WO2016093085A1

WO2016093085A1 - Magnetic resonance imaging apparatus and method for controlling operation of refrigerator

Info

Publication number: WO2016093085A1
Application number: PCT/JP2015/083409
Authority: WO
Inventors: 津田　宗孝
Original assignee: 株式会社日立製作所
Priority date: 2014-12-09
Filing date: 2015-11-27
Publication date: 2016-06-16
Also published as: JPWO2016093085A1

Abstract

The purpose of the present invention is to prevent image degradation due to an error field in the time axis direction caused by a refrigerator of a superconducting magnet. The superconducting magnet includes the refrigerator, the refrigerator is provided with a movable section for providing cooling and a drive section for operating the movable section, and a control section operates the movable section of the refrigerator in cycles synchronized with the repetition time (TR) of a pulse sequence at least during execution of the pulse sequence. This suppresses the influence of an error field caused by operation of the refrigerator, and allows for accurate detection of NMR signals.

Description

Magnetic resonance imaging apparatus and operation control method for refrigerator

The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI) apparatus, and more particularly to an MRI apparatus that achieves a high magnetic field intensity using a superconducting magnet and obtains a high-definition diagnostic image.

The MRI apparatus has a stronger nuclear magnetic resonance signal intensity in proportion to the magnetic field intensity. Therefore, a superconducting magnet that can generate a strong magnetic field is often used as the magnetic field generating means. For example, an MRI apparatus having a magnetic field strength of 1 Tesla to 3 Tesla is used for diagnosis in medical facilities.

The superconducting magnet has a structure in which a superconducting coil that has been formed into a coil after processing a NbTi superconducting material into a wire is cooled to a critical temperature of 10 Kelvin temperature (−263 ° C.) or lower. In the cooled superconducting coil, a permanent current having a target value flows and generates a magnetic field.

The superconducting magnet has a structure in which the superconducting coil is housed in a cryostat having excellent heat insulation characteristics in order to keep the superconducting coil below the critical temperature. The space around the superconducting coil in the cryostat is filled with liquid helium. Even if the cryostat has an excellent heat insulating structure, the liquid helium with a small heat of vaporization is vaporized by a slight conduction heat or radiant heat from the room temperature outside the cryostat and released into the atmosphere. For this reason, in order to operate an MRI apparatus using a superconducting magnet, it is necessary to periodically replenish liquid helium.

Therefore, in order to reduce the replenishment frequency of liquid helium, a superconducting magnet that incorporates a refrigerator into a cryostat, cools and condenses the vaporized helium gas in the refrigerator, and reuses it as liquid helium is the mainstream of current MRI systems. It has become. In general, a heater is disposed inside a cryostat in which a refrigerator is incorporated, and a weak current is supplied. By controlling the amount of weak current, the amount of helium gas cooled and condensed by the refrigerator matches the amount of helium gas evaporated from liquid helium, the pressure in the cryostat is kept constant, and the magnetic field is stabilized.

Patent Document 1 discloses a superconducting magnet that controls the frequency of electric power supplied to a motor that drives a refrigerator in accordance with the pressure in the helium vessel of the superconducting magnet to adjust the cooling capacity. Thereby, without using a heater, the evaporation of the refrigerant of the superconducting magnet and the liquefaction by the refrigerator are equalized, and the pressure of the helium container (vessel) of the superconducting magnet is made constant.

JP 2011-5091

The MRI apparatus is required to have a high magnetic field free from errors in the time axis direction and the space axis direction in order to be able to display high-definition images. The inventor measured the error magnetic field in the time axis direction of the superconducting magnet with high accuracy, and found that the periodic error magnetic field accompanying the operation of the refrigerator is about 5 nanotesla. It is considered that the main cause of the error magnetic field is that mechanical vibration accompanying the operation of the refrigerator is transmitted to the superconducting magnet and that the movement of the magnetic regenerator used in the refrigerator disturbs the magnetic field. In a high-definition image, this slight error magnetic field causes blurring of the outline of the diagnostic image and a false image. Since the refrigerator is driven by a synchronous motor that rotates in synchronization with a commercial power source, a periodic error magnetic field is generated. Depending on the imaging mode (timing of the imaging pulse sequence), the nuclear magnetic resonance (hereinafter referred to as NMR) signal of the subject is regularly modulated due to this error magnetic field, and sideband images are formed on both sides of the original image. May occur.

