WO2013038960A1 - 磁気共鳴イメージング装置、その運転方法およびクエンチ防止装置 - Google Patents
磁気共鳴イメージング装置、その運転方法およびクエンチ防止装置 Download PDFInfo
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- WO2013038960A1 WO2013038960A1 PCT/JP2012/072534 JP2012072534W WO2013038960A1 WO 2013038960 A1 WO2013038960 A1 WO 2013038960A1 JP 2012072534 W JP2012072534 W JP 2012072534W WO 2013038960 A1 WO2013038960 A1 WO 2013038960A1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F6/00—Superconducting magnets; Superconducting coils
- H01F6/02—Quenching; Protection arrangements during quenching
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/288—Provisions within MR facilities for enhancing safety during MR, e.g. reduction of the specific absorption rate [SAR], detection of ferromagnetic objects in the scanner room
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/381—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets
- G01R33/3815—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets with superconducting coils, e.g. power supply therefor
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/385—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using gradient magnetic field coils
Definitions
- the present invention relates to a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) using a superconducting magnet, and more particularly to an MRI apparatus using a superconducting magnet storing high energy.
- an MRI apparatus magnetic resonance imaging apparatus
- a superconducting magnet is used as a source of a strong static magnetic field.
- the superconducting magnet is a superconducting coil in which a superconducting wire is wound around a coil bobbin. (-268.8 ° C)).
- a current is passed through the superconducting coil, and in a state where the rated magnetic field has been reached, a circuit called a superconducting switch is closed to a closed loop state in which a permanent current flows. Thereby, a superconducting state can be maintained.
- An object of the present invention is to prevent quenching that occurs accidentally in a superconducting magnet.
- a superconducting magnet a gradient magnetic field coil that applies a gradient magnetic field to an imaging space, a high frequency coil that applies a high frequency magnetic field to the imaging space, a gradient magnetic field coil, and the high frequency coil
- a quench prevention sequence execution unit that executes a quench prevention sequence that vibrates the superconducting magnet during a time period when the control unit does not execute the imaging sequence.
- a magnetic resonance imaging apparatus is provided.
- FIG. 1 is a block diagram showing the overall configuration of an MRI apparatus according to a first embodiment of the present invention. Sectional drawing of the superconducting magnet of the MRI apparatus of 1st Embodiment. The operation
- the block diagram which shows the structure of the quenching prevention apparatus of 2nd Embodiment. The operation
- the inventors conducted a high-field superconducting magnet with a high magnetic field strength and a high energy accumulated in the superconducting coil. It was also understood that the superconducting magnet has an open structure in which a pair of superconducting magnets are arranged to face each other. In addition, it was found that the time zone and day of the week when accidental quenching is likely to occur are nights and holidays when the MRI apparatus is suspended.
- the main cause of accidental quenching was the creep of the resin filling the gap of the superconducting wire (for example, thermosetting epoxy resin), and sudden cracks in the resin suddenly occurred. It turns out that it is to occur.
- the resin that is filled and fixed with the superconducting wire of the superconducting coil is subjected to heat shrinkage stress by being cooled with liquid helium, and creep occurs.
- the resin cracks. Since the superconducting magnet is left stationary while the MRI apparatus is at rest, the creep strain of the resin gradually increases, and the increased strain causes a large crack. With the energy generated when a large crack occurs, the superconducting coil near the crack is warmed and a quench occurs. For example, when a crack with a length of 30 mm occurs, the temperature rises to 6 Kelvin (-267 ° C). With a strong magnetic field strength and a large permanent current, 6 Kelvin is approximately equal to the critical temperature at which the superconducting wire transitions to normal conduction.
- the reason why accidental quenching often occurs at night and on holidays is as follows.
- a pulsed gradient magnetic field is repeatedly applied by executing an imaging sequence, and the superconducting magnet constantly vibrates minutely due to the electromagnetic force action. Due to this minute vibration, an elastic strain is superimposed on the resin in addition to the creep strain, and this serves as a trigger, and a crack occurs in the resin when the creep strain is small. If the crack is small (30 mm or less), the temperature rise is small, so even a superconducting coil that accumulates high energy does not exceed the critical temperature at which the superconducting wire transitions to normal conduction.
- the present invention prevents accidental quenching by the MRI apparatus having the following configuration, and stabilizes the MRI apparatus and improves the reliability of inspection.
- the MRI apparatus of the present invention controls a superconducting magnet, a gradient magnetic field coil that applies a gradient magnetic field to the imaging space, a high frequency coil that applies a high frequency magnetic field to the imaging space, and controls the operations of the gradient magnetic field coil and the high frequency coil. And a quench prevention sequence execution unit for executing a quench prevention sequence for vibrating the superconducting magnet in a time zone during which the control unit does not execute the photography sequence.
- the superconducting magnet may be vibrated by driving the gradient magnetic field coil and applying the gradient magnetic field to the superconducting magnet, or separately from the gradient magnetic field coil, a gradient magnetic field for vibrating the superconducting magnet may be used. You may arrange
- the superconducting magnet can be vibrated by executing a sequence in which an alternating gradient magnetic field whose gradient direction is reversed is applied to the superconducting magnet. At this time, when the frequency of the alternating gradient magnetic field or the harmonic frequency thereof matches the resonance frequency of the superconducting magnet, the superconducting magnet can be resonated, so that vibration can be generated effectively.
