Superconducting magnets and magnetic resonance imaging apparatus

Abstract

To reduce the current in the superconducting coil without causing a quench, even when the external power supply is cut off. [Solution] One embodiment of the superconducting magnet comprises at least one superconducting coil that generates a static magnetic field, a refrigerator for cooling the superconducting coil, a radiation shield for shielding radiant heat, and a vacuum vessel. At least one first diode and at least one second diode are provided in parallel with the superconducting coil. The second diode is installed at a position where the steady state temperature is higher than that of the superconducting coil, and the on-voltage of the second diode is lower than that of the first diode when it is in a closed circuit.

Description

TECHNICAL FIELD

[0001] Embodiments disclosed in the present specification and the drawings relate to a superconducting magnet and a Magnetic Resonance Imaging (MRI) apparatus.

BACKGROUND ART

[0002] An MRI apparatus is an imaging apparatus that executes a scan for exciting nuclear spins of a subject placed in a static magnetic field with a Radio Frequency (RF) pulse of the Larmor frequency and acquiring Magnetic Resonance (MR) signals generated from the subject in association with the excitation, and generates an MR image based on the MR signals acquired by the scan.

[0003] Some MRI apparatuses include a static magnetic field magnet that generates a static magnetic field, which is configured as a superconducting magnet. A superconducting magnet incorporates a superconducting coil, and conventionally, the superconducting coil is cooled to an extremely low temperature by liquid helium. When the superconducting coil is cooled by liquid helium, in the process leading to quenching of the superconducting coil, the latent heat of liquid helium absorbs the magnetic energy accumulated in the superconducting coil, so that the temperature rise of the superconducting coil can be suppressed. Therefore, even if the superconducting coil is quenched, the MRI apparatus can be quickly restarted by injecting replenished liquid helium. Note that quenching refers to a phenomenon in which a superconducting substance such as a superconducting coil transitions from a superconducting state to a normal conducting state.

[0004] However, due to the soaring price and difficulty in obtaining helium, MRI devices are being developed that do not use liquid helium, or use only a few liters of helium to cool the superconducting coils. In the case of superconducting coils that do not use liquid helium, the superconducting magnet is not filled with liquid helium. Therefore, if the external power supply to the refrigerator is cut off due to a power outage or malfunction, the temperature of the superconducting coil will rise due to heat intrusion from the outside, and the superconducting coil may quench. Furthermore, once quenching occurs, it becomes necessary to recool the superconducting coil, which takes time before the MRI device can be restarted.

[0005] Furthermore, even with a structure that cools the superconducting coil with a few liters of helium, once a quench occurs, the temperature of the superconducting coil rises, necessitating recooling, which delays the restart of the MRI device. Note that the external power supply refers to the power supplied to the MRI device, including the refrigerator, from an external source, and does not include the power generated by the magnetic energy stored in the superconducting coil. Therefore, if there is a risk of a quench occurring, the current flowing through the superconducting coil is reduced before the quench occurs. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2024-115549 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] One of the problems that the superconducting magnets of the embodiments disclosed herein and in the drawings aim to solve is to reduce the current in the superconducting coil without generating a quench, even when the external power supply is cut off. However, the problems that the embodiments disclosed herein and in the drawings aim to solve are not limited to the above problem. Problems corresponding to the effects of each configuration shown in each embodiment described later can also be positioned as other problems. [Means for solving the problem]

[0008] One embodiment of the superconducting magnet comprises at least one superconducting coil that generates a static magnetic field, a refrigerator for cooling the superconducting coil, a radiation shield for shielding from radiant heat, and a vacuum vessel. At least one first diode and at least one second diode are provided in parallel with the superconducting coil. The second diode is installed at a position where the steady-state temperature is higher than that of the superconducting coil, and the on-voltage of the second diode is lower than that of the first diode when it is in a closed circuit. [Brief explanation of the drawing]

[0009] [Figure 1] A block diagram showing an example of the overall configuration of an MRI apparatus according to this embodiment. [Figure 2] Circuit diagram showing a superconducting magnet related to a comparative example. [Figure 3] A block diagram showing an example of the internal configuration of a superconducting magnet according to the first embodiment. [Figure 4] A diagram illustrating the concepts of (A) persistent current mode and (B) demagnetization mode in a superconducting magnet according to the first embodiment. [Figure 5] A circuit diagram showing an example of a superconducting magnet according to a modified example of the first embodiment. [Figure 6] A block diagram showing an example of the internal configuration of a superconducting magnet according to the second embodiment. [Modes for carrying out the invention]

[0010] The superconducting magnet 10 and MRI apparatus 1 according to this embodiment will be described below with reference to the attached drawings. In each figure, the same elements are denoted by the same reference numerals, and redundant explanations are omitted. In the following description, an MRI apparatus 1 equipped with the superconducting magnet 10 is shown, but the superconducting magnet 10 according to this embodiment can also be applied to devices other than MRI apparatuses, such as NMR devices.

