Magnetic resonance imaging apparatus and its operating method

Abstract

By extending the lifespan of the refrigeration unit and reducing the frequency of replacement, the operating rate of the MRI system will be improved. [Solution] The cold head life extension mode is implemented. In the cold head life extension mode, the refrigerator's displayer is operated at a constant frequency lower than a predetermined upper frequency, regardless of the temperature of the superconducting coil, and the compressor inverter controls the drive frequency of the compressor drive unit according to the temperature of the superconducting coil.

Description

Technical Field

[0001] The present invention relates to a Magnetic Resonance Imaging (hereinafter referred to as MRI) apparatus.

Background Art

[0002] An MRI apparatus that cools a superconducting magnet using liquid helium is known. The superconducting coil is disposed within a liquid helium container, and the liquid helium container is equipped with a cold head of a refrigerator that cools the vaporized helium and liquefies it again.

[0003] Patent Document 1 discloses a technique for varying the cooling capacity by changing the driving frequency of a compressor (compressor) of a refrigerator in order to keep the pressure within the liquid helium container constant.

[0004] On the other hand, MRI apparatuses that do not use liquid helium are also known. For example, Patent Document 2 discloses an MRI apparatus that conducts cooling of a bobbin of a superconducting coil disposed within a vacuum container using cold heads of two or more refrigerators. An inverter is connected to the compressor of the refrigerator, and the inverter is controlled based on the temperature of the superconducting coil unit. Thereby, the capacity of the compressor is changed to keep the temperature of the superconducting coil unit constant.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] MRI systems using superconducting coils require a refrigerator to cool the coils. When replacing the refrigerator, the MRI system must be shut down.

[0007] The objective of this invention is to improve the operating rate of MRI equipment by extending the replacement life of the refrigerator and reducing the frequency of refrigerator replacement. [Means for solving the problem]

[0008] According to one aspect of the present invention, a magnetic resonance imaging apparatus is provided having a superconducting magnet that generates a static magnetic field in the imaging space. The superconducting magnet includes a superconducting coil, a container housing the superconducting coil, a cold head attached to the container, a compressor that supplies compressed refrigerant gas to the cold head, and a processor. The compressor includes a mechanism, a compressor drive unit that periodically moves the mechanism and compresses the refrigerant gas, and a compressor inverter that adjusts the drive frequency of the compressor drive unit. The cold head includes a cylinder into which the refrigerant gas compressed by the compressor is supplied, a displayer disposed within the cylinder, and a displayer drive unit that periodically moves the displayer within the cylinder. The cylinder of the cold head is connected to the superconducting coil by a metallic heat conductive member to cool the superconducting coil. The processor has a cold head life extension mode, in which the displayer is operated at a constant frequency lower than a predetermined upper frequency, regardless of the temperature of the superconducting coil, and the compressor inverter controls the drive frequency of the compressor drive unit according to the temperature of the superconducting coil. [Effects of the Invention]

[0009] According to the present invention, by operating the MRI system in cold head life extension mode, the number of displayer movements is reduced, thereby extending the replacement life of the chiller. This reduces the frequency of chiller replacement in the MRI system and improves the operating rate of the MRI system. [Brief explanation of the drawing]

[0010] [Figure 1] A block diagram showing the overall configuration of an MRI apparatus according to an embodiment of the present invention. [Figure 2] A cross-sectional view showing the arrangement of superconducting magnets and other components in the MRI apparatus shown in Figure 1. [Figure 3] Figure 1 shows the cross-sectional structure of the superconducting magnet in the MRI device. [Figure 4] Figure 3 is a cross-sectional view showing the configuration of the cold head 107 and compressor 108 of the superconducting magnet. [Figure 5] A flowchart illustrating the control of the cold head 107 and compressor 108 by the processor of the superconducting magnet in the first embodiment. [Figure 6] A flowchart illustrating the control of the cold head 107 and compressor 108 by a processor in a modified version of the first embodiment of a superconducting magnet. [Figure 7] A flowchart illustrating the control of the cold head 107 and compressor 108 by the processor of the superconducting magnet in the second embodiment. [Figure 8] (a) A graph showing the heat input and compressor operating frequency of the superconducting magnet in the fourth embodiment, and (b) A graph showing the compressor operating frequency and magnet temperature of the superconducting magnet in the fourth embodiment. [Figure 9] A flowchart illustrating the control of the cold head 107 and compressor 108 by the processor of the superconducting magnet in the fourth embodiment. [Figure 10] A flowchart illustrating the control of the cold head 107 and compressor 108 by a processor in a modified example of the fourth embodiment. [Figure 11] A graph showing the amount of heat input from imaging the superconducting magnet in the fifth embodiment and the state of the MRI device. [Figure 12] A flowchart illustrating the control of the cold head 107 and compressor 108 by the processor of the superconducting magnet in the fifth embodiment. [Modes for carrying out the invention]

[0011] An MRI apparatus according to an embodiment of the present invention will be described based on the drawings. In all the drawings for explaining the embodiments of the invention, those having the same function are denoted by the same reference numerals, and the repeated description thereof will be omitted.

[0012] <Overall Configuration of the MRI Apparatus of the Present Embodiment> First, the overall configuration of the MRI apparatus operated in the present embodiment will be described.

[0013] FIG. 1 shows the overall configuration of the MRI apparatus of the present embodiment installed in a medical facility.

[0014] First, an overall outline of the MRI apparatus to which the present invention is applied will be described. As shown in FIG. 1, the MRI apparatus 1 includes an imaging unit 10 that generates nuclear magnetic resonance in the atomic nuclei of the atoms constituting the tissue of the subject 102 and thereby collects nuclear magnetic resonance signals generated from the subject, a processor 20 that processes the nuclear magnetic resonance signals collected by the imaging unit 10 and controls the imaging unit 10, and a user interface unit (hereinafter, UI unit) 30 for setting imaging conditions, inputting commands necessary for processing, and displaying images and GUIs obtained by the MRI apparatus 1 between the MRI apparatus 1 and an operator such as a doctor or a technician (hereinafter, referred to as a user). The MRI apparatus 1 may also include an external storage device 60 for storing the generated images and other information and an interface (not shown) for communicating with an external device.

[0015] The imaging unit 10 includes a superconducting magnet 101 that generates a uniform static magnetic field in the examination space where the subject 102 is placed, a gradient magnetic field coil 112 that applies a magnetic field gradient to the static magnetic field, an RF transmission coil 115 that applies a predetermined high-frequency magnetic field to the subject, and an RF reception coil 117 that receives a nuclear magnetic resonance signal (hereinafter also referred to as an echo signal) generated from the subject. The gradient magnetic field coil 112 is connected to a gradient magnetic field power supply 113, the RF transmission coil 115 is connected to an RF transmission unit 116 composed of an RF transmitter and an RF amplifier, etc., and the RF reception coil 117 is connected to an RF reception unit 118 equipped with a QD detector and an AD converter, respectively. Note that the RF transmission coil 115 and the RF reception coil 117 may be combined by one RF coil in some cases. Generally, the RF transmission coil 115 is housed in a gantry (not shown) so as to surround the examination space together with the superconducting magnet 101 and the gradient magnetic field coil 112, and the RF reception coil 117 is arranged in the examination space while being attached to the subject 102.