Patent Document 1 does not consider the error magnetic field accompanying the driving of the refrigerator at all.

In order to prevent image degradation due to the periodic error magnetic field accompanying the drive of the refrigerator, a method of temporarily stopping the operation of the refrigerator during imaging can be considered, but the cooling capacity of the refrigerator during shooting decreases, The consumption of liquid helium is inevitable. In particular, when shooting takes a long time, the frequency of replenishment of liquid helium increases, and there is a problem that the cost of liquid helium for replenishment and the maintenance cost for replenishment work increase. Also, in a conduction-cooling superconducting magnet that does not use liquid helium, when the refrigerator is stopped, the temperature of the superconducting coil rises, resulting in a quenching risk of the superconducting coil.

An object of the present invention is to suppress image degradation caused by an error magnetic field in the time axis direction caused by a superconducting magnet refrigerator.

In order to solve the above problems, the MRI apparatus of the present invention has the following configuration. That is, the MRI apparatus includes a superconducting magnet that generates a static magnetic field in an imaging space, a gradient magnetic field generator that applies a gradient magnetic field pulse to the imaging space, a high-frequency magnetic field generator that irradiates the imaging space with a high-frequency magnetic field pulse, and an imaging space A detection unit for detecting a nuclear magnetic resonance signal from the subject arranged in the head, a gradient magnetic field generation unit, and a control unit for controlling the operation of the high-frequency magnetic field generation unit to execute a predetermined pulse sequence. The superconducting magnet includes a refrigerator, and the refrigerator includes a movable part for realizing cooling and a drive part for operating the movable part. The control unit controls the driving unit to operate the movable unit of the refrigerator at a period synchronized with the pulse sequence repetition time (TR) at least during execution of the pulse sequence.

Further, the operation control method for a refrigerator of the present invention includes a superconducting magnet that includes a refrigerator and generates a static magnetic field in an imaging space, and a control unit that executes a predetermined pulse sequence for detecting a nuclear magnetic resonance signal from a subject. The refrigerator is a method for controlling the operation of a refrigerator in a magnetic resonance imaging apparatus having a movable part for realizing cooling and a drive part for operating the movable part, and at least during execution of a pulse sequence Has a step of controlling the drive unit so as to operate the movable unit of the refrigerator in a cycle synchronized with the repetition time (TR) of the pulse sequence.

According to the present invention, it is possible to suppress image deterioration due to an error magnetic field in the time axis direction caused by a superconducting magnet refrigerator.

The block diagram which shows the structure of the MRI apparatus using the superconducting magnet of embodiment. FIG. 2 is a cross-sectional view of an open superconducting magnet used in the MRI apparatus of FIG. The graph which shows the measurement result of the error magnetic field of a superconducting magnet. The flowchart which shows the control operation of the refrigerator by the computer of an MRI apparatus. The timing chart figure which shows the imaging | photography pulse sequence of embodiment, and the alternating voltage supplied to a refrigerator. FIG. 2 is a cross-sectional view of a conduction cooling type open superconducting magnet used in the MRI apparatus of FIG.

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

<Configuration of MRI system using superconducting magnet>
The configuration of the MRI apparatus using the superconducting magnet of the present embodiment will be described with reference to FIGS. FIG. 1 shows the overall configuration of the MRI apparatus, and FIG. 2 shows the structure of the superconducting magnet.

As shown in FIG. 1, a superconducting magnet 103 that generates a uniform static magnetic field in the imaging space 102, a gradient magnetic field generator (106, 107) that applies one or more gradient magnetic field pulses to the imaging space 102, and an imaging space 102 A high-frequency magnetic field generator (108, 109) for irradiating a high-frequency magnetic field pulse; a detection unit (110, 111) for detecting a nuclear magnetic resonance signal from the subject 101 arranged in the imaging space 102; and a gradient magnetic field generator ( 106, 107) and a control unit (112) for controlling the operation of the high-frequency magnetic field generation unit (108, 109) to execute a predetermined pulse sequence.