[0011] (Overall configuration of the MRI system) Figure 1 is a block diagram showing an example of the overall configuration of MRI apparatus 1. MRI apparatus 1 comprises a pallet unit 100, a control cabinet 300, a console 400, and a patient table unit 500.

[0012] The mounting device 100 comprises a superconducting magnet 10, a gradient magnetic field coil 11, and a whole-body RF coil (WB coil) 12. These components are housed in a cylindrical casing.

[0013] The superconducting magnet 10 has a roughly cylindrical shape and generates a static magnetic field in the imaging area within the bore into which the patient subject is brought. The bore is the examination space inside the cylinder of the superconducting magnet 10. The superconducting magnet 10 contains at least one superconducting coil 21 (see Figure 3). The superconducting coil 21 is a coil wound with superconducting wire, cooled to a temperature at which it becomes superconducting, and is a superconducting magnet that stably generates a static magnetic field without supplying current from outside the magnet.

[0014] The superconducting magnet 10 of this embodiment is a conduction-cooled superconducting magnet that cools the superconducting coil 21 through a heat transfer path from a refrigerator without using liquid helium or other refrigerants. The superconducting wire used in the superconducting magnet can be of any type, including low-temperature superconducting wires such as niobium titanium and niobium tin, and high-temperature superconducting wires such as copper oxide superconducting materials (REBCO), Bi-based materials (Bi-2212 and Bi-2223, etc.), and MgB2. The superconducting coil 21 is cooled, for example, by a refrigerator 101 (see Figure 3) to maintain its superconducting state. The superconducting magnet 10 may also be a superconducting magnet immersed in liquid helium or other refrigerants, or it may be a thermosiphon-type superconducting coil that cools the superconducting coil 21 by circulating a refrigerant through a tube. Further details about the superconducting magnet 10 will be described later.

[0015] The gradient magnetic field coil 11 has a substantially cylindrical shape, and is fixed inside the superconducting magnet 10 in the radial direction of the cylindrical shape. The gradient magnetic field coil 11 receives current supply from the gradient magnetic field power supply 31 to generate a gradient magnetic field. The gradient magnetic field coil 11 incorporates coils that generate magnetic fields corresponding to the mutually orthogonal X-axis, Y-axis, and Z-axis, and the coils for each axis are individually supplied with current from the gradient magnetic field power supply 31 to superimpose the magnetic fields corresponding to the X-axis, Y-axis, and Z-axis on the static magnetic field, thereby generating the gradient magnetic field.

[0016] Here, the Z-axis direction is a direction set along the magnetic flux of the static magnetic field generated by the superconducting magnet 10, and is the same direction as the longitudinal direction of the top plate 51. The Y-axis direction is a vertical direction orthogonal to the Z-axis direction, and is a direction perpendicular to the top plate 51. The X-axis direction is a direction orthogonal to both the Z-axis and the Y-axis, and is the same direction as the lateral direction of the top plate 51.

[0017] The WB coil 12 is an RF coil that has a substantially cylindrical shape and is fixed inside the gradient magnetic field coil 11 so as to surround the subject. The WB coil 12 transmits RF pulses transmitted from the transmitter 32 to the subject, and receives MR signals emitted from the subject P by excitation of hydrogen atomic nuclei.

[0018] The MRI apparatus 1 may have a local coil 20 in addition to the WB coil 12. The local coil 20 is an RF coil disposed close to the subject, and receives MR signals emitted from the subject at a position close to the subject. The local coil 20 may transmit the RF pulses transmitted from the transmitter 32 to the subject. The local coil 20 has various types corresponding to the imaging region of the subject, such as for the head, chest (for example, Fig. 1), spine, lower limbs, and whole body.

[0019] The control cabinet 300 includes a gradient magnetic field power supply 31, a transmitter 32, a receiver 33, and a sequence controller 34.

[0020] The gradient magnetic field power supply 31 supplies current to the gradient magnetic field coil 11 under the control of the sequence controller 34, and generates gradient magnetic fields corresponding to the respective axes of the X-axis, Y-axis, and Z-axis.

