WO2014199793A1 - Magnetic resonance imaging device and method for operating same - Google Patents

Abstract

In order to provide a magnetic resonance imaging device that does not use liquid helium and that can stably operate a superconducting magnet operating in driven mode, the present invention is, at a power source unit that supplies current to a superconducting coil, provided with a control unit that controls the current supplied to the superconducting coil on the basis of at least one unit of information among the power supply state from the outside to the power source unit, the operating state of a refrigerator, the temperature of the superconducting coil, the magnetostatic field strength, the current value flowing to the superconducting coil, and the nuclear magnetic resonance signal detected by a detector.

Description

Magnetic resonance imaging apparatus and operation method thereof

The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI) apparatus using a superconducting magnet and an operation method thereof, and in particular, in a driven mode in which a power source for applying current to a superconducting coil is always connected and operated. The present invention relates to an MRI apparatus using a superconducting magnet and an operation method of the MRI apparatus.

In order for the superconducting magnet used in the MRI apparatus to generate a magnetic field stably, the temperature of the superconducting coil must be kept below the critical temperature inherent to the wire that maintains the superconducting state of the superconducting coil. The superconducting coil wires currently used in commercial use are NbTi and Nb ₃ Sn, and it is necessary to cool the superconducting coil to 10 Kelvin (−263 ° C.) or lower in order to pass a current stably in the superconducting state.

Therefore, as described in Patent Document 1, a superconducting coil is disposed in a low-temperature tank combined with liquid helium or a refrigerator kept at a critical temperature or lower. Then, a permanent current switch (hereinafter referred to as PCS) is connected to the superconducting coil to form a closed loop circuit of the superconducting coil and operated in the permanent current mode. When the supply of liquid helium is insufficient or the refrigerator is stopped due to a power failure, the temperature of the superconducting coil is detected. When the temperature of the superconducting coil reaches a predetermined upper limit temperature, the PCS is opened, and the current flowing through the superconducting coil is caused to flow through the protective resistor to be consumed, thereby reducing the current flowing through the superconducting coil to zero. With this configuration, the temperature of the superconducting coil does not rise above a predetermined temperature, so that a recooling process at the time of re-operation is unnecessary, and a superconducting magnetic resonance imaging apparatus capable of rapid re-operation is provided. .

On the other hand, Patent Document 2 proposes a superconducting magnet using a high-temperature superconducting wire in order to obtain a stronger magnetic field strength than a superconducting magnet composed of a conventional low-temperature superconductor. Since the high-temperature superconducting wire has a slight residual resistance, in Patent Document 2, the power supply is always connected, and the superconducting magnet is operated in a driven mode in which current is continuously applied to compensate for the current attenuation due to the minute residual resistance. In Patent Document 2, the superconducting coil of the high-temperature superconducting wire is arranged in a liquid helium tank.

Non-Patent Document 1 discloses a superconducting magnet for MRI in which a MgB ₂ high-temperature superconducting wire is wound around 12 pancake-shaped coils and combined with a U-shaped iron yoke. This superconducting magnet cools the coil to a temperature of 20 Kelvin with two refrigerators without using liquid helium. Then, in a driven mode operation in which 90 amperes are energized from the power source, a magnetic field of 0.5 Tesla is generated in a U-shaped 60 cm opening. The human head and lumbar vertebra are arranged in the space of the opening and their MRI images are obtained.

JP 2005-124721 JP JP 2008-020266 A

IEEE Transaction on Applied Superconductivity, June 2008, Vol.18, No.2, p.882-886

In the superconducting magnets of Patent Document 2 and Non-Patent Document 1 described above, no consideration is given to the case where the refrigerator is stopped due to a power failure or system failure during the driven mode operation. In the conventional superconducting magnet, when the replenishment of liquid helium is not in time or the refrigerator is stopped, the superconducting coil exceeds the critical temperature and transitions from superconducting to normal conducting. When the superconducting coil transitions to normal conduction, the energy stored in the superconducting coil becomes heat, and the temperature of the superconducting coil rises. With superconducting magnets such as Patent Documents 1 and 2 that use liquid helium, as long as liquid helium remains, the heat of the superconducting coil can be taken away by vaporization of liquid helium, preventing the temperature of the superconducting coil from rising. it can.

However, unlike the superconducting magnet of Non-Patent Document 1, liquid helium is not used, the superconducting coil is cooled by a refrigerator, and the superconducting magnet operating in the driven mode cannot cool the superconducting coil when the refrigerator is stopped. Moreover, since a current is continuously applied from the power source, the superconducting coil is thermally damaged. Even if thermal damage does not occur, the temperature of the superconducting coil rises, so even if the refrigerator is restarted, it takes a long time to recool the superconducting coil whose temperature has risen below the critical temperature. To do.

The present invention has been made in view of the above problems, and its object is to use a superconducting magnet that does not use liquid helium and that is operated in a driven mode, even when the refrigerator is stopped, the time to restart. An object of the present invention is to provide an MRI apparatus that can be operated with a reduced length.

The present invention provides an external power supply state to a power supply unit that supplies current to the superconducting coil, an operating state of the refrigerator, a temperature of the superconducting coil, a static magnetic field strength, a current value flowing through the superconducting coil, and detection of the detection unit The current supplied to the superconducting coil is controlled based on at least one of the nuclear magnetic resonance signals.

According to the present invention, even if a power failure or system failure occurs, it is possible to prevent the superconducting magnet operated in the driven mode from becoming high temperature, so that the time required for recooling can be shortened and the operation can be resumed early. Thereby, the interruption time of the MRI examination can be shortened as much as possible.

Explanatory drawing which shows the whole structure of the MRI apparatus of embodiment. FIG. 2 is a cross-sectional view of the superconducting magnet 103 of FIG. FIG. 2 is a circuit diagram of a superconducting magnet 103 and a magnet power source 106 of the MRI apparatus of FIG. FIG. 2 is a flowchart showing a method of operating the superconducting magnet 103 when the superconducting magnet is activated by the MRI apparatus of FIG. FIG. 2 is a flowchart showing a method of operating a superconducting magnet 103 during MRI examination with the MRI apparatus of FIG. FIG. 2 is a flowchart showing a method of operating a superconducting magnet 103 when the refrigerator 108 is stopped in the MRI apparatus of FIG. FIG. 4 is a flowchart showing another example of the operation method of the superconducting magnet 103 when the refrigerator 108 is stopped in the MRI apparatus of FIG. (a) A graph showing the relationship between elapsed time and energy consumption in a comparative example in which the current flowing through the superconducting coil is reduced by the protective resistance, (b) When the current of the superconducting coil 105 is reduced at the constant reduction rate of this embodiment The graph which shows the relationship between elapsed time and energy consumption. FIG. 2 is a flowchart showing a method of operating the superconducting magnet 103 when the operation of the refrigerator is resumed in the MRI apparatus of FIG.

The MRI apparatus of the present invention detects a nuclear magnetic resonance signal from a superconducting magnet 103 that generates a static magnetic field as shown in FIGS. 1 to 3 and a subject 101 that is disposed in a static magnetic field space generated by the superconducting magnet 103. A detection unit (109 to 114) and an image construction unit (115) that configures an image of the subject 101 from a nuclear magnetic resonance signal are provided. The superconducting magnet 103 includes a superconducting coil 105, a refrigerator 108, a heat conduction member 208 that cools the superconducting coil 105 by connecting the cooled portion 206 of the refrigerator 108 and the superconducting coil 105, and at least a detection unit (109). 114) includes a power supply unit (106) for supplying current to the superconducting coil 105 and maintaining the current flowing in the superconducting coil 105 while detecting the nuclear magnetic resonance signal.

