WO2013172148A1 - Magnetic resonance imaging device, gas recovery unit for magnetic resonance imaging device, and method for operating magnetic resonance imaging device - Google Patents

Abstract

Provided is technology with which a coolant that has gasified due to power stoppage, and the like can be returned to a superconducting magnet with high efficiency. By means of this technology, a gas bag (125) that maintains a constant internal pressure by expanding and contracting in response to the volume of gas on the inside is connected to a coolant vessel of a superconducting magnet (101) and the coolant gas generated when a cooling unit (107) stops during power stoppage is reserved inside the coolant vessel. Thus, when power is restored and the cooling unit (107) is restarted, the coolant gas returns to the coolant vessel as the gas bag (125) contracts and all of the gas that has flowed out to the gas bag can thereby be returned to the coolant vessel. As a result, replenishing of coolant is unnecessary.

Description

Magnetic resonance imaging apparatus, gas recovery apparatus for magnetic resonance imaging apparatus, and method of operating magnetic resonance imaging apparatus

The present invention relates to a magnetic resonance imaging apparatus (Magnetic Resonance Imaging apparatus, hereinafter referred to as an MRI apparatus), and more particularly to an MRI apparatus using a superconducting magnet equipped with a cryocooler.

The MRI device arranges the subject in a magnetic field space of uniform intensity, detects a nuclear magnetic resonance signal (Nuclear Magnetic Resonance Signal, hereinafter referred to as NMR signal) from the examination site of the subject, and calculates the signal It is a diagnostic device that obtains medical diagnostic information by processing and obtaining a tomographic image of an examination site.

Currently, one of the two methods of permanent magnets and superconducting magnets is adopted for the magnetic field generation unit of the MRI apparatus. An MRI apparatus using a permanent magnet has the advantages of easy maintenance and low running costs. On the other hand, an MRI apparatus using a superconducting magnet has a merit that a high magnetic field strength (0.5 Tesla or higher) can be obtained, and an advanced diagnostic function can be used by using a high-sensitivity NMR signal.

A superconducting magnet can generate a stable high magnetic field semipermanently by passing a current through a coil in a superconducting state. In order to bring this coil into a superconducting state, it is necessary to keep the coil cooled below the critical temperature inherent to the wire used for the coil. Ni ₃ Sn and NbTi are generally used as superconducting wires. All of these require liquid helium as a refrigerant.

Since liquid helium is difficult to transport over long distances or for long periods of time, MRI systems using superconducting magnets are installed in areas where a network of refrigerant services for transporting liquid helium from a helium liquefaction base by truck or other means of transportation is established. It becomes necessary to do. For this reason, the area where the MRI apparatus using the superconducting magnet can be installed is limited.

Patent Document 1 discloses that a cryocooler is incorporated in a superconducting magnet, vaporized helium gas is cooled, recondensed, and reused as liquid helium. Thereby, reduction of liquid helium in a superconducting magnet can be controlled. However, in the event of a power failure or system failure that causes the cryocooler to lose its cooling capacity, helium gas cannot be condensed into liquid helium, and helium gas is released into the atmosphere through the pressure relief valve of the superconducting magnet. Is done. For this reason, a service is also required in which liquid helium is transported by a truck or the like and supplied to the superconducting magnet.

In Patent Document 2, a large gas tank that stores helium gas released from a superconducting magnet at room temperature in the event of a power outage or system failure is connected to the superconducting magnet, and after recovery from a power outage or system failure, Proposed a configuration in which the helium gas is liquefied and returned to the superconducting magnet for reuse.

JP-A-6-69030 JP 2007-134703

As in Patent Document 2, a huge gas tank is required to store the helium gas released from the superconducting magnet at the time of a power failure or the like in the gas tank. Specifically, when the cooling capacity of the cryocooler stops, a heat quantity of several hundred milliwatts to 1 watt enters the helium vessel. One watt of heat vaporizes 1.25 liters of liquid helium per hour, resulting in approximately 10 liters of helium gas. With this volume expansion, the internal pressure of the helium container rises, and when a predetermined pressure is exceeded, the safety valve opens and is released from the helium container into the atmosphere. In the process of being released into the atmosphere, the helium gas is warmed to room temperature and further expanded, and the volume of 10 liters in the helium container expands to about 700 liters.

国 It is not uncommon for countries to have frequent power outages for about half a day due to planned power outages. In the case of a system failure, the service call, the arrival of a service person, the cause investigation, and the recovery of the countermeasure usually take half a day to one day. During this time, the cooling capacity of the cryocooler stops. Therefore, 8400 liters of helium gas is released into the atmosphere in 12 hours and 16800 liters in 24 hours.

Therefore, as proposed in Patent Document 2, when storing the released helium gas in the gas tank without releasing it all into the atmosphere, prepare a 3 meter square room (18 m ³ as 2 meters in height). In this case, a gas tank must be placed in advance. For this reason, the problem that the room for arrange | positioning a gas tank must be prepared besides the room which arrange | positions an MRI apparatus.

In addition, since a huge gas tank with a fixed volume at normal temperature and pressure is always connected to the cryocooler, helium gas at atmospheric pressure is stored in the gas tank even when the cryocooler is operating normally. become. That is, the 18 m ³ atmospheric pressure helium gas in the gas tank does not return to the superconducting magnet, and the helium gas cannot be used effectively.

In addition, a fixed volume gas tank at room temperature and normal pressure is always connected to the cryocooler at low temperature, so natural convection occurs between the space in the superconducting magnet and the space in the gas tank, and helium gas at room temperature is always in the cryocooler. A small amount of helium gas leaks from the cryocooler into the gas tank. For this reason, the cryocooler must always cool the normal temperature helium gas that flows in by convection, which may increase the load and promote deterioration. In Patent Document 2, natural convection is reduced by making the shape of the connection pipe into a bow shape, but it cannot be completely eliminated. It has also been proposed to use a pair of anti-parallel valves between the cryocooler and the gas tank to reduce natural convection (paragraph 0020). However, there is no description of a specific configuration beyond this.

In addition, since a gas tank with a large fixed volume at room temperature is connected to the cryocooler, there is a risk that the atmosphere will be trapped in the helium gas from the connection point between the gas tank and the cryocooler, a safety leak valve provided in the gas tank, etc. is there. When the atmosphere enters the cryocooler or the superconducting magnet, it becomes solid and may block the pipe connected to the gas tank or hinder the control of the internal pressure of the superconducting magnet.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a technique capable of returning a refrigerant evaporated by a power failure or the like to a superconducting magnet with high efficiency.

In the present invention, a gas bag having a structure in which the internal pressure is maintained constant by expanding and contracting according to the amount of gas inside is connected to the refrigerant container, the cooler is stopped during a power failure, and the refrigerant gas generated in the refrigerant container Store. Thereby, when the power failure is restored and the cooler is restarted, the refrigerant gas is returned to the refrigerant container while the gas bag is contracted, so that all the gas flowing out to the gas bag can be returned to the refrigerant container.

According to the present invention, since the refrigerant vaporized due to a power failure or the like can be returned to the superconducting magnet with high efficiency, the MRI apparatus can be used even in an area where the network of the refrigerant service is not developed.

The block diagram which shows the whole structure of the MRI apparatus of 1st embodiment. FIG. 2 is a cross-sectional view of the upper cryostat 104 of the superconducting magnet 101 constituting the MRI apparatus of FIG. FIG. 2 is an explanatory diagram showing the connection between the superconducting magnet 101 and the gas back 125 of FIG. The flowchart which shows the operation | movement before and after the power failure of the MRI apparatus of this embodiment. FIG. 5 is a graph showing the relationship between the pressure of the helium container and the volume of the gas bag during the operation of the flowchart of FIG. (a) Cross-sectional view of the upper cryostat 104 of the superconducting magnet 101 constituting the MRI apparatus of the second embodiment, (b) A cross-sectional view enlarging a part of FIG. The block diagram which shows the structure of the MRI apparatus of 3rd embodiment. Explanatory drawing which shows the example of the structure which connects a some gas back | bag in parallel in 3rd embodiment. FIG. 10 is a view taken in the direction of an arrow A in FIG. 1 showing a mark 801 indicating the position when the gas bag 125 is most expanded in the fifth embodiment.

An MRI apparatus according to a first aspect of the present invention includes a superconducting coil, a refrigerant container containing the superconducting coil, a vacuum container covering the refrigerant container, and a cooler having a tip inserted into the refrigerant container. Use a magnet. The superconducting magnet preferably includes a radiation shield plate between the refrigerant container and the vacuum container.

The refrigerant container is connected with a gas bag having a structure that maintains the internal pressure constant by expanding and contracting according to the amount of gas inside. For example, the refrigerant container is connected to an exhaust pipe that guides the refrigerant gas generated in the refrigerant container to the outside of the vacuum container when the cooler is stopped, and the gas bag is connected to the refrigerant container via the exhaust pipe. . As a result, the gas bag expands and contracts according to the amount of gas inside while maintaining a constant internal pressure, so that all the gas stored in the gas bag can be returned to the refrigerant container.