[0021] The transmitter 32 generates an RF pulse train in the Larmor frequency band as an RF transmission wave based on an instruction from the sequence controller 34, and outputs it to the RF coil to excite the subject P.

[0022] The receiver 33 performs analog-to-digital (AD: Analog to Digital) conversion on the MR signal received by the RF coil, and outputs raw data, which is the digitized MR signal, to the sequence controller 34.

[0023] The sequence controller 34 executes a scan of the subject P by respectively driving the gradient magnetic field power supply 31, the transmitter 32, and the receiver 33 under the control of the console 400. The sequence controller 34 receives the raw data via the receiver 33, and transmits the raw data to the console 400.

[0024] The sequence controller 34 includes a processing circuit (not illustrated). The processing circuit of the sequence controller 34 is configured by, for example, hardware such as a processor that executes a predetermined program, an FPGA (Field Programmable Gate Array), or an ASIC (Application Specific Integrated Circuit).

[0025] The couch device 500 includes a couch main body 50 and a top plate 51. The couch main body 51 is capable of moving the top plate 51 in the vertical direction and the horizontal direction, and moves the subject P placed on the top plate 51 to a predetermined height and into the bore.

[0026] The console 400 comprises a processing circuit 41, a storage circuit 42, an input interface 43, a network interface 44, and a display 45. The console 400 is an image processing device configured with a computer.

[0027] The processing circuit 41 provides overall control for the entire MRI device 1. The processing circuit 41 is a processor that realizes various functions by reading and executing various processing programs stored in the memory circuit 42 or directly incorporated into the processing circuit 41.

[0028] The memory circuit 42 is a recording medium readable by a processor, such as a semiconductor memory element like RAM (Random Access Memory) or flash memory, a hard disk, or an optical disc drive. The memory circuit 42 may also be a portable medium such as a USB (Universal Serial Bus) memory or a DVD (Digital Versatile Disk). The memory circuit 42 stores various information and data. In addition, the memory circuit 42 stores various programs executed by the processor of the processing circuit 41, as well as data necessary for program execution.

[0029] The input interface 43 includes an input device that can be operated by the user and an input circuit that processes signals from the input device. The input device is, for example, a trackball, a switch, a mouse, a keyboard, a touchpad that performs input operations by touching the scanning surface, a touchscreen that integrates a display screen and a touchpad, a non-contact input device using an optical sensor, or an audio input device. When the user operates the input device, the input circuit generates a signal corresponding to that operation and outputs it to the processing circuit 41.

[0030] The network interface 44 implements various information communication protocols according to the network configuration. The network interface 44 performs communication control according to the various protocols and communicates with various devices connected to the network via wired or wireless means, exchanging various information and data.

[0031] The display 45 is a display device such as a liquid crystal display, an OLED (Organic Light Emitting Diode) display panel, a plasma display panel, or an organic EL (Electro Luminescence) panel. The display 45 displays various information and images according to the control of the processing circuit 41. The display 45 may also be a GUI (Graphical User Interface) that functions as both a display device and an input device.

[0032] These components allow the console 400 to control the entire MRI device 1. Specifically, the processing circuit 41 receives instructions regarding imaging conditions and other various information from a user, such as a medical technologist, via the input interface 43. The processing circuit 41 then instructs the sequence controller 34 to perform a scan based on the input imaging conditions. The processing circuit 41 also reconstructs the MR image based on the raw data transmitted from the sequence controller 34. The generated MR image is displayed on the display 45 and stored in the memory circuit 42.

[0033] As mentioned above, the superconducting magnet 10 according to this embodiment is a conduction-cooled superconducting magnet. Therefore, if the external power supply is cut off due to a power outage or malfunction, the refrigerator cannot be driven, and the temperature of the superconducting coil rises due to heat intrusion from the outside, which may cause the superconducting coil to quench.

[0034] Furthermore, if the external power supply is cut off due to a power outage or malfunction, the cooling device for cooling the load may not be able to be driven, potentially preventing the load from being adequately cooled. If the load is not adequately cooled, it may burn out, which could lead to the superconducting coil quenching. Therefore, the current flowing through the superconducting coil is reduced before quenching occurs. This operation is called demagnetization. During this demagnetization process, for example, all or part of the magnetic energy stored in the superconducting coil (e.g., several to tens of MJ (Mega Joules)) is consumed by the load, etc.

[0035] Here, Figure 2 shows a circuit configuration of a superconducting magnet according to a comparative example (prior art). The superconducting magnet according to the comparative example comprises a superconducting coil, a persistent current switch, a load, a cooling device, and a heat sink. Conventionally, a protection diode (not shown) is electrically connected to the superconducting coil in parallel with the superconducting coil and in parallel with the persistent current switch. Here, for example, the load is a diode that converts the forward current flowing during demagnetization into heat through its internal resistance, the cooling device is a cooling fan, and the heat sink is a heat sink fin.