The power supply unit (106) is an external power supply state to the power supply unit (106), the operating state of the refrigerator 108, the temperature of the superconducting coil 105, the static magnetic field strength of the superconducting magnet 103, the current value flowing through the superconducting coil 105, And a control unit (115, 304 to 307) for controlling a current supplied to the superconducting coil 105 based on at least one piece of information among the nuclear magnetic resonance signals detected by the detection unit (109 to 114).

For example, the control unit (115, 304 to 307) stops supplying power from the outside to the power source unit 106, stops the operation of the refrigerator 108, and the temperature of the superconducting coil 105 is higher than a predetermined temperature. When at least one of these occurs, the supply of current to the superconducting coil 105 is stopped. As a result, the temperature of the superconducting coil 105 of the superconducting magnet 103 operating in the driven mode is prevented from rising, and the transition to the normal state is prevented. Further, the time spent for cooling the superconducting coil 105 when the operation is resumed can be shortened.

In addition, when the control unit (115, 304 to 307) stops the current to the superconducting coil 105 as described above, it is preferable to reduce the current at a reduction rate equal to or less than a predetermined value. When the control unit (115, 304 to 307) reduces the current, the current is consumed by some means (for example, electrical resistance). Since the current reducing means generates heat as the current is consumed, it is necessary to arrange a cooling structure (for example, a heat radiating fin or a heat radiating fan) in the current reducing means. When the control unit controls the current reduction rate to be equal to or less than a predetermined value, the cooling structure can be miniaturized to the minimum necessary.

As an example, when the operation of the refrigerator 108 stops, the control unit (115, 304 to 307) predicts the time when the temperature of the superconducting coil 105 rises from a predetermined temperature, and until the predicted time is reached. In the meantime, the current can be reduced at a reduction rate at which the supply current to the superconducting coil becomes zero. As another example, when the operation of the refrigerator 108 stops, the control unit (115, 304 to 307) waits until a predetermined time elapses, and then reduces the supply current to the superconducting coil at a predetermined reduction rate. Reduce with.

Further, the control unit (115, 304 to 307) is a case where the external power supply to the power supply unit 106 and the operation of the refrigerator 108 are started, and the temperature of the superconducting coil 105 is set in advance. When the temperature reaches a predetermined temperature or lower, control is performed so that supply of current to the superconducting coil 105 is started.

In addition, when the current supply is started, it is desirable that the control unit (115, 304 to 307) increases the current value of the superconducting coil 105 at a predetermined rate. Further, it is desirable that the control unit determines whether or not the temperature of the superconducting coil is equal to or lower than a preset value at a predetermined time interval after the start of current supply to the superconducting coil 105.

Also, the control unit (115, 304 to 307) can stop supplying the current to the superconducting coil 105 when the detection of the nuclear magnetic resonance signals of all the subjects 101 is completed. This makes it possible to more easily perform safety management of the MRI apparatus in the medical facility. For example, since the superconducting magnet 103 does not generate a static magnetic field during non-inspection time such as at night, it is possible to prevent an accident that a magnetic body such as a cleaning tool is attracted by the static magnetic field and sticks to the superconducting magnet 103 strongly. .

In addition, when the detection of the nuclear magnetic resonance signals of all the subjects 101 is completed, and when the supply of current to the superconducting coil 105 is stopped, the control unit (115, 304 to 307), as described above, the superconducting coil When stopping the current to 105, it is preferable to reduce the current at a reduction rate of a predetermined value or less. The control unit (115, 304 to 307) controls the current reduction rate to be equal to or less than a predetermined value, whereby the cooling structure can be miniaturized to the minimum necessary.

In addition, when the detection of the nuclear magnetic resonance signals of all the subjects 101 is completed, or when the supply of current to the superconducting coil 105 is stopped, the control unit (115, 304 to 307) receives an instruction from the operator. If the temperature of the superconducting coil 105 is equal to or lower than the predetermined temperature, the supply of current to the superconducting coil 105 is started.

The superconducting coil 105 is an open loop, and a power supply unit 106 connected to both ends can be used. As an example, the power source unit 106 includes a current value control unit (304 to 307) that changes a current value flowing through the superconducting coil 105. It is desirable that the current value control units (304 to 307) increase or decrease the current supplied to the superconducting coil at a predetermined rate.

The control unit (115, 304 to 307) has at least one of the static magnetic field strength of the superconducting magnet 103, the current value flowing through the superconducting coil 105, and the nuclear magnetic resonance signal detected by the detection unit (109 to 114). If it is outside the predetermined range, the current value supplied to the superconducting coil 105 is adjusted.

Further, the power supply unit 106 may include a resistance element inside, and may be configured to reduce the current supplied to the superconducting coil 105 by consuming current with the resistance element.

The superconducting coil 105 can be brought into a superconducting state even at a temperature higher than the boiling point of He by using the superconducting coil 105 of a high-temperature superconducting material. As a result, the driven-mode superconducting magnet 103 can be configured without using liquid helium.

It is desirable that the superconducting magnet 103 includes a temperature sensor 212 that detects the temperature of the superconducting coil 105. When there are a plurality of temperature sensors 212, it is more desirable because it can measure the temperatures of a plurality of parts of the superconducting coil. The control unit (115) acquires the temperature of the superconducting coil 105 from the output of the temperature sensor 212.

Embodiments of the present invention will be specifically described below with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment of the invention, and the repetitive description thereof is omitted.
(Embodiment 1)
<Overall Configuration of MRI Apparatus of Embodiment 1>
FIG. 1 is an explanatory diagram showing the overall configuration of an MRI apparatus according to an embodiment of the present invention, showing a state in which the MRI apparatus is installed in a medical facility and a medical diagnostic image of a patient who is a subject is taken .

As shown in FIG. 1, the MRI apparatus of this embodiment includes a superconducting magnet 103, a detection unit (109 to 114) that detects a nuclear magnetic resonance signal from the subject 101, and an image of the subject 101 from the nuclear magnetic resonance signal. An image configuration unit (115) to be configured is provided.

The superconducting magnet 103 has a structure in which a pair of superconducting coils 105 arranged opposite to each other is cooled by conduction cooling with a refrigerator 108 without using liquid helium. A pair of superconducting coils 105 arranged opposite to each other is supported by an iron yoke 104 having a C-shaped side surface. The iron yoke 104 constitutes a magnetic circuit, and the vicinity of the connection portion between the pair of superconducting coils 105 is a magnetic pole.

Also, a magnet power source 106 is always connected to the superconducting coil 105, and a current is always supplied when a static magnetic field is generated, so that the superconducting coil 105 is operated in a so-called driven mode.

Thereby, the superconducting magnet 103 generates a uniform static magnetic field in the z-axis direction in the imaging space 102 between the pair of superconducting coils 105. The examination region of the subject 101 is disposed at the center of the imaging space 102.

