MRI apparatus

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 using a cold storage material used at extremely low temperatures. [Background technology]

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

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

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

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

[0006] In addition, in refrigerators such as GM refrigerators, pulse tube refrigerators, and Stirling refrigerators, high-pressure working gas flows back and forth through the gaps between 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 multiple manufacturing processes such as synthesis of raw materials, granulation, sintering at high temperatures, and polishing to make them spherical, intermetallic compounds, which can be manufactured by the simple process of melting and solidifying, are preferable from the perspective of manufacturing cold accumulators. As a candidate for intermetallic cold accumulators, RCu2X2 (R = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, X = Si, Ge) is known to have a large specific heat at extremely low temperatures.

[0008] However, RCu2X2-based intermetallic compounds are produced, for example, by melting the raw materials using an arc melting method, and then subjecting the resulting ingot to a high-temperature, long-term homogenizing heat treatment in a vacuum (for example, at 800 degrees for one week). The need for a high-temperature, long-term heat treatment process after melting and solidification in this way 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 device equipped with a refrigerator that is highly efficient and has excellent cooling performance by filling a cold storage material that has a large specific heat in the cryogenic temperature range, small magnetization, and good manufacturability, and further provides an MRI device that can reduce the effects of magnetic noise originating from the cold storage 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 grains made of an intermetallic compound in which the ThCr2Si2 type structure accounts for 80 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 description of the drawings]

[0013] [Figure 1] 1 is a schematic diagram of a ThCr2Si2 type structure showing a crystal structure of a regenerator material according to a first embodiment. [Diagram 2] FIG. 2 is an explanatory diagram of the granular shape of the cold storage material according to the first embodiment. [Diagram 3] FIG. 11 is a cross-sectional view of a two-stage expansion GM refrigerator, which is 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 incorporated device according to the third embodiment. [Diagram 5]Graph showing the measurement results of Example 1 (upper part) and Comparative Example 1 (lower part) by powder X-ray diffraction method. [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] Graph showing the measurement results of Example 1 (upper part) and Comparative Example 2 (lower part) by powder X-ray diffraction method. [Figure 8] A table showing the crystallite size, volume percentage of ThCr2Si2 type structure, proportion of finely powdered samples, peak specific heat temperature, and peak specific heat value 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 a cryogenic temperature region in Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] (First embodiment) Hereinafter, the embodiments will be described in detail. Fig. 1 is a schematic diagram of a ThCr2Si2 type structure 11 showing the crystal structure of the cold storage material according to the first embodiment. The cold storage material according to the first embodiment is a grain made of an intermetallic compound in which the ThCr2Si2 type structure 11 occupies 80 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 between the regenerator material filled in the regenerator, and the heat generated by the gas compression-expansion cycle is stored in the regenerator material, thereby cooling from room temperature to extremely low temperatures. Therefore, the regenerator material installed in the refrigerator is required to have a large specific heat characteristic within the operating temperature range.

[0017] By having the ThCr2Si2 type structure 11 occupy 80% by volume or more of this intermetallic compound, a cold storage material having high specific heat characteristics in the cryogenic temperature range can be obtained. If the ThCr2Si2 type structure 11 occupies less than 80% by volume of the intermetallic compound, the specific heat characteristics may be deteriorated compared to general substances listed as cold storage materials in the cryogenic temperature range. The volume percentage of the ThCr2Si2 type structure can be calculated by Rietveld analysis of the powder X-ray diffraction method or by evaluating the ratio of phases in multiple fields of view by scanning electron microscope observation.