[0036] The protection diode is installed to prevent the potential difference across the superconducting coil and the persistent current switch from exceeding a predetermined value, that is, to protect the superconducting coil and the persistent current switch from overvoltage. This has the effect of preventing the superconducting coil and the persistent current switch from burning out or failing.

[0037] However, if the turn-on voltage, which is the minimum voltage required for a diode with a load to begin flowing current in the forward direction, is greater than the turn-on voltage of the protection diode, then even if the permanent current switch is changed from the superconducting state (closed circuit state) to the normal conducting state (open circuit state) to demagnetize it, no current will flow to the load. Instead, current will flow to the protection diode inside the cooling container, causing the temperature inside the cooling container, including the superconducting coil, to rise.

[0038] Therefore, in the embodiment, the superconducting magnet 10, even when the external power supply is cut off during the demagnetization process, is configured to allow the magnetic energy stored in the superconducting coil to be consumed by a load or the like, regardless of the potential difference between the superconducting coil and the permanent current switch, thereby suppressing the temperature rise of the superconducting coil and the quenching of the superconducting coil.

[0039] (First Embodiment) Figure 3 is a block diagram showing an example of the internal configuration of a superconducting magnet 10 according to the first embodiment. The superconducting magnet 10 of the MRI apparatus 1 comprises a cooling container 103 that houses the superconducting magnet 10, a vacuum container 102 that houses the cooling container 103, and a refrigerator 101 attached to the vacuum container 102. The refrigerator 101 is, for example, a two-stage refrigerator and has a first cooling stage and a second cooling stage 104 that is at a lower temperature than the first cooling stage. The cooling container 103 is, for example, made of an aluminum thermal radiation shield plate and is an example of a radiation shield that blocks radiant heat.

[0040] The superconducting magnet 10 comprises a superconducting coil 21, a first diode section 22, a persistent current switch (PCS) 23, a cooling device 24, a load 25, a heat sink 26, and a control device 27. The superconducting coil 21, the first diode section 22, and the persistent current switch 23 are enclosed in a vacuum vessel 102 called a cryostat, which prevents heat intrusion in order to cool them to extremely low temperatures. The load 25 may be installed outside the cooling vessel 103 so as not to affect the temperature of the superconducting coil 21, or it may be installed inside the vacuum vessel 102 (low temperature section), or it may be installed inside the vacuum vessel 102, or it may be installed outside the vacuum vessel 102 (room temperature section). The load 25 may be installed, for example, on the outer wall or thermal radiation shield of the room temperature section of the vacuum vessel 102. In other words, the load 25 is installed at a location where the steady state temperature is higher than that of the superconducting coil 21. A steady state is a state in which the magnetic field generated by the superconducting magnet 10 remains stable and maintains a constant strength. This is due to the advantages of the large heat capacity of the vacuum container 102 and the expectation of heat dissipation into the air, as well as the ease of replacement of the load 25 in the event of a failure.

[0041] Figure 3 is an example of an equivalent circuit diagram showing the electrical connection relationships of a superconducting magnet 10. The superconducting coil 21 generates a static magnetic field by a persistent current. Although Figure 3 schematically shows one superconducting coil 21, the superconducting coil 21 may be composed of multiple superconducting coils. When the superconducting coil 21 is composed of multiple superconducting coils, all of the superconducting coils may be electrically connected in series, all of them may be electrically connected in parallel, or a circuit configuration in which series and parallel connections are combined may be used.

[0042] The persistent current switch 23 is electrically connected in parallel with the superconducting coil 21, that is, between the current input terminal 211 and the current output terminal 212 of the superconducting coil 21. The persistent current switch 23 can switch between a superconducting state (closed circuit state) and a normal conducting state (open circuit state) by thermal control. The persistent current switch 23 is configured, for example, with a superconducting member and a heater 28 disposed in close proximity to the superconducting member. When the heater 28 is off, the persistent current switch 23 is in a closed circuit state, and the superconducting member is maintained in a superconducting state. When the heater 28 is on, the superconducting member is in a normal conducting state, and the persistent current switch 23 is in an open circuit state. The persistent current switch 23, which is configured with a superconducting member and a heater 231 disposed in close proximity to the superconducting member, performs its switching function based on the presence or absence of electrical resistance, and even in the open circuit state, both ends of the persistent current switch 23 are electrically connected. Furthermore, in a normal conducting state, the persistent current switch is almost entirely resistive, so a shunt current flows depending on the circuit state.