[0016] The imaging unit 10 further includes a sequencer 128 that operates the RF transmission unit 116, the gradient magnetic field power supply 113, and the RF reception unit 118 according to a predetermined pulse sequence, and imaging is performed according to the imaging sequence set in the sequencer 128. The operations related to imaging are the same as those of a general MRI device.

[0017] The imaging unit 10 is further provided with a bed device 122 for placing the subject.

[0018] The processor 20 controls the imaging unit 10 and performs signal processing and various calculations on the nuclear magnetic resonance signal collected by the imaging unit 10.

[0019] To realize the above processing, the processor 20 includes an imaging control unit 21 that controls imaging, a display control unit 25 that controls the display in the UI unit 30, an image generation unit 22 that performs various calculations related to image generation such as image reconstruction, and a magnet control unit 110 that controls the superconducting magnet 101.

[0020] In this embodiment, each of the above processes of the processor 20 can be executed on any computer. Furthermore, any computer may execute these processes using a processor as hardware, a program as software, or a combination thereof. In that case, the processor is configured to work in cooperation with the program to execute the various processes in this embodiment, and can function as the units 21, 22, 230, 25, etc., or the means in this embodiment. Also, the execution order of the processes by the processor is not limited to the order described and may be changed as appropriate. Any computer may be a general-purpose computer, a computer designed for a specific purpose, a workstation, or any other system capable of executing each of the processes.

[0021] A processor may consist of one or more pieces of hardware, and the type of hardware is not limited. For example, a processor may be a CPU (Centr A processor can be composed of a programmable logic device such as an al Processing Unit (AL), MPU (Micro Processing Unit), FPGA (Field Programmable Gate Array), dedicated circuits for executing specific processes such as an ASIC (Application Specific Integrated Circuit), GPU (Graphic Processing Unit), or NPU (Neural Processing Unit). The types of hardware may also be combinations of different types of hardware. When multiple hardware components are configured to execute one or more processes of a processor, these components may reside in physically separate devices or in the same device. Furthermore, in any embodiment, the order of each process performed by the processor is not limited to the order described above and may be changed as appropriate. Hardware is composed of electrical circuits (circuitry) that combine circuit elements such as semiconductor elements.

[0022] Furthermore, the program may be firmware or software such as microcode. Alternatively, the program may be, for example, a set of program modules, each function of which may be implemented by a processor configured to perform its respective function. The program may be program code or multiple code segments stored on one or more non-temporary computer-readable media (e.g., storage media or other storage). The program may be divided and stored on multiple non-temporary computer-readable media located on physically separate devices. Program code or code segments may represent any combination of procedures, functions, subprograms, routines, subroutines, modules, software packages, classes, or instructions, data structures, or program statements. Program code or code segments may be connected to other code segments or hardware circuits by sending and receiving information, data, arguments, parameters, or memory contents.

[0023] The configuration of the MRI apparatus 1 will be further explained using Figures 2 to 4. Figure 2 shows the arrangement of the static magnetic field magnets, etc., Figure 3 shows the cross-sectional structure of the superconducting magnet 11, and Figure 5 shows the structure of the refrigerator 70.

[0024] <Superconducting Magnet 101> This MRI device 1 is equipped with a superconducting magnet 101 as a magnet that generates a static magnetic field. The superconducting magnet 101 may be a magnet that generates a static magnetic field in the vertical direction or a magnet that generates a static magnetic field in the horizontal direction, but the explanation will be given using a superconducting magnet 101 that generates a static magnetic field in the horizontal direction as an example. As shown in Figure 3, the superconducting magnet 101 that generates a static magnetic field in the horizontal direction uses a superconducting coil 203 in which a coil is wound in a cylindrical shape to generate a static magnetic field in the horizontal direction (the axis direction of the subject 102's body). The cylindrically wound superconducting coil 203 is placed inside a cylindrical vacuum container 201.

[0025] The cylindrical vacuum vessel 201 has a double-walled structure consisting of an outer vessel 201a and an inner vessel 201b. The space between the outer vessel 201a and the inner vessel 201b is evacuated to a predetermined pressure, and a radiant heat shield plate 216 is placed there. A superconducting coil 203 is placed inside the inner vessel 21b.

[0026] The space within the inner container 201b where the superconducting coil 203 is located is evacuated to a predetermined pressure.

[0027] The magnetic field space 103 in which the subject 102 is placed is formed in the space near the central axis of the cylindrical vacuum container 201.

[0028] The vacuum vessel 201 is equipped with a cold head 107. The tip of the cold head 107 is inserted into the inner vessel 21b of the vacuum vessel 201. A compressor 108 is connected to the cold head 107, and compressed refrigerant gas is supplied from the compressor 108. The cold head 107 and the compressor 108 constitute a cryocooler 70 that cools the superconducting magnet 101.

[0029] The cold head 107 generates a cooling effect due to the adiabatic expansion of the refrigerant gas inside, which cools the tip of the cold head 107. A highly thermally conductive heat conduction member 225 is placed between the tip of the cold head 107 and the superconducting coil 203, with one end of the heat conduction member 225 connected to the cold head 107 and the other end connected to the superconducting coil 203 (see Figures 3 and 4). As a result, the cold head 107 cools the superconducting coil 203 through the heat conduction of the heat conduction member 225.

[0030] Furthermore, a portion of the cold head 107 is connected to the radiant heat shield plate 216 of the vacuum vessel 201 and the inner vessel 201b, and cools them.

[0031] In this way, the refrigerator maintains the thermal equilibrium state of the superconducting magnet 101, realizing a closed-type superconducting magnet. That is, it is controlled to have a cooling capacity that cools the amount of heat that enters the vacuum container 201 without excess or deficiency.

[0032] Furthermore, the superconducting magnet 101 incorporates multiple temperature sensors 206 and pressure sensors for monitoring its operating state, and the sensor connection terminals 109 are connected to the magnet control unit 110.

[0033] The magnet control unit 110 monitors the operating status of the superconducting magnet 101 and controls the cold head 107 and the compressor 108.

[0034] As a result, the superconducting coil 203 is cooled below its critical temperature, enters a superconducting state, and by passing a predetermined persistent current through it, a stable static magnetic field with a magnetic field strength of 1.5 Tesla can be generated in the magnetic field space (imaging space) 103.