[0018] In addition, in GM and Stirling refrigerators, the regenerator filled with the regenerator material vibrates, so the regenerator material is required to have mechanical strength. Therefore, by making the crystallite size of the regenerator material fine at 70 nm or less, the regenerator material has excellent mechanical strength. The crystallite size L is calculated by evaluating the peak width (half-width) β in the X-ray diffraction pattern and using Scherrer's formula (formula (1)). 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, 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 cold storage material is larger than 70 nm, the mechanical strength is poor, and the particles are fatigue-fractured and pulverized 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 the particle diameter 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, φmax and φmin are in the range of 0.01 mm to 1 mm, and more preferably in the range of 0.05 mm to 0.5 mm. If the area of ​​the projection image 15 of this cold storage material is A and the area of ​​the smallest circumscribing circle 16 circumscribing this projection image 15 is M, the shape factor represented by M / A is in the range of 1.0 to 5.0 in all projection directions.

[0021] By making the particle diameter of the cold storage material fall within the range of 0.01 mm to 1 mm, the flow of the working gas (He gas) reciprocating in the cold storage unit filled with the cold storage material in the refrigerator described below is not impeded, and good heat exchange between the working gas and the cold storage material is realized. If the particle diameter of the cold storage material is less than 0.01 mm (10 μm), the gap between the particles of the cold storage material, i.e., the space through which the working gas flows, becomes narrow, and the pressure loss of the gas may increase. If the particle diameter of the cold storage material is larger than 1 mm, the filling rate of the cold storage material in the cold storage unit may decrease, and the heat exchange between the working gas and the cold storage material may decrease.

[0022] The manufacture of such a cold storage material is carried out by at least going through a process of mixing and melting the component elements of the intermetallic compound that can have the above-mentioned ThCr2Si2 type structure 11 in their stoichiometric ratio, and a process of injecting this molten liquid into a dynamic cooling medium to rapidly cool and solidify it into granules.

[0023] That is, elemental metals mixed to obtain a stoichiometric ratio of the ThCr2Si2 type structure 11 are melted by high-frequency induction heating or the like. The molten metal is then supplied to the running surface of a high-speed rotating body placed in a vacuum or inert gas atmosphere. The molten metal is finely dispersed by the motion of the rotating body and is simultaneously quenched and solidified to form spherical particles. Alternatively, the above-mentioned molten metal is flowed into a vacuum or inert gas atmosphere and a non-oxidizing atomizing gas is applied. As a result, the molten metal is atomized and dispersed, and is simultaneously quenched and solidified to form spherical particles.

[0024] Specific methods for rapidly solidifying the above-mentioned molten metal include a rotating disk process (RDP: Rotary Disc Process), a single roll process, a twin roll process, an inert gas atomization process, a rotating nozzle process, and the like. 5 ~10 6 The material can be rapidly cooled at a cooling 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 the rapid solidification method for molten metal are explained in Patent No. 2609747, etc.

[0025] By the way, by adding an intermetallic compound with a different magnetic phase transition temperature to an intermetallic compound with a ThCr2Si2 type structure, the specific heat characteristics per unit volume of the cold storage material can be improved. For example, if an intermetallic compound with a ThCr2Si2 type structure contains phases with AlB2 type and LiGaGe type structures, the specific heat can be increased in the 4-20K range. Also, if an intermetallic compound with a ThCr2Si2 type structure contains a phase with a Gd3Cu4Ge4 type structure, the specific heat can be increased in the vicinity of 7-50K. However, if a phase other than the ThCr2Si2 type is present at 20% by volume or more, the volumetric specific heat derived from the ThCr2Si2 type phase becomes small. Also, since the intermetallic compound is composed of phases with different crystal structures, the mechanical strength of the cold storage 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 first cylinder 31 with a large diameter and a second cylinder 32 with a small diameter that is coaxially connected to the first cylinder 31. A first cold storage unit 34 is disposed in the first cylinder 31 so as to be capable of reciprocating, and a second cold storage unit 35 is disposed in the second cylinder 32 so as to be capable of reciprocating. Seal rings 36 and 37 are disposed between the first cylinder 31 and the first cold storage unit 34, and between the second cylinder 32 and the second cold storage unit 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 tip 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 is at 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 while securing a passage 33 for the working gas (He gas, etc.). The first regenerator material 38 may be a stainless steel mesh in addition to the copper alloy mesh, or may use both. The second regenerator 35 is filled with a second regenerator material 40 while securing a passage 39 for the working gas. Although the regenerators 34 and 35 are shown in which the first regenerator material 38 and the second regenerator material 40 are filled separately, they may 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. Taking into consideration the fluidity of the working gas, the filling rate of the second cold storage materials 40a, 40b in the space separated by the mesh 48 is preferably 50 to 75%, and more preferably 55 to 65%.