Here, the superconducting magnet 103 is described as an example in which it has an open structure as shown in FIG. 1, but it is of course possible to have other shapes such as a cylindrical shape. The superconducting magnet 103 having an open structure includes a pair of cryostats 104 and connecting columns 105 disposed above and below an imaging space (hereinafter referred to as an inspection space) 102. A superconducting coil 202 is incorporated in the cryostat 104. The superconducting magnet 103 having the structure of FIG. 1 can provide an open inspection environment because the front (y axis) and the left and right (x axis) of the inspection space 102 are not blocked.

For example, a permanent current of 450 amperes flows through the superconducting coil 202 in the cryostat 104, and a uniform 1.2 Tesla magnetic field can be generated in the examination space 102 by this permanent current. In order for the superconducting coil 202 to stably maintain the superconducting state, the internal space of the cryostat 104 is filled with liquid helium 203 as a refrigerant as shown in FIG. A refrigerator 105 is incorporated in the cryostat 104 disposed on the upper side. The refrigerator 105 reuses the vaporized helium gas as a liquid refrigerant in the cryostat 104 by recondensing it into liquid helium.

A gradient coil assembly 106 is attached to the examination space 102 side of the cryostat 104. The gradient coil assembly 106 can generate a magnetic field gradient in the examination space 102 along three axial directions orthogonal to each other. In the gradient coil assembly 106, three types of coils (not shown) of x, y, and z are laminated to form a disk shape. Due to this disc shape, the open characteristics of the inspection space 102 are maintained. To the x coil, y coil, and z coil of the gradient coil assembly 106, a three-channel gradient magnetic field power source 107 that can independently apply a current is connected.

The functions of the gradient coil assembly 106 and the gradient magnetic field power source 107 are, for example, that when a positive current is applied to the z coil, the z coil attached to the upper cryostat 104 is in the same direction as the magnetic flux generated by the superconducting magnet 103. A magnetic flux along a certain + z axis is generated and superimposed on the magnetic field of the superconducting magnet 103, and the magnetic field strength of the examination space 102 is increased.

On the other hand, the z coil attached to the lower cryostat 104 generates a magnetic flux along the −z axis in the direction opposite to the magnetic flux generated by the superconducting magnet 103, and weakens the magnetic field strength of the examination space 102. As a result, a magnetic field gradient in which the magnetic field strength increases from bottom to top along the z axis of the examination space 102 can be created.
Similarly, the x coil changes the magnetic flux density generated by the superconducting magnet 103 along the x axis, and generates a magnetic field gradient along the x axis of the examination space 102. Further, the y coil generates a magnetic field that gradients along the y axis of the examination space 102.

Furthermore, a pair of high-frequency transmitter coils 108 are attached to the cryostat 104 on the inspection space 102 side. The high-frequency transmitter coil 108 has a planar shape so as to maintain an open examination space 102. A high frequency power amplifier 109 is connected to the high frequency transmitter coil 108. The high frequency transmitter coil 108 has a coil circuit configured to generate a magnetic flux parallel to the xy plane of the examination space 102.

This coil circuit is configured to match the output impedance of the high-frequency power amplifier 109. For this reason, when a high frequency current of 51 megahertz causing proton nuclear spin to cause NMR phenomenon with a magnetic field intensity of 1.2 Tesla, for example, is applied from the high frequency power amplifier 109 to the high frequency transmitter coil 108, magnetic flux is generated in the examination space 102 on the xy plane. A high-frequency magnetic field that is parallel and rotates at 51 MHz is generated.

Due to the above-described uniform static magnetic field strength of, for example, 1.2 Tesla generated by the superconducting magnet 103, the magnetic field gradient of x, y, and z generated by the gradient coil assembly 106, and the high-frequency magnetic field generated by the high-frequency transmitter coil 108 The proton nuclear spin at a specific part of the subject 101 arranged in the examination space 102 causes an NMR phenomenon.