[0035] <Shim Plate 111> As shown in Figure 2, a shim plate 111 is attached to the side of the vacuum vessel 201 facing the magnetic field space 103. The shim plate 111 has multiple screw holes (not shown in the figure), and small magnetic screws are embedded in the appropriate positions. The magnetic field generated by the small magnetic screws changes the magnetic flux distribution generated by the superconducting magnet 101, thereby adjusting the magnetic field uniformity of the magnetic field space 103 to a target value (e.g., 3 ppm or less).

[0036] <Gradient coil 112> On the magnetic field space 103 surface of the shim plate 111, gradient magnetic field coils 112 are arranged to generate gradient magnetic fields. Each of the gradient magnetic field coils 112 has a stacked structure consisting of three types of coils (not distinguished in Figure 1) that generate gradient magnetic fields in three mutually orthogonal axis directions: x, y, and z. A gradient magnetic field power supply 113 is connected to each of the x, y, and z coils to independently apply current.

[0037] In addition to the x-coil, y-coil, and z-coil, the gradient magnetic field coil 112 includes a Bo coil that compensates for the magnetic field strength generated by the superconducting magnet 101, and higher-order modes of x, y, z, such as ZX, ZY, XY, Z 2 and x 2 +y 2 and a shim coil that generates a magnetic field are incorporated. A current is applied to these by a shim power supply 114. <QQ

[0038] <QQ <RF transmission coil 115> An RF transmission coil 115 is attached to the magnetic field space 103 side of the gradient magnetic field coil 112. An RF transmission unit 116 is connected to the RF transmission coil 115, and a high-frequency current is supplied. This generates a high-frequency magnetic field necessary for nuclear magnetic resonance of the nuclear spins in the imaging region of the subject 102.

[0039] By combining the above-described stable and highly uniform static magnetic field, gradient magnetic field, and high-frequency magnetic field, it is possible to accurately and selectively cause the hydrogen nuclei in the imaging region of the subject 102 to undergo nuclear magnetic resonance (NMR) phenomenon. Then, three-dimensional position information is added by applying the gradient magnetic field pulsatively during the subsequent precession motion process of the nuclear spins. ;

[0040] ; ;<RF reception coil 117> An RF reception coil 117 is disposed at approximately the center position of the magnetic field space 103, that is, at the imaging region of the subject 102. This RF reception coil 117 detects slight magnetic field fluctuations due to the precession motion of the aforementioned nuclear spins as an electrical signal (NMR signal) by an induced current on the RF reception coil 117. The detected NMR signal is passed to an RF reception unit 118 connected to the RF reception coil 117. The RF reception unit 118 performs signal processing of amplification and detection on the NMR signal and then converts it into a digital signal.

[0041] <Processor 20> The image generation unit 22 of the processor 20 generates MRI images and spectral charts from NMR signals converted into digital signals. The generated images, etc., are stored in the external storage device 60. The display control unit 25 displays the MRI images, etc., on the display 120 of the UI unit 30.

[0042] Furthermore, the imaging control unit 21 of the processor 20 controls the application of a gradient magnetic field and a high-frequency magnetic field from the gradient magnetic field coil 112 and the RF transmitting coil 115 to the subject 102 at timings determined by a predetermined imaging sequence, in order to realize a predetermined imaging method, and to detect the resulting NMR signal at a predetermined timing. The imaging sequence differs depending on the imaging method and imaging conditions. The imaging control unit 21 generates an imaging sequence that realizes the imaging method and imaging conditions input by the operator via the input device 121 of the UI unit 30 by executing a program pre-stored in its built-in memory device. This makes it possible to realize multiple types of imaging methods under various imaging conditions. The input device 121 is, for example, a keyboard or mouse.

[0043] The operating status of the magnet control unit 110, gradient magnetic field power supply 113, shim power supply 114, RF transmitter 116, RF receiver 118, etc., is recorded in a memory device built into the processor 20. The processor 20 can also output this operating status information to an external device via a communication control device (not shown in the diagram). This makes it possible to remotely monitor the MRI device.

[0044] <Bed device 122> The MRI device is equipped with a patient table device 122 that transports the area of ​​the patient 102 being scanned to the center of the magnetic field space 103.

[0045] The superconducting magnet 101 and the examination table device 122 are installed in an electromagnetically shielded examination room 123. This prevents electromagnetic waves generated by external equipment from entering the RF receiving coil 117 as noise and degrading the quality of the diagnostic images.

[0046] <Detailed structure of vacuum vessel 201 for superconducting magnet> The detailed structure of the vacuum vessel 201 of the superconducting magnet 101 described above will be further explained using Figure 3.

[0047] Load supports (not shown) are attached to the outer container 201a, radiant heat shield plate 216, and inner container 201b of the vacuum vessel 201 of the superconducting magnet 101 in order to fix their relative positions. The load supports are made of stainless steel and resin to minimize conductive heat.

[0048] The outer container 201b of the vacuum vessel 201 is made of stainless steel, for example, 10 millimeters thick. The inner container 201b is made of stainless steel, for example, 15 millimeters thick, and has the rigidity to withstand the electromagnetic force applied to the superconducting coil 203 and the pressure difference between the inside and outside.

[0049] The superconducting coil 203 inside the inner container 201b is composed of multiple coils. The superconducting coil 203 is fixed to the inner container 201b.

[0050] A temperature sensor 206 is installed on the superconducting coil 203 to measure its temperature. The output signal line of the temperature sensor 206 is led out from the sensor connection terminal 109 to the outside of the superconducting magnet 101.

[0051] The radiant heat shield plate 216 is made of 5 mm thick aluminum. Its surface is mirror-polished to suppress radiant heat. In addition, a super insulator (not shown) is laid in the gap between the vacuum vessel 201 and the radiant heat shield plate 216. The super insulator 217 is composed of multiple layers of polyethylene sheets with a thin aluminum film deposited on them, and reduces radiant heat.

[0052] The inside of the inner container 201b is evacuated to a predetermined vacuum level.

[0053] <Details of the vacuum vessel 201 for the superconducting magnet> A cold head 107 is located in the vacuum vessel 201. As shown in Figure 4, the cold head 107 has two cooling stages: the first cooling stage 219 is 43 Kelvin (-230°C), and the second cooling stage 220 is 4 Kelvin (-269°C).

[0054] The first cooling stage 219 of the cold head 107 is in thermal contact with the radiant heat shield plate 216 and cools the radiant heat shield plate 216. The tip of the second cooling stage 220 of the cold head 107 is inserted into the inner container 201b.

[0055] The tip of the second cooling stage 220 is connected to the superconducting coil 203 by a heat conductive member 225. This cools the superconducting coil 203 to 4 Kelvin.