- an imaging sequence may be executed as a quench prevention sequence.
- a phantom imaging sequence for capturing a phantom and performing a predetermined image analysis is generally performed to adjust the apparatus before execution of the predetermined imaging sequence
- the same imaging sequence is executed as a quench prevention sequence. May be.
- the phantom imaging sequence is also used as the quench prevention sequence, and the control unit can perform apparatus adjustment in a short time from the start-up by performing image analysis using an image captured in advance in the quench prevention sequence.
- the quench prevention sequence execution unit may be configured to repeatedly execute the quench prevention sequence at a predetermined time interval during a time period when the imaging sequence is not performed, or may execute the quench prevention sequence at some timing.
- the quench prevention sequence execution unit informs the subject or operator that the quench prevention sequence will be executed before the execution of the quench prevention sequence. You can avoid being surprised by the vibration of the magnet.
- the quench prevention sequence execution unit may include a detection unit that detects whether the subject or the operator is in a predetermined range around the superconducting magnet before the execution of the quench prevention sequence. When it is detected that the subject or the operator is in the vicinity, it is possible to postpone the execution of the quench prevention sequence.
- the MRI apparatus includes a superconducting magnet, a gradient magnetic field coil that applies a gradient magnetic field to the imaging space, a high frequency coil that applies a high frequency magnetic field to the imaging space, and operations of the gradient magnetic field coil and the high frequency coil. And a control unit that executes a predetermined imaging sequence, and the control unit executes a predetermined imaging sequence for vibration to vibrate the superconducting magnet during a time period when the imaging sequence is not executed. be able to. As a result, the superconducting magnet vibrates due to the execution of the imaging sequence for vibration, so that accidental quenching can be prevented.
- the present invention as a method of operating an MRI apparatus that includes a superconducting magnet and images a subject by executing a predetermined imaging sequence, quenching of the superconducting magnet is prevented during a time period when the predetermined imaging sequence is not performed.
- a method for vibrating a superconducting magnet is provided. Thereby, accidental quenching can be prevented.
- the superconducting magnet may be vibrated by generating a gradient magnetic field in the gradient magnetic field coil for the imaging sequence of the MRI apparatus, or the gradient for vibration provided separately from the gradient magnetic field coil for the imaging sequence of the MRI apparatus
- a gradient magnetic field may be generated using a magnetic field coil.
- the superconducting magnet is prevented from quenching when the predetermined imaging sequence is not executed. Therefore, a driving method for imaging a phantom is provided. Since the superconducting magnet vibrates during imaging of the phantom, accidental quenching can be prevented.
- a method for operating a magnetic resonance imaging apparatus including a superconducting magnet, in which a step of performing imaging of a subject, a step of performing an operation of pausing the imaging, and an operation of pausing are performed. And a step of vibrating the superconducting magnet is provided. Accidental quenching can be prevented by vibrating the superconducting magnet.
- the present invention also provides a quench prevention device for a magnetic resonance imaging apparatus provided with a superconducting magnet.
- This apparatus includes a vibration gradient magnetic field coil that generates a gradient magnetic field for vibrating a superconducting magnet, and a current supply unit that repeatedly performs an operation of supplying an alternating current to the vibration gradient magnetic field coil at a predetermined time interval. It is what you have.
- this quench prevention device By arranging this quench prevention device in the MRI apparatus or the examination room, it is possible to prevent accidental quenching of the superconducting magnet of the MRI apparatus.
- FIG. 1 shows an overall configuration in a state where the MRI apparatus of the present embodiment is installed in a medical facility.
- This MRI apparatus includes a superconducting magnet 101 having an open structure as a magnet for generating a static magnetic field.
- the open-structure superconducting magnet 101 includes an upper cryostat 104 and a lower cryostat 105 that are arranged vertically with an imaging space 103 in which the subject 102 is arranged.
- a superconducting coil (not shown in FIG. 1) serving as a magnetomotive force source is incorporated in each of the upper cryostat 104 and the lower cryostat 105.
- the upper cryostat 104 and the lower cryostat 105 are connected by a connecting pipe 106 serving as a support, and the upper cryostat 104 is supported by the connecting pipe 106 with respect to the lower cryostat 105.
- the superconducting magnet 101 has an open structure in which the imaging space 103 is open in the front, rear, left and right, so that the feeling of pressure applied to the subject 102 can be relieved and a gentle examination environment can be provided.
- the outermost part of the upper cryostat 104 and the lower cryostat 105 of the superconducting magnet 101 is a vacuum vessel 401 made of a stainless steel plate (for example, 18 mm thick). Inside the vacuum vessel 401, a helium vessel 403 made of a stainless steel plate (for example, 15 mm thick) is disposed via a vacuum layer 402. In order to fix the helium container 403 to the vacuum container 401, a plurality of load supports 404 are attached.
- the load support 404 is made of a highly heat-insulating material (for example, FRP (fiber reinforced plastic resin)) so as not to transmit heat to the helium vessel 403.
- FRP fiber reinforced plastic resin
- each helium vessel 403 of the upper cryostat 104 and the lower cryostat 105 one or more superconducting coils 405 are arranged so as to have a symmetrical positional relationship across the imaging space 103.
- the superconducting coil 405 is firmly fixed to the helium vessel 403 by a support metal fitting 406.