[0043] The control device 27 performs thermal control to switch between the superconducting state (closed circuit state) and the normal conducting state (open circuit state) of the persistent current switch 23. Specifically, the control device 27 controls the on / off state of the heater 28, and when it is on, the heater 28 is heated. The control device 27 is composed of, for example, a processor that executes a predetermined program, or hardware such as an FPGA or ASIC.

[0044] The first diode section 22 is comprised of at least one diode. The first diode section 22 has, for example, at least one pair of diodes 221 and 222 that are installed bidirectionally. The first diode section 22 is a protection diode as described above for protecting the superconducting coil 21 and the persistent current switch 23. The bidirectional installation of the pair of diodes 221 and 222 protects the superconducting coil 21 and the persistent current switch 23 in the event of an unexpected quench.

[0045] Between the two ends of the superconducting coil 21, that is, between the current input terminal 211 and the current output terminal 212, high voltages of opposite polarity can be generated when the current flowing through the superconducting coil 21 decreases rapidly and when it increases rapidly, respectively. Therefore, by providing a bidirectionally connected first diode section 22 (i.e., a protection diode), the generation of high voltages of either positive or negative polarity can be prevented.

[0046] In Figure 3, an example is shown where a pair of diodes 221 and 222 are electrically connected in parallel with each other. However, multiple diodes that are electrically connected in series may be installed bidirectionally and electrically connected in parallel. The first diode section 22 is electrically connected in parallel with the superconducting coil 21.

[0047] The load 25 is electrically connected in parallel with the superconducting coil 21. In the first embodiment, the load 25 is a second diode section 25 consisting of at least one diode 251 and 252. The second diode section 25 is electrically connected in parallel with the superconducting coil 21. The cathode of the second diode section 25 is connected to the current input terminal 211 of the superconducting coil 21, and the anode of the second diode section 25 is connected to the current output terminal 212 of the superconducting coil 21.

[0048] In other words, at least one diode 251 and 252 constituting the second diode section 25 is connected to the superconducting coil 21 such that no current flows through the second diode section 25 during the excitation process of the superconducting coil 21, and current flows through the second diode section 25 during the demagnetization process of the superconducting coil 21.

[0049] Here, the excitation process of the superconducting coil 21 refers to the process of supplying current to the superconducting coil using a static magnetic field power source (not shown) to generate a magnetic field, and the demagnetization process of the superconducting coil 21 refers to the process of reducing the current flowing through the superconducting coil 21 and thereby reducing the magnetic field generated by the superconducting coil 21.

[0050] In Figure 3, the case in which the two diodes of the second diode section 25 are electrically connected in series is shown as an example. However, the second diode section 25 may have one or more diodes. If the second diode section 25 is composed of multiple diodes, all of the diodes may be electrically connected in series, all of them may be electrically connected in parallel, or a circuit configuration in which series and parallel connections are combined may be used.

[0051] In this embodiment of the superconducting magnet 10, the second diode section 25 and the first diode section 22 are configured such that the on-voltage (i.e., turn-on voltage) at which the second diode section 25 becomes a closed circuit is lower than that of the first diode section 22.

[0052] When diodes are electrically connected in parallel, current will flow through the diode with the lower turn-on voltage among the electrically connected diodes in parallel. In other words, because the turn-on voltage of the second diode section 25 is lower than the turn-on voltage of the first diode section 22, even when the external power supply is cut off during the demagnetization process, the current flowing through the superconducting coil 21 flows through the second diode section (i.e., the load) 25, not the first diode section (i.e., the protection diode) 22, regardless of the potential difference between the superconducting coil and the permanent current switch. Therefore, the magnetic energy stored in the superconducting coil 21 is reliably and efficiently consumed by the load 25.

[0053] Furthermore, the superconducting magnet 10 may also be equipped with radio wave shielding. It is preferable that all (for example, area A1 in Figure 3) or part of the wiring electrically connected to the load 25 and the load 25 itself are surrounded by radio wave shielding. For example, it is preferable that the wiring installed outside the vacuum vessel 102 is surrounded by radio wave shielding. This makes it possible to suppress noise from the transmitter 32 or external electromagnetic waves from causing unexpected currents to flow through the load 25. Also, from the viewpoint of repair and maintenance, it is preferable that all or part of the load 25 is configured to be detachable from the vacuum vessel 102. Note that part of the load 25 is, for example, one or more of the at least one diodes that constitute the second diode section.