[0030] In the two-stage refrigerator 30, the 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 of the first cold storage material 38 housed in the first cold storage device 34, reaches the first expansion chamber 41, and cools the first cooling stage 43 by expanding. Next, the working gas passes through the gaps of the second cold storage material 40 housed in the second cold storage device 35, reaches the second expansion chamber 42, and cools the second cooling stage 44 by expanding.

[0031] The low-pressure working gas passes through the second regenerator 35 and the first regenerator 34 in that 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 38, 40 exchanges thermal energy with the working gas to store and hold cold heat, and also to regenerate heat.

[0032] Next, the cycle will be described with a focus 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 300K), and is pre-cooled by the first cold storage material 38 as it passes through the first cold storage device 34, and reaches the first expansion chamber 41. The temperature of the working gas further drops as it expands in the first expansion chamber 41, and cools the first cooling stage 43. Next, the working gas is pre-cooled by the second cold storage material 40 as it passes through the second cold storage device 35, and reaches the second expansion chamber 42. The temperature of the working gas further drops as it expands in the second expansion chamber 42, and cools the second cooling stage 44.

[0033] The low-pressure working gas passes through the second cold storage device 35 while storing cold energy in the second cold storage material 40 (while the working gas itself is being heated). The working gas then passes through the first cold storage device 34 while storing cold energy in the first cold storage 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 cold storage materials 38, 40 inside the cold storage units 34, 35. In such a refrigeration cycle, the larger the specific heat of the cold storage 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, it is effective to selectively arrange the second cold storage material 40 having good heat recovery characteristics in each temperature range according to the temperature gradient. Therefore, the second cold storage unit 35 is filled with a plurality of second cold storage materials 40 (40a, 40b) having different heat recovery characteristics.

[0036] In order 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 100K or less, Cu is selected because it has a large specific heat per unit volume in this temperature range, and a 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 60K or lower, Pb or Bi having 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 8K or lower, a cold storage material having a ThCr2Si2 type structure according to the first embodiment having 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. In this way, it is preferable that the cold storage materials 38, 40 of the GM refrigerator are arranged by selecting a material having a large volumetric specific heat in the operating temperature range of each part, taking into consideration the temperature gradient inside the cold storage units 34, 35. 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 HoCu2, Er3Ni, etc. may be arranged, and the second cold storage material 40 is not limited to the above-mentioned two layers, and may be formed into three layers or more.

[0038] Moreover, the refrigerator equipped with the regenerator material according to the first embodiment is not limited to the above-mentioned GM refrigerator. 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 a boundary region between a cold part and a hot 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 incorporating apparatus according to the third embodiment. In a diagnosis using this MRI apparatus 50, a movable table (not shown) on which a subject 52 lies is moved into a tunnel-shaped bore space 51. Then, 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 subject 52 is reconstructed.

[0041] Currently, in mainstream MRI devices 50, a superconducting coil that generates a high magnetic field such as 1.5T or 3T is used for the first electromagnet 53. The stronger the magnetic field, the better the S / N (signal / noise) ratio of the magnetic resonance response signal, and the clearer the image can 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] Since these wires need to be kept below the critical temperature for superconducting transition, the first electromagnet 53 is placed in a He bath 56 filled with liquid He that liquefies at 4.2 K (approximately -269°C) or less under 1 atmosphere. Since 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, 59 are provided in the insulating vacuum layer 57 to reduce the influence of heat penetration from the environment (room temperature: approximately 300 K) in which the MRI device 50 is installed. The shield 58 is cooled to approximately 4 K, and the shield 59 is cooled to approximately 40 K by the installed refrigerator 30.