- FIG. 2 shows an example in which one superconducting coil 405 is disposed in each of the upper and lower helium containers 403.
- the inside of the helium vessel 403 is filled with liquid helium 407, and the superconducting coil 405 is cooled to 4.2 Kelvin ( ⁇ 268.8 ° C.) which is the boiling temperature thereof. By this cooling, the superconducting coil 405 maintains a superconducting state, a permanent current (for example, 450 A) flows stably, and a magnetic field (for example, 1 Tesla) is generated in the imaging space 103.
- a permanent current for example, 450 A
- a magnetic field for example, 1 Tesla
- a radiation shield plate 408 (for example, 5 mm thick aluminum) is disposed at an intermediate position.
- the shield plate 408 suppresses radiant heat from the vacuum container 401 to the helium container 403 and suppresses consumption of the liquid helium 407.
- the load support 404 and the radiation shield plate 408 having the above-described heat insulation structure, the amount of heat entering the helium vessel 403 is suppressed to about several watts.
- a cryocooler 107 is provided on the upper cryostat 104 of the superconducting magnet 101.
- the cryocooler 107 acts to cool the helium gas vaporized in the upper cryostat 104 and the lower cryostat 105 to liquid helium again and return it to the upper cryostat 104.
- the cooling unit of the cryocooler 107 cools the radiant heat shield plate 408 and the helium vessel 403.
- the cooling capacity of the cryocooler 107 compensates for the amount of heat entering the helium vessel 403 to constitute the closed-type superconducting magnet 101, thereby suppressing the consumption of liquid helium for cooling the superconducting coil.
- the MRI apparatus also includes a magnet control unit 109, a gradient magnetic field power amplifier 112, a high frequency power amplifier 114, a high frequency amplifier circuit 116, a computer 117, and a display 118 as a control / calculation system.
- the computer 117 controls the operations of the gradient magnetic field coil 111 and the high frequency coil 113 to execute a predetermined imaging sequence, and quenches to vibrate the superconducting magnet during a time period when the control unit does not execute the imaging sequence.
- a quench prevention sequence execution unit (hereinafter referred to as a quench prevention unit) 117a for executing the prevention sequence is configured.
- the superconducting magnet 101 incorporates a plurality of temperature sensors and pressure sensors (not shown in the figure) for monitoring its operating state, and the connection terminals 108 of these sensors are pulled out to the upper part of the upper cryostat 104.
- the magnet control unit 109 is connected.
- a pipe called a service port 409 for replenishing liquid helium 407 and discharging helium gas is installed above the vacuum vessel 401 of the upper cryostat 104.
- the shim plate 110 and the inclined ground coil 111 are fixed by fixing bolts 410 to the surfaces of the upper and lower cryostats 104 and 105 on the imaging space 103 side of the vacuum vessel 401.
- the shim plate 110 has a plurality of screw holes (not shown), and magnetic screws are embedded in screw holes at appropriate positions among the plurality of screw holes.
- the shim plate 110 improves the magnetic field uniformity to a target value (for example, 3 ppm or less) by minutely changing the distribution of magnetic flux generated by the superconducting magnet 101 and adjusting the magnetic field uniformity.
- the pair of gradient coils 111 are disposed on the surface of the imaging space 103 of the pair of shim plates 110.
- the gradient magnetic field coil 111 has a flat plate structure so as not to hinder the open structure of the superconducting magnet 101.
- Each of the pair of gradient magnetic field coils 111 has a structure in which three types of coils that generate gradient magnetic fields in three axial directions of xyz orthogonal to each other are stacked.
- Gradient magnetic field power amplifiers 112 are connected to the three types of coils, and currents are independently applied thereto. For example, when a positive current is applied to the upper z coil and the lower z coil from the gradient power amplifier 112, 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 A magnetic flux in the opposite direction is generated. 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 imaging space 103 is created.
- the x-coil and the y-coil have a function of giving gradients that change the density of the magnetic flux generated by the superconducting magnet 101 along the x-axis and the y-axis (both horizontal axes), respectively.
- the gradient magnetic field coil 111 is configured such that when a current of 500 A, for example, is applied from the gradient magnetic field power amplifier 112, a gradient of 50 millitesla per meter is independently generated in the three axes of xyz. Yes.
- the pair of high-frequency coils 113 are flat-plate coils, and are attached to the imaging space 103 side of the pair of gradient magnetic field coils 111, respectively.
- a high frequency power amplifier 114 is connected to the pair of upper and lower high frequency coils 113 and supplied with a high frequency current.
- the pair of high-frequency coils 113 generate a high-frequency magnetic field that causes a nuclear magnetic resonance phenomenon to occur in the nuclear spin at the examination site of the subject 102.
- a high frequency magnetic field of 42 MHz is generated in which hydrogen nuclei cause a nuclear magnetic resonance phenomenon with a magnetic field intensity of 1 Tesla.
- the detection coil 115 is located at the center position of the imaging space 103 and is arranged at the examination site of the subject 102, and detects the precession of the nuclear spin as a nuclear magnetic resonance signal.
- the output signal of the detection coil 115 is amplified and detected by the high frequency amplifier circuit 116 and converted into a digital signal.
- the computer 117 converts the nuclear magnetic resonance signal converted into a digital signal into an image or spectrum chart for use in medical diagnosis, stores it in a built-in storage device (not shown), and displays the display 118. To display.