It is desirable that the exhaust pipe is branched into a first transport pipe and a second transport pipe, and each is connected to a gas bag. In this case, the first transport pipe is provided with a first one-way valve that opens when the internal pressure of the refrigerant container is higher than the internal pressure of the gas bag by the first pressure difference or more to flow the refrigerant gas from the refrigerant container to the gas bag. Deploy. The second transport pipe is provided with a second one-way valve that opens when the internal pressure of the gas bag is higher than the internal pressure of the refrigerant container by a second pressure difference or more and allows the refrigerant gas to flow from the gas bag to the refrigerant container.

For example, the first pressure difference is set to a pressure difference larger than the second pressure difference.

The second transport pipe is preferably connected to the upper part of the gas bag. This is to prevent the impurity gas from returning to the refrigerant container.

It is preferable that a part of the exhaust pipe is disposed along the radiation shield plate so as to be in thermal contact with the radiation shield plate over a predetermined length. Thereby, the radiation shield plate can be cooled by the refrigerant gas flowing out of the refrigerant container.

The exhaust pipe can be provided with a removal section that separates impurity gas mixed in the refrigerant gas. For example, the removal unit is configured to have a liquid reservoir for collecting an impurity liquid in which the impurity gas is liquefied in the middle of the exhaust pipe. The liquid reservoir is preferably provided in a region where the temperature of the exhaust pipe is not higher than the boiling point where the impurity gas is liquefied and is not lower than the melting point. Further, the liquid reservoir can be provided with a discharge portion for discharging the accumulated impurity liquid to the outside of the superconducting magnet. For example, the discharge unit includes a heater that heats and vaporizes the impurity liquid in the liquid reservoir.

The gas bag described above may be divided into a plurality of parts.

The exhaust pipe may have a configuration in which a plurality of gas bags are connected in parallel. In this case, the first and second transport pipes and the first and second one-way valves are arranged for each of the plurality of gas bags. The first and second one-way valves arranged for each of the plurality of gas bags are configured so that the set first pressure difference and second pressure difference are different for each of the plurality of gas bags arranged. Also good. Thereby, it is possible to rank which gas bag is preferentially stored with the refrigerant gas.

In addition, as a second aspect of the present invention, the following MRI apparatus is provided. The MRI apparatus includes a superconducting magnet including a refrigerant container that houses a superconducting coil, a radiation shield plate that covers the refrigerant container, a vacuum container that covers the radiation shield plate, and a cooler with a tip inserted inside the refrigerant container. Use. The refrigerant container is connected to an exhaust pipe that guides refrigerant gas generated in the refrigerant container to a gas bag outside the vacuum container when the cooler is stopped. The exhaust pipe is provided with a removing unit that separates impurity gas mixed in the refrigerant gas.

The exhaust pipe of the MRI apparatus of the second aspect can be provided with a removing unit that separates impurity gas mixed in the refrigerant gas. For example, the removal unit is configured to have a liquid reservoir for collecting an impurity liquid in which the impurity gas is liquefied in the middle of the exhaust pipe. The liquid reservoir is preferably provided in a region where the temperature of the exhaust pipe is not higher than the boiling point where the impurity gas is liquefied and is not lower than the melting point. Further, the liquid reservoir can be provided with a discharge portion for discharging the accumulated impurity liquid to the outside of the superconducting magnet. For example, the discharge unit includes a heater that heats and vaporizes the impurity liquid in the liquid reservoir.

Also, as a third aspect of the present invention, an MRI apparatus gas recovery apparatus is provided. This apparatus is connected to a superconducting magnet of the MRI apparatus, and has a transport pipe that guides the refrigerant gas generated from the refrigerant container that houses the superconducting coil, and a gas bag that stores the refrigerant gas connected to the transport pipe. The gas bag is a structure that keeps the internal pressure constant by expanding and contracting according to the amount of gas inside.

It is desirable that the transport pipe of the gas recovery device according to the third aspect is divided into a first transport pipe and a second transport pipe, and each is connected to a gas bag. The first transport pipe is provided with a first one-way valve that opens when the internal pressure of the refrigerant container is higher than the internal pressure of the gas bag by a first pressure difference or more to flow the refrigerant gas from the refrigerant container to the gas bag. ing. The second transport pipe is provided with a second one-way valve that opens when the internal pressure of the gas bag is higher than the internal pressure of the refrigerant container by a second pressure difference or more and allows the refrigerant gas to flow from the gas bag to the refrigerant container. Yes.

Also, as a fourth aspect of the present invention, a method for operating an MRI apparatus provided with a superconducting magnet is provided. When the cooler provided in the superconducting magnet stops, the first step of stopping the photographing operation, and if the internal pressure of the refrigerant container rises and reaches a predetermined pressure, the refrigerant gas in the refrigerant container The second step of storing and guiding the gas bag with a structure that expands and contracts according to the amount of refrigerant gas while maintaining the temperature, and if the cooler restarts and the internal pressure in the refrigerant container drops below a predetermined pressure, the gas And a third step of returning the refrigerant gas in the bag back to the refrigerant container, liquefying it by cooling of the cooler, and storing it in the refrigerant container.

In the third step, the photographing operation can be performed in parallel with the liquefaction by the cooler.

In the second step, the refrigerant gas in the refrigerant container is preferably guided to the gas bag through the one-way valve, and in the third step, the refrigerant gas in the gas bag is preferably returned to the refrigerant container through the one-way valve.

In the third step, it is preferable to correct the variation in the uniformity of the static magnetic field of the superconducting magnet caused by the pressure change in the refrigerant container before performing the photographing operation.

According to each aspect of the present invention described above, the following effects can be obtained.
(1) An MRI apparatus using a superconducting magnet that consumes little refrigerant is provided.
(2) There is no need to replenish the refrigerant even during a power failure, system failure or maintenance of the cooler.
(3) Since a gas bag for storing refrigerant gas can be added flexibly, it can be operated flexibly according to the power situation of medical facilities and the area.

By (1) to (3) above, it is possible to perform diagnostic imaging by installing MRI equipment using superconducting magnets even in areas where the refrigerant service network is not developed or where power outages occur frequently become.

Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings. In all the drawings for explaining the embodiments, parts having the same function are given the same reference numerals, and repeated explanation thereof is omitted.

(First embodiment)
<Overall configuration of MRI apparatus of this embodiment>
First, the overall configuration of the MRI apparatus operated in the first embodiment will be described.

FIG. 1 shows an overall configuration in a state where the MRI apparatus of the present embodiment is installed in a medical facility.

An open-structure superconducting magnet 101 is used as a magnet for generating a static magnetic field of this MRI apparatus. In the superconducting magnet 101 having an open structure, an upper cryostat 104 and a lower cryostat 105 are disposed opposite to each other across a magnetic field space 103 in which the subject 102 is disposed. Each of the internal spaces of the upper cryostat 104 and the lower cryostat 105 contains a superconducting coil serving as a magnetomotive force source and is filled with liquid helium as a refrigerant. The upper cryostat 104 is connected to the lower cryostat 105 by a connecting pipe 106 and is supported by the connecting pipe 106 as a support, thereby forming an open structure in which the front, rear, right and left of the magnetic field space 103 are vacant. As a result, the feeling of pressure applied to the subject 102 is relieved and a gentle examination environment is provided. Detailed configurations of the upper cryostat 104 and the lower cryostat 105 will be described in detail later.

A cold head 107 is incorporated in the superconducting magnet 101. A compressor unit 108 is connected to the cold head 107, and compressed refrigerant gas (hereinafter also referred to as helium gas) is supplied from the compressor unit 108. The cold head 107 and the compressor unit 108 constitute a cooler for the superconducting magnet 101 (hereinafter also referred to as a cryocooler). The cold head 107 cools the superconducting magnet 101 by a cooling effect during adiabatic expansion of the compressed refrigerant gas. As a result, the cold head 107 cools a radiation shield, which will be described later, in the superconducting magnet 101, and in the upper cryostat 104 and the lower cryostat 105, the refrigerant gas (helium gas) generated by the vaporization of the refrigerant (liquid helium). It is cooled, condensed again into liquid helium, and returned to the upper cryostat 104.

Further, the cold head 107 is controlled so as to have a cooling capacity necessary for recondensing the helium vaporized by the amount of heat that has entered the upper cryostat 104 and the lower cryostat 105 without excess or deficiency. Therefore, since the vaporized helium gas is not released into the atmosphere, a closed type superconducting magnet is realized. For this control, the superconducting magnet 101 is provided with a control system including various sensors and circuits. Specifically, the superconducting magnet 101 incorporates a plurality of temperature sensors and pressure sensors for monitoring its operating state, and its sensor connection terminal 109 is connected to the magnet control unit 110. The magnet control unit 110 monitors the operating state of the superconducting magnet 101 and outputs a signal necessary for controlling the cryocooler to a heater and a compressor unit 108 incorporated in the superconducting magnet 101.