Next, a description will be given of the high-frequency receiver coil 110 that detects precession of proton nuclear spin that has caused an NMR phenomenon as an NMR signal by electromagnetic induction. The high frequency receiver coil 110 is attached in the vicinity of the examination site of the subject 101. The high frequency receiver coil 110, like the high frequency transmitter coil 108, is tuned to a frequency of 51 megahertz. However, in order to efficiently convert the precession of proton nuclear spins into electrical signals, the high-frequency receiver coil 110 has a different shape for each examination site so that it is mounted as close as possible to the examination site of the subject 101. Are prepared and selected according to the examination site. In FIG. 1, a high-frequency receiver coil 110 for the abdomen is used.

The signal processing unit 111 is connected to the high frequency receiver coil 110, and the output signal of the high frequency receiver coil 110 is input to the signal processing unit 111. In the signal processing unit 111, NMR signal amplification, detection, and analog / digital conversion are performed. A computer 112 is connected to the signal processing unit 111. The computer 112 reconstructs the NMR signal converted into the digital electrical signal into an image suitable for diagnosis, and causes the display device 114 to display the reconstructed image. In addition, the computer 112 records the NMR signal and the reconstructed image in the storage device 113.

In order to obtain an image suitable for the purpose of diagnosis, it is necessary to apply a gradient magnetic field and a high-frequency magnetic field at an appropriate timing so that an NMR signal can be detected from a specific part of the subject 101. Therefore, various imaging mode programs are incorporated in the computer 112, and an input device 115 for operating the MRI apparatus such as selection of the imaging mode is connected.

Further, a sequence controller 116 is connected to the computer 112. The sequence controller 116 controls the operations of the gradient magnetic field power source 107, the high frequency power source 109, and the signal processing unit 111 based on the selected shooting mode program. The sequence controller 116 has an interface function between the computer 112 and these units, and not only controls the operation of each unit but also has a function of monitoring the NMR signal and the operation state of the unit.

<Magnet internal structure and refrigerator>
Next, the structure of the superconducting magnet 103 will be described with reference to FIG. Here, the superconducting magnet 103 having an open structure will be described, but a cylindrical superconducting magnet or the like that generates a magnetic field strength of 1.5 to 3 Tesla can be similarly configured except for the shape.
Although only the upper cryostat 104 is shown in FIG. 2, the lower cryostat 104 has a vertically symmetrical structure with respect to the upper cryostat 104 except for the portion where the refrigerator 105 is incorporated.

As shown in FIG. 2, the superconducting coil 202 is wound around a coil bobbin 201 made of stainless steel and disposed in a helium vessel 204 filled with liquid helium 203.

A vacuum vessel 205 is disposed outside the helium vessel 204, and a vacuum layer 206 is formed in the gap between the vacuum vessel 205 and the helium vessel 204. The helium vessel 204 is fixed to the vacuum vessel 205 by a plurality of supports 207. The helium vessel 204 and the vacuum vessel 205 are made of, for example, 8 mm thick stainless steel so as to sufficiently withstand the electromagnetic force applied to the superconducting coil 202 or the like. The support 207 is made of carbon fiber or FRP (fiber reinforced plastic) in order to reduce the conduction heat transferred from the vacuum vessel 205 to the helium vessel 204 as much as possible.

In the vacuum layer 206, a radiation shield plate 208 and a super insulator 209 are arranged so as to surround the helium vessel 204 with a predetermined interval from the helium vessel 204. The radiation shield plate 208 is made of, for example, an aluminum plate having a thickness of 5 mm, and is in contact with the refrigerator 105 through a plurality of copper mesh wires 213. As a result, the radiation shield plate 208 is cooled to a temperature of about 50 Kelvin and realizes a function of reducing radiant heat to the helium vessel 204. The super insulator 209 has a structure in which several tens of specular sheets each having an aluminum vapor deposited thin film are laminated on a polyethylene film, and is attached to the outer surface of the radiation shield plate 208 to reduce radiant heat.