- the storage device in the computer 117 stores a program for executing a predetermined imaging sequence such as diffusion weighted echo planar imaging.
- the CPU in the computer 117 reads and executes this program to control the operations of the magnet control unit 109, the gradient magnetic field power amplifier 112, the high frequency power amplifier 114, and the high frequency amplifier circuit 116, respectively. Thereby, imaging can be performed according to a predetermined imaging sequence.
- the computer 117 also includes a program for realizing a spectrum analysis function.
- the computer 117 can record the operation state of each unit and can remotely monitor the MRI apparatus via a communication control device (not shown).
- the superconducting magnet 101 and the bed 119 are installed in an examination room 120 provided with an electromagnetic shield, and prevent electromagnetic waves generated by external devices from entering the detection coil 115 as noise, thereby preventing the quality of the photographed image from deteriorating. .
- a method of forcibly quenching by repeating excitation and demagnetization of the superconducting magnet a plurality of times (at least three times), applying an overcurrent to the superconducting coil, or applying heat from the outside is used.
- This training is performed in advance at the manufacturing site before the MRI device is set up in a medical institution, etc., thereby releasing the heat shrinkage strain energy of the superconducting coil and preventing quenching after installation in the medical institution. To do.
- a quench prevention sequence is executed in order to prevent a quench that occurs accidentally.
- FIG. 3 is an operation flowchart of the MRI apparatus of FIGS. 1 and 2. 3 is realized by executing a program stored in advance in a storage device built in the computer 117.
- a predetermined activation operation of the MRI apparatus is performed (step 201). This activation operation is executed when the activation switch is turned on by the operator or when the time reserved by the operator in advance in the computer 117 is reached.
- the computer 117 turns on the power of each part of the MRI apparatus and confirms the operation.
- an imaging test called a QA (Quality Assurance) test is performed (step 202).
- a test sample called a phantom is mounted on the bed 119 by an operator and placed in the imaging space 103, the computer 117 images the phantom by a predetermined imaging sequence, and the computer 117 stores the obtained image in a predetermined program.
- This is a test for verifying the signal-to-noise ratio, density uniformity, spatial resolution, image position accuracy, etc. by performing image analysis according to the above and adjusting the imaging conditions.
- the QA test (step 202) can be omitted.
- the operator places the first subject 102 on the day on the bed 119, places the examination site in the imaging space 103, and operates the operation unit to notify the computer 117 of the completion of the placement.
- the computer 117 causes each unit to execute an imaging sequence suitable for diagnosis set by the operator, and images the subject 102 (steps 203 and 204).
- the bed 119 is operated to remove the subject 102 from the imaging space 103 (step 205), and the MRI examination of the first subject 102 is completed.
- the computer 117 proceeds to the determination process for today's inspection, and displays a display on the display 118 asking the operator whether or not the current inspection has ended (step 206).
- the process proceeds to the termination process of the MRI apparatus (step 207). If the operator inputs NO, that is, if the next subject 102 is waiting, the process returns to step 203, the next subject 102 is placed on the bed 119, and the examination site is arranged in the imaging space 103.
- the end processing in step 207 is not to turn off the power of all units of the MRI apparatus, but to shift some functions to a state in which the operating state continues.
- This state is generally called a sleep mode, and continues the monitoring function for monitoring a part of the MRI apparatus, the cooling operation of the cryocooler 107, the start of step 201 by a timer, and the operating state of the remote communication function.
- the functions of the gradient magnetic field power amplifier 112 and the quench prevention unit 117a of the computer 117 that enables the gradient magnetic field power amplifier 112 to operate in a predetermined quench prevention sequence described later are continued. This function is referred to as sleep mode II.
- the timing at which the computer 117 operates the gradient magnetic field power amplifier 112 in the quench prevention sequence will be described below in the case of a predetermined constant time interval (for example, 1 hour).
- the quench prevention sequence may be operated at some timing.
- the computer 117 determines whether or not a new device has been activated by the operator while maintaining the sleep mode II, or whether or not the device activation has been reserved by a timer (step 208). When the reserved activation time comes, the process returns to the first step 201 to perform the activation operation of the MRI apparatus. If there is no activation instruction from the operator, the elapsed time is measured by the timer function programmed in the computer 117, and the MRI apparatus maintains the sleep mode II (step 209).
- step 210 If the timer measures that the predetermined time for executing the quench prevention sequence determined by the operator has elapsed while maintaining the sleep mode II, the process proceeds to step 210, and the quench prevention unit 117a in the computer 117 is reached. Controls the gradient power amplifier 112 so that the quench prevention sequence of FIG. 4 is executed. As a result, the alternating gradient magnetic field shown in FIG. The operation of the quench prevention unit 117a in step 210 is realized by the CPU executing a program stored in the computer 117 in advance.
- the computer 117 After executing the quench prevention sequence in step 210, the computer 117 maintains the sleep mode II again, returns to step 208, and performs a new apparatus activation determination process by the operator.
- the MRI apparatus will stop the quenching sequence shown in FIG. 4 at a fixed time until the operator starts the apparatus or the apparatus is started with a timer.
- the gradient magnetic field power amplifier 112 is driven to generate a gradient magnetic field.