With the configuration of the closed-type superconducting magnet 101 as described above, a stable static magnetic field having a high magnetic field strength (for example, 1 Tesla) is generated in the magnetic field space 103. This stable magnetic field is realized by flowing a permanent current (for example, 450 amperes) through a superconducting coil cooled to a temperature lower than the critical temperature at which it becomes a superconducting state by liquid helium (temperature 4.2 Kelvin).

Also, a pair of shim plates 111 are attached to the surfaces of the upper cryostat 104 and the lower cryostat 105 on the magnetic field space 103 side of the superconducting magnet 101. The shim plate 111 generates a magnetic field that improves the magnetic field uniformity of a spherical space (for example, 40 centimeters in diameter) in the center of the magnetic field space 103. The shim plate 111 includes a plate member having a plurality of screw holes and magnetic small screws embedded in predetermined screw holes among the plurality of screw holes. By adjusting the position of the small screw of the magnetic body according to the nonuniformity of the magnetic field, the magnetic field uniformity of the above-mentioned spherical space can be adjusted to a target value (eg, 3 ppm or less).

A pair of gradient magnetic field coils 112 that generate a gradient magnetic field are arranged on the surface of the magnetic field space 103 of the shim plate 111. The gradient coil 112 has a flat plate structure so as not to hinder the open structure of the superconducting magnet 101. Each of the pair of upper and lower gradient magnetic field coils 112 includes three types of stacked coils of x, y, and z, and these generate gradient magnetic fields in three axial directions orthogonal to each other. A gradient magnetic field power supply 113 for applying a current independently is connected to each of the x coil, the y coil, and the z coil. When a positive current is applied to the z coil from the gradient power amplifier 113, the upper z coil generates a magnetic flux in the same direction as the magnetic flux generated by the superconducting magnet 101, and the lower z coil generates a magnetic flux in the opposite direction. To do. As a result, a gradient in which the magnetic flux density gradually decreases from the top to the bottom of the z-axis (vertical axis) of the magnetic field space 103 can be formed.

Similarly, the x coil and the y coil give a gradient magnetic field that makes the magnetic flux density generated by the superconducting magnet 101 gradient along the x axis and the y axis (both horizontal axes), respectively. These x coils, y coils, and z coils can also function as shim coils of primary components of x, y, and z with non-uniform magnetic fields, respectively. In other words, the gradient magnetic field power supply 113 can output the current for generating the gradient by superimposing the shim current for improving the magnetic field uniformity.

In addition to the x coil, the y coil, and the z coil, the gradient coil 112 generates a Bo coil that adjusts the magnetic field intensity generated by the superconducting magnet 101 and a high-order mode magnetic field in the x, y, and z directions. A shim coil, for example, a coil that generates a magnetic field of x ² , y ² , x ³ , x ² + y ² is provided. A shim power source 114 is connected to these coils and a current is applied thereto.

A pair of high-frequency coils 115 are attached to the magnetic field space 103 side of the gradient coil 112. The high-frequency coil 115 is also a flat-plate coil so as not to hinder the open structure of the superconducting magnet 101. A high frequency power source 116 is connected to the pair of upper and lower high frequency coils 115, and a high frequency current is supplied. As a result, a high-frequency magnetic field necessary for nuclear magnetic resonance of the nuclear spin at the inspection site of the subject 102 is generated. For example, it generates a 42 megahertz high-frequency magnetic field in which hydrogen nuclei cause nuclear magnetic resonance with a static magnetic field strength of 1 Tesla.

By combining the above-mentioned stable and highly uniform static magnetic field, gradient magnetic field, and high-frequency magnetic field, it is possible to cause a nuclear magnetic resonance phenomenon to occur accurately and selectively in the hydrogen nuclei at the examination site of the subject 102. it can. Three-dimensional positional information can be added by applying a gradient magnetic field in a pulsed manner to the subsequent precession process of nuclear spins. The precession of the nuclear spin is detected as an NMR signal at the examination site, and an image or the like is generated. A configuration for detection and image generation will be described below.

A detection coil 117 for detecting an NMR signal is disposed at substantially the center position of the magnetic field space 103, that is, at the examination site of the subject 102. This detection coil 117 detects a slight magnetic field fluctuation due to the above-described precession of the nuclear spin as an induced current (electric signal). The NMR signal is passed to the high frequency amplifier 118 connected to the detection coil 117. In the high frequency amplifier 118, the NMR signal is subjected to amplification / detection signal processing to be converted into a digital signal suitable for computer arithmetic processing.

The computer 119 receives the NMR signal converted into a digital signal from the high-frequency amplifier 118, converts it into an image or spectrum chart for use in medical diagnosis, and stores it in a storage device (not shown in the figure) in the computer 119. ) And displayed on the display 120. The computer 119 stores programs such as an image analysis function useful for diagnosis. Operation instructions to the computer 119 are input via an input device 121 such as a keyboard.

Further, the computer 119 stores the control program for controlling the operation of each unit and executing various MRI examination (imaging) modes, for example, the high-speed spin echo method and the diffusion weighted echo planar method. That is, the magnet control unit 110, the gradient magnetic field power supply 113, the shim power supply 114, the high frequency power supply 116, and the high frequency amplifier 118 are connected to the computer 119, respectively, and their operations are controlled with programmed contents. Thereby, a gradient magnetic field or a high-frequency magnetic field is applied at a predetermined timing, and an NMR signal is acquired by a predetermined MRI examination (imaging) method. The operating states of these units can be recorded in the memory of the computer 119. Also, the operation status information of each unit recorded in the computer 119 is transmitted to an external monitoring base via a communication control device (not shown in the figure), enabling remote monitoring from the external monitoring base. .

As another main unit, a patient table 122 that transports the subject 102 to the center of the magnetic field space 103 is incorporated in the front surface of the superconducting magnet 101. The superconducting magnet 101 and the patient table 122 are installed in an examination room 123 provided with an electromagnetic shield. The electromagnetic shield prevents electromagnetic waves generated by an external device from entering the detection coil 117 as noise.

Furthermore, an extendable gas back 125 is connected to the superconducting magnet 101 via an exhaust pipe (hereinafter referred to as a helium exhaust pipe) 124. Details of the function of the gas bag 125 will be described later.

The computer 119 of the MRI apparatus according to the present embodiment incorporates a function program for analyzing and correcting the magnetic field performance of the magnetic field space 103. This program executes the following steps.
(1) The NMR signal of the subject 102 disposed in the magnetic field space 103 is measured in a state where no current is supplied to the gradient magnetic field coil 112, the Bo coil, and all the shim coils (examination mode).
(2) The measured NMR signal is Fourier transformed by the computer 119, and the frequency component of the NMR signal is obtained.
(3) A magnetic field corresponding to the difference between the nuclear magnetic resonance frequency of 42 megahertz of the hydrogen nuclear spin and the frequency obtained in the above step is obtained by calculation with a magnetic field intensity of 1 Tesla. The shim power supply 114 is controlled to generate a differential magnetic field from the Bo coil.
(4) Next, the NMR signal of the subject 102 is measured in a state where a current of, for example, 10 amperes is applied to the x coil.
(5) The measured NMR signal is expanded with a spherical harmonic function, and the error magnetic field in the x-axis direction of the imaging space 103 is analyzed.
(6) Similarly, the error magnetic field component is analyzed for the y-axis and the z-axis.
(7) Gradient magnetic field power supply 113 so that a shim current that corrects the error magnetic field components of the x-axis, y-axis, and z-axis with the magnetic fields of the x-coil, y-coil, z-coil, and shim coil of the gradient coil 112 flows. The shim power supply 114 is controlled.

By using this function, the static magnetic field in the magnetic field space 103 is kept in an optimum state for MRI examination (imaging). According to this function, a magnetic field change due to a change in internal pressure of the refrigerant container of the superconducting magnet 101 due to a power failure, a magnetic field change over time, and an error magnetic field due to the magnetic susceptibility of the examination site of the subject 102 (for example, It is possible to correct the magnetic medical implant influence).

It is also possible to measure in advance the relationship between the change in the pressure of the refrigerant container of the superconducting magnet 103 due to a power failure or the like and the change in the magnetic field in the magnetic field space 103 and input it to the computer 119. Based on the pressure signal from the magnetic field control unit 110, the computer 119 refers to the relationship measured in advance, controls the gradient magnetic field power supply 113 and the shim power supply 114, and compensates for the magnetic field change accompanying the pressure change. This function is suitable for, for example, an MRI examination of an emergency patient because it is not necessary to measure an NMR signal for magnetic field correction before the MRI examination and can be corrected in real time.