[0056] <Details of Refrigeration Unit 70> Next, the detailed structure of the refrigerator 70 will be explained using Figure 4.

[0057] The cold head 107 comprises two-stage cylinders 304a and 304b, a first cooling stage 219 and a second cooling stage 220; two-stage displayers 303a and 303b; and a displayer drive unit 302 that causes the displayers 303a and 303b to reciprocate within the cylinders 304a and 304b. The displayer 303 is packed with lead spheres in the first stage and copper holobium spheres in the second stage as a cold storage material, and is structured to undergo heat exchange as the refrigerant gas passes through them.

[0058] The displayer drive unit 302 is equipped with an intake valve 305 for drawing in refrigerant gas compressed by the compressor 108, and an exhaust valve 306 for exhausting it. These valves open and close in synchronization with the reciprocating motion of the displayer 303. The intake valve 305 and exhaust valve 306 are connected to the compressor 108 via pressure gas hoses 307 and 308, respectively.

[0059] As shown in Figure 4, the refrigerator 70 is equipped with a GM cycle control unit 301 (not shown in Figure 2) between the cold head 107 and the magnet control unit 1. The GM cycle control unit 301 operates the motor of the displayer drive unit 302 of the cold head 107 at a frequency instructed by the magnet control unit 110. Specifically, the GM cycle control unit 301 controls the frequency of the drive power output to the displayer drive unit 302 of the cold head 107. To this end, for example, the GM cycle control unit 301 has the function of a variable inverter and converts power input from an external source such as a commercial power supply into multiple frequencies, including frequencies lower than the frequency of the input power supply (and also converts the voltage as needed), and supplies it to the displayer drive unit 302.

[0060] The compressor 108 includes a mechanism 131 consisting of a cylinder and a piston, a compressor drive unit 132 that periodically moves the piston of the mechanism 131 to compress the refrigerant gas in the cylinder, and a compressor inverter 133 that adjusts the drive frequency of the compressor drive unit 132.

[0061] The cold head 107 generates a cooling effect by operating in the following steps (1) to (3).

[0062] (1) When the displayers 303a and 303b are moved toward the tip (downward) of the cold head 7, the compressed refrigerant gas drawn in from the compressor 108 fills the upper space 309 inside the cylinder 304a.

[0063] (2) Next, when the displayers 303a and 303b are moved upward, the compressed refrigerant gas moves to the lower space 310 while passing through the cold storage agent in the displayers 303a and 303b.

[0064] (3) As the displayers 303a and 303b reach their highest point, the exhaust valve 306 opens. The refrigerant gas in cylinders 304a and 304b undergoes adiabatic expansion due to the pressure drop, causing its temperature to decrease. It then returns to the compressor 108 through the pressure gas hose 308 from the exhaust valve 306.

[0065] By repeating this cycle from (1) to (3), the refrigerant gas continuously absorbs heat from the lower space 310 of the cylinder 304. This thermal cycle is called the Gifford-MacMahon cycle (GM), and cooling devices that use this GM cycle are called GM type refrigerators.

[0066] <First Embodiment> In the first embodiment, the processor 20 has a cold head life extension mode. In the cold head life extension mode, the magnet control unit 110 of the processor 20 operates the displayers 303a and 303b at a constant frequency lower than a predetermined upper frequency, regardless of the temperature of the superconducting coil 203, and controls the drive frequency of the compressor drive unit 132, which is adjusted by the compressor inverter 133, according to the temperature of the superconducting coil 203. It is preferable that the operating frequency of the displayers 303a and 303b be set to a frequency lower than the drive frequency of the compressor drive unit 132.

[0067] The cold head life extension mode can be set by a user or service worker operating the input device 121 of the UI unit 30 in the magnet control unit 110 of the processor 20. Alternatively, the cold head life extension mode can be set by a user or service worker remotely operating the magnet control unit 110 of the processor 20 via a communication control device (not shown in Figures 1 and 2) provided in the MRI device 1.

[0068] The operation of the magnet control unit 110 of the processor 20 will be specifically explained using the flowchart in Figure 5.

[0069] (Step 501) First, the magnet control unit 110 proceeds to step 502, where it obtains the temperature of the superconducting coil 203 from the temperature sensor 206 inside the inner container 201b.

[0070] (Step 502) The magnet control unit 110 determines whether the "cold head life extension mode" has been set by the user or service personnel. If the "cold head life extension mode" has been set by the user, the process proceeds to step 503; otherwise, the process proceeds to step 507.

[0071] (Step 503) The magnet control unit 110 instructs the GM cycle control unit 301 to set the frequency at which the displayers 303a and 303b of the cold head 107 operate to a constant drive frequency lower than a predetermined upper limit frequency. To this end, for example, the GM cycle control unit 301 converts power with a constant frequency input from an external source such as a commercial power supply to a lower frequency (and also converts the voltage as needed) and supplies it to the displayer drive unit 302.

[0072] As a result, the displayers 303a and 303b of the cold head 107 operate at a predetermined constant frequency, regardless of the temperature of the superconducting coil 203.

[0073] The magnet control unit 110 proceeds to step 504.

[0074] (Steps 504-506) Next, if the temperature detected by the temperature sensor 206 is higher than the set temperature (step 504), the magnet control unit 110 proceeds to step 505, instructing the compressor inverter 133 of the compressor 108 to increase the drive frequency of the compressor drive unit 132. As a result, the drive frequency of the compressor drive unit 132 increases, which increases the pressure of the refrigerant gas sent from the compressor 108 to the cold head 107.

[0075] On the other hand, if the temperature obtained from the temperature sensor 206 is below the set temperature, the magnet control unit 110 proceeds to step 506 and instructs the compressor inverter 133 of the compressor 108 to decrease the drive frequency of the compressor drive unit 132. As a result, the drive frequency of the compressor drive unit 132 increases, and the pressure of the refrigerant gas sent from the compressor 108 to the cold head 107 decreases.

[0076] In this way, the magnet control unit 110 increases or decreases the drive frequency of the compressor drive unit 132 according to the temperature of the superconducting coil 203, thereby changing the pressure of the refrigerant gas sent to the cold head 107. As a result, even if the drive frequencies of the displayers 303a and 303b of the cold head 107 remain constant, the cooling capacity of the cold head 107 can be adjusted, and the temperature of the superconducting coil 203 can be maintained at a constant level.

[0077] The set temperature in step 504 may be a specific temperature such as 5 Kelvin or 4 Kelvin, or it may be a temperature range such as 4 Kelvin or more and 5 Kelvin or less. When a temperature range (4 to 5 Kelvin) is used as the set temperature, the magnet control unit 110 proceeds to step 505 if the temperature taken from the temperature sensor 206 is higher than the upper limit of the set temperature range (5 Kelvin), and proceeds to step 506 if it is lower than or equal to the lower limit of the set temperature range (4 Kelvin). If the temperature taken from the temperature sensor 206 is greater than 4 Kelvin and 5 Kelvin or less, the magnet control unit 110 does not control the compressor 108 and returns to step 501.

