regenerator material in the refrigerator described below is not impeded, and good heat exchange between the working gas and the regenerator material is achieved. If the particle size of the regenerator material is less than 0.01 mm (10 μm), the gaps between the particles of the regenerator material, i.e., the space through which the working gas flows, become narrow, which may increase the pressure loss of the gas. Furthermore, if the particle size of the regenerator material is greater than 1 mm, the filling rate of the regenerator material may decrease, which may reduce the heat exchange between the working gas and the regenerator material.

[0022] Such a regenerator material is manufactured by at least the steps of blending and melting the component elements of the intermetallic compound that can have the above-mentioned ThCr2Si2 type structure 11 in their stoichiometric ratio, and pouring this molten liquid into a dynamic cooling medium to rapidly cool and solidify it into granules.

[0023] That is, elemental metals blended to achieve the stoichiometric ratio of the ThCr2Si2 structure 11 are melted using high-frequency induction heating or the like. The molten metal is then supplied to the running surface of a high-speed rotor placed in a vacuum or inert gas atmosphere. This molten metal is finely dispersed by the motion of the rotor and simultaneously rapidly cooled and solidified to form spherical granules. Alternatively, the molten metal is flowed into a vacuum or inert gas atmosphere and a non-oxidizing atomizing gas is applied. This causes the molten metal to be atomized and dispersed, simultaneously rapidly cooled and solidified to form spherical granules.

[0024] Specific methods for rapidly solidifying the above-mentioned molten metal include the rotary disc process (RDP), single roll process, twin roll process, inert gas atomization process, and rotary nozzle process. 5 ~10 6 This method allows for rapid solidification of the molten metal at a rate of 1000 °C / s. This method makes it possible to produce granular intermetallic compounds with a ThCr2Si2 structure very simply and at low cost. Details of this method for rapid solidification of molten metal are explained in Japanese Patent No. 2609747, etc.

[0025] By adding an intermetallic compound with a different magnetic phase transition temperature to a ThCr2Si2-type intermetallic compound, the specific heat per unit volume of the regenerator material can be improved. For example, if a ThCr2Si2-type intermetallic compound contains phases with AlB2 and LiGaGe structures, the specific heat can be increased in the 4-20 K range. Furthermore, if a ThCr2Si2-type intermetallic compound contains a phase with a Gd3Cu4Ge4 structure, the specific heat can be increased in the 7-50 K range. However, if more than 20% by volume of a phase other than the ThCr2Si2-type phase is present, the volumetric specific heat derived from the ThCr2Si2-type phase decreases. Furthermore, by constructing an intermetallic compound with phases with different crystal structures, the mechanical strength of the regenerator material can be increased.

[0026] (Second embodiment) 3 is a cross-sectional view of a two-stage expansion GM refrigerator exemplified as a refrigerator 30 according to the second embodiment. The refrigerator 30 has a large-diameter first cylinder 31 and a small-diameter second cylinder 32 coaxially connected to the first cylinder 31. A first regenerator 34 is disposed in the first cylinder 31 so as to be able to reciprocate, and a second regenerator 35 is disposed in the second cylinder 32 so as to be able to reciprocate. Seal rings 36 and 37 are disposed between the first cylinder 31 and the first regenerator 34, and between the second cylinder 32 and the second regenerator 35, respectively.

[0027] A first expansion chamber 41 is provided between the connecting portion of the first regenerator 34 and the second regenerator 35 and the inner wall of the first cylinder 31. A second expansion chamber 42 is provided between the second regenerator 35 and the front end wall of the second cylinder 32. A first cooling stage 43 is formed at the bottom of the first expansion chamber 41, and a second cooling stage 44, which has a lower temperature than the first cooling stage 43, is formed at the bottom of the second expansion chamber 42.

[0028] The first regenerator 34 contains a first regenerator material 38 such as a copper alloy mesh, with a passage 33 for the working gas (e.g., He gas). Note that the first regenerator material 38 may be a stainless steel mesh instead of a copper alloy mesh, or both may be used. The second regenerator 35 is filled with a second regenerator material 40, with a passage 39 for the working gas. Note that the regenerators 34, 35 are shown in which the first regenerator material 38 and the second regenerator material 40 are filled separately, but they may also be filled in a single regenerator.