<Structure and function of cryocooler>
FIG. 2 is a diagram showing details of the upper cryostat 104 and the cold head 107 shown in FIG. In the superconducting magnet 101, the internal structure of the upper cryostat 104 and the lower cryostat 105 is basically vertically symmetric about the magnetic field space 103, so only the upper cryostat 104 will be described here.

The upper cryostat 104 is covered with a vacuum vessel 201 on the outside. The vacuum vessel 201 is made of stainless steel having a thickness of 10 millimeters, for example, and has rigidity capable of withstanding the weight of the main body and the internal vacuum pressure. A refrigerant container (hereinafter referred to as a helium container) 202 is disposed inside the vacuum container 201. A plurality of superconducting coils 203 (only one is shown in the figure) are arranged inside the helium vessel 202 and fixed to the helium vessel 202. The helium vessel 202 is made of stainless steel having a thickness of 15 millimeters, for example, and has rigidity capable of withstanding the electromagnetic force applied to the superconducting coil 203 and the pressure difference between the inside and outside.

The inside of the helium vessel 202 is normally filled with liquid helium 204 to approximately 90% of its volume, and the superconducting coil 203 is immersed in the liquid helium 204. As a result, the superconducting coil 203 is cooled to 4.2 Kelvin (−268.8 ° C.), which is the boiling point temperature of the liquid helium 204, and maintains the superconducting state. Further, a second cooling stage 216 of the cold head 107 is inserted into the helium gas reservoir in the upper part of the helium container 202, and the helium gas in the helium container 202 is directly cooled and liquefied (condensed).

The gap between the vacuum vessel 201 and the helium vessel 202 is a vacuum layer, and a radiation shield plate 212 is disposed in the middle. The radiation shield plate 212 is made of, for example, aluminum having a thickness of 5 mm, and its surface is polished to a mirror surface to suppress radiant heat. The radiation shield plate 212 is cooled by being in thermal contact with the first cooling stage 213 of the cold head 107 and functions to further reduce radiant heat.

Also, a super insulator 214 (only a part is shown in FIG. 2) is arranged in the gap between the vacuum vessel 201 and the radiation shield plate 212. The super insulator 214 is composed of, for example, multiple layers of polyethylene sheets on which an aluminum thin film is deposited, and is effective in reducing radiant heat.

Between the vacuum vessel 201, the radiation shield plate 212, and the helium vessel 202, load supports 215 are attached at a plurality of locations in order to fix their positions. In order to minimize the heat conducted from the vacuum vessel 201 to the radiation shield plate 212 and from the radiation shield plate to the helium vessel 202 via the load support 215, the load support 215 is reinforced with a material having low heat conductivity, such as stainless steel. It is made of carbon resin or reinforced plastic resin.

With such a configuration, the amount of heat applied from the vacuum vessel 201 at room temperature (about 300 Kelvin) to the helium vessel 202 at the boiling point of helium (about 4 Kelvin) is suppressed to about 1 watt in total of radiant heat and conduction heat. It has been.

The cold head 107 is adjusted so that the cooling capacity is almost in thermal equilibrium with the heat applied to the helium vessel 202. The first cooling stage 213 of the cold head 107 is 43 Kelvin (−230 ° C.), has a cooling capacity of about 45 watts, and cools the radiant heat shield plate 212 as described above. The second cooling stage 216 is 4 Kelvin (−269 ° C.) and has a cooling capacity of about 1.4 watts, and the helium gas is directly cooled and condensed in the helium vessel 202.

Inside the helium vessel 202, a liquid level sensor 205 for measuring the liquid level of the liquid helium 204 and a pressure sensor 206 for measuring the pressure of the helium gas evaporated from the liquid helium 204 are incorporated. In addition, a heater element 207 for heating liquid helium is also arranged in the helium vessel 202. When the pressure detected by the pressure sensor 206 is lower than a predetermined value (for example, about 5 KPa), liquid helium is slightly added. The pressure is kept constant by heating and vaporizing. This control is performed by the magnet control unit 110. The output signal lines of the sensors 205 and 206 and the heater element 207 are connected from the sensor connection terminal 109 to the magnet control unit 110 via the hermetic seal 208 so as not to cause a vacuum leak.

A tubular service port 209 is connected to the upper part of the helium vessel 202. This service port 209 is used when liquid helium transported from the outside is injected into the helium container 202. Specifically, liquid helium can be injected into the helium vessel 202 by removing the stopper 210 at the top of the service port 209 and inserting a liquid injection pipe (not shown in the figure). The pipe branches from the middle of the service port 209, and a rupture plate 211 that ruptures at a predetermined pressure (for example, 40 KPa) is connected to the branched pipe.

In the normal operation state of the MRI apparatus, the magnet control unit 110 described above supplies a current to the heater element 207 to maintain the pressure of the helium vessel 202 at about 5 KPa. When quenching or urgent magnetic field decay, liquid helium vaporizes all at once, and when the internal pressure of the helium vessel 202 reaches a predetermined pressure (for example, 40 KPa), the rupture plate 211 is ruptured, and helium gas is safely released to the outside. . As a result, no pressure higher than a predetermined pressure is generated in the helium vessel 202, and safety is maintained.

On the other hand, when the cooling performance of the cold head 107 is stopped due to a power failure or a system failure, or when the control of the heater element 207 stops functioning and continues to generate heat, the liquid helium gradually vaporizes. This vaporization rate is not as great as during quenching or emergency magnetic field decay. In the present invention, the generated helium gas is allowed to flow through the helium exhaust pipe 124 to the extendable gas bag 125 and stored. Thereby, the pressure of the helium vessel 202 is maintained so as not to exceed a predetermined pressure (for example, 20 KPa). If it is within the pressure range of 20 KPa or less, it is confirmed that the change in the magnetic field performance of the magnetic field space 103 can be corrected by the gradient coil 112, the Bo coil, and the shim coil. Can do. Further, when the power failure or system failure is recovered and the cold head 107 is restarted, the helium gas stored in the gas bag 125 can be returned to the helium vessel 202 and liquefied. This will be described in detail below.

<Configuration of helium exhaust pipe and gas bag>
One end of a helium exhaust pipe 124 is connected to the upper portion of the helium vessel 202 as shown in FIG. A portion of the helium exhaust pipe 124 is disposed so as to contact the outer surface of the radiation shield plate 212 over a predetermined length, and constitutes a thermal contact portion 217. Thus, the radiation shield plate 212 can be cooled by the latent heat of the helium gas vaporized in the helium vessel 202.

The temperature of the radiation shield plate 212 has the lowest temperature at the thermal contact portion of the cold head 107 with the first cooling stage 213, and the temperature increases as the distance from the first cooling stage 213 increases. Since the temperature of the helium exhaust pipe 124 drawn out from the helium vessel 202 is the lowest, the position where the helium exhaust pipe 124 starts to contact the outer surface of the radiation shield plate 212 through the radiation shield plate 212 is as shown in FIG. It is arranged near the position. From this position over a predetermined length, the helium exhaust pipe 124 is disposed along the radiation shield plate 212 so that the outer peripheral surface of the helium exhaust pipe 124 contacts the outer surface of the radiation shield plate 212. It is composed. In the thermal contact portion 217, the helium exhaust pipe 124 is disposed so as to gradually move away from the cold head 107, away from the radiation shield plate 212 at the end of the thermal contact portion 217 farthest from the cold head 107, and In the direction, it passes through the vacuum vessel 201 and is drawn out. Thus, the helium exhaust pipe 124 can effectively cool the radiation shield plate 212 with the latent heat of the helium gas at the thermal contact portion 217.

Although not shown in FIG. 1, the helium exhaust pipe 124 branches in two directions outside the superconducting magnet 101 as shown in FIG. One is a transport pipe (hereinafter referred to as a helium transport pipe) (A) 301, which is connected to a helium transport pipe (B) 303 via a one-way valve (hereinafter referred to as a one-way relief valve) 302, and gas. Connected to the bottom of the back 125. The other is connected from the helium transport pipe (C) 304 to the helium transport pipe (D) 306 via the one-way relief valve 305. The helium transport pipe (D) 306 is connected to the upper part of the gas bag 125.

Here, the one-way relief valve 302 is opened when the gas pressure in the helium transport pipe (A) 301 is higher than the gas pressure in the helium transport pipe (B) 303 by 10 KPa or more, and helium gas is removed from the helium transport pipe ( It is configured to flow in the direction from A) 301 to the helium transport pipe (B) 303. Backflow is not possible with any pressure difference.

On the other hand, the one-way relief valve 305 is opened when the gas pressure in the helium transport pipe (D) 306 is higher than the gas pressure in the helium transport pipe (C) 304 by 5 KPa or more, and the helium gas is supplied to the helium transport pipe (D ) 306 to the helium transport pipe (C) 304. Backflow is not possible with any pressure difference.