A refrigerator 105 is mounted on the vacuum vessel 205 of the upper cryostat 104. The refrigerator 105 includes a first cooling unit 210 having a 50 Kelvin temperature, a second cooling unit 211 having a 4 Kelvin temperature, and a motor driving unit 212. Although not shown, inside the refrigerator 105, a first displacer (movable part) packed with a cold storage agent of copper and lead and a second displacer (movable part) packed with a cold storage agent of a horobium alloy are arranged. These movable parts are driven by a motor drive part 212 and reciprocate in the vertical direction in the example of FIG. Two valves for injecting and discharging the refrigerant gas in conjunction with the reciprocating motion are arranged in the motor driving unit 212. As the first and second displacers reciprocate, the refrigerant gas repeats adiabatic expansion and absorbs heat from the respective regenerators. Thereby, cooling is implement | achieved.

冷凍 Mechanical vibration is generated in the refrigerator 105 due to the reciprocating motion of the displacer and the expansion of the high-pressure refrigerant gas. Since the first cooling unit 210 of the refrigerator 105 is connected to the radiation shield plate 208 by a plurality of copper mesh wires 213, the vibration of the first cooling unit 210 is transmitted to the radiation shield plate 208 and vibrates. As the radiation shield plate 208 vibrates in the changing region of the magnetic flux density generated by the superconducting coil 202, an eddy current as a demagnetizing field is induced in the radiation shield plate 208.

In order to reduce this eddy current, the radiation shield plate 208 has a plurality of slits (not visible in the figure).

The second cooling unit 211 is installed in the upper space of the helium vessel 204 to directly cool the vaporized helium gas. A copper heat exchanger 214 is incorporated at the tip of the second cooling unit 211 so as to increase the contact area with the helium gas, efficiently cooling the helium gas and condensing it into liquid helium. The condensed liquid helium is united with the liquid layer of liquid helium 203 by natural fall. As described above, the superconducting magnet 103 realizes the complete circulation of helium: heat penetration into the helium vessel 204 → vaporization of liquid helium → cooling of helium gas with a refrigerator → reuse as liquid helium.

The reciprocating motion of the displacer and the suction, expansion and exhaust operations of the high-pressure refrigerant gas are performed by a synchronous motor and a drive mechanism (both not visible in the figure) of the motor drive unit 212. The AC power for driving the synchronous motor is supplied from the drive power supply unit 118.

The superconducting magnet 103 incorporates a plurality of sensors for monitoring its operating state. A pressure sensor 215 is incorporated in the upper part of the helium container 204. The pressure sensor 215 can indirectly measure the ratio of liquid helium to be vaporized and helium gas to be liquefied by utilizing the fact that the volume ratio of liquid helium to 4 Kelvin helium gas is about 10 times. That is, the cooling capacity of the second cooling unit 211 of the refrigerator 105 with respect to the amount of heat transferred to the helium container 204 can be known.

Furthermore, a temperature sensor 216 is attached to the radiation shield plate 208. The temperature sensor 216 can indirectly measure the cooling capacity of the first cooling unit 210 of the refrigerator 105, and is a material for determining whether or not the refrigerator 105 needs to be maintained. The sensor 217 is a sensor that detects the numerical level of liquid helium.

The computer 112 and the sequence controller 116 have a function for monitoring the cooling state of the superconducting magnet 103 and controlling the cooling capacity of the refrigerator 105. A magnet monitoring unit 117 is connected to the

sensors

215, 216, and 217 described above, and output signals from the

sensors

215, 216, and 217 are input to the computer 112 via the magnet monitoring unit 117 and the sequence controller 116. . The computer 112 determines whether the values of the

sensors

215, 216, and 217 are within an appropriate range, and controls the drive power supply unit 118 that supplies an alternating current to the refrigerator 105 and the compressor unit 119 that supplies compressed gas. .

For example, if the computer 112 detects that the pressure of the cryostat 104 is increasing from the signal of the pressure sensor 215 and determines that the rate of vaporization of liquid helium is higher than