[0029] The second cold storage material 40 housed inside the second cold storage unit 35 is filled with a plurality of types of second cold storage materials 40a, 40b separated by a mesh 48. The filling rate of the second cold storage materials 40a, 40b in the space separated by the mesh 48 is preferably 50 to 75%, more preferably 55 to 65%, in consideration of the fluidity of the working gas.

[0030] In the two-stage refrigerator 30, working gas (He gas, etc.) is compressed by a compressor 45 and supplied to the refrigerator 30 through a high-pressure line 46. The supplied working gas passes through the gaps in the first regenerator material 38 housed in the first regenerator 34, reaches the first expansion chamber 41, and cools the first cooling stage 43 by expansion. Next, the working gas passes through the gaps in the second regenerator material 40 housed in the second regenerator 35, reaches the second expansion chamber 42, and cools the second cooling stage 44 by expansion.

[0031] The low-pressure working gas passes through the second regenerator 35 and the first regenerator 34 in this order (in the opposite direction to when it was at high pressure), and is returned to the compressor 45 through the low-pressure line 47. It is then compressed by the compressor 45, and the above cycle is repeated. The expansion of each expansion chamber 41, 42 is achieved by the reciprocating movement of the regenerators 34, 35. At this time, each regenerator material 38, 40 exchanges thermal energy with the working gas to store and hold cold and also to regenerate heat.

[0032] Next, the cycle will be explained, focusing on the flow of heat. The high-pressure working gas supplied from the compressor 45 to the refrigerator 30 is at room temperature (up to about 300 K). As it passes through the first regenerator 34, it is pre-cooled by the first regenerator material 38 and reaches the first expansion chamber 41. As it expands in the first expansion chamber 41, the temperature of the working gas further drops, cooling the first cooling stage 43. Next, as it passes through the second regenerator 35, the working gas is pre-cooled by the second regenerator material 40 and reaches the second expansion chamber 42. As it expands in the second expansion chamber 42, the temperature of the working gas further drops, cooling the second cooling stage 44.

[0033] The low-pressure working gas passes through the second regenerator 35 while storing cold energy in the second regenerator material 40 (while the working gas itself is being heated). The working gas then passes through the first regenerator 34 while storing cold energy in the first regenerator material 38 (while the working gas itself is being heated), where it is heated to near room temperature, and returns to the compressor 45 through the low-pressure line 47.

[0034] During steady-state operation of the refrigeration cycle, a temperature gradient occurs in the regenerator materials 38, 40 inside the regenerators 34, 35. In such a refrigeration cycle, the larger the specific heat of the regenerator material at the operating temperature, the more the thermal efficiency of the working gas cycle improves, thereby achieving even lower temperatures and higher refrigeration performance.

[0035] Generally, the specific heat of a solid changes depending on the temperature. Therefore, in order to enhance the heat recovery effect of the second cold storage material 40 in particular, it is effective to selectively arrange second cold storage materials 40 that have good heat recovery characteristics in each temperature range in accordance with the temperature gradient. Therefore, the second cold storage unit 35 is filled with a plurality of second cold storage materials 40 (40a, 40b) with different heat recovery characteristics.

[0036] To obtain a good heat recovery effect, it is important that the heat capacity (specific heat) of the regenerator material is large at the operating temperature of each part in the cycle process, and that the heat exchange between the regenerator material 40, 38 and the working gas is good. Since the main operating temperature range of the first regenerator 34 is from room temperature to 100 K or less, Cu is selected because it has a large specific heat per unit volume in this temperature range, and Cu mesh is widely used as the first regenerator material 38 because wire-drawn mesh is easy to use industrially.

[0037] When the temperature is below 60 K, Pb or Bi, which has a larger specific heat than Cu, is selected as the second cold storage material 40a on the high-temperature side of the second cold storage unit 35. Furthermore, when the temperature is below 8 K, a cold storage material having a ThCr2Si2 structure according to the first embodiment, which has a larger specific heat than Pb or Bi, is selected as the second cold storage material 40b on the low-temperature side of the second cold storage unit 35. As described above, it is preferable that the cold storage materials 38 and 40 of the GM refrigerator are selected and arranged using a material having a large volumetric specific heat in the operating temperature range of each component, taking into consideration the temperature gradient inside the cold storage units 34 and 35. Note that the second cold storage material 40a arranged on the high-temperature side of the second cold storage unit 35 is not limited to Pb or Bi, and may be HoCu2 or Er3Ni, for example. Furthermore, the second cold storage material 40 is not limited to the two-layer structure described above, but may be formed into three or more layers.