Since the iron yoke 104 has a C-shaped side surface, there is nothing to block the field of view of the subject 101 in the front (y-axis direction) and the left and right sides (x-axis direction) of the imaging space 102, and an open examination environment It is possible to provide. Further, the magnetic flux that spreads outside the superconducting magnet 103 is suppressed to a minimum by the magnetic circuit by the iron yoke 104.

The structure of the superconducting magnet 103 will be described in detail later.

The detection unit (109 to 114) for detecting a nuclear magnetic resonance signal from the subject 101 includes a gradient magnetic field coil assembly 109, a gradient magnetic field power source 110, a high frequency transmitter coil 111, a high frequency power source 112, a high frequency receiver coil 113, and signal processing. A unit 114 is provided.

The pair of gradient magnetic field coil assemblies 109 has a flat plate shape, is disposed on the imaging space 102 side of the pair of superconducting coils 105, and is supplied with current from the gradient magnetic field power supply 110, thereby being orthogonal to each other in the imaging space 102. A gradient magnetic field having a gradient in intensity in the axial direction (x, y, z direction) is generated. The gradient coil assembly 109 is supported by the iron yoke 104.

Although not distinguished in FIG. 1, the gradient coil assembly 109 is laminated with three types of coils, x, y, and z. For example, when a positive current is supplied to the z gradient magnetic field coil, the z gradient magnetic field coil attached to the upper magnetic pole of the iron yoke 104 generates a magnetic flux in the + z axis direction that is the same as the magnetic flux generated by the superconducting coil 105. The density is increased by being superposed on the magnetic flux generated by the superconducting coil 105. On the other hand, the z gradient magnetic field coil attached to the lower magnetic pole of the iron yoke 104 generates a magnetic flux along the −z axis in the opposite direction to the magnetic flux generated by the superconducting coil 105, and the density of the magnetic flux generated by the superconducting coil 105 is reduced. Decrease. As a result, a gradient magnetic field in which the magnetic flux density increases from bottom to top along the z-axis of the imaging space 102 can be created.

The x gradient coil changes the magnetic flux density generated by the superconducting coil 105 along the x axis of the imaging space 102, and the y gradient coil changes the y axis of the imaging space 102. The gradient magnetic field power supply 110 supplies current to the x, y, and z gradient magnetic field coils independently. For example, by supplying a current of 500 amperes, it is possible to generate a 25 mT / m gradient magnetic field in which the magnetic field strength of 25 millitesla changes in one meter.

The pair of high-frequency transmitter coils 111 has a flat plate shape so as not to hinder an open inspection environment, and is disposed on the imaging space 102 side of the pair of gradient coil assemblies 109. The high frequency power source 112 supplies a current to the pair of high frequency transmitter coils 111.

The high-frequency transmitter coil 111 is printed with a coil conductor so that a magnetic flux parallel to the xy plane of the imaging space 102 is generated. A plurality of capacitive elements are incorporated (not shown in FIG. 1), for example, a 21 MHz LC resonance circuit. A high frequency magnetic field is generated in the imaging space 102 by flowing a high frequency current of, for example, 21 megahertz from the high frequency power source 112.

By applying a gradient magnetic field to a subject 101 placed in a static magnetic field by irradiating a high frequency magnetic field to cause a nuclear magnetic resonance (hereinafter referred to as NMR) phenomenon in a hydrogen nuclear spin at a specific part of the subject 101 Spatial information of x, y, and z can be added to the precession of the hydrogen nuclear spin.

The high-frequency receiver coil 113 is given spatial information and detects the precession of the hydrogen nuclear spin as an NMR electrical signal. The high frequency receiver coil 113 is attached to the examination site of the subject 101. As with the high-frequency transmitter coil 111, the high-frequency receiver coil 113 includes a capacitive element (not shown in FIG. 1), and is, for example, a resonance circuit of 21 megahertz. The difference from the high-frequency transmitter coil 111 is that it fits the body shape of the examination site so as to detect spin precession as an electric signal by electromagnetic induction with high efficiency. In FIG. 1, a coil for detecting the cervical vertebra site of the subject 101 is described.

The NMR signal detected by the high-frequency receiver coil 113 is input to the signal processing unit 114, subjected to amplification processing, detection processing, analog / digital conversion processing, and the like, and then delivered to the computer 115.

The computer 115 operates as an image reconstruction unit 115a by reading and executing a program for image reconstruction stored in a built-in memory. The image reconstruction unit 115a performs arithmetic processing such as Fourier transform on the NMR signal, and generates a tomographic image and a spectrum distribution map effective for medical diagnosis. These data are stored in a storage device (not shown) of the computer 115 and displayed on the display 116.

On the other hand, the computer 115 reads a program for executing a pulse sequence and also operates as the imaging control unit 115b. The imaging control unit 115b operates the gradient magnetic field power source 110 and the high frequency power source 112 according to a timing chart called a pulse sequence so that a target NMR signal can be obtained from the examination region of the subject 101. Therefore, a sequencer 120 is arranged between the computer 115 and the high-frequency power source 112, the gradient magnetic field power source 110, etc. as shown in FIG. In addition, an input device 117 for an operator of the MRI apparatus to input selection of a pulse sequence type or the like to the imaging control unit 115b is connected to the computer 115.

Furthermore, the computer 115 also operates as the superconducting magnet control unit 115c by reading and executing a control program for operating the superconducting magnet 103. The superconducting magnet control unit 115c, together with the control circuit (304 to 307) in the magnet power supply 106, the external power supply state to the power supply unit (106), the operating state of the refrigerator 108, the temperature of the superconducting coil 105, the superconducting magnet The current supplied to the superconducting coil 105 is controlled based on at least one of the information of the static magnetic field strength of 103, the value of the current flowing in the superconducting coil 105, and the nuclear magnetic resonance signal detected by the detection unit (109 to 114). To do. This prevents the superconducting coil 105 from reaching a high temperature even if a power failure, system failure, or the like occurs, shortens the time required for recooling, and allows the operation to be resumed early. This operation will be described in detail later.

In addition, a patient table 118 is mounted in front of the superconducting magnet 103 in order to carry the inspection site of the subject 101 into and out of the center of the imaging space 102. The superconducting magnet 103, the patient table 108, the gradient magnetic field coil assembly 109, the high-frequency transmitter coil 111, and the high-frequency receiver coil 113 are installed in an examination room 119 that is shielded from electromagnetic waves. Also, the signal lines connecting the superconducting magnet 103, the gradient magnetic field coil assembly 109, the high frequency transmitter coil 111 and the high frequency receiver coil 113 to the magnet power source 106, the gradient magnetic field power source 110, the high frequency power source 112 and the signal processing unit 114 are respectively It is drawn into the examination room 119 via a noise filter 121 provided on the wall surface of the examination room 119. This prevents electromagnetic waves generated by the computer 115 and other devices from entering the high frequency receiver coil 113 as noise.

<Structure of conduction cooled superconducting magnet 103>
The structure of the superconducting magnet 103 will be described in detail. FIG. 2 is a cross-sectional view of the superconducting magnet 103.

The superconducting magnet 103 has a structure in which a pair of superconducting coils 105 opposed to each other with the imaging space 102 interposed therebetween is supported by an iron yoke 104 having a C-shaped side surface. Two (N and S poles) magnetic poles 201 are fixed to the opening of the C-shaped iron yoke 104 so as to protrude inside the pair of superconducting coils 105. The iron yoke 104 is provided with a through hole from the back toward the C-shaped opening, and the refrigerator 108 is inserted into the through hole from the back side.