[0078] (Steps 507-509) On the other hand, in step 502, if the magnet control unit 110 determines that it is not in cold head life extension mode, it proceeds to step 507. If the temperature obtained from the temperature sensor 206 is higher than the set temperature, it proceeds to step 508. In step 508, the magnet control unit 110 instructs the GM cycle control unit 301 to increase the frequency at which the displayer drive unit 302 of the cold head 107 operates. As a result, the drive frequency of the displayers 303a and 303b increases, thereby increasing the cooling capacity of the cold head 107.

[0079] On the other hand, if the temperature obtained from the temperature sensor 206 in step 507 is lower than the set temperature, the magnet control unit 110 proceeds to step 509. In step 509, the magnet control unit 110 instructs the GM cycle control unit 301 to reduce the frequency at which the displayer drive unit 302 of the cold head 107 operates. As a result, the drive frequency of the displayers 303a and 303b is reduced, thereby reducing the cooling capacity of the cold head 107.

[0080] As a result, in steps 507 to 509, the magnet control unit 110 can adjust the cooling capacity of the cold head 107 by increasing or decreasing the driving frequency of the displayers 303a and 303b, thereby maintaining a constant temperature for the superconducting coil 203.

[0081] The temperature set in step 507 may be a specific temperature or a temperature range, similar to the temperature set in step 504.

[0082] As described above, in the cold head life extension mode, in step 503, the operating frequency of the displayers 303a and 303b of the cold head 107 is set to a constant value smaller than a predetermined upper limit. Therefore, the reciprocating frequency of the displayers 303a and 303b is reduced compared to when the drive frequency of the displayers 303a and 303b is increased or decreased according to the temperature, as in steps 507 to 509. This reduces wear and failure of the displayers 303a and 303b and extends the life of the cold head 107.

[0083] The effects of this embodiment will be described in more detail.

[0084] Conventional MRI systems use a synchronous motor as the motor for the cold head 107's displayer drive unit 302. This synchronous motor rotates at a constant rotational speed (synchronous speed) if the power supply frequency is constant, and the displayers 303a and 303b are driven at a frequency corresponding to the commercial AC power supply frequency. For example, a synchronous motor that operates the displayers 303a and 303b at 1Hz for a 50Hz power supply frequency was used as the displayer drive unit 302. In areas where the commercial AC power supply is 60Hz, the displayers 303a and 303b of this MRI system were driven at 1.2Hz.

[0085] In contrast, in this embodiment, the GM cycle control unit 301 controls the displayers 303a and 303b in step 503 to operate at a frequency of 0.8Hz, which is lower than 1Hz or 1.2Hz. To this end, for example, the GM cycle control unit 301 has the function of a variable inverter and has the function of converting the 50Hz commercial power supply not only to 50Hz but also to 40Hz and other frequencies. By converting the frequency to 40Hz and supplying it to the displayer drive unit 302, it becomes possible to operate the displayers 303a and 303b at a frequency of 0.8Hz, which is lower than 1Hz or 1.2Hz.

[0086] The reduced frequency of displayers 303a and 303b reduces the frequency of reciprocating motion, thereby extending the lifespan of the cold head 107.

[0087] Therefore, in step 503, the upper limit of the frequency at which the magnet control unit 110 instructs the GM cycle control unit 301 to operate the displayers 303a and 303b is set to be lower than the operating frequency of the refrigerator displayer in a conventional MRI device (1 Hz or 1.2 Hz in the example above). To this end, for example, the GM cycle control unit 301 has a function to lower the frequency of the input power supply and supply it to the displayer drive unit 302.

[0088] The upper limit frequency of operation for the displayers 303a and 303b is preferably less than or equal to the value obtained by dividing the frequency of the input power, such as the commercial power supply, by the intrinsic value of the displayer drive unit 302. Specifically, for example, if the commercial power supply is 50Hz and the intrinsic value of the displayer drive unit 302 having a synchronous motor is 50, the upper limit is preferably 1Hz or less, and particularly preferably 0.8Hz or less.

[0089] The configuration of the displayer drive unit 302 and the GM cycle control unit 301 of the cold head 107 in this embodiment is not limited to the example described above. Any configuration is acceptable as long as the operating frequency of the displayers 303a and 303b can be set to a constant value smaller than a predetermined upper limit.

[0090] In this embodiment, the magnet control unit 110 reduces the operating frequency of the displayers 303a and 303b to a constant value by instructing the GM cycle control unit 301 (step 503), while at the same time variably controlling the drive frequency of the compressor 108 (steps 505, 506), so as not to reduce the overall cooling performance of the refrigerator 70.

[0091] Furthermore, it is preferable to set the drive frequency of the displayer drive unit 302 of the cold head 107 and the operating frequencies of the displayers 303a and 303b to avoid the natural frequency of the superconducting magnet 101. This is because if the displayer drive unit 302 and the displayers 303a and 303b operate near the natural frequency of the superconducting magnet 101, vibrations will propagate within the superconducting magnet 101, causing unintended structural vibrations and potentially degrading the image quality of the MRI. For example, if a synchronous motor is used as the displayer drive unit 302 to operate the displayers 303a and 303b at a frequency of 1 / 50 of the input power supply frequency, it is preferable to set the power supply frequency input to the displayer drive unit 302 to avoid 50 times the natural frequency of the superconducting magnet 101.

[0092] Furthermore, while the control in steps 504-506 and 507-509 describes an example of increasing or decreasing the drive frequency of the compressor 108 and the operating frequencies of the displayers 303a and 303b depending on whether the temperature is higher or lower than the set temperature, this embodiment is not limited to this control method. Various known control methods exist for controlling the cooling capacity of the cold head 107 (tip temperature of the cold head 107), so any desired method can be used. For example, PID control, a type of feedback control, can be used.

[0093] As described above, in the cold head life extension mode, the MRI apparatus 1 of this embodiment can reduce the number of reciprocating motions compared to conventional displayers by moving the displayers 303a and 303b at a constant frequency lower than a predetermined upper frequency, regardless of the temperature of the superconducting coil 203, thereby extending the replacement life of the cold head 107. Furthermore, by controlling the drive frequency of the displayer drive unit 302, the overall cooling capacity of the refrigerator 70 can be maintained.

[0094] Furthermore, in the cold head life extension mode of the MRI apparatus 1 of this embodiment, the displayers 303a and 303b are operated at a constant frequency lower than a predetermined upper frequency regardless of the temperature of the superconducting coil 203. Therefore, even when an imaging sequence is executed, the operating frequency of the displayers 303a and 303b does not change during the imaging sequence.