- step 210 when a predetermined time has elapsed since entering the sleep mode II state in step 207, the quench prevention unit 117a of the computer 117 sends the activation signal 301 to the gradient power amplifier 112 in order to execute a quench prevention sequence. (See FIG. 4 (a)).
- the gradient magnetic field power amplifier 112 Upon receipt of the activation signal 301, the gradient magnetic field power amplifier 112 turns on its own power supply, performs the cooling water circulation of the gradient magnetic field coil 111, and performs a self circuit check, and enters the standby mode after about 2 minutes.
- the control signal from the quench prevention unit 117a of the computer 117 is transferred again to the gradient magnetic field power amplifier 112, and the gradient magnetic field power amplifier 112 supplies a current to the x coil of the gradient magnetic field coil 111 for a predetermined time ( A period 302 for driving for 5 minutes, a period 303 for supplying current to the y coil for a predetermined time (5 minutes), and a period 304 for supplying a current to the z coil for driving for a predetermined time (5 minutes) are sequentially performed.
- This series of operations is a quench prevention sequence, which is a 17-minute sequence.
- the x coil is first supplied with a current pulse 305 of a predetermined current value and time, and subsequently, the current pulse 305 and Is supplied with a current pulse 306 with the polarity reversed.
- the current pulse 305 is set to +500 A for 2 seconds
- the current pulse 306 is set to ⁇ 500 A for 2 seconds.
- the operation of repeatedly supplying the current pulse 305 and the current pulse 306 having the reversed polarity alternately for a predetermined period (for example, 20 seconds) is repeated a plurality of times (for example, 5 times) with a pause period 307 (for example, 40 seconds) interposed therebetween.
- the x coil alternates between the gradient magnetic field pulse in the x-axis direction and the gradient magnetic field pulse in which the direction of the gradient in the x-axis direction is reversed during the period in which the current pulse 305 and the current pulse 306 are alternately and repeatedly supplied. It occurs repeatedly.
- the driving period 303 for the y coil and the driving period 304 for the z coil are configured in the same manner as the driving period 302 for the x coil.
- the gradient magnetic field coil 111 When the gradient magnetic field coil 111 generates a gradient magnetic field, a repulsive force or an attractive force is generated between the superconducting coil and the gradient magnetic field coil 111.
- a gradient magnetic field is generated in a pair of upper and lower z coils
- the upper z coil electromagnetically repels the superconducting coil 405 of the upper cryostat 104
- the lower z coil generates an electromagnetic force that attracts the superconducting coil 405 of the lower cryostat 105.
- the x coil and y coil change the density of magnetic flux generated by the superconducting magnet 101 along the x axis and y axis (both horizontal axes), respectively. Since a gradient is applied, reaction force and attractive force are generated along the x-axis and y-axis, respectively.
- the superconducting coil 405 is formed by winding a superconducting wire (about 1 mm ⁇ ) at room temperature and filling the gap with a resin (for example, thermosetting epoxy resin) and fixing it. After fixing in the helium vessel 403, 4.2 Kelvin When cooled to ( ⁇ 268.8 ° C.), the superconducting wire and the resin are distorted due to thermal contraction during the cooling process. Although this distortion is released by the above-described training before installation, it cannot be completely released, so that the resin creep gradually changes.
- a resin for example, thermosetting epoxy resin
- the MRI apparatus of this embodiment can execute the quench prevention sequence by changing the software configuration without changing the hardware configuration of the apparatus.
- the quench prevention sequence is executed at regular time intervals after the MRI apparatus shifts to the sleep mode II, but the waiting time until the first subject MRI examination is completed and the next subject MRI examination is started.
- the quench prevention sequence may also be executed when it is longer than one hour. In this case, it is possible to surprise the subject or operator who is still on the bed 119 with the vibration sound that drives the gradient coil other than during imaging in the quench prevention sequence, so take measures to prevent it. Can be incorporated into the program.
- an advance announcement can be made to the subject 102 and the operator.
- the quench prevention sequence can be prevented from being executed while the subject or the operator is in the examination room.
- the subject 102 is mounted on the subject 119 by determining the presence of a moving object by image analysis by the computer 117 from images of a patient monitor camera with an MRI device or a camera placed in the examination room. It can be automatically detected that the subject or the operator is within a predetermined distance from the examination room or the superconducting magnet 101. In this case, the execution of the quench prevention sequence can be postponed.
- the quench prevention sequence of FIG. 4 is configured to drive the x coil, y coil, and z coil in order, but the quench prevention sequence of the present invention is not limited to the sequence of FIG. It is possible to use a sequence that drives simultaneously, or a sequence that drives any two types of coils of xyz coils simultaneously. In this case, the same quenching prevention effect is expected.
- the number of repetitions of the current pulses 305 and 306 with reversed polarity and the number of pauses can be changed to an arbitrary number.
- the quench prevention sequence shown in FIG. 4 is an example in which a current pulse 305 of +500 A for 2 seconds and a current pulse 306 of ⁇ 500 A for 2 seconds are alternately applied to the gradient coil 111, so that the current pulse has a frequency of 0.25. It includes the fundamental wave of Hertz and its harmonics.
- the superconducting magnet 101 having the structure shown in FIG. 2 has a resonance frequency determined by the structure.
- the resonance frequency of the superconducting coil 405 incorporated therein is 8 hertz, 25 hertz, and 45 hertz.