[0038] Furthermore, the refrigerator equipped with the regenerator material according to the first embodiment is not limited to the GM refrigerator described above. In refrigerators that generate extremely low temperatures from room temperature, such as pulse tube refrigerators, Claude refrigerators, and Stirling refrigerators, the regenerator material is installed in a location where a large thermal impedance is required, such as the boundary region between a low-temperature part and a high-temperature part generated by the compression-expansion cycle of the working gas.

[0039] (Third embodiment) 4 is a cross-sectional view of a magnetic resonance imaging (MRI) apparatus 50 showing an example of a superconducting coil incorporated apparatus according to the third embodiment. Diagnosis using this MRI apparatus 50 involves moving a movable table (not shown) on which a subject 52 lies down into a tunnel-shaped bore space 51. A static magnetic field is applied by a first electromagnet 53, and a gradient magnetic field is applied by a second electromagnet 54.

[0040] Furthermore, radio waves are transmitted from the RF coil 55, and a magnetic resonance response signal is received from the subject 52. Due to the presence of a gradient magnetic field, information on the position where the response signal is generated is also received at the same time. The received response signal is analyzed by a signal processing system (not shown), and an image of the inside of the body of the subject 52 is reconstructed.

[0041] Currently, mainstream MRI devices 50 use a superconducting coil for the first electromagnet 53 that generates a high magnetic field such as 1.5 T or 3 T. The stronger the magnetic field, the better the S / N (signal / noise) ratio of the magnetic resonance response signal, allowing for clearer images to be captured. The superconducting coil used for the first electromagnet 53 is usually a solenoid coil wound with a metallic low-temperature superconducting wire such as NbTi or Nb3Sn.

[0042] These wires must be kept below the critical temperature for superconducting transition, so the first electromagnet 53 is placed in a He bath 56 filled with liquid He, which liquefies at 4.2 K (approximately -269°C) or below at 1 atmosphere. Because liquid He is rare and expensive, an insulating vacuum layer 57 is provided outside the He bath 56 to suppress evaporation of the liquid He. Furthermore, two radiation shields 58 and 59 are provided within the insulating vacuum layer 57 to reduce the effects of heat penetration from the environment in which the MRI device 50 is installed (room temperature: approximately 300 K). The installed refrigerator 30 cools the shield 58 to approximately 4 K and the shield 59 to approximately 40 K.

[0043] The refrigerator 30 is not particularly limited, and may be a combination of a GM refrigerator and a JT refrigerator, or may be a single refrigerator such as a GM refrigerator, a pulse tube refrigerator, a Claude refrigerator, or a Stirling refrigerator. In particular, the refrigeration performance of GM refrigerators was dramatically improved in the 1990s by incorporating a magnetic regenerator material, making it possible to generate extremely low temperatures below the temperature of liquid He using only a GM refrigerator. Therefore, GM refrigerators are widely used in MRI devices 50 that are in widespread use at the time of filing of this application.

[0044] As shown in Figure 4, the first cooling stage 43 (Figure 3) of the GM refrigerator 30 is connected to a shield 59, and the second cooling stage 44 (Figure 3) is connected to a shield 58. At the time of filing, GM refrigerators capable of stably obtaining a cooling capacity of 1 W or more at 4 K are in widespread use. Therefore, by balancing the heat penetration into the He bath 56 and the cooling by the GM refrigerator 30, it is possible to maintain extremely low temperatures and almost completely suppress the evaporation of liquid He.

[0045] As a result, if a hospital or other medical institution injects liquid He into the MRI system 50 during initial startup, there is no need to periodically replenish the expensive and difficult-to-handle liquid He during subsequent operation. This significant improvement in convenience has led to the widespread adoption of MRI systems 50 in small and medium-sized hospitals. Furthermore, MRI systems incorporating direct-cooling superconducting coils, in which the superconducting coils are conduction-cooled using a refrigerator, without using liquid He, are also being commercialized. In this case, the liquid He bath 6 can be omitted.

[0046] In recent years, MRI systems have been developed that use high-temperature superconducting wires such as Y-based, Bi-based, and MgB2. As with MRI systems that use low-temperature superconducting materials, the superconducting coils in these systems must be cooled to 10 to 30 K (approximately -257°C) or below, which is the temperature below the critical temperature for superconducting transition and allows the current required to generate a magnetic field to flow.