The gas bag 125 has a structure that starts to expand when the internal pressure reaches a predetermined pressure (10 KPa) due to the elastic characteristics of the material, and expands and contracts while maintaining the predetermined pressure (10 KPa) according to the amount of inflowing helium gas. That is, the gas bag 125 is not a fixed volume, but is configured to expand and contract according to the amount of helium gas and the gas pressure inside, and the internal volume changes. For example, a gas bag 125 made of a film material having a high gas barrier property against helium gas and folded in a bellows shape can be used. When the internal pressure of the gas bag 125 reaches a predetermined pressure (10 KPa), the volume increases due to the bellows extending. The maximum capacity of the gas bag 125 is set to a capacity that can sufficiently store the helium gas vaporized by the amount of heat that enters the helium vessel 202 during that time based on the predicted time when the cold head 107 stops due to a power failure or system failure. As an example, if it can be predicted that the cold head 107 will stop for 40 hours due to a power failure etc., it will depend on the size and configuration of the helium vessel 202, but about 700 liters of helium gas is generated per hour, so the maximum capacity is 28 m Connect ³ gas bags 125. The gas bag 125 has a compact configuration with the bellows folded in the initial state, but expands to a maximum of 28 m ³ due to the inflow of helium gas, so a 28 m ³ space can be prepared in the direction in which the gas bag 125 expands during a power failure. Place gas bag 125 in place.

In FIG. 1, the gas bag 125 is arranged in the immediate vicinity of the examination room 123, but it is also possible to extend the helium exhaust pipe 124 and arrange the gas bag 125 in a room away from the examination room 123.

If the cooling performance of the cold head 107 stops functioning due to a power failure or system failure, the amount of heat entering the helium vessel 202 from the vacuum vessel 201 is structurally suppressed to about 1 watt, but this amount of heat is about 1 watt. Then, liquid helium is vaporized and the pressure in the helium vessel 202 starts to rise. Each part operates as follows, and the pressure of the helium vessel 202 is maintained at an allowable pressure of 20 KPa.

That is, when the pressure in the helium vessel 202 reaches 20 KPa, the one-way relief valve (10 KPa) 302 is opened and helium gas flows out to the gas back 125. The gas bag 125 increases in volume according to the amount of helium gas flowing in. As a result, the helium vessel 202 maintains 20 KPa, and the gas bag 125 maintains 10 KPa.

In this process, the flow of helium gas comes into contact with the radiation shield plate 212 at the thermal contact portion 217. Therefore, the radiation shield plate 212 is cooled by the latent heat of the helium gas, the radiant heat is lowered, and the liquid helium 204 in the helium vessel 202 is vaporized. Work to prevent. Further, the heat exchange has an effect of reducing the helium gas that has been cooled to the outside of the superconducting magnet 101 and reducing frost on the helium exhaust pipe 124 and the helium transport pipe (A) 301.

When the power failure or system failure is restored, the operation of the cold head 107 resumes, and helium gas condenses in the gas reservoir at the top of the helium vessel 202 with approximately 1.4 watts of cooling heat from the second cooling stage 216 of the cold head 107. Liquefied. As a result, the pressure in the helium vessel 202 that has been maintained at 20 KPa begins to decrease, and the difference from the internal pressure of the gas bag 125 at which 10 KPa is maintained becomes less than 10 KPa. As a result, the one-way relief valve 302 is closed and the inflow of gas from the helium vessel 202 to the gas bag 125 is stopped.

Furthermore, if the cold head 107 continues to liquefy the helium gas in the helium vessel 202 and the internal pressure of the helium vessel 202 decreases to 5 KPa or less, the pressure difference from the internal pressure 10 KPa of the gas back 125 becomes 5 KPa or more. The valve (5 KPa) 305 is opened, and helium gas flows from the gas back 125 into the helium vessel 202. When the gas pressure in the helium vessel 202 is heated by the heater element 207 under the control of the magnet control unit 110 and is maintained at around 5 KPa, the pressure difference from the gas back 125 that maintains 10 KPa becomes less than 5 KPa. At this point, the one-way relief valve 305 is closed, and the inflow of helium gas from the gas back 125 to the helium vessel 202 is completed. At this time, the internal pressure of the gas bag 124 is maintained at 10 KPa, but since it has returned to the size before starting to expand, almost all of the helium gas stored in the gas bag 125 can be returned to the helium container 202. .

In the process of returning the helium gas from the gas bag 125 to the helium vessel 202, the helium gas at room temperature comes into contact with the radiation shield plate 212 cooled by the first cooling stage 213 of the cold head 107 at the thermal contact portion 217. Precooled. Therefore, liquefaction of helium gas in the helium vessel 202 is promoted.

The operation of the MRI apparatus when a power failure occurs is described in more detail using the flowchart of FIG.

When a power failure occurs, the MRI system stops and the cooling capacity of the cold head 107 also stops. In the case of a planned power outage, the operator cancels the MRI inspection in advance by inputting the input device 121 in advance. In the case of an unexpected power outage, the operator carries out the subject 102 under examination from the MRI apparatus by manual operation of the patient table 122 (step 401).

As the power failure time elapses, the internal pressure of the helium vessel 202 increases (step 402). This is because 1.25 liters of liquid helium per hour is vaporized into 10 times (about 10 liters) helium gas with an amount of heat of about 1 watt entering the helium vessel 202 from the vacuum vessel 201. The pressure increase rate depends on the space volume of the gas reservoir at the top of the helium vessel 202, but in the case of the helium vessel 202 filled with the usual 90% liquid helium, it rises at a rate of 4 KPa per hour.

After about 4 hours, the internal pressure of the helium vessel 202 reaches 20 KPa (step 403). When the internal pressure reaches 20 KPa, the pressure difference with the gas back 125 maintained at the internal pressure of 10 KPa reaches 10 KPa, so the one-way relief valve (10 KPa) 302 is opened.

Thereby, the helium gas in the helium vessel 202 flows out from the helium exhaust pipe 124 through the one-way relief valve (10 KPa) 302 to the gas bag 125 and is stored in the gas bag 125. Thereby, the internal pressure of the helium vessel 202 is maintained at 20 KPa (steps 404 and 405). When helium gas flows out of the helium vessel 202, the helium gas in the helium exhaust pipe 124 is heated by heat exchange between the radiation shield plate 212 and the thermal contact portion 217, and helium having a temperature close to room temperature expanded about 70 times. It flows out as gas.

As the internal helium gas volume increases, the gas bag 125 expands against the contraction force of the gas bag itself and maintains a gas pressure of about 10 KPa (step 405).

When the power failure is restored, the MRI system starts operation. The cold head 107 also resumes cooling (step 406). Even if a part of the liquid helium is vaporized, the superconducting coil 203 is immersed in the liquid helium, and the superconducting state is maintained.

In about 30 minutes of operation, the cold head 107 reaches the normal cooling capacity, and the helium gas in the upper gas reservoir of the helium vessel 202 is cooled. As helium gas condenses into 1/10 volume ratio of liquid helium, the pressure drops from 20 KPa at a rate of about 2 KPa per hour. When the pressure in the helium vessel 202 is less than 20 KPa, the differential pressure with respect to the internal pressure 10 KPa of the gas bag 125 is less than 10 KPa, so the one-way relief valve (10 KPa) 302 is closed. During this time, since the superconducting state of the superconducting magnet 101 is maintained, the operator can place the subject 102 in the magnetic field space 103 of the MRI apparatus, perform magnetic field correction, and perform an inspection (step 407).

About 8 hours after the power failure recovery, the helium tank pressure drops to 5 KPa due to the cooling and condensation of the helium gas in the helium vessel 202. Then, helium gas flows from the gas back 125 into the helium vessel 202 through the one-way relief valve (5 KPa) 305. The pressure in the helium vessel 202 is maintained at 5 KPa. On the other hand, the gas bag 125 contracts in volume corresponding to the volume of the helium gas that has flowed out, and its internal pressure is maintained at 10 KPa (step 408).

ヘ リ ウ ム All the helium gas in the gas bag 125 flows out, and the gas bag 125 reaches its most contracted state before the volume starts to expand. The internal pressure of the helium vessel 202 is controlled by heat generation of the heater element 207 installed in the helium vessel 202 by applying current controlled by the magnet control unit 110 so that the internal pressure does not become 5 KPa or less. As a result, the difference between the internal pressure 10 KPa of the gas bag 125 and the internal pressure of the helium vessel 202 becomes less than 5 KPa, and the one-way relief valve (5 KPa) 505 is closed. The helium vessel 202 maintains a thermal equilibrium state (step 409).

In the flow of FIG. 4, the pressure change of the helium vessel 202 and the volume change of the gas bag 125 are as shown in FIG.