- a gradient magnetic field can be applied as a more effective vibration source.
- the superconducting coil 405 inside the superconducting magnet 101 can be effectively vibrated by making the gradient magnetic field power amplifier 112 drive in a sequence driven by, for example, an alternating current pulse of 100 A.
- the quench prevention sequence is configured to supply an alternating current to the gradient coil 111 and vibrate, but in the second embodiment, the quench prevention apparatus is attached to the MRI apparatus.
- a vibration generating coil 501 for executing a quench prevention sequence is installed separately from the gradient magnetic field coil 111.
- a gradient magnetic field coil (hereinafter referred to as a vibration generating coil) 501 of the quench prevention device is provided on the outer surface of the vacuum vessel 401 of the upper cryostat 104 and the lower cryostat 105 of the superconducting magnet 101.
- the quench prevention apparatus includes a dedicated current supply unit (hereinafter referred to as a drive power source) 502 connected to the vibration generating coil 501.
- the driving power source 502 includes an amplifier circuit that applies a current necessary for the vibration generating coil 501 to the vibration generating coil 502, and a timer circuit for operating the amplifier circuit at predetermined time intervals. After the MRI examination is completed, the operator turns on the drive power supply 502, or the computer 117 turns on the drive power supply 502, so that the amplifier circuit is connected to the vibration generating coil 501, for example, every hour. An alternating current having a predetermined frequency (for example, a basic frequency of 0.25 hertz) is supplied for a predetermined time to vibrate. Thereby, the quench prevention sequence can be executed.
- a predetermined frequency for example, a basic frequency of 0.25 hertz
- the gradient magnetic field coil 111 of the MRI apparatus is designed to avoid electromagnetic interference with the superconducting magnet 101 as much as possible, it has a structure in which vibration is not generated in the superconducting coil.
- the shield coil is often arranged in combination with the gradient magnetic field coil 111, and the required gradient magnetic field is effectively generated in the imaging space 103, but the gradient magnetic field is generated in a space in the cryostat other than the imaging space. This is because the shield coil is actively shielded.
- the vibration generating coil 501 since the vibration generating coil 501 different from the gradient magnetic field coil 111 is used, the vibration generating coil 501 is arranged at an optimal position according to the position of the superconducting coil 405, Since the vibration generating coil 501 can be attached in the direction in which vibration is to be applied, the superconducting coil 405 can be effectively vibrated.
- the configuration of the second embodiment can arrange the vibration generating coil 501 and the drive power source 502 as an independent configuration different from the MRI apparatus, by attaching these to the existing MRI apparatus It is also possible to prevent accidental quenching of existing MRI equipment.
- the vibration generating coil 501 is attached to the superconducting magnet 101, but the attachment position is not limited to this position. That is, it can be attached to the wall of the room where the superconducting magnet 101 is installed, or attached to the bed 119, and the magnetic field generating coil 501 is installed at the center of the superconducting magnet 101 by the same operation as the loading of the subject 102. It is also possible.
- a phantom imaging sequence is executed as a quench prevention sequence.
- FIG. 6 shows the operation flow of the MRI apparatus of the third embodiment.
- the flow in FIG. 6 is the same as steps 201 to 206 in the first embodiment in steps 201 to 206.
- step 206 when the operator inputs the end of the inspection, the process proceeds to step 601, and the computer 117 displays a display on the display 118 that prompts the operator to install the phantom in the imaging space 103. The operator sees this display, mounts the phantom on the bed 109, places it in the imaging space, and inputs the fact that the placement has been completed to the computer 117 via the operation unit or the like. In response to this, the computer 117 performs the operation of the termination process II of the MRI apparatus. If the operator inputs NO in step 206, the process returns to step 203.
- a transition to the sleep mode III is performed.
- sleep mode III the monitoring function for monitoring some units of the MRI machine, the cooling operation of the cryocooler 107, the activation by the timer, and the remote communication function continue operating, and the MRI machine is restarted at the necessary timing.
- This mode continues the timer function for taking phantom images.
- the computer 117 maintains the sleep mode III and determines whether or not a new device has been activated (step 208). If there is an activation instruction, the process returns to the first step 201. When there is no activation instruction, the sleep mode III is maintained while measuring the elapsed time by the timer function programmed in the computer 117 (step 602). For example, the program proceeds to the next step 603 after 1 hour has elapsed according to the program.
- step 603 the quench prevention unit 117a of the computer 117 captures a phantom image in accordance with a pre-programmed imaging sequence as a quench prevention sequence.
- the MRI apparatus When imaging of the phantom image is completed, the MRI apparatus maintains the sleep mode III again, and returns to step 208 for performing a new apparatus activation presence determination process.
- the MRI apparatus takes a phantom image every hour.
- step 603 since a phantom image is taken as a quench prevention sequence, the computer 117 can store the image in a built-in storage device.
- the quench prevention sequence in step 603 can also serve as the QA test imaging sequence in step 202. Therefore, the phantom imaging of the QA test in step 202 can be omitted.
- step 202 the phantom image photographed in step 603 is analyzed and a QA test is performed. Therefore, the time from the start of step 201 to the step 204 of imaging of the subject 102 can be shortened.
- the quench prevention sequence of the present embodiment can be applied to a superconducting magnet having another shape such as a cylindrical shape.