[0095] If the operation of the displayers 303a and 303b stops, starts, or changes in frequency during the shooting sequence, the vibration state of the superconducting magnet 101 changes, which can cause degradation of the captured image.

[0096] In this embodiment, in the cold head life extension mode, the displayers 303a and 303b always operate at a constant frequency, which has the advantage of preventing degradation of the captured image.

[0097] As shown in Figure 6, the cold head life extension mode may include both a mode for use during imaging and a mode for use when not imaging. That is, if the user has set the cold head life extension mode in step 502, the magnet control unit 110 proceeds to step 611 to determine whether imaging is currently being performed.

[0098] If imaging is being performed, the magnet control unit 110 proceeds to step 612 and, as described in step 503 above, operates the displayers 303a and 303b continuously at a constant first frequency, regardless of the temperature of the superconducting coil, in imaging mode.

[0099] On the other hand, if no image is being taken in step 611, the magnet control unit 110 proceeds to step 613 and operates the displayers 303a and 303b continuously at a constant second frequency as a non-image-taking mode.

[0100] The second frequency is set lower than the first frequency in the imaging mode, taking into consideration that the amount of heat input to the superconducting magnet 101 is small when not imaging. The second frequency may also be set by the magnet control unit 110 according to the temperature acquired in step 501.

[0101] As a result, the number of operations is further reduced in non-shooting mode than in shooting mode, thus extending the lifespan of the cold head 107.

[0102] <Second Embodiment> Next, the second embodiment will be explained using the flowchart in Figure 7.

[0103] The MRI apparatus 1 of the second embodiment has the same configuration as the first embodiment, but as shown in Figure 7, based on the temperature obtained from the temperature sensor 206 (step 501), the magnet control unit 110 determines whether accelerated cooling is necessary (step 601), and if accelerated cooling is necessary, proceeds to step 602.

[0104] In step 602, the magnet control unit 110 controls the compressor inverter 133 to set the drive frequency of the compressor mechanism 131 of the compressor 108 to the maximum possible value. At the same time, the magnet control unit 110 instructs the GM cycle control unit 301 to also set the operating frequencies of the cold head 107's displayers 303a and 303b to the maximum possible value.

[0105] This allows the superconducting coil 203 to be cooled as quickly as possible.

[0106] If the magnet control unit 110 determines in step 601 that accelerated cooling is unnecessary, steps 503 and onward are executed in the same manner as in the first embodiment.

[0107] In step 601, the magnet control unit 110 determines that accelerated cooling is necessary in the following cases:

[0108] When using an MRI system, there may be situations where transient cooling is required. In particular, in superconducting magnets 101 that are not cooled by liquid helium but by conduction cooling, the transient heat of the superconducting magnet 101 cannot be dissipated by the latent heat of vaporization of liquid helium, so accelerated cooling is often required. For example, this may occur immediately after transporting the MRI system 1, when a quench occurs in the superconducting coil 203, when a power outage occurs, or when the refrigerator 70 is replaced. In these situations, a steady static magnetic field cannot be generated, so the MRI system cannot perform imaging, and it is required to complete the cooling of the superconducting coil 203 as quickly as possible.

[0109] Therefore, in the second embodiment, in step 602, the magnet control unit 110 controls the system to set the drive frequency of the mechanism 131 of the compressor 108 to the maximum possible value, and also to set the operating frequencies of the displayers 303a and 303b of the cold head 107 to the maximum possible value.

[0110] This may increase power consumption and potentially increase static magnetic field oscillations, but the priority is to complete the cooling of the superconducting coil 203.

[0111] Since the aforementioned conditions requiring non-steady-state cooling capacity may occur at night, it is desirable for the magnet control unit 110 to automatically determine this. Therefore, in step 601, if the temperature of the superconducting coil 203 detected by the temperature sensor 206 is determined to have exceeded a predetermined temperature that cannot occur in a steady state (critical temperature), the process proceeds to step 602 to perform accelerated cooling.

[0112] The critical point for superconductivity is determined by three factors: temperature, empirical magnetic field, and current value. Beyond this critical point, superconductivity cannot be maintained. Since the empirical magnetic field and current value are known at the design stage, the critical temperature is determined in advance based on these factors.

[0113] In step 601, if the temperature of the superconducting coil 203 obtained in step 501 is above a predetermined critical temperature, the magnet control unit 110 determines that the superconducting coil 203 is no longer in a superconducting state and proceeds to step 602 to perform accelerated cooling.

[0114] In step 501, it is also possible to acquire temperature sensors for the superconducting magnet 101 other than the temperature sensor 206. Alternatively, the determination in step 601 may be made using detection results other than the temperature of the superconducting magnet 101. For example, a magnetic field sensor for detecting the magnetic field of the superconducting magnet 101 may be placed, and if no magnetic field is detected in step 601, the process may proceed to step 602 to maximize the cooling capacity.

[0115] The configuration and operation of the MRI apparatus 1 of the second embodiment, other than those described above, are the same as those of the first embodiment, so their description will be omitted.

[0116] <Third Embodiment> Next, a third embodiment will be described.

[0117] The MRI apparatus 1 of the third embodiment has the same configuration as the first embodiment, but the processor 20 matches the frequency determined from the parameters of the imaging sequence executed by the imaging unit 10 for imaging with a constant frequency for operating the displayers 303a and 303b. Specifically, the operating frequencies of the displayers 303a and 303b and the driving frequency of the displayer drive unit 302 are set to be n times the reciprocal of the imaging sequence repetition time (TR) (1 / (TR)) (where n is an integer).

[0118] Since the cold head 107 of the refrigerator 70 is inserted inside the superconducting magnet 101, vibrations generated by the operation of the displayers 303a and 303b and the displayer drive unit 302 cause minute vibrations in the superconducting magnet 101. For this reason, in the first embodiment, it was explained that it is desirable to set the frequencies of the operation of the displayers 303a and 303b and the displayer drive unit 302 so as not to match the natural frequency of the superconducting magnet 101.

[0119] In the third embodiment, the operating frequencies of the cold head 107's displayers 303a and 303b and the displayer drive unit 302 are set to be n times (where n is an integer) the reciprocal of the repetition time (TR) of the imaging sequence (1 / (TR)). This allows the fluctuations in the magnetic field caused by the vibration of the superconducting magnet 101 to be synchronized with the imaging sequence, thereby minimizing the influence of the cold head 107's vibrations on imaging.

[0120] Magnetic field fluctuations that are not synchronized with the TR of the imaging sequence cannot be corrected even by post-processing the NMR signal acquired during imaging. However, if the magnetic field fluctuations are synchronized with the imaging sequence, their effects can be mitigated.