The pair of superconducting coils 105 has a structure in which, for example, a high-temperature superconducting material MgB ₂ wire is wound in a donut shape around a coil bobbin 202 that serves both for cold storage and heat transfer. The upper and lower superconducting coils 105 are connected in series as shown in the circuit configuration of FIG. 3, and have an open loop structure in which both ends are open. Both ends of the superconducting coil 105 are always connected to the magnet power source 106 via lead wires 209a and 209b, and a superconducting current is always supplied from the magnet power source 106 when a static magnetic field is generated.

Superconducting coils 105 are housed in ring-shaped vacuum vessels 107, respectively. A part of the pair of vacuum vessels 107 is connected, and a tip portion 206 of the refrigerator 108 is inserted into the connecting portion 220. The tip portion 206 of the refrigerator 108 is cooled, and is thermally connected to the superconducting coil 105 by the heat conducting member 208 and the coil bobbin 202.
The refrigerator 108 cools the superconducting coil 105 to a temperature of 20 Kelvin by heat conduction of the heat conducting member 208. For the heat conductive member 208, for example, a copper mesh wire is used.

The high-temperature superconducting material MgB ₂ constituting the superconducting coil 105 exhibits stable superconducting characteristics at 20 Kelvin (−253 ° C.). Around the superconducting coil 105, an insulator 203 (only a part of which is shown in the drawing and the others are omitted) for preventing radiant heat from the inner wall surface of the vacuum vessel 107 is wound. As the insulator 203, for example, a mirror-coated polyethylene sheet deposited with aluminum is used. The coil bobbin 202 is composed of a composite material so as to have a reciprocal characteristic that is a good heat conductor, a large heat capacity, and an electric nonconductor. Furthermore, the coil bobbin 202 has sufficient rigidity to withstand the electromagnetic force applied to the superconducting coil 105.

Also, support members 204 for fixing the relative position to the vacuum vessel 107 are attached to the four locations of the coil bobbin 202. The support member 204 is made of fiber reinforced plastic (FRP) for suppressing conduction heat. With such a structure, the radiant heat and conduction heat transmitted to the superconducting coil 105 during the steady operation of the refrigerator 108 are 5 watts or less.

Also, temperature sensors 212 are attached to a plurality of locations (one location is shown in FIG. 2) of the coil bobbin 202. The temperature sensor 212 is connected to a sensor terminal 213 (see FIG. 3) disposed on the surface of the vacuum vessel 107 by a phosphor bronze wire lead wire to minimize heat conduction.

As a specific example, the refrigerator 108 has a part 206 cooled to 20 Kelvin and a part 207 cooled to 77 Kelvin, each having a cooling capacity of 6 watts and 65 watts. Can do. As an example, model CH-208R manufactured by Sumitomo Heavy Industries, Ltd. can be used. The 20 Kelvin cooling portion 206 is in thermal contact with the upper and lower coil bobbins 202 and the heat conducting member 208 as described above. The 77 Kelvin cooling portion 207 is in thermal contact with the lead wires 209a and 209b of the superconducting coil 105.

The lead wires 209a and 209b are guided to the coil bobbin 202 along the heat conducting member 208. As a result, the amount of heat transferred to the superconducting coil 105 via the lead wires 209a and 209b is minimized to about 5 watts, and the superconducting coil can be kept below the critical temperature even when the magnet power supply 106 is always connected. Yes. The lead wires 209a and 209b are led to terminals 210a and 210b outside the vacuum vessel 107 through thermal contact with the 77 Kelvin cooling portion 207 described above. The terminal 210 is connected to the current terminals 301a and 301b of the magnet power supply 106 by cables 211a and 211b.

The magnet power source 106 supplies a current of, for example, 160 amperes to the superconducting coil 105 via the lead wires 209a and 209b when a static magnetic field is generated. Thereby, the superconducting coil 105 generates a magnetic flux having a strength of 0.5 Tesla in the imaging space 102 in the z-axis direction.

Also, specifically, the iron yoke 104 has a weight of, for example, 14 tons and the opening has a height of 55 cm. The shape of the iron yoke 104 is designed to secure a magnetic flux density that generates a magnetic field strength of 0.5 Tesla in the imaging space 102 and to reduce the magnetic flux leaking out of the iron yoke 104 as much as possible.

Further, since the magnetic pole 201 provided in the opening of the iron yoke 104 generates a uniform magnetic field, the surface on the imaging space 102 side is processed into a concave surface as shown in FIG.

<Circuit configuration of magnet power supply 106>
Next, the circuit configuration of the magnet power supply 106 will be described with reference to FIG. The magnet power supply 106 includes a DC power supply unit 302, an uninterruptible power supply unit 303, a low current control unit 304, a current detection unit 305, a reference current setting unit 306, an error amplification circuit unit 307, and a refrigerator operation detection unit. 308, a temperature detection unit 309, and an interface circuit 310.

The DC power supply unit 302 is supplied with power from the outside via the terminal 302a, and generates DC power required by the circuit units 303 to 307. The electric power supplied from the DC power supply unit 302 to the uninterruptible power supply unit 303 is a current source that flows through the superconducting coil 105. The uninterruptible power supply unit 303 is mainly composed of a lithium battery or a lead storage battery, and constantly stores 160 AH of electric power necessary for operating the superconducting magnet 103 for 2 hours or more. The anode 303a of the uninterruptible power supply unit 303 is connected to the current terminal 301a via the constant current control unit 304, and the cathode 303b is connected to the current terminal 301b via the current detection unit 305.

The constant current control unit 304 controls the current supplied to the superconducting coil 105. The low current control unit 304 is composed of, for example, a transistor element capable of flowing a large current, and controls the current flowing between the collector and the emitter according to the current value input from the error amplification circuit unit 307 to the base terminal thereof. Thus, the current value supplied to the superconducting coil 105 is controlled.

The error amplification circuit unit 307 receives the reference voltage output from the reference current setting unit 306 and the voltage corresponding to the current value of the superconducting coil 105 detected by the current detection unit 305. The reference current setting unit 306 amplifies the difference with a predetermined amplification factor and inputs the difference to the base terminal of the transistor element constituting the constant current control unit 304. As a result, the constant current control unit 304 controls the supply current to the superconducting coil 105 to match the current value (160A) corresponding to the reference voltage of the reference current setting unit 306.

The current detection unit 305 is a unit that detects the current supplied to the superconducting coil 105, and is composed of, for example, a resistance element (for example, 0.1 ohm) having extremely good temperature characteristics and an accurate resistance value. The voltage signals at both ends correspond to the value of the current flowing through the resistance element. As an example, when a current of 161 amperes greater than 160 amperes is flowing between the current terminals 301a and 301b, the current detection unit 305 generates a voltage of 16.1 volts at both ends thereof. The error amplifying circuit unit 307 outputs a signal in a direction to reduce the current flowing through the constant current control unit 304, being 0.1V larger than the output reference voltage (for example, 16V).

The reference current setting unit 306 receives the output of the refrigerator operation detection unit 308 and the temperature detection unit 309 that processes the output voltage of the thermometer. The reference current setting unit 306 switches the value of the reference voltage to be output based on the output signals from both detection units.