[0054] The heat sink 26 is, for example, positioned in contact with the load 25 to release heat from the load 25 by thermal conduction. The heat sink 26 may release a portion of the heat from the load 25. The heat sink 26 may be, for example, a heat sink, and may include a phase change material housed within the heat sink. The heat sink is made of, for example, a metal with excellent thermal conductivity such as aluminum or copper. The phase change material is, for example, a material that undergoes at least one phase change within a predetermined temperature range, such as paraffin wax.

[0055] In other words, if the heat sink 26 is equipped with a phase change material, the latent heat (absorption or release of thermal energy) generated when a phase change occurs within a predetermined temperature range is used to release heat from the load 25.

[0056] The cooling device 24 cools all or part of the load 25 and the heat sink 26. The cooling device 24 may cool all or part of the load 25, or all or part of the heat sink 26, or all or part of both the load 25 and the heat sink 26. The cooling device 24 includes a cooling unit 241 that operates using the magnetic energy stored in the superconducting coil 21 as a power source during the demagnetization process of the superconducting coil 21. The cooling unit 241 has a cooling device 242 and a control circuit 243 that controls the cooling device 242. The cooling device 242 is, for example, a cooling fan, an air compressor, a circulation pump, or a cooler. The cooling unit 241 enhances the cooling of the heat sink 26 by increasing or strengthening the flow rate of a coolant, such as a gas (e.g., air) or liquid (e.g., water), flowing to the heat sink 26 via the cooling device 242, thereby indirectly or directly assisting in the cooling of the load 25.

[0057] In other words, during the demagnetization process, the cooling unit 241 consumes the power (electrical energy) generated by the magnetic energy stored in the superconducting coil 21 and converts it into mechanical energy, thereby suppressing the temperature rise of the load 25 and promoting its temperature drop without relying on an external power source. The cooling unit 241 may have multiple cooling devices 242.

[0058] The cooling device 24 is electrically connected in parallel with the load 25. The cooling device 24 has a diode 244 electrically connected in series with the cooling unit 241. The cathode of the diode 244 is connected to the current input terminal 211 of the superconducting coil 21, and the anode of the diode 244 is connected to the current output terminal 212 of the superconducting coil 21. In other words, the cooling device 24 is installed in such a way that no current flows through it during the excitation process of the superconducting coil 21, and current flows through it during the demagnetization process of the superconducting coil 21.

[0059] Next, we will explain the operation of the superconducting magnet 10 during the excitation process (i.e., excitation mode), the persistent current mode, and the demagnetization process (i.e., demagnetization mode).

[0060] In excitation mode, a current supplied from a static magnetic field power supply (not shown) is applied to the superconducting coil 21. In excitation mode, the control device 27 controls the heater 28 of the permanent current switch 23 to turn on, and the permanent current switch 23 enters a normal conducting state (open circuit state). In excitation mode, the excitation current supplied from the static magnetic field power supply mainly flows through the superconducting coil 21. When the excitation current is gradually increased from zero and reaches the rated value, the system transitions from excitation mode to permanent current mode. In some cases, the system transitions from excitation mode to permanent current mode after an operation called overshoot, in which the excitation current exceeds the rated value and then decreases to the rated value.

[0061] Figure 4(A) is a diagram illustrating the concept of the persistent current mode in the superconducting magnet 10. When the excitation current reaches the rated value, the control device 27 turns off the heater 28 of the persistent current switch 23. When the heater 28 of the persistent current switch 23 is turned off, the superconducting material of the persistent current switch 23 cools down and transitions from a normal conducting state to a superconducting state, and the persistent current switch 23 enters a superconducting state (closed circuit state).

[0062] As a result, a persistent current loop is formed between the persistent current switch 23 and the superconducting coil 21, causing a persistent current to flow through the superconducting coil 21 and generating a static magnetic field. Alternatively, once the superconducting magnet 10 enters persistent current mode, the current of the static magnetic field power supply may be reduced to 0A before disconnecting the static magnetic field power supply. Since both the superconducting coil 21 and the persistent current switch 23 are superconducting and have zero electrical resistance, even if the static magnetic field power supply is removed from the superconducting magnet 10, the persistent current remains in persistent current mode, maintaining the persistent current loop.

[0063] Figure 4(B) is a diagram illustrating the concept of the demagnetization mode in the superconducting magnet 10. In the demagnetization mode, the control device 27 controls the heater 28 of the permanent current switch 23 to turn on, and the permanent current switch 23 enters a normal conducting state (open circuit state).