[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 cooling performance of the GM refrigerator was dramatically improved in the 1990s by installing a magnetic regenerator material, and it became possible to generate extremely low temperatures below the liquid He temperature using only the GM refrigerator, and therefore GM refrigerators are widely used in MRI devices 50 that are in widespread use at the time of filing of the present application.

[0044] As shown in Fig. 4, the first cooling stage 43 (Fig. 3) of the GM refrigerator 30 is connected to a shield 59, and the second cooling stage 44 (Fig. 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. For this reason, 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 medical institution such as a hospital injects liquid He during the initial startup of the MRI device 50, there is no need to periodically replenish liquid He, which is expensive and difficult to handle, during subsequent operation. This significant improvement in convenience has led to the widespread introduction of MRI devices 50 into small and medium-sized hospitals. MRI devices incorporating direct-cooling superconducting coils that use a refrigerator to cool the superconducting coils by conduction, without using liquid He, have also been commercialized. In this case, the liquid He bath 6 can be omitted.

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

[0047] Therefore, in MRI devices using high-temperature superconducting materials, it is necessary to 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 less at 1 atmosphere, or to conductionally cool the superconducting coil with a refrigerator. Even in the former method, it is preferable to cool using a refrigerator to prevent the evaporation of liquid He, H2, and Ne. To improve the performance of the refrigerator at 10 to 30 K, it is preferable to install a cold storage material with a large specific heat in the same temperature range in the refrigerator.

[0048] The superconducting coil assembly 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 5K or less. Since the magnetization of the regenerator material is small in this way, the influence of magnetic noise originating from the regenerator material can be reduced, and high-quality images can be obtained. The superconducting coil assembly device according to the third embodiment is not limited to the above-mentioned MRI device 50, and other examples include superconducting magnets for magnetic levitation 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. For this reason, the performance of cryopump devices can be improved by installing a cold storage material with a large specific heat at around 10 K in the refrigerator. EXAMPLES

[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 roll quenching at a distance of 0.5 mm between the nozzle and the roll and a cooling rate of 10 5 ~10 6As Comparative Example 1, the mixture and melting conditions were the same as those in Example 1, and a thin-flak sample was prepared by using an arc melting method at a cooling rate of 10 °C / sec. 2 Bulk samples were prepared by slow cooling and solidification at ℃ / sec.

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

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

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

[0054] FIG. 6 is a graph showing the specific heat characteristics in the cryogenic range of Example 1 and Comparative Example 1. The specific heat characteristics were measured using a Physical Property Measurement System (PPMS) manufactured by Nippon Quantum Design Co., Ltd. As shown in FIG. 6, it can be seen that 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. Thus, by using the intermetallic compound of Example 1 as a cold storage material filled in the cold storage unit of a refrigerator, the cooling capacity of the refrigerator is improved.

[0055] Comparative Example 2 The compounding 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 the above-mentioned 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 four 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 four days.

[0058] Comparative Example 5 A bulk sample was prepared in the same manner as in Comparative Example 2, except that the heat treatment was carried out at 700° C., which is below the solidification point, for four days.

[0059] Comparative Example 6 The compounding conditions were the same as in Example 1, and the high-frequency melting method was used at a cooling rate of 10 2 Bulk samples were prepared by slow cooling and solidification at ℃ / sec.

[0060] Example 4 A flake-shaped 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 four days.

[0063] Example 5 A flake-shaped 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 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 four days.

[0066] Example 6 A flake-shaped 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 four days.

[0069] Example 7 A flake-shaped 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 heat treatment was carried out at 900° C., which is below the solidification point, for four days.