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Abstract
Description
<第1の実施形態>
以下、本発明の第1の実施形態のMRI装置を添付図面に基づいて具体的に説明する。
なお、発明の実施形態を説明するための全図において、同一機能を有するものは同一符号を付け、その繰り返しの説明は省略する。
まず、本実施形態のMRI装置の構成について説明する。
図1は、本実施形態のMRI装置が医療施設に据付けられた状態での全体構成を示す。
このMRI装置は、静磁場を発生する磁石として開放構造を有する超電導磁石101を備えている。
室温で組み立てられた超電導磁石101内には、液体ヘリウム407が供給され、4.2ケルビン(-268.8℃)まで冷却され、永久電流を流し、閉ループとすることにより予め励磁される。超電導磁石101の超電導線と樹脂の夫々の熱収縮率の違いから、それぞれの部材には熱収縮歪が発生しているため、医療機関等に設置される前に、熱収縮歪を開放する作業が行われる。この開放作業は、超電導コイルのトレーニングと称される。
図3は、図1および図2のMRI装置の動作フロー図である。
図3のフローの動作は、コンピュータ117が内蔵する記憶装置に予め格納されているプログラムを実行することにより実現する。まず、1日の検査の開始に当たり、MRI装置の所定の起動動作を行なう(ステップ201)。この起動動作は、オペレータによる起動スイッチが投入された時点もしくは、コンピュータ117に予めオペレータが予約した時刻になった時点で実行される。起動動作としては、コンピュータ117はMRI装置の各部の電源を投入し、動作確認を行う。
ここで、上述のステップ210が行うクエンチ防止シーケンスについて図4(a),(b)を用いて説明する。ステップ210では、ステップ207でスリープモードIIの状態に入ってから所定時間が経過したならば、クエンチ防止シーケンスを実行するため、コンピュータ117のクエンチ防止部117aは、傾斜磁場パワーアンプ112に起動信号301(図4(a)参照)を出力する。起動信号301を受けて傾斜磁場パワーアンプ112は、自らの電源を投入し、傾斜磁場コイル111の冷却水の循環、セルフ回路チェックを実施し、約2分後にスタンバイモードに入る。この時点で、コンピュータ117のクエンチ防止部117aからのコントロール信号が傾斜磁場パワーアンプ112に再び受け渡され、傾斜磁場パワーアンプ112は、傾斜磁場コイル111のxコイルに電流を供給して所定時間(5分間)駆動させる期間302、yコイルに電流を供給して所定時間(5分間)駆動させる期間303、zコイルに電流を供給して所定時間(5分間)駆動させる期間304を順に実施する。この一連の動作がクエンチ防止シーケンスであり、17分間のシーケンスである。
図4に示したクエンチ防止シーケンスは2秒間の+500Aの電流パルス305と、2秒間の-500Aの電流パルス306が交互に傾斜磁場コイル111に印加される例であるため、電流パルスは、周波数0.25ヘルツの基本波と、その高調波を含むことになる。
第1の実施形態では、クエンチ防止シーケンスとして、傾斜磁場コイル111に交番電流を供給し振動させる構成であったが、第2の実施形態では、MRI装置にクエンチ防止装置を取り付ける構成とする。クエンチ防止装置として、傾斜磁場コイル111とは別に、クエンチ防止シーケンス実行用の振動発生用コイル501を設置する。
第3の実施形態では、クエンチ防止シーケンスとして、ファントムの撮像シーケンスを実行う。
起動指示がない場合は、コンピュータ117にプログラムされたタイマー機能により、経過時間を計測しながら、スリープモードIIIを維持する(ステップ602)。例えば、プログラムにより1時間経過後に次のステップ603に移行する。
Claims (20)
- 超電導磁石と、撮像空間に傾斜磁場を印加する傾斜磁場コイルと、撮像空間に高周波磁場を印加する高周波コイルと、前記傾斜磁場コイルと前記高周波コイルの動作を制御して所定の撮影シーケンスを実行させる制御部とを備えた磁気共鳴イメージング装置であって、
前記制御部が前記撮影シーケンスを実行していない時間帯に、前記超電導磁石を振動させるクエンチ防止シーケンスを実行させるクエンチ防止シーケンス実行部を備えることを特徴とする磁気共鳴イメージング装置。 - 請求項1に記載の磁気共鳴イメージング装置において、前記クエンチ防止シーケンスは、前記傾斜磁場コイルを駆動して前記超電導磁石に傾斜磁場を印加するシーケンスであることを特徴とする磁気共鳴イメージング装置。
- 請求項1に記載の磁気共鳴イメージング装置において、前記クエンチ防止シーケンス実行部は、前記傾斜磁場コイルとは別に、前記超電導磁石を振動させるための傾斜磁場を前記超電導磁石に印加する振動用コイルを備え、
前記クエンチ防止シーケンスは、前記振動用コイルを振動させることを特徴とする磁気共鳴イメージング装置。 - 請求項1に記載の磁気共鳴イメージング装置において、前記クエンチ防止シーケンスは、勾配の向きが反転する交番傾斜磁場を前記超電導磁石に印加するシーケンスであることを特徴とする磁気共鳴イメージング装置。