[0064] As a result, in demagnetization mode, a current loop is formed between the load 25 and the superconducting coil 21, and the magnetic energy stored in the superconducting coil 21 is gradually consumed by the resistive load 25 until demagnetization is complete. Therefore, the current flowing through the current loop decreases, and the magnetic field gradually decreases. In addition, in demagnetization mode, a current loop is formed between the cooling device 24 and the superconducting coil 21, and the magnetic energy stored in the superconducting coil 21 is consumed as power (electrical energy), causing the cooling unit 241 of the cooling device 24 to operate.

[0065] Thus, according to the superconducting magnet 10 of this embodiment, even if the external power supply is cut off due to a power outage or refrigerator failure, the magnetic field generated by the superconducting coil 21 can be demagnetized. Furthermore, during this demagnetization process, the cooling unit 241 of the cooling device 24 can be driven without relying on an external power supply by consuming the power generated by the magnetic energy stored in the superconducting coil 21. As a result, the heat sink and the load can be cooled, and even in cases where conventional methods failed to cool the load, causing it to burn out and resulting in a quench, demagnetization can be performed without burning out the load or causing a quench. In addition, the time required to re-excite the superconducting coil can be shortened, which in turn shortens the time required to start a rescan by the MRI device 1.

[0066] (Modification of the first embodiment) Figure 5 is a circuit diagram showing an example of a superconducting magnet 10 according to a modification of the first embodiment. The modification of the first embodiment differs from the first embodiment shown in Figure 3, in that a plurality of (for example, three in Figure 5) superconducting coils 21a, 21b, and 21c are electrically connected in series. The plurality of superconducting coils 21a, 21b, and 21c are electrically connected in series with each other, and the plurality of first diode sections 22a, 22b, and 22c are each electrically connected in parallel with the plurality of superconducting coils.

[0067] (Second Embodiment) Figure 6 is a block diagram showing an example of the internal configuration of a superconducting magnet 10 according to the second embodiment. In the second embodiment, the load 25 is configured to have at least one resistor 253 and an on / off switch 254 electrically connected in series with at least one resistor 253, which is different from the first embodiment in that the load 25 is configured to have a second diode section. The on / off switch 254 is a device that can switch the current flowing through the resistor by mechanical, electrical, or both mechanical and electrical means, that is, a device for switching the connection of an electrical circuit. The resistor 253 may be configured to have a diode.

[0068] The on / off switch 254 is set to an open circuit state when the control device 27 supplies current to the superconducting coil 21 (i.e., during the excitation process). Conversely, the on / off switch 254 is set to a closed circuit state when the control device 27 reduces the current flowing through the superconducting coil 21 (i.e., during the demagnetization process). The on / off switch 254 is configured to be set to an open circuit state or a closed circuit state using the magnetic energy stored in the superconducting coil 21 as a power source. This ensures that the superconducting coil 21 can be reliably demagnetized even when the external power supply is cut off during the demagnetization process.

[0069] As mentioned above, the "load 25" in the first embodiment was at least one diode (second diode section), whereas the "load 25" in the second embodiment is at least one resistor and an on / off switch connected in series with at least one resistor. In the description of the first embodiment, "load 25" can be read as at least one resistor and an on / off switch connected in series with at least one resistor in the second embodiment. In addition, part of the load 25 may be at least one resistor, the individual resistors constituting at least one resistor, or an on / off switch.

[0070] According to the superconducting magnet and MRI apparatus of the embodiments described above, even if the external power supply is cut off, the current in the superconducting coil can be reduced without generating a quench.

[0071] In the above embodiment, the term "processor" refers to circuits such as a dedicated or general-purpose CPU (Central Processing Unit), GPU (Graphics Processing Unit), or Application Specific Integrated Circuit (ASIC), or a programmable logic device (e.g., a Simple Programmable Logic Device (SPLD), a Complex Programmable Logic Device (CPLD), and a Field Programmable Gate Array (FPGA)).

[0072] If the processor is, for example, a CPU, it implements various functions by reading and executing programs stored in memory circuits. Alternatively, if the processor is, for example, an ASIC, instead of storing programs in memory circuits, functions equivalent to those programs are directly incorporated as logic circuits within the processor's circuitry. In this case, the processor implements various functions through hardware processing that reads and executes the programs incorporated within the circuitry. Furthermore, a processor can also implement various functions by combining software and hardware processing.

[0073] Furthermore, although the above embodiment shows an example where a single processor in the processing circuit implements each function, a processing circuit may be configured by combining multiple independent processors, with each processor implementing each function. Also, when multiple processors are provided, the memory circuit for storing programs may be provided individually for each processor, or a single memory circuit may store programs corresponding to the functions of all processors together.