[0047] Therefore, in MRI systems using high-temperature superconducting materials, it is necessary to either cool the superconducting coil by immersing it in liquid He, H2, or Ne, which has a liquefaction temperature of 4 to 30 K (approximately -269°C) or lower at 1 atmosphere, or to conductionally cool the superconducting coil using a refrigerator. Even in the former method, it is preferable to use a refrigerator to cool the liquid He, H2, or Ne to prevent evaporation. To improve the performance of a refrigerator at temperatures between 10 and 30 K, it is desirable to install a regenerator material with a large specific heat capacity in the same temperature range.

[0048] The superconducting coil embedded device according to the third embodiment is equipped with the refrigerator according to the second embodiment, which is equipped with the regenerator material according to the first embodiment. The magnetization of this regenerator material is preferably 10 emu / g or less, more preferably 5 emu / g or less, and even more preferably 2 emu / g or less, at an external magnetic field of 1000 Oe and a temperature of 5 K or less. Such a low magnetization of the regenerator material reduces the influence of magnetic noise originating from the regenerator material, enabling high-quality images to be obtained. The superconducting coil embedded device according to the third embodiment is not limited to the MRI device 50 described above, and other examples include superconducting magnets for magnetically levitated trains, superconducting electromagnet devices, cryopump devices, Josephson voltage standard devices, and magnetic field application type single crystal pulling devices.

[0049] In particular, cryopump devices achieve a high degree of vacuum by cooling to approximately 10 K. Therefore, the performance of cryopump devices can be improved by installing a regenerator material with a large specific heat capacity around 10 K in the refrigerator. [Example]

[0050] (Example 1, Comparative Example 1) Next, Example 1 will be described in more detail. The elemental metals that are components of the intermetallic compound DyCu2Ge2 were used as raw materials, mixed in a stoichiometric ratio, melted, and cooled by a roll quenching method with the distance between the nozzle and the roll set to 0.5 mm and a cooling rate of 10 5 ~10 6As Comparative Example 1, the blending and melting conditions were the same as those of Example 1, and a thin flake sample was prepared by arc melting at a cooling rate of 10 °C / sec. 2 Bulk samples were prepared by slow cooling and solidification at ℃ / sec.

[0051] Example 2 A thin flake sample was prepared under the same conditions as in Example 1, except that the distance between the nozzle and the roll was set to 0.6 mm.

[0052] Example 3 A thin flake sample was prepared under the same conditions as in Example 1, except that the distance between the nozzle and the roll was set to 0.7 mm.

[0053] 5 is a graph showing the measurement results of Example 1 (top) and Comparative Example 1 (bottom) by powder X-ray diffraction. The powder X-ray diffraction was measured using a SmartLab manufactured by Rigaku Corporation. From the X-ray diffraction patterns in this graph, it can be seen that the intermetallic compound of Example 1 obtained by rapid solidification treatment has a crystal structure that is mostly DyCu2Ge2. On the other hand, it can be seen that the intermetallic compound of Comparative Example 1 obtained by slow solidification treatment also contains multiple subphases.

[0054] FIG. 6 is a graph showing the specific heat characteristics in the cryogenic temperature range of Example 1 and Comparative Example 1. The specific heat characteristics were measured using a Physical Property Measurement System (PPMS) manufactured by Japan Quantum Design Co., Ltd. As shown in FIG. 6, Example 1, which was subjected to rapid solidification treatment, has a larger maximum value of specific heat in the low temperature range than Comparative Example 1, which was subjected to slow solidification treatment. This shows that by using the intermetallic compound of Example 1 as a refrigerant to be filled in the refrigerant storage unit of a refrigerator, the cooling capacity of the refrigerator is improved.

[0055] (Comparative Example 2) The blending and melting conditions were the same as those of Comparative Example 1, and the mixture was heat-treated at 800°C, which is below the solidification point, for one week to prepare a bulk sample. The preparation conditions for the sample of Comparative Example 2 were the same as those disclosed in Non-Patent Document 1.

[0056] (Comparative Example 3) A bulk sample was prepared in the same manner as in Comparative Example 2, except that the sample was heat-treated at 900° C., which is below the solidification point, for 4 days.