[0121] Let me explain the specific vibration frequencies. In conventional MRI systems, the cold head's displayer drive unit uses a synchronous motor and operates the displayer at 1Hz or 1.2Hz, receiving a commercial AC power supply of 50Hz or 60Hz. As a result, vibrations occurred at 1Hz or 1.2Hz, causing magnetic field fluctuations of 1Hz or 1.2Hz in the superconducting magnet 101.

[0122] In the third embodiment, the drive frequency of the displayer drive unit 302 of the cold head 107 is changed, for example, within a range of 40Hz to 70Hz, depending on the shooting sequence. If a synchronous motor similar to that of the conventional is used as the displayer drive unit 302, when the supplied AC power is commercial AC power at 50Hz, the operating frequency of the displayers 303a and 303b becomes 1Hz. Therefore, if the drive frequency of the displayer drive unit 302 is set to a range of 40Hz to 70Hz, for example, the operating frequency of the displayers 303a and 303b becomes 0.8Hz to 1.4Hz. This corresponds to a cycle of 1250ms to 714ms.

[0123] For example, in the case of an imaging sequence with a repeat time (TR) of 800ms, the frequency is 1 / 0.8 = 1.25Hz. Therefore, by selecting 1.25 * 50 = 62.5Hz as the drive frequency for the displayer drive unit 302, the repeat time (TR) of the imaging sequence can be synchronized with the operating frequency of the displayers 303a and 303b of the cold head 107.

[0124] Thus, according to the third embodiment, the fluctuations in the magnetic field caused by the vibration of the superconducting magnet 101 can be synchronized with the imaging sequence, thereby reducing the influence of the vibration of the cold head 107 on imaging.

[0125] <Fourth Embodiment> Next, a fourth embodiment will be described using Figures 8 to 10.

[0126] The MRI apparatus 1 of the fourth embodiment has the same configuration and operation as the first embodiment, but reduces energy consumption compared to the first embodiment. To achieve this, when operating in life extension mode, if the temperature of the superconducting coil 203 drops at night or otherwise, and the drive frequency of the compressor drive unit 132 of the compressor 108 reaches a predetermined lower limit, the operation of the compressor drive unit 132 of the compressor 108 and the displayers 303a and 303b of the cold head 107 is stopped. This will be explained in detail below.

[0127] In the flow chart of Figure 5 of the first embodiment, if the life extension mode is set, steps 503 to 506 are executed, the displayers 303a and 303b operate at a constant frequency, and the temperature of the cold head 107 is controlled to remain constant by increasing or decreasing the drive frequency of the compressor drive unit 132 of the compressor 108 according to the temperature of the superconducting coil 203.

[0128] By performing this control, if an imaging sequence is executed in the MRI device 1 during the daytime and heat is input to the superconducting magnet 101, the temperature can be controlled to a constant level by steps 503 to 506 (Figure 8(a)).

[0129] On the other hand, at night or when the heat input is small, the drive frequency of the compressor drive unit 132 of the compressor 108 may reach the lower limit frequency, as shown in Figure 8(a). If the compressor drive unit 132 remains at the lower limit frequency, the temperature of the superconducting coil 203 will drop too low, as shown in Figure 8(b).

[0130] Therefore, in the fourth embodiment, control is performed to temporarily stop the operation of the compressor drive unit 132 of the compressor 108 and the displayers 303a and 303b of the cold head 107.

[0131] The control of the magnet control unit 110 in the fourth embodiment will be explained using the flowchart in Figure 9.

[0132] (Steps 501-506) In the flowchart of Figure 9, steps 501 to 506 are the same as in the first embodiment.

[0133] (Step 701) In step 504, the temperature of the superconducting coil 203 is below the set temperature, and in step 506, the magnet control unit 110 performs control to reduce the drive frequency of the compressor drive unit 132. As a result, if the drive frequency of the compressor drive unit 132 reaches the lower limit, the magnet control unit 110 sets this time as a1 (step 701).

[0134] (Steps 702, 703) The cold head 107 has a cooling capacity that exceeds the heat input even when the drive frequency of the compressor drive unit 132 reaches its lower limit, so the temperature of the superconducting coil 203 begins to decrease, as shown in Figure 8(b). The magnet control unit 110 proceeds to step 703 when a predetermined X1 hours have elapsed from time a1 and it is time a2 (step 702), and stops the operation of the compressor drive unit 132 of the compressor 108 and the displayers 303a and 303b of the cold head 107. At this time, the temperature of the superconducting coil 203 has decreased to T2.

[0135] (Steps 704, 705) Because the magnetic control unit 110 stopped the operation of the compressor 108 and the cold head 107, cooling no longer occurs. As a result, the temperature rises, as shown in Figure 8(b).

[0136] Therefore, after stopping the compressor 108 and cold head 107 in step 703, when time a3 arrives (X2 hours later), the magnet control unit 110 restarts the compressor 108 and cold head 107 and returns to step 501.

[0137] At this time, the temperature is rising, so in steps 501 to 505, the temperature of the superconducting coil 203 is controlled again by controlling the drive frequency of the compressor drive unit 132 of the compressor 108 so that it approaches the set temperature.

[0138] Instead of controlling based on time as shown in the flow diagram of Figure 9 above, it is also possible to control based on the temperature of the superconducting magnet 101, as shown in the flow diagram of Figure 10.

[0139] (Steps 501-506) In the flow chart of Figure 10, steps 501 to 506 are the same as in the first embodiment.

[0140] (Steps 801, 802) In step 506, if the drive frequency of the compressor drive unit 132 reaches a lower limit, the magnet control unit 110 continuously monitors the temperature of the superconducting coil 203, and when the temperature reaches the lower limit T2, it stops the operation of the compressor drive unit 132 of the compressor 108 and the displayers 303a and 303b of the cold head 107.

[0141] (Steps 803, 804) The magnet control unit 110 constantly monitors the temperature of the superconducting coil 203, and when it rises to the upper limit temperature T3, it restarts the compressor drive unit 132 of the compressor 108 and the displayers 303a and 303b of the cold head 107, and returns to step 501.

[0142] In the flow shown in Figure 9, it is necessary to pre-set appropriate times X1 and X2 so that restarts in step 705 do not occur frequently, and so that T3 does not exceed T1 excessively. Similarly, in the flow shown in Figure 10, it is necessary to pre-set appropriate times T2 and T3 so that restarts in step 804 do not occur frequently, and so that T3 does not exceed T1 excessively.

[0143] As described above, in the fourth embodiment, when the drive frequency of the compressor drive unit 132 of the compressor 108 reaches a predetermined lower limit, the operation of the compressor 108 and the cold head 107 is stopped, thereby reducing energy consumption.

[0144] <Fifth Embodiment> A fifth embodiment will be described with reference to Figures 11 and 12.

[0145] In the fifth embodiment, a cold head life extension mode is executed depending on the state of the device.