As shown in FIG. 5, when the MRI apparatus is operating normally, the pressure of the helium vessel 202 is maintained at 5 KPa, and the volume of the gas bag 125 is 0% that is the most contracted (pre-expansion state). When a power outage occurs at time A, the pressure in the helium vessel 202 rises at a rate of 4 KPa per hour (the rate at which liquid helium vaporizes due to the amount of heat entering the helium tank of about 1 watt), and toward the time B at 20 KPa. . At time B, the one-way relief valve (10 KPa) 302 is opened, so that helium gas is sent to the gas back 125 by 700 liters per hour, and the pressure in the helium vessel 202 is maintained at 20 KPa. On the other hand, the volume of the gas bag 125 starts to expand due to the helium gas sent. Since the upper limit of the volume is 28 m ³ , it is possible to continue to expand by accepting the inflow of helium gas even if the power failure continues for about 40 hours.

From the time point C when the power failure is restored, the pressure in the helium vessel 202 decreases at a rate of 2 KPa / hour (condensation of helium gas due to a difference of about 0.5 watts between the amount of heat entering and the cooling capacity of the cryocooler), and moves toward the point D. From time D, helium gas flows from the gas back 125 through the one-way relief valve (5 KPa) 305, and the pressure of the helium vessel 202 is maintained at 5 KPa. On the other hand, the gas bag 125 starts to shrink in accordance with the volume of the helium gas that has flowed out. At time E, the gas bag is completely contracted to return to the state before expansion, and the pressure in the helium vessel 202 returns to the normal operation state that maintains 5 KPa.

As described above, in this embodiment, instead of a fixed volume, a constant internal pressure is maintained, and the gas bag 125 that expands and contracts according to the amount of gas flowing in is paired with a pair of different open pressure differences. It is connected to the helium vessel 202 via a directional relief valve. As a result, using the internal pressure difference between the helium vessel 202 and the gas bag 125, the helium gas stored in the gas bag 125 when the cold head (cryo-cooler) stops due to a power failure or the like is returned to the helium vessel 202 almost completely after restoration. Can do. Therefore, the helium gas in the gas bag 125 can be condensed and reused as a refrigerant without wasting it.

In addition, when the cold head is not stopped due to a power failure, etc., the gas bag 125 does not expand, so the space necessary for the gas bag 125 to expand during a power failure should be used for other purposes during a non-power failure. And the use efficiency of the space can be increased. For example, a space necessary for the gas bag to expand can be used as a space for a subject or an operator to prepare an examination.

In addition, since the gas bag 125 is contracted during a non-power failure and the pair of one- way relief valves 302 and 305 having different pressure differences are closed during a non-power failure, the helium gas is interposed between the gas bag 125 and the helium vessel 202. No convection occurs. Therefore, it is not necessary for the cryocooler to cool the convective helium gas, the load can be reduced, and the cryocooler can have a long life.

As described above, since the helium transport pipe (D) 306 is connected to the uppermost part of the helium gas bag 125, even if an impurity gas such as air is mixed in the gas bag 125, it is the lightest gas. The helium gas accumulates in the upper part of the gas bag 125, and when returning to the helium container, only helium gas not containing impure gas can flow into the helium transport pipe (D) 306. Therefore, it is possible to prevent the impurity gas from freezing in the helium container 202.

On the other hand, the helium transport pipe (B) 303 is not limited to the lowermost part of the gas bag 125, and may be connected to any position of the gas bag 125, but is connected to the position of the drain plug 307 at the lowermost part of the gas bag 125. By doing so, the opening of the gas bag 124 can be reduced as much as possible, and the effect of reducing the failure potential factor such as leakage can be obtained.

In addition, in the configuration of FIG. 2, the inner part of the vacuum vessel 201 of the helium exhaust pipe 124 may be merged with the service port 209 inside the vacuum vessel 201 or may be configured to be shared with the service port 209. is there. When the service port 209 is also used as the helium exhaust pipe 124, the thermal contact portion 217 between the helium exhaust pipe 124 and the radiation shield 212 can be omitted. The helium exhaust pipe 124 is branched from the service port 209 again outside the vacuum vessel 201. Accordingly, it is not necessary to draw out two pipes of the helium exhaust pipe 124 and the service port 209 from the vacuum container 201, and the number of the pipes can be reduced to one, so that the airtightness of the vacuum container 201 can be easily maintained. In addition, access from the plug 210 of the service port 209 to the helium exhaust pipe 124 is facilitated.

Note that new helium gas can be injected into the gas bag 125 from the drain plug 307. If for some reason the liquid helium in the helium vessel 202 is reduced, instead of injecting liquid helium from the service port 209, helium gas is injected into the gas bag 125 and condensed into liquid helium with the cold head 107. Can do. Liquid helium is difficult to transport and store for a long time, but helium gas packed in a cylinder can be transported and stored for a long time. As a result, the MRI apparatus using the superconducting magnet can be used even in areas outside the liquid helium service network, and the convenience of the MRI apparatus using the superconducting magnet is increased.

As described above, in this embodiment, when the cold head 107 stops in the event of a power failure or system failure, the pressure in the helium vessel 202 reaches 20 KPa and is maintained until the power failure is restored (step 403). By utilizing this, when a helium gas back or a helium exhaust pipe is damaged during a power failure or the like, it is possible to detect this and issue an alarm. Specifically, when the helium exhaust pipe 124 or the gas back 125 is damaged, the helium gas leaks and the pressure in the helium vessel 202 becomes 20 KPa or less. The magnet control unit 110 of the superconducting magnet 101 is driven by a battery or the like even during a power failure, and detects the pressure in the helium vessel 202 by the pressure sensor 206.

Therefore, when the pressure in the helium vessel 202 falls below 20 KPa at the time of a power failure or system failure, the magnet control unit 110 can notify the operator or administrator by an alarm or the like. In addition, since the magnet control unit 110 transmits the pressure detection result to the remote monitoring base via the network or the like via the remote monitoring system, it detects a pressure drop at the remote monitoring base and issues an alarm or the like. Is also possible. Since the operation of the magnet control unit 110 detecting the pressure in the helium vessel 202 by the pressure sensor 206 has been performed conventionally, the operation of detecting damage such as a gas back to 20 KPa or less at the time of a power failure is a conventional MRI apparatus. Even so, it can be easily added.

Here, the procedure for installing the MRI apparatus of the present embodiment in a medical facility or the like will be briefly described below.
(1) Install an MRI apparatus other than the gas bag 125 in the medical facility in the same manner as the conventional MRI apparatus.
(2) The transport pipes 301, 303, 304, 306 provided with the one- way relief valves 302, 305 as shown in FIG. 3 are connected to the exhaust pipe 124 at the upper part of the superconducting magnet 101. However, the one- way relief valves 302 and 305 are opened manually.
(3) The gas bag 125 in a contracted state is disposed at a predetermined installation position.
(4) Helium gas flows out from the helium vessel 202 of the superconducting magnet 101 through the transport pipes 301, 303, 304, and 306. This can be easily realized by stopping the cold head 107 for a certain time and setting the internal pressure of the helium vessel 202 to 10 KPa.
(5) Next, the drain valve 307 of the gas bag 125 is opened, a helium gas cylinder is connected to the opening to which the transport pipe 306 above the gas bag 125 is connected, and helium gas is injected. This is continued for 10 minutes until the air in the gas bag 125 is sufficiently replaced with helium gas.
(6) Next, a helium gas cylinder is connected to the drain valve 307 of the gas bag 125 so that helium gas flows out from the opening to which the transport pipe 306 is connected and the opening to which the transport pipe 303 is connected. This can be realized by maintaining the internal pressure of the gas bag 125 with helium gas at about 1 to 2 KPa from 10 KPa.
(7) Helium gas flows out from the opening to which the transport pipes 303 and 306 of the gas bag 125 are connected by the above (6), and helium also flows from the transport pipes 303 and 306 connected to the superconducting magnet 101 by the above (4). With the gas flowing out, the transport pipes 303 and 306 are connected to the gas bag 125. Thereby, it is possible to prevent the atmosphere from being mixed into the transport pipes 301, 303, 304, 306 and the gas bag 125.
(8) Helium gas is injected from the gas cylinder connected to the drain valve 307 until the internal pressure of the gas bag reaches 10 KPa.
(9) Close the drain valve 307 and restart the operation of the cold head 107.
(10) Due to the cooling capacity of the cold head 107, the helium gas in the gas bag 125 moves to the helium vessel 202 via the transport pipes 304 and 306, and is stored as liquid helium.

¡By installing according to the above procedure, atmospheric gas is not mixed in the gas bag 125. Further, the gas bag 125 can be installed separately from the installation work of the MRI apparatus. In addition, gas bags can be easily added or replaced depending on the power situation.

(Second embodiment)
An MRI apparatus according to the second embodiment will be described with reference to FIGS. 6 (a) and 6 (b).