[0057] Comparative Example 4 A bulk sample was prepared in the same manner as in Comparative Example 2, except that the sample was heat-treated at 800° C., which is below the solidification point, for 4 days.

[0058] (Comparative Example 5) A bulk sample was prepared in the same manner as in Comparative Example 2, except that the sample was heat-treated at 700° C., which is below the solidification point, for 4 days.

[0059] (Comparative Example 6) The compounding conditions were the same as in Example 1, and the high-frequency melting method was used. 2 Bulk samples were prepared by slow cooling and solidification at ℃ / sec.

[0060] Example 4 A thin flake sample was prepared under the same conditions as in Example 1, except that the composition was changed to DyCu2Si2.

[0061] (Comparative Example 7) A bulk sample was prepared under the same conditions as in Comparative Example 1, except that the composition was changed to DyCu2Si2.

[0062] (Comparative Example 8) A bulk sample was prepared in the same manner as in Comparative Example 7, except that the sample was heat-treated at 900° C., which is below the solidification point, for 4 days.

[0063] Example 5 A thin flake sample was prepared under the same conditions as in Example 1, except that the composition was changed to GdCu2Si2.

[0064] (Comparative Example 9) A bulk sample was prepared under the same conditions as in Comparative Example 1, except that the composition was changed to GdCu2Si2.

[0065] (Comparative Example 10) A bulk sample was prepared in the same manner as in Comparative Example 9, except that the sample was heat-treated at 900° C., which is below the solidification point, for 4 days.

[0066] Example 6 A thin flake sample was prepared under the same conditions as in Example 1, except that the composition was changed to PrCu2Si2.

[0067] (Comparative Example 11) A bulk sample was prepared under the same conditions as in Comparative Example 1, except that the composition was changed to PrCu2Si2.

[0068] (Comparative Example 12) A bulk sample was prepared in the same manner as in Comparative Example 11, except that the sample was heat-treated at 900° C., which is below the solidification point, for 4 days.

[0069] Example 7 A thin flake sample was prepared under the same conditions as in Example 1, except that the composition was changed to NdCu2Si2.

[0070] (Comparative Example 13) A bulk sample was prepared under the same conditions as in Comparative Example 1, except that the composition was changed to NdCu2Si2.

[0071] (Comparative Example 14) A bulk sample was prepared under the same conditions as in Comparative Example 13, except that the sample was heat-treated at 900° C., which is below the solidification point, for 4 days.

[0072] 7 is a graph showing the measurement results of Example 1 (top) and Comparative Example 2 (bottom) by powder X-ray diffraction. It should be noted that Example 1 (top) in FIG. 7 and Example 1 (top) in FIG. 5 are identical data, with the only difference being the scale displayed on the horizontal axis. As shown in FIG. 7, in Comparative Example 2, by maintaining the solid phase at a high temperature and heat-treating it, the X-ray diffraction pattern of the unintended crystal structure present in Comparative Example 1 disappears, and it can be seen that the majority of the crystal structure is DyCu2Ge2, as in Example 1.

[0073] Furthermore, comparing the X-ray diffraction patterns in Figure 7 between Example 1 and Comparative Example 2, it can be seen that the peak broadening is greater in Example 1. Using the peak identified as a ThCr2Si2 type, the crystallite size was calculated from its half-value width β. Even though the crystal structure of the intermetallic compound is the same, Example 1, which was subjected to rapid solidification treatment, has a smaller crystallite size than Comparative Example 2, which was subjected to high-temperature heat treatment in the solid phase, and therefore can be said to have superior mechanical properties.

[0074] The sample was placed in a vibration test container (D = 15 mm, h = 14 mm) and subjected to a maximum acceleration of 300 m / s 2 Simple harmonic motion of 1×10 6 After the test, the samples were subjected to appropriate shape classification and sieving, and the weight ratio of the finely powdered sample was determined to evaluate the mechanical strength of the sample.

[0075] The table shown in Figure 8 shows the results of the crystallite size, content of ThCr2Si2 type structure, proportion of pulverized sample, specific heat peak temperature, and specific heat peak value of the sample for Examples 1 to 7 and Comparative Examples 1 to 14. When the crystallite size is larger than 70 nm, the proportion of pulverized sample increases significantly, and the mechanical strength decreases. When the content of ThCr2Si2 type structure is less than 80% by volume, the specific heat peak value decreases significantly.