Further, each unit of the magnet power source 106 is connected to the computer 115 via the interface circuit 310.

Thus, the magnet power source 106 is always connected to the superconducting coil 105, and operates the superconducting magnet 103 in a driven mode that controls the current supplied to the superconducting coil. As a result, when a static magnetic field is generated, the current value supplied to the superconducting coil 105 can be controlled to be constant so that the static magnetic field can be stabilized, and in the event of a power failure or system failure, the current to the superconducting coil Can be stopped, the temperature of the superconducting coil 105 can be prevented from rising, and the operation can be restarted at an early stage.

Hereinafter, the operation method of the superconducting magnet 103 by the magnet power source 106 and the computer 115 will be described in detail.

<Excitation of superconducting magnet>
The operation method at the time of starting the superconducting magnet 103 will be described according to the flow of FIG.

First, the operator performs an operation to instruct activation of the superconducting magnet 103 from the input device 117 of the computer 115 (step 401). In response to this, the computer 115 operates as follows as the superconducting magnet control unit 115c by reading and executing a control program for starting operation of the superconducting magnet 103.

The computer 115 causes the temperature detection unit 309 to measure the temperature of each part of the superconducting coil 105 from the output signals of the plurality of temperature sensors 212 incorporated in the superconducting magnet 103, and receives the measurement result (step 402). From the measurement result, it is determined whether the temperature of each part falls within a predetermined value range (step 403). If the determination is normal (within a predetermined range), the units constituting the magnet power supply 106 are instructed to start operation, and the process proceeds to step 406 (step 404). On the other hand, if the determination is abnormal, the contents are displayed on the display 116 and the process ends (step 405).

If the temperature is within the predetermined range, the computer 115 increases the supply current to the superconducting coil 105 at a rate of 5 amperes / minute until the rated current reaches 160 amperes (steps 406 to 414). Specifically, the computer 115 gradually increases the reference voltage output from the reference current setting unit 306 so that the current supplied to the superconducting coil 105 increases at an increase rate of 5 amperes / minute (step 406). During this time, a current is supplied from the magnet power source 106 to the superconducting coil 105, and a current flows through the superconducting coil 105.Therefore, the computer 115 causes the temperature detection unit 309 to measure the temperature of the superconducting coil 105 at intervals of 5 minutes (step 407, 408).

The computer 115 determines whether the measured temperature is within a predetermined value range (step 409). Based on this temperature measurement and determination, it is confirmed whether the cooling by the refrigerator 109 is functioning normally during the startup of the superconducting magnet 103 and whether the heat generated by the power loss in the superconducting coil 105 itself is within a predetermined range. And can be done. This power loss is called AC loss (AC loss), and is caused by the fact that the current of the superconducting coil 105 having inductance changes at 5 amperes / minute.

If the temperature determination in step 409 is normal (within a predetermined range), the computer 115 receives the current value supplied to the superconducting coil 105 from the current detection unit 305, and whether it has reached the rated 160 amperes. Is determined (step 414). If 160 ampere has not been reached, the process returns to step 406 and the operation of increasing the output current of the magnet power source 106 is continued. Thus, the current supplied to the superconducting coil 105 increases at an increase rate of 5 amperes / minute until the current supplied to 160 amperes is reached. When 160 ampere is reached in step 414, the reference voltage of the reference current setting unit 306 is held so as to maintain the current value of 160A (step 415). As a result, the supply current to the superconducting coil 105 can reach 160 amperes and can be held approximately 32 minutes after the start of starting the superconducting magnet 103. Thereby, excitation of the superconducting magnet is completed, and a stable static magnetic field of 0.5 Tesla can be generated in the imaging space 102.

On the other hand, if the temperature determination is abnormal (out of the predetermined range) in step 409, the computer 115 immediately stops increasing the reference current of the reference current setting unit 306 and stops increasing the output current of the magnet power supply 106 (step 411). ), A message notifying that the temperature is outside the predetermined range is displayed on the display 116 (step 412). Then, an operation of setting the current output to the superconducting coil 105 to zero is performed, and the operation is finished (step 413).

As described above, in this embodiment, by monitoring the temperature while increasing the current flowing in the superconducting coil 105 at a constant rate at the time of starting the superconducting magnet 103, the superconducting coil 105 is heated by heat generated by the superconducting coil 105 itself. Temperature rise can be prevented.

<Execution of MRI examination>
The operation method when the excitation of the superconducting magnet 103 is completed as described above and the MRI examination is performed will be described with reference to the flow of FIG. When the operator performs an operation to instruct the execution of the MRI examination from the input device 117 of the computer 115, and the operator arranges the test object 101 called a phantom in the imaging space 102 of the superconducting magnet 103, the computer 115 The superconducting magnet controller 115c operates as follows by reading and executing a control program for MRI inspection of the superconducting magnet 103.

First, the computer 115 measures the phantom NMR signal (step 501). Specifically, the computer 115 causes the sequencer 120, the gradient magnetic field power supply 110, the high-frequency power supply 112, and the signal processing unit 114 to execute a predetermined pulse sequence for phantom measurement and measure the phantom NMR signal. The computer 115 performs arithmetic processing on the measured NMR signal to obtain the magnetic field strength Bo and the magnetic field uniformity ΔBo of the imaging space 102 (step 502).

The computer 115 determines whether the obtained values of Bo and δBo are within a predetermined range (step 503). If the determination is normal (the magnetic field strength Bo and the magnetic field uniformity ΔBo are within a predetermined range), an MRI examination of the subject 101 is performed (step 504). On the other hand, if the determination in step 503 is abnormal (at least one of the magnetic field strength Bo and the magnetic field uniformity δBo is outside the predetermined range), both the output current of the magnet power supply 106 and the output current of the gradient magnetic field power supply 110, In some cases, one of them is adjusted (step 505). After the adjustment, the process returns to the step (501) of measuring the NMR signal of the phantom again. As a result, if the magnetic field strength Bo and the magnetic field uniformity δ are within the predetermined ranges, the process proceeds to step 504 to start the MRI examination of the subject.

In step 504, the computer 115 reads the program for executing the pulse sequence and operates as the imaging control unit 115b, so that the target NMR signal can be obtained from the examination site of the subject 101 and the gradient magnetic field power source 110 and the high frequency power source. Operate 112 according to the pulse sequence. The obtained NMR signal is processed by the signal processing unit 114 and the computer 115 to reconstruct an image.

Even during the execution of the pulse sequence, the computer 115 measures the temperature of the superconducting coil 105 at intervals of 5 minutes (step 508) and determines whether the measured temperature is within a predetermined value range (step 509). ). If the determination is normal (measured temperature is within a predetermined range), the MRI examination is continued until a series of pulse sequences is completed (steps 510 and 504). On the other hand, if the temperature judgment is abnormal (the measured temperature is outside the predetermined range), a message notifying the temperature abnormality is displayed on the display 116 (step 511), and the process proceeds to step 507, where the output current of the magnet power source 106 is set. Perform zeroing and exit. The specific operation in step 507 is the same as the operation in step 703 in FIG. 7 described later, and thus detailed description thereof is omitted here.

Further, when the MRI examination (pulse sequence) of the subject 101 is completed, the computer 115 determines whether or not to perform the MRI examination on the next subject (step 506). For example, whether or not the operator has already input the information of the next subject 101 to the input device 117 and whether or not the operator asks whether the next subject has an MRI examination are displayed on the display 116 to prompt the input. To confirm. If there is an MRI examination of the next subject, the process returns to step 504 to conduct the examination. If there is no examination of the next subject, that is, if all MRI examinations for one day are completed, the process proceeds to step 507.

In step 507, the computer 115 lowers the output current of the magnet power source 106 at a decreasing rate of 5 amperes per minute, and sets it to zero, and then ends. Specifically, the computer 115 reduces the output current of the constant current control unit 304 at a decrease rate of 5 amperes per minute by decreasing the reference voltage of the reference current setting unit 306 at a predetermined decrease rate.

As described above, when the MRI examination of all the subjects 101 is completed, setting the output current of the magnet power source 106 to zero makes the static magnetic field generated by the superconducting magnet 103 zero. There is a big merit. For example, in a medical facility, cleaning work is often performed at night, but the static magnetic field generated by the superconducting magnet 103 is not visible, so the cleaning worker is not fully aware of the danger to the strong magnetic field potential. May cause a suction accident that the cleaning tool (made of a magnetic material such as iron) brought into the examination room 119 is strongly attracted and stuck to the superconducting magnet 103 and cannot be pulled apart.

In the present invention, when all the MRI examinations of the subject 101 are completed, the output current of the magnet power source 106 is set to zero and the current flowing through the superconducting coil 105 is set to zero. It does not occur and no suction accident occurs. Therefore, compared with the MRI apparatus which always generates a static magnetic field, the MRI apparatus of the present embodiment can easily perform safety management.
<When the refrigerator stops>
Next, an operation method of the superconducting magnet 103 at the time of an abnormality in which the refrigerator 108 is stopped due to a system failure or a power failure will be described with reference to FIGS. In this MRI system, when the cooling capacity of the refrigerator 108 decreases due to a system failure or power failure, the current flowing in the superconducting coil 105 is consumed outside the superconducting magnet 103 before the temperature of the superconducting coil 105 reaches the critical temperature. The stored electrical energy of the superconducting coil 105 is released. This avoids temperature rise and burnout damage of the superconducting coil 105.

For this reason, it is necessary to grasp the time until the superconducting coil 105 reaches the critical temperature and consume the current flowing in the superconducting coil 105 within that time.In this embodiment, (1) the coil bobbin 202 is assembled. The temperature sensor 211 output is monitored and the critical temperature arrival time is estimated from the rate of temperature change (Fig. 6), or (2) determined by the heat capacity and heat penetration of the vacuum vessel 107 incorporating the superconducting coil 105 Any one of the methods (FIG. 7) for obtaining in advance the time to reach the critical temperature from the stop of the refrigerator 108 from the thermal time constant is used.

In the operation method shown in the flow of FIG. 6, specifically, when the computer 115 detects the stop of the refrigerator 108 (step 601), the computer 115 continuously measures the temperature of the superconducting coil 105 from the signal of the temperature sensor 212. (Step 602). From the rate of temperature rise of the superconducting coil 105, a time T1 until the critical temperature of the superconducting coil 105 is reached is obtained by calculation (step 603). Within the obtained time T1, a constant reduction rate of the current for making the output current of the magnet power supply zero is obtained by calculation, and the reference current setting unit 306 outputs with the reduction rate of the reference voltage corresponding to this current reduction rate. The reference voltage is reduced (step 604). Thereby, the constant current control unit 304 reduces the current flowing through the superconducting coil 105 at the current reduction rate.

The constant current control unit 304 generates heat because it consumes current so as to achieve a constant current reduction rate, but the cooling device (for example, an air cooling fan and fins or a water cooling jacket) provided in the constant current control unit 304 cools it. To do. Since the current reduction rate is constant, the heat generation amount per predetermined time is also constant, and it is sufficient to install a cooling device having a scale corresponding to the heat generation amount in advance. Therefore, it is not necessary to install a large cooling device in preparation for a sudden large amount of heat generation, and the cooling device can be downsized.

When the refrigerator 108 stops due to a power failure, the above steps are performed by operating the computer 115 and each unit in the magnet power source 106 using the uninterruptible power supply unit 303 incorporated in the magnet power source 106.

Further, in step 604, the constant reduction rate of the current for making the output current of the magnet power supply zero within the time T1 is obtained by calculation. However, the calculation method is not limited to this, and a predetermined constant value is obtained. You may reduce by a reduction rate. In this case, the current reduction operation is performed until the time (T1−T2) obtained by subtracting the time T2 required until the current reaches zero when the current is reduced at a predetermined reduction rate from the time T1 until reaching the criticality elapses. It is also possible to adopt a configuration that does not. In this case, the current in the superconducting coil 105 is not greatly reduced until the remaining time until the time when the critical temperature is reached reaches the time T2 required to make the current zero. Therefore, when the operation of the refrigerator 108 is restarted because the power failure is eliminated during this period, there is an advantage that the operation can be restarted in a short time.

Next, a flow for reducing the current flowing in the superconducting coil 105 in the time until the superconducting coil 105 reaches the critical temperature by the operation method shown in FIG. 7 will be described.

First, when the computer 115 detects the stop of the refrigerator 108 (step 601), the computer 115 starts counting the elapsed time (stop duration) from the stop (step 701). It is determined whether the stop continuation time has reached a predetermined allowable time (for example, 2 hours) (step 702). If not, the process returns to step 701 to continue counting. When a predetermined allowable time (for example, 2 hours) is reached, the output current of the magnet power source 106 is reduced at a predetermined constant reduction rate, and the current of the superconducting coil 105 is made zero (step 703). Specifically, the reference voltage output from the reference current setting unit 306 is reduced at a reference voltage reduction rate corresponding to a constant current reduction rate.

The allowable duration of the stop duration described above (for example, 2 hours) is determined by the operation of Step 703 from the time T3 required until the temperature of the superconducting coil 105 exceeds the critical temperature of the superconducting coil 105 after the refrigerator 108 stops. This is a time (T3−T2) obtained by subtracting the time T2 required to make the current of the coil 105 zero. The time T3 is obtained in advance from the heat capacity of the vacuum vessel 107 incorporating the superconducting coil 105 and the heat time constant determined by the heat penetration amount when the refrigerator 108 is stopped.

In the operation method of FIG. 7, the current of the superconducting coil 105 is not greatly reduced until the allowable time is reached. Therefore, when the operation of the refrigerator 108 is resumed due to a power failure, etc., the operation is resumed in a short time. There is a merit that you can.

Further, since the calculation method of FIG. 7 does not require a reduction rate calculation, the computer 115 is operated by adding a timer function and a function of reducing the reference current at a predetermined reduction rate to the reference current setting unit 306. Even if not, it can be realized by the operation of the reference current setting unit 306. In this case, since there is no need to operate the computer 115 by the uninterruptible power generation unit 303 at the time of a power failure, there is also an advantage that the capacity of the uninterruptible power supply unit 303 can be reduced.

Further, in the flow of FIG. 6 and FIG. 7, as a method of reducing the current of the superconducting coil 105 to zero, a method of reducing the reference current of the reference current setting unit 306 to be consumed by the constant current control unit 304 is used. However, the present invention is not limited to this method. For example, it is possible to use a method in which current is consumed by a resistance element separately incorporated in the magnet power source 106.

Here, a calculation method for obtaining in advance the time T3 required until the temperature of the superconducting coil 105 exceeds the critical temperature of the superconducting coil 105 after the refrigerator 108 is stopped will be described. This time T3 is the product of the heat capacity of the vacuum vessel 107 and the temperature difference from the temperature when the refrigerator 108 is stopped to the critical temperature, and the amount of heat intrusion per unit time when the refrigerator 108 is stopped It can be obtained by dividing by.

In order to lengthen the time T3, it is preferable to increase the heat capacity of the vacuum vessel 107 by adding a heat storage effect by making the coil bobbin 202 the main component of copper or lead. Furthermore, a heat storage material using a compound of a heavy rare earth element such as Dy or Er can be added to the vacuum vessel 107. In this case, since this type of heat storage material is a magnetic material, the heat storage material is arranged so as to be advantageous for the superconducting magnet 103 of the present embodiment. As a specific example, it is possible to dispose a magnetic heat storage body inside the coil bobbin 202 so as to function as a part of the function of the magnetic pole 201 for improving the magnetic field uniformity. *

Alternatively, it can be made to function as a part of the iron yoke 104 by being incorporated in the connecting portion 220 of the upper and lower vacuum vessels 107. Thereby, there is an advantage that the weight of the iron yoke 104 can be reduced and the shape can be optimized. Thus, in the case of the superconducting magnet 103 having a heat storage effect at a low temperature (20 Kelvin), even if the operation of the refrigerator 108 is stopped, the temperature of the superconducting coil 105 can be kept below the critical temperature for a certain time.

For example, if the cold accumulating material of DyNi ₂ to the connecting portion 220 of the vacuum chamber 107 (specific heat at 20 Kelvin to 800kJ / m ³ · K) that incorporates 0.1 m ^3, the thermal capacity becomes 80k Joules. On the other hand, when the refrigerator 108 is stopped, the amount of heat entering the superconducting coil 105 due to radiant heat from the surface of the vacuum vessel 107 and conduction heat from the support material 204 of the superconducting coil 105 and the lead wires 209a and 209b of the superconducting coil is Average 30 joules per second (30 watts).

The reason for the 30 watts is that the heat intrusion immediately after the stop of the refrigerator 108 is 5 watts, but the 6 watt cooling stop at the 20 Kelvin cooling part 206 of the refrigerator 108 and the superconducting coil lead wires 209a and 209b This is because the conduction heat gradually increases due to the cooling stop, etc., and on average it becomes about 30 watts. In this heat balance state, it takes 2,667 seconds (= 80 k joules ÷ 30 joules) to raise the temperature of the superconducting coil 105 supported by the vacuum vessel 107 equipped with a cold storage material having a heat capacity of 80 k joules by 1 Kelvin. Need. If the upper limit of the critical temperature of the superconducting coil 105 is 23 Kelvin, the time T3 until the initial temperature of the superconducting coil 105 at the start of the stop of the refrigerator 108 rises from 20 Kelvin to 23 Kelvin is 2,667 seconds × 3 Kelvin = 8,000 Second (about 2.2 hours). Actually, since the heat capacities of the coil bobbin 202 and the superconducting coil 105 are taken into consideration, this time T3 is further increased.

The allowable time (about 2 hours) can be obtained by subtracting the time T2 required to make the current of the superconducting coil 105 zero in step 703 from this time T3 (about 2.2 hours).

As described above, in Step 604 of FIG. 6 and Step 703 of FIG. 7, it has already been described that the cooling device provided in the low current control unit 304 can be reduced in size by reducing the current at a constant current reduction rate. However, this will be described in more detail.

As a comparative example, the PCS is opened when the operation of the refrigerator is stopped as described in Patent Document 1 above, and the current flowing through the superconducting coil is reduced by passing the current through the protective resistance. The relationship is shown in the graph of FIG. As shown in FIG. 8 (a), the energy consumption of the protective resistor attenuates exponentially, so that the current attenuation rate of the superconducting coil 105 increases immediately after the PCS is opened. For this reason, the superconducting coil 105 receives a large electromagnetic force stress, the stress accumulated when the superconducting coil is re-excited is released, and there is a possibility that the superconducting coil 105 may be quenched by heat generated when the stress is released. In addition, a protective resistance element that can withstand a large amount of energy immediately after the PCS is opened and a system that cools the heat generated by the large energy consumption are required, resulting in an increase in the size of the device. Further, since the current decays exponentially, it takes a long time to completely reduce the current to zero.

On the other hand, in this embodiment, utilizing the fact that the current flowing through the superconducting coil 105 is driven mode operation in which the magnet power source 106 always controls, the current is reduced at a constant current reduction rate in the above steps 604 and 703. is doing. For this reason, as shown in FIG. 8 (b), the energy consumed by the constant current control unit 304 is reduced at a constant rate, so that there is little electromagnetic force stress generated in the superconducting coil 105, and quenching is performed during re-excitation. Hard to occur. Further, since a small transistor can be used as the transistor of the low current control unit 304 and the cooling system may be small, the apparatus can be downsized. Further, there is an advantage that the current can be completely zero within a certain time.

<Re-excitation of superconducting magnet when refrigerating machine restarts>
FIG. 9 is a flowchart for explaining the operation method of the superconducting magnet when the operation of the refrigerator 108 is resumed.

When the refrigerator is stopped due to a power failure or a system failure, the refrigerator 108 is configured to automatically restart operation when the cause of the stop is removed. In addition, since the MRI inspection is interrupted due to a power failure or system failure, it is desirable to restart the MRI inspection as quickly as possible. Since the MRI apparatus of the present embodiment consumes the current of the superconducting coil 105 outside the superconducting magnet 103 by the operation method of FIG. 6 or FIG. 7 while the refrigerator 108 is stopped, it becomes zero. The coil 105 does not generate heat, and the temperature does not increase greatly. Therefore, when the operation of the refrigerator is resumed, the superconducting magnet 103 can be automatically returned to the state before the stop of the refrigerator 108 in the shortest time according to the flow of FIG.

Specifically, as described in the flow of FIG. 6 and FIG. 7, the computer 115 sets the output current of the magnet power supply 106 to zero, and then enters a standby mode waiting for a status signal for restarting the refrigerator 108 (step 801). When receiving a signal for restarting operation of the refrigerator 108, the computer 115 measures the temperature of each part of the superconducting coil 105 from the output signals of the plurality of temperature sensors 212 in the superconducting magnet 103 (step 802). Then, it is determined whether the temperature of each part is within a predetermined value range (step 803). If the temperature of one or more parts is out of the predetermined temperature range, cooling by the refrigerator 108 is performed. Is not sufficient, the process returns to step 802 and the temperature of the superconducting coil 105 is measured again.

When all the temperatures of the respective parts are within the predetermined temperature range, the process proceeds to step 406, and the temperature of the superconducting coil 105 is measured while increasing the output current of the magnet power supply 106 in increments of 5 amperes / minute. The temperature determination is continued (steps 406 to 409), and the current value is increased until the current value of the superconducting coil 105 reaches the value before the stop of the refrigerator 108 (step 807). Then, the current value of the magnet power supply 106 is held (step 415). Thereby, the current value of the superconducting coil 105 can be returned to the state before the refrigerator 108 is stopped.

These steps 406 to 409 are the same as steps 406 to 409 of the operation at the time of starting the superconducting magnet described in FIG. If the temperature of the superconducting coil 105 is outside the predetermined range in Step 409, Steps 411 to 413 described in FIG. 4 are executed to reduce the current to zero and notify the display 412 of the temperature or higher. A message is displayed and informed, and the process ends.

As described above, the MRI apparatus according to the present embodiment monitors the temperature while the computer 115 increases the current at which the operation of the refrigerator is resumed at a constant rate, so that the superconducting coil 105 is heated by heat generated in the superconducting coil 105. The superconducting magnet 103 can be returned to the state before the refrigerator 108 is stopped in the shortest time while preventing the temperature rise. Therefore, the interruption time of the MRI examination can be shortened as much as possible.

103 Superconducting magnet, 104 Iron yoke, 105 Superconducting coil, 106 Magnet power supply, 107 Vacuum vessel, 108 Refrigerator, 115 Computer, 116 Display, 202 Coil bobbin, 211 Temperature sensor, 302 Constant current circuit unit, 303 Control circuit, 304 Resistance element

Claims (19)

A superconducting magnet that generates a static magnetic field, a detection unit that detects a nuclear magnetic resonance signal from a subject disposed in a static magnetic field space generated by the superconducting magnet, and an image of the subject are composed of the nuclear magnetic resonance signal An image composition unit
The superconducting magnet includes a superconducting coil, a refrigerator, a heat conducting member that cools the superconducting coil by connecting a cooled portion of the refrigerator to the superconducting coil, and at least the detection unit includes the nuclear magnetic resonance signal. A power supply unit that supplies a current to the superconducting coil and maintains the current flowing in the superconducting coil,
The power supply unit includes an external power supply state to the power supply unit, an operating state of the refrigerator, a temperature of the superconducting coil, a strength of the static magnetic field, a current value flowing through the superconducting coil, and detection of the detection unit A magnetic resonance imaging apparatus comprising: a control unit that controls a current supplied to the superconducting coil based on at least one of the nuclear magnetic resonance signals. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit is configured to stop power supply from the outside to the power supply unit, stop operation of the refrigerator, and temperature of the superconducting coil. A magnetic resonance imaging apparatus characterized in that, when at least one of the states above the temperature rises, the supply of current to the superconducting coil is stopped. 3. The magnetic resonance imaging apparatus according to claim 2, wherein the control unit reduces the current at a reduction rate equal to or less than a predetermined value when stopping the current to the superconducting coil. Resonance imaging device. 3. The magnetic resonance imaging apparatus according to claim 2, wherein when the operation of the refrigerator is stopped, the control unit predicts a time when the temperature of the superconducting coil rises from a predetermined temperature, and the prediction is performed. A magnetic resonance imaging apparatus characterized by reducing the current at a reduction rate at which the supply current to the superconducting coil becomes zero before reaching the time. 3. The magnetic resonance imaging apparatus according to claim 2, wherein when the operation of the refrigerator is stopped, the control unit waits until a predetermined time elapses, and then supplies a supply current to the superconducting coil to a predetermined value. The magnetic resonance imaging apparatus is characterized by being reduced at a reduction rate of. 3. The magnetic resonance imaging apparatus according to claim 2, wherein the control unit is a case where power supply from the outside to the power supply unit is started and operation of the refrigerator is started, A magnetic resonance imaging apparatus characterized in that, when the temperature of the superconducting coil reaches a predetermined temperature or less, supply of current to the superconducting coil is started. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit stops the supply of current to the superconducting coil when the detection of the nuclear magnetic resonance signals of all the subjects is completed. Magnetic resonance imaging device. 8. The magnetic resonance imaging apparatus according to claim 7, wherein the control unit reduces the current at a reduction rate equal to or less than a predetermined value when stopping the current to the superconducting coil. Resonance imaging device. 8. The magnetic resonance imaging apparatus according to claim 7, wherein the controller is instructed by an operator or when a predetermined time is reached, the temperature of the superconducting coil is equal to or lower than a predetermined temperature. If so, the magnetic resonance imaging apparatus starts to supply current to the superconducting coil. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit is a case where an external power supply to the power supply unit is started and an operation of the refrigerator is started, A magnetic resonance imaging apparatus characterized in that, when the temperature of the superconducting coil reaches a predetermined temperature or less, supply of current to the superconducting coil is started. 11. The magnetic resonance imaging apparatus according to claim 10, wherein the control unit increases the current value of the superconducting coil at a predetermined rate. 11. The magnetic resonance imaging apparatus according to claim 10, wherein the control unit has a temperature of the superconducting coil equal to or lower than a preset value at a predetermined time interval after the start of current supply to the superconducting coil. The magnetic resonance imaging apparatus characterized by determining. The magnetic resonance imaging apparatus according to claim 1, wherein the superconducting coil is an open loop, and the power supply unit is connected to both ends.
The magnetic resonance imaging apparatus, wherein the power supply unit includes a current value control unit that changes a current value flowing through the superconducting coil. 14. The magnetic resonance imaging apparatus according to claim 13, wherein the power supply unit increases or decreases a current supplied to the superconducting coil at a predetermined rate by the current value control unit. apparatus. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit includes at least one of the static magnetic field strength, a current value flowing through the superconducting coil, and a nuclear magnetic resonance signal detected by the detection unit. A magnetic resonance imaging apparatus characterized by adjusting a current value to be supplied to the superconducting coil when it is out of a predetermined range. The magnetic resonance imaging apparatus according to claim 1, wherein the superconducting coil is an open loop, and the power supply unit is connected to both ends.
The magnetic resonance imaging apparatus, wherein the power supply unit includes a resistance element, and the current supplied to the superconducting coil is reduced by consuming current at the resistance element. 2. The magnetic resonance imaging apparatus according to claim 1, wherein a temperature of the superconducting coil cooled by the refrigerator is higher than a boiling point of He. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the superconducting magnet includes a temperature sensor that detects a temperature of the superconducting coil, and the control unit obtains the temperature of the superconducting coil from an output of the temperature sensor. A magnetic resonance imaging apparatus. A superconducting magnet that generates a static magnetic field, a detection unit that detects a nuclear magnetic resonance signal from a subject disposed in a static magnetic field space generated by the superconducting magnet, and an image of the subject are composed of the nuclear magnetic resonance signal The superconducting magnet includes a superconducting coil, a refrigerator, a heat conductive member that thermally connects the cooled cold head of the refrigerator to the superconducting coil, and the detection unit A method of operating a magnetic resonance imaging apparatus comprising a power supply unit that supplies a current to the superconducting coil while detecting the nuclear magnetic resonance signal and maintains the current flowing in the superconducting coil,
The power supply state from the outside to the power supply unit, the operating state of the refrigerator, the temperature of the superconducting coil, the static magnetic field intensity, the current value flowing through the superconducting coil, and the nuclear magnetic resonance signal detected by the detection unit A method for operating a magnetic resonance imaging apparatus, comprising: controlling a current supplied from the power source to the superconducting coil based on at least one piece of information.

Images