- 請求項4に記載の磁気共鳴イメージング装置において、前記交番傾斜磁場の周波数または、その高調波の周波数は、前記超電導磁石の共振周波数に一致していることを特徴とする磁気共鳴イメージング装置。
- 請求項1に記載の磁気共鳴イメージング装置において、前記クエンチ防止シーケンスは、撮影シーケンスであることを特徴とする磁気共鳴イメージング装置。
- 請求項6に記載の磁気共鳴イメージング装置において、前記制御部は、前記所定の撮像シーケンスの実行前に、ファントムを撮影して所定の画像解析を行うファントム撮像シーケンスを実行し、前記クエンチ防止シーケンスは、前記ファントム撮像シーケンスと同じ撮像条件であることを特徴とする磁気共鳴イメージング装置。
- 請求項6に記載の磁気共鳴イメージング装置において、前記クエンチ防止シーケンスは、ファントム撮像シーケンスを兼用しており、前記制御部は、前記クエンチ防止シーケンスで撮像した画像を用いて前記画像解析を行うことを特徴とする磁気共鳴イメージング装置。
- 請求項1に記載の磁気共鳴イメージング装置において、前記クエンチ防止シーケンス実行部は、前記クエンチ防止シーケンスを所定の時間間隔で繰り返し実行することを特徴とする磁気共鳴イメージング装置。
- 請求項1に記載の磁気共鳴イメージング装置において、前記クエンチ防止シーケンス実行部は、前記クエンチ防止シーケンスの実行前に、被検体またはオペレータに対して、これから実行することを報知することを特徴とする磁気共鳴イメージング装置。
- 請求項1に記載の磁気共鳴イメージング装置において、前記クエンチ防止シーケンス実行部は、前記クエンチ防止シーケンスの実行前に、被検体またはオペレータが前記超電導磁石の周囲の所定の範囲にいることを検出する検出部を有することを特徴とする磁気共鳴イメージング装置。
- 請求項11に記載の磁気共鳴イメージング装置において、前記検出部が、被検体またはオペレータが前記範囲にいることを検出した場合には、前記クエンチ防止シーケンスの実行を延期することを特徴とする磁気共鳴イメージング装置。
- 超電導磁石と、撮像空間に傾斜磁場を印加する傾斜磁場コイルと、撮像空間に高周波磁場を印加する高周波コイルと、前記傾斜磁場コイルと前記高周波コイルの動作を制御して所定の撮影シーケンスを実行させる制御部とを備えた磁気共鳴イメージング装置であって、
前記制御部は、前記撮影シーケンスを実行していない時間帯に、前記超電導磁石を振動させるために所定の振動用撮像シーケンスを実行することを特徴とする磁気共鳴イメージング装置。 - 超電導磁石を備え、所定の撮像シーケンスを実行することにより被検体を撮像する磁気共鳴イメージング装置の運転方法であって、
前記所定の撮像シーケンスが実行されてない時間帯に、前記超電導磁石のクエンチ防止のために前記超電導磁石を振動させることを特徴とする磁気共鳴イメージング装置の運転方法。 - 請求項14に記載の磁気共鳴イメージング装置の運転方法であって、前記磁気共鳴イメージング装置は、傾斜磁場コイルを備え、前記クエンチ防止のために前記傾斜磁場コイルに傾斜磁場を発生させることにより前記超電導磁石を振動させることを特徴とする磁気共鳴イメージング装置の運転方法。
- [規則91に基づく訂正 18.10.2012]
請求項14に記載の磁気共鳴イメージング装置の運転方法であって、前記磁気共鳴イメージング装置は、前記撮像シーケンスに用いる傾斜磁場コイルとは別に、前記超電導磁石に傾斜磁場を印加するための傾斜磁場を発生する振動用傾斜磁場コイルを備え、
前記振動用傾斜磁場コイルに傾斜磁場を発生させることにより前記超電導磁石を振動させることを特徴とする磁気共鳴イメージング装置の運転方法。 - 超電導磁石を備え、所定の撮像シーケンスを実行することにより被検体を撮像する磁気共鳴イメージング装置の運転方法であって、
前記所定の撮像シーケンスが実行されてない時間帯に、前記超電導磁石のクエンチ防止のためにファントムを撮像することを特徴とする磁気共鳴イメージング装置の運転方法。 - 超電導磁石を備えた磁気共鳴イメージング装置の運転方法であって、
被検体の撮像を実施するステップと、
前記撮像の実施を休止する動作を行うステップと、
前記休止する動作が行われた後に、前記超電導磁石を振動させるステップとを
行うことを特徴とする磁気共鳴イメージング装置の運転方法。 - 超電導磁石を備えた磁気共鳴イメージング装置のクエンチ防止装置であって、
前記超電導磁石を振動させるための傾斜磁場を発生する振動用傾斜磁場コイルと、
前記振動用傾斜磁場コイルに交番電流を所定時間供給する動作を所定の時間間隔で繰り返し行う電流供給部とを有することを特徴とするクエンチ防止装置。 - 超電導磁石と、撮像空間に傾斜磁場を印加する傾斜磁場コイルと、撮像空間に高周波磁場を印加する高周波コイルと、前記傾斜磁場コイルと前記高周波コイルの動作を制御して所定の撮影シーケンスを実行させる制御部とを備えた磁気共鳴イメージング装置であって、
前記制御部が、1日の4分の3以上の時間で、撮影シーケンスあるいはクエンチ防止シーケンスのいずれか一方を実行することを特徴とする磁気共鳴イメージング装置。
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