[0146] In hospitals where MRI scans are only performed during the daytime, as shown in Figure 11, the power to the scanning unit 10 of the MRI device 1 is turned OFF at night (state (1)), and only the minimum necessary units, such as the refrigerator 70 for the superconducting magnet 101 and the device status monitor, are powered.

[0147] When the hospital opens for business, the power to the imaging unit 10 is turned ON. This power-on energizes all units, including the gradient magnetic field power supply 113 and the RF transmitter 116, putting the system into standby mode for imaging (State (2)). After the operators position the patient 102 and the imaging protocol is set up, imaging is performed (State (3)).

[0148] In the fifth embodiment, as shown in the flowchart of Figure 12, in step 1101, it is determined whether the period is either the standby state (2) or the shooting state (3). If it is state (2) or (3), the cold head life extension mode is executed in steps 503 to 506. As a result, the operating frequencies of the displayers 303a and 303b are set to a constant value, and the temperature is controlled to remain constant by changing the drive frequency of the compressor drive unit 132 of the compressor 108.

[0149] In the nighttime state (1), assuming that no shooting is taking place, steps 507 to 509 are performed to increase or decrease the frequencies of displayers 303a and 303b. Although not shown in the flow chart in Figure 12, in addition to steps 507 to 509, it is also possible to stop and restart the compressor 108 and displayers 303a and 303b by performing steps 703 and 705 in Figure 9, or steps 802 and 804 in Figure 10.

[0150] By performing the control as in the fifth embodiment, it is possible to minimize the impact on shooting while maximizing the energy-saving effect. [Explanation of Symbols]

[0151] 1 MRI machine 10. Photography Department 11 Superconducting Magnets 20 processors 21. Image capture control unit 22 Image generation unit 25 Display Control Unit 30 UI section 60 External storage device 70 Refrigeration unit 101 Superconducting Magnet 102 Subjects 103 Magnetic field space 107 Coldhead 108 Compressor 109 Sensor connection terminal 110 Magnet Control Unit 111 Shim Plate 112 Gradient field coil 113 Gradient magnetic field power supply 114 SIM power supply 115 RF Transmitter Coil 116 RF Transmitter 117 RF receiving coil 118 RF Receiver 120 displays 121 Input device 122 Bed equipment 123 Laboratory 128 Sequencer 131 Mechanism Department 132 Compressor drive unit 133 Inverter for Compressor 201 Vacuum container 201a Outer container 201b Inner container 203 Superconducting Coil 204 Liquid Helium 205 Liquid level sensor 206 Temperature Sensor 216 Radiant Heat Shielding Plate 217 Super Insulator 219 First Cooling Stage 220 Second Cooling Stage 225 Heat Conducting Material 301 GM Cycle Control Unit 302 Display drive unit 303 Display 303a, 303b Display 304 Cylinder 304a, 304b cylinders 305 Intake Valve 306 Exhaust Valve 307 Pressure Gas Hose 308 Pressure Gas Hose 309 Upper space 310 Lower space

Claims

1. It has a superconducting magnet that generates a static magnetic field in the shooting space, The superconducting magnet includes a superconducting coil, a container housing the superconducting coil, a cold head attached to the container, a compressor supplying compressed refrigerant gas to the cold head, and a processor. The compressor includes a mechanism, a compressor drive unit that periodically moves the mechanism and compresses the refrigerant gas, and a compressor inverter that adjusts the drive frequency of the compressor drive unit. The cold head includes a cylinder into which the refrigerant gas compressed by the compressor is supplied, a displayer disposed within the cylinder, and a displayer drive unit that periodically moves the displayer within the cylinder. The cylinder of the cold head is connected to the superconducting coil by a metal heat conductive member, and cools the superconducting coil. The magnetic resonance imaging apparatus is characterized in that the processor has a cold head life extension mode, in which the displayer is operated at a constant frequency lower than a predetermined upper frequency regardless of the temperature of the superconducting coil, and the drive frequency of the compressor drive unit adjusted by the compressor inverter is controlled according to the temperature of the superconducting coil.

2. A magnetic resonance imaging apparatus according to claim 1, characterized in that the displayer drive unit and the compressor inverter are supplied with power of a constant frequency from a commercial AC power source.

3. A magnetic resonance imaging apparatus according to claim 1, wherein the cold head further comprises a cold head inverter for adjusting the drive frequency of the displayer drive unit, The processor controls the inverter for the cold head in addition to the inverter for the compressor. The cold head life extension mode of the aforementioned processor includes a mode for use during shooting and a mode for use when not shooting. In the aforementioned imaging mode, the magnetic resonance imaging apparatus is characterized in that the displayer is operated continuously at the constant frequency regardless of the temperature of the superconducting coil.

4. A magnetic resonance imaging apparatus according to claim 3, wherein the processor operates the displayer at a frequency smaller than the constant frequency of the imaging mode in the non-imaging mode.

5. A magnetic resonance imaging apparatus according to claim 3, wherein the processor stops the displayer in the non-imaging mode according to the temperature of the superconducting coil.

6. A magnetic resonance imaging apparatus according to claim 3, wherein the processor determines the constant frequency for moving the displayer based on the repetition frequency used for the parameters of the imaging sequence performed by the magnetic resonance imaging apparatus for imaging.

7. A magnetic resonance imaging apparatus according to claim 1, wherein the processor controls the drive frequency of the compressor drive unit according to the temperature of the superconducting coil, and stops the compressor and the displayer when the drive frequency of the compressor drive unit reaches a predetermined lower limit frequency.

8. A magnetic resonance imaging apparatus according to claim 1, wherein the processor controls the drive frequency of the compressor drive unit according to the temperature of the superconducting coil, and stops the compressor and the displayer when the temperature of the superconducting coil reaches a predetermined lower limit temperature.

9. A magnetic resonance imaging apparatus according to claim 7, characterized in that the processor restarts the compressor and the displayer when a predetermined time has elapsed since the compressor and the displayer were stopped, or when the temperature of the superconducting coil reaches a predetermined upper limit temperature, or when the pressure inside the container reaches a predetermined upper limit pressure.

10. A magnetic resonance imaging apparatus according to claim 1, wherein the processor executes the cold head life extension mode when the magnetic resonance imaging apparatus is performing imaging or in an imaging standby state.

11. A method for controlling a refrigerator provided in a superconducting magnet of a magnetic resonance imaging apparatus, The refrigerator includes a cold head provided on a superconducting magnet and a compressor that supplies compressed refrigerant gas to the cold head. A control method characterized by operating the cold head displayer at a constant frequency lower than a predetermined upper frequency, regardless of the temperature of the superconducting magnet, and controlling the drive frequency of the compressor drive unit of the compressor according to the temperature of the superconducting magnet.

Images