[0076] FIG. 9 is a graph showing the magnetization characteristics in the cryogenic temperature range of Example 1. The magnetization characteristics were measured using a magnetic property measurement system (MPMS) manufactured by Nippon Quantum Design Co., Ltd. At an external magnetic field of 1000 Oe, the magnetization in the temperature range of 2 to 5 K was 0.97 emu / g or less. Note that the magnetization of GOS, which has high specific heat characteristics similar to Examples 1 to 3, in the temperature range around 5 K was 1.5 emu / g. HoCu2, used on the low-temperature side of the second-stage regenerator other than GOS, had a magnetization of 3.5 emu / g, and Er3Ni had a magnetization of 7 emu / g. Therefore, the regenerator materials of Examples 1 to 3 have small magnetization characteristics, and therefore, when installed in an MRI device, they contribute to improving the image quality and reducing magnetic noise in superconducting coil-integrated devices.

[0077] If the granular particle diameter of the cold storage material described in Example 1 is less than 0.01 mm (10 μm), the gaps between the particles of the cold storage material, i.e., the space through which the working gas flows, become narrow, increasing the pressure loss of the gas and resulting in a decrease in refrigeration performance. Also, if the granular particle diameter of the cold storage material is greater than 1 mm, the filling rate of the cold storage material in the cold storage unit decreases, resulting in a decrease in refrigeration performance.

[0078] According to at least one of the above-described embodiments, an MRI apparatus can be provided that is equipped with a cryogenic refrigerator that is highly efficient and has excellent cooling performance by being filled with a regenerator material that has a large specific heat capacity in a cryogenic temperature range, small magnetization, and good manufacturability.Furthermore, an MRI apparatus can be provided that can reduce the influence of magnetic noise originating from the regenerator material.

[0079] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as the inventions described in the claims and their equivalents. [Explanation of symbols]

[0080] 11...ThCr2Si2 type structure, 12...Th site, 13...Cr site, 14...Si site, 15...projection image, 16...circumscribed circle, 30...refrigerating machine, 31...first cylinder, 32...second cylinder, 33...passage of working gas, 34...first regenerator, 35...second regenerator, 36, 37...seal ring, 38...first regenerator material, 39...passage of working gas, 40 (40a, 40b)...second Cooling material, 41...first expansion chamber, 42...second expansion chamber, 43...first cooling stage, 44...second cooling stage, 45...compressor, 46...high pressure line, 47...low pressure line, 48...mesh, 50...MRI device, 51...bore space, 52...subject, 53...first electromagnet, 54...second electromagnet, 55...RF coil, 56...He bath, 57...insulating vacuum layer, 58, 59...shield.

Claims

1. ThCr 2 Si 2 An MRI apparatus equipped with a refrigerator containing a regenerator material including particles of an intermetallic compound in which a type structure accounts for 80% by volume or more and the crystallite size is 70 nm or less, The cold storage material is the ThCr 2 Si 2 In the type structure, the Th site is at least one element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y, the Cr site is at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, and the Si site is at least one element selected from Si and Ge.

2. 2. The MRI apparatus according to claim 1, An MRI apparatus comprising a superconducting coil using a solenoid coil wound with low-temperature superconducting wire.

3. 3. The MRI apparatus according to claim 2, wherein the low-temperature superconducting wire is made of NbTi or Nb ３ An MRI device characterized in that it is Sn.

4. 2. The MRI apparatus according to claim 1, An MRI apparatus comprising a superconducting coil using high-temperature superconducting wire.

5. 5. The MRI apparatus according to claim 4, wherein the high-temperature superconducting wire is Y-based, Bi-based, or MgB 2 An MRI apparatus characterized in that:

6. 2. The MRI apparatus according to claim 1, The MRI apparatus is characterized in that the refrigerator is a GM refrigerator, a pulse tube refrigerator, a Claude refrigerator, or a Stirling refrigerator.

7. 2. The MRI apparatus according to claim 1, The MRI apparatus is characterized in that the refrigerator uses a combination of a GM refrigerator and a JT refrigerator.

8. 2. The MRI apparatus according to claim 1, An MRI device characterized by using liquid He.

9. 2. The MRI apparatus according to claim 1, An MRI device incorporating a direct-cooling type superconducting coil in which the superconducting coil is electrically cooled by a refrigerator.

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