[0072] 7 is a graph showing the measurement results of Example 1 (upper row) and Comparative Example 2 (lower row) by powder X-ray diffraction method. Note that Example 1 (upper row) of FIG. 7 and Example 1 (upper row) of FIG. 5 are the same data with only the scale indication of the horizontal axis being different. As shown in FIG. 7, in Comparative Example 2, by holding at high temperature and heat treating the solid phase, the X-ray diffraction pattern of the unintended crystal structure that existed in Comparative Example 1 disappears, and it can be seen that most of the crystal structure becomes DyCu2Ge2 as in Example 1.

[0073] Furthermore, when comparing the X-ray diffraction patterns in FIG. 7 between Example 1 and Comparative Example 2, it can be seen that the peak broadening is large in Example 1. The crystallite size was calculated from the half-width β of the peak identified as ThCr2Si2 type. 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 has superior mechanical properties.

[0074] The sample was placed in a vibration test container (D=15mm, h=14mm) and subjected to a maximum acceleration of 300m / s 2 A simple harmonic motion of 1×10 6 After the test, the samples were classified by shape and sieved appropriately, 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 of the sample, the content of the ThCr2Si2 type structure, the proportion of the pulverized sample, the specific heat peak temperature, and the specific heat peak value in Examples 1 to 7 and Comparative Examples 1 to 14. When the crystallite size is larger than 70 nm, the proportion of the pulverized sample increases significantly, and the mechanical strength decreases. When the content of the ThCr2Si2 type structure is less than 80 volume %, the specific heat peak value decreases significantly.

[0076] FIG. 9 is a graph showing the magnetization characteristics in the cryogenic region of Example 1. The magnetization characteristics were measured using a magnetic property evaluation device (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 is 0.97 emu / g or less. Note that the magnetization of GOS, which has high specific heat characteristics similar to those of Examples 1 to 3 in the temperature range near 5 K, is 1.5 emu / g, while the magnetization of HoCu2 used on the low-temperature side of the second-stage regenerator other than GOS is 3.5 emu / g, and the magnetization of Er3Ni is 7 emu / g. Therefore, the regenerator materials of Examples 1 to 3 have small magnetization characteristics, and therefore contribute to high image quality when installed in an MRI device, and contribute to reducing magnetic noise in a superconducting coil-embedded device.

[0077] In the case of the cold storage material described in Example 1, if the particle diameter is less than 0.01 mm (10 μm), the gap between the particles of the cold storage material, i.e., the space through which the working gas flows, becomes narrow, and the pressure loss of the gas increases, so the refrigeration performance decreases. Also, if the particle diameter of the cold storage material is larger than 1 mm, the filling rate of the cold storage material in the cold storage unit decreases, so the refrigeration performance decreases.

[0078] According to at least one of the above-described embodiments, it is possible to provide an MRI apparatus equipped with a cryogenic refrigerator that is highly efficient and has excellent cooling performance by filling the cold storage material that has a large specific heat in a cryogenic temperature range, a small magnetization, and good manufacturability. Furthermore, it is possible to provide an MRI apparatus that can reduce the influence of magnetic noise caused by the cold storage material.

[0079] Although some 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 in the scope of the invention and its equivalents described in the claims, as well as in the scope and spirit of the invention. [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 S 2 The MRI apparatus is equipped with a refrigerator containing a cold storage material including particles of an intermetallic compound in which a type structure occupies 80% by volume or more and the crystallite size is 70 nm or less, The cold storage material is the ThCr 2 S 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 a 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 apparatus characterized in that the material 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 The MRI apparatus according to claim 1,

6. 2. The MRI apparatus according to claim 1, The MRI apparatus, wherein 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 apparatus characterized by using liquid He.

9. 2. The MRI apparatus according to claim 1, An MRI apparatus incorporating a directly cooled superconducting coil in which a refrigerator cools the superconducting coil electrically.

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