In the second embodiment, before the helium gas stored in the gas bag 125 is returned to the helium vessel 202, a mechanism for removing impurity gas such as air mixed in the helium gas is provided.

As described in FIG. 3 in the first embodiment, the helium transport pipe (D) 306 is connected to the top of the helium gas bag 125, and is configured so that only the lightest helium gas returns to the helium vessel. Yes. However, in the process of returning the helium gas to the helium vessel, the air that has leaked due to a failure in the connection part of the helium transport pipe (D) 306, the gas back 125, or the helium exhaust pipe 124 cannot be separated by this configuration alone. .

Therefore, in the second embodiment, as shown in FIG. 6, a liquid reservoir 901 that stores liquid air is disposed in a part of the thermal contact portion 217 in which the helium exhaust pipe 214 exchanges heat with the radiant heat shield plate 212. The liquid reservoir 901 has a configuration in which a part of the helium exhaust pipe 214 is expanded downward. The liquid reservoir 901 is formed of stainless steel having a thickness of 2 mm, for example. In the portion of the liquid reservoir 901, an opening is provided in the radiation shield 212, and the liquid reservoir 901 protrudes from this opening into the space on the helium container 202 side. Even if a small amount of impurity gas such as air is mixed into the helium gas at the connection part of the helium exhaust pipe 124 or the gas back 125, the impurity gas such as air is liquefied before the helium gas in the thermal contact portion 217. It is possible to collect and remove the liquid in the liquid reservoir 901 using the property of This will be further described below.

When the cooling capacity of the cold head 107 is restored and the helium gas stored in the gas bag 125 returns to the helium vessel 202 through the helium exhaust pipe 124, the air mixed with the helium gas is cooled by the thermal contact portion 217. The In the thermal contact portion 217, in the process of moving from the side closer to the gas bag 125 to the portion closer to the cold head 107, from room temperature to around 43 Kelvin (−230 ° C.) that is the temperature of the first cooling stage 213 of the cold head 107 To be cooled. In this process, the oxygen mixed in the helium gas is liquefied when the boiling point of oxygen is about −183 ° C., and the nitrogen component is liquefied when the boiling point of nitrogen is about −196 ° C. The liquefied air falls into the liquid reservoir 901 and is accumulated. Therefore, only helium gas flows into the helium container 202 and is liquefied in the helium container 202.

Since the temperatures (melting points) at which liquid oxygen and liquid nitrogen solidify are about −218 ° C. and −210 ° C., respectively, the sump 901 is higher than the heat contact portion 217 where air is completely liquefied and solidified. Install in the temperature range of -196 ° C to -210 ° C.

Whenever a power failure or system failure occurs during the long-term operation of the superconducting magnet 101, the liquid helium 204 turns into helium gas and moves back and forth between the helium vessel 202 and the gas bag 125. Therefore, impurity gas such as air is gradually accumulated in the liquid reservoir 901. Therefore, in the second embodiment, a configuration for exhausting the impurity gas accumulated in the liquid reservoir 901 is further provided.

As shown in FIG. 6B, the heater 902 of the liquid reservoir 901 is incorporated as a configuration for exhausting the impurity gas. The heater 902 is connected to a wiring 903 that supplies current. The wiring 903 is routed through the helium exhaust pipe 124 to a position outside the vacuum vessel 201, and is drawn out from the helium exhaust pipe 124 through an airtight seal or the like. Alternatively, the wiring 903 is routed to the vicinity of the drain line 307 of the gas back 125.

The timing for exhausting the impurity gas is preferably during a periodic inspection of the superconducting magnet 101 once a year. At this timing, the drain plug 307 and the one-way relief valve (10 KPa) 302 at the bottom of the gas bag 125 are manually opened, and current is applied to the heater 902. With this heat generation, the liquid air is vaporized and expanded, and can be discharged into the atmosphere from the drain plug 307 at the bottom of the gas bag 125 through the helium discharge pipe 124.

Since the liquid reservoir 901 is made of stainless steel having a thickness of 2 mm, heat propagation to the outside is small, and the heat generation action of the heater 902 for a short time is limited to local heat generation inside the liquid reservoir 901. Therefore, even when the liquid air is released, the air (impurity gas) can be exhausted with little influence on the helium vessel 202 and the radiant heat shield plate 212.

When the liquid air in the liquid reservoir 901 is completely vaporized and released, the temperature of the heater 902 suddenly rises and its electric resistance changes suddenly, so the operator applies it to the terminal of the lead wire of the heater 902. By monitoring the voltage value and the current value, it is possible to grasp that the discharge operation of the liquid air has been completed. When the liquid air (impurity gas liquid) is completely vaporized, the energization is stopped, and the drain plug 307 and the one-way relief valve (10 KPa) 302 are manually closed.

As described above, according to the second embodiment, since the liquid reservoir 901 and the heater 902 are provided, even if an impurity gas such as air is slightly involved in the helium gas, it can be removed. Therefore, it is possible to eliminate the trouble that air enters the helium vessel 202, freezes in the vicinity of the opening of the helium discharge pipe 124, and becomes blocked.

Since the configuration, operation, and effects other than those described above are the same as those in the first embodiment, description thereof will be omitted.

(Third embodiment)
As a third embodiment, a superconducting MRI apparatus in which a gas bag 125 for storing helium gas is divided into a plurality of parts will be described with reference to FIG. FIG. 7 shows a part of the MRI apparatus, and various power supply units and the computer 109 are omitted because they have the same configuration as FIG. By dividing the gas bag 125, the space required when the gas bag 125 is inflated can be divided into several parts. improves.

As shown in FIG. 7, the gas bag 125 is divided into two gas bags 601,602. The two gas bags 601 and 602 are connected by a helium gas transport pipe 603. The helium exhaust pipe 124 is connected to the first gas back 601. The connection structure between the first gas back 601 and the helium exhaust pipe 124 is the same as the connection structure (FIG. 3) between the gas back 125 and the helium exhaust pipe 124 in the first embodiment. A second gas bag 602 is connected from the first gas bag 601 through a helium gas transport pipe (E) 603.

In the example of FIG. 7, the first gas bag 601 is disposed on the outer side surface of the examination room 123, and the second gas bag 602 is disposed on the ceiling of the examination room 123.

The structure of the first gas bag 601 and the second gas bag 602 is a structure in which the volume is expanded when the internal pressure reaches a predetermined pressure (about 10 KPa), and the predetermined pressure is maintained, like the gas bag 125 of the first embodiment. It is. The capacity of the two gas bags 601 and 602 can be combined with the capacity of the gas bag 125, and the combined capacity can be larger than that of the gas bag 125. It is also possible to respond.

In the present embodiment, the use of an empty space such as the back of the ceiling of the examination room 123 for the expansion space of the gas bag makes it easy to secure the expansion space of the gas bag, and has an effect of reducing the space burden on the building. In addition, it is easy to increase the capacity by connecting a plurality of gas bags, so that it is possible to cope with a facility where a power outage takes a long time.

Fig. 8 shows another example of the arrangement of multiple gas bags. The configuration of FIG. 8 is a configuration in which a plurality of gas bags 701, 702, 703,... Are connected in parallel to the helium exhaust pipe 125. A one-way relief valve 302 and a one-way relief valve 305 are disposed between the gas bags 701, 702, 703 and the like and the helium exhaust pipe 124, respectively. As a result, the connection structure between the individual gas bags 701, 702, 703, etc. and the helium exhaust pipe 124 is equivalent to the connection structure (FIG. 3) between the gas bag 125 and the helium exhaust pipe 124 of the first embodiment. I have to. Therefore, helium gas generated in the helium vessel 202 flows out and is stored in the gas bags 701, 702, 703, etc. in the event of a power failure or system failure, and helium gas is transferred from the gas bags 701, 702, 703, etc. to the helium vessel 202 when the power failure is restored. Can be returned and liquefied.

In the configuration of FIG. 8, the operating pressures of the one-way relief valve 302 and the one-way relief valve 305 arranged for each of the plurality of gas bags 701, 702, 703, etc. are different for each gas bag 701, 702, 703, etc. By setting the operating pressure, the operation of the gas bag can be ordered. For example, the operating pressures of the one-way relief valve 302 and the one-way relief valve 305 of the gas bag 701 are set to 9 KPa and 4 KPa, respectively, and the one-way relief valve 302 and the one-way relief valve 305 of the gas bag 702 are set to 10 KPa and 5 KPa, respectively. The operating pressures of the one-way relief valve 302 and the one-way relief valve 305 are set to 11 KPa and 6 KPa.

Thus, when helium gas is generated from the helium vessel 202 at the time of a power failure and the internal pressure rises, the one-way relief valve 302 opens in the order of the gas back 701, the gas back 702, and the gas back 703, and helium gas is stored. Also at the time of recovery, the one-way relief valve 305 opens in the order of the gas back 701, the gas back 702, and the gas back 703, and the internal gas returns to the helium vessel 202.