[0074] While several embodiments have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These embodiments can be implemented in a variety of other forms, and various omissions, substitutions, modifications, and combinations of embodiments are possible without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of Symbols]

[0075] 1…Magnetic Resonance Imaging (MRI) device 10…Superconducting magnet 21…Superconducting coil 22…First diode section 23…Permanent current switch 24…Cooling device 25…Load 26…Heat sink 27…Control unit 241…Cooling unit

Claims

1. A superconducting magnet comprising at least one superconducting coil for generating a static magnetic field, a refrigerator for cooling the superconducting coil, a radiation shield for shielding from radiant heat, and a vacuum vessel, The superconducting coil is provided in parallel with at least one first diode and at least one second diode, wherein the second diode is installed at a position where the steady-state temperature is higher than that of the superconducting coil, and the on-voltage of the second diode is lower than that of the first diode when it becomes a closed circuit. Superconducting magnet.

2. A superconducting magnet comprising at least one superconducting coil for generating a static magnetic field, a refrigerator for cooling the superconducting coil, a radiation shield for shielding from radiant heat, and a vacuum vessel, The superconducting coil is characterized in that at least one first diode, at least one resistor, and a switch connected in series with the at least one resistor, which can switch the current flowing through the resistor mechanically, electrically, or both mechanically and electrically, are provided in parallel with the superconducting coil. Superconducting magnet.

3. The resistor is characterized in that it is a diode. The superconducting magnet according to claim 2.

4. When supplying current to the superconducting coil, the switching switch is in an open circuit state. The superconducting magnet according to claim 2 or 3.

5. When the current flowing through the superconducting coil is reduced, the switching switch is configured to be in a closed-circuit state. The superconducting magnet according to claim 2 or 3.

6. The aforementioned switch operates using the magnetic energy of the superconducting coil as its power source. The superconducting magnet according to claim 2 or 3.

7. The at least one second diode is installed in the vacuum container. The superconducting magnet according to claim 1.

8. The at least one resistor and the on / off switch connected in series to the at least one resistor are installed in the vacuum container. The superconducting magnet according to claim 2 or 3.

9. The at least one second diode is configured to be detachable from the vacuum container, in whole or in part. The superconducting magnet according to claim 1.

10. The at least one resistor and the on / off switch, in whole or in part, are configured to be detachably attached to the vacuum container. The superconducting magnet according to claim 2 or 3.

11. The invention further comprises a heat sink for dissipating heat from at least one of the second diodes. The superconducting magnet according to claim 1.

12. The invention further comprises the aforementioned at least one resistor and a heat sink for dissipating heat from the on / off switch. The superconducting magnet according to claim 2.

13. The heat sink contains a phase change material that undergoes at least one phase change within a predetermined temperature range. The superconducting magnet according to claim 11 or 12.

14. The system further comprises a cooling device for cooling all or part of the at least one second diode and the heat sink. The superconducting magnet according to claim 11.

15. The system further comprises a cooling device for cooling all or part of the at least one resistor, the on / off switch, and the heat sink. The superconducting magnet according to claim 12.

16. The cooling device operates using the magnetic energy of the superconducting coil as its power source. The superconducting magnet according to claim 14 or 15.

17. The superconducting coil and the first diode are each a plurality, The plurality of superconducting coils are electrically connected in series with each other, and the plurality of first diodes are electrically connected in parallel with each of the plurality of superconducting coils. A superconducting magnet according to claim 1 or 7.

18. The device further comprises wiring electrically connected to at least one second diode, All or part of the aforementioned wiring and the at least one second diode are surrounded by an electromagnetic shield. A superconducting magnet according to claim 1 or 7.

19. The device further comprises wiring electrically connected to the at least one resistor, All or part of the aforementioned wiring, the at least one resistor, and the on / off switch are surrounded by an electromagnetic shield. The superconducting magnet according to claim 2 or 3.

20. A magnetic resonance imaging apparatus using a superconducting magnet, The superconducting magnet comprises at least one superconducting coil that generates a static magnetic field, a refrigerator for cooling the superconducting coil, a radiation shield for shielding from radiant heat, and a vacuum vessel. At least one first diode and at least one second diode are provided in parallel with the superconducting coil, the second diode is installed at a position where the steady-state temperature is higher than that of the superconducting coil, and the second diode has a lower closed-circuit on-voltage than the first diode. Magnetic resonance imaging device.

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