7 and 8 makes it possible to install a gas bag of a desired capacity even in a facility where it is difficult to install a large gas bag. In addition, a small space under the ceiling or under the floor can be used effectively. Further, by adding an order to the operation of the gas bag, it is possible to accumulate gas from a desired gas bag according to the use situation of each room or space where the gas bag is installed. Therefore, the gas bag was arranged for the prolonged power outage, but if the power outage is short, if possible, if you do not want to expand the gas bag in the room (space), lower the order of the gas back (relief) This can be done by increasing the operating pressure of the valve).

(Fourth embodiment)
In the first to third embodiments, in preparation for a case where a system failure or power outage lasts for a long time, and as a result, the helium gas exhausted from the superconducting magnet 101 fills all of the gas back with its operating pressure of 10 KPa. In the fourth embodiment, a discharge valve that leaks at a predetermined pressure is provided in a part of the gas bag.

The drain valve may be provided separately from the drain valve 307, or the drain valve 307 can have the function of the drain valve. Specifically, the drain valve 307 is configured to leak helium gas when the internal pressure of the gas bag reaches a predetermined pressure (for example, 15 KPa).

The predetermined pressure causing the leak is a pressure that the gas bag 125, 601, 602, 701, 702, 703, etc. can withstand structurally, and the connection structure between the gas bag and the helium vessel 202 is Set the pressure to withstand. For example, if the gas bag can withstand a pressure of 15 KPa, the leak pressure of the exhaust valve can be set to 15 KPa. When the leak pressure of the discharge valve is set to 15 KPa, the pressure in the helium vessel 202 rises to 25 KPa with the operating pressure of the one-way relief valve 302 added to 10 KPa. The discharge valve leaks. Therefore, the gas bag and the connection structure are not overloaded, and the gas bag can be prevented from being damaged.

The helium vessel 202 has a structure capable of withstanding up to a pressure (for example, 40 KPa) at which the rupturable plate 211 bursts, so that the helium vessel 202 is not damaged even if the pressure rises to 25 KPa.

Since the configuration and operational effects other than those described above are the same as those in the first to third embodiments, description thereof will be omitted.

(Fifth embodiment)
In the MRI apparatus of the first to fourth embodiments, the position of the gas bag at the maximum expansion as shown in FIG. 9 is placed on the floor of the room where the gas bags 125, 601, 602, 701, 702, 703, etc. are installed. It is possible to indicate the mark 801 indicating the mark with a marking tape or the like. FIG. 9 is a view taken in the direction of arrow A in FIG.

In this way, the area at the time of maximum expansion is indicated on the floor with the mark 801, so that it is possible to notify the operator and the subject not to enter the area of the mark 801 in the event of a power failure or system failure. Safety can be ensured. In addition, it is possible to prevent acts such as inadvertently placing objects in this area at the time of a power failure, etc., and to recognize that this area can be used at times other than at the time of a power failure, etc., so that the space can be used effectively.

In FIG. 9, only the floor area is shown, but the area can be shown together with the wall surface. In addition to the marking tape, it is also effective to show the expansion region of the gas bag three-dimensionally, such as hanging a chain from the ceiling.

101 superconducting magnet, 102 subject, 107 cold head, 110 magnet control unit, 112 gradient magnetic field coil, 113 gradient magnetic field power source, 114 shim power source, 119 computer, 124 helium exhaust pipe, 125 gas back, 202 helium container, 204 liquid helium , 206 Pressure sensor, 207 Heater element, 212 Radiation shield plate, 213 First cooling stage, 216 Second cooling stage, 217 Thermal contact section, 301 Helium transport pipe (A), 302 One-way relief valve (10 KPa), 303 Helium Transport pipe (B), 304 Helium transport pipe (C), 305 One-way relief valve (5 KPa), 306 Helium transport pipe (D), 601 First gas bag, 602 Second gas bag.

Claims (15)

1. A refrigerant container that accommodates the superconducting coil, a vacuum container that covers the refrigerant container, a cooler having a tip inserted into the refrigerant container, and the refrigerant container connected to the refrigerant container when the cooler is stopped. An exhaust pipe for guiding the generated refrigerant gas to the outside of the vacuum vessel, and a superconducting magnet,
   A gas bag connected to the refrigerant container via the exhaust pipe and configured to maintain an internal pressure constant by expanding and contracting according to the amount of gas inside;
   A magnetic resonance imaging apparatus comprising:
2. The magnetic resonance imaging apparatus according to claim 1, wherein the exhaust pipe branches into a first transport pipe and a second transport pipe, each connected to the gas bag,
   The first transport pipe is opened when the internal pressure of the refrigerant container is higher than the internal pressure of the gas bag by a first pressure difference or more and allows the refrigerant gas to flow from the refrigerant container to the gas bag. A directional valve is provided,
   The second transport pipe opens when the internal pressure of the gas bag is higher than the internal pressure of the refrigerant container by a second pressure difference or more, and causes the refrigerant gas to flow from the gas bag to the refrigerant container. A magnetic resonance imaging apparatus comprising a directional valve.
3. 3. The magnetic resonance imaging apparatus according to claim 2, wherein the first pressure difference is larger than the second pressure difference.
4. 3. The magnetic resonance imaging apparatus according to claim 2, wherein the second transport pipe is connected to an upper portion of the gas bag.
5. 2. The magnetic resonance imaging apparatus according to claim 1, further comprising a radiation shield plate between the refrigerant container and the vacuum container, wherein a part of the exhaust pipe is heated to the radiation shield plate over a predetermined length. The magnetic resonance imaging apparatus is disposed along the radiation shield plate so as to come into contact with each other.
6. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the exhaust pipe is provided with a removal unit that separates impurity gas mixed in the refrigerant gas.
7. 7. The magnetic resonance imaging apparatus according to claim 6, wherein the removing unit is a liquid reservoir for collecting an impurity liquid obtained by liquefying the impurity gas in the middle of the exhaust pipe.
8. 8. The magnetic resonance imaging apparatus according to claim 7, wherein the liquid reservoir is provided with a discharge unit for discharging the accumulated impurity liquid to the outside of the superconducting magnet.
9. 9. The magnetic resonance imaging apparatus according to claim 8, wherein the discharge unit includes a heater that heats and vaporizes the impurity liquid in the liquid reservoir.
10. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the gas bag is divided into a plurality of parts.
11. The magnetic resonance imaging apparatus according to claim 2, wherein a plurality of gas backs are connected in parallel to the exhaust pipe,
    The magnetic resonance imaging apparatus, wherein the first and second transport pipes and the first and second one-way valves are arranged for each of the plurality of gas bags.
12. 12. The magnetic resonance imaging apparatus according to claim 11, wherein the first and second one-way valves arranged for each of the plurality of gas bags have the set first pressure difference and second pressure difference, A magnetic resonance imaging apparatus characterized by being different for each of a plurality of gas bags arranged.
13. A transport pipe connected to the superconducting magnet of the magnetic resonance imaging apparatus and guiding the refrigerant gas generated from the refrigerant container containing the superconducting coil;
    A gas bag for storing refrigerant gas connected to the transport pipe;
    Have
    The gas recovery device for a magnetic resonance imaging apparatus, wherein the gas bag has a structure that maintains an internal pressure constant by expanding and contracting according to the amount of gas inside.
14. The gas recovery apparatus for a magnetic resonance imaging apparatus according to claim 13, wherein the transport pipe branches into a first transport pipe and a second transport pipe, each connected to the gas bag,
    The first transport pipe is opened when the internal pressure of the refrigerant container is higher than the internal pressure of the gas bag by a first pressure difference or more and allows the refrigerant gas to flow from the refrigerant container to the gas bag. A directional valve is provided,
    The second transport pipe opens when the internal pressure of the gas bag is higher than the internal pressure of the refrigerant container by a second pressure difference or more, and causes the refrigerant gas to flow from the gas bag to the refrigerant container. A gas recovery apparatus for a magnetic resonance imaging apparatus, comprising a directional valve.
15. An operation method of a magnetic resonance imaging apparatus provided with a superconducting magnet,
    When the cooler provided in the superconducting magnet stops, the first step of stopping the shooting operation,
    When the internal pressure of the refrigerant container rises and reaches a predetermined pressure, the refrigerant gas in the refrigerant container is stored in the gas bag having a structure that expands and contracts according to the amount of the refrigerant gas while maintaining a constant internal pressure. 2 processes,
    When the cooler is restarted and the internal pressure in the refrigerant container drops below a predetermined pressure, the refrigerant gas in the gas bag is returned to the refrigerant container and liquefied by cooling the cooler. And a third step of storing in the refrigerant container. A method for operating the magnetic resonance imaging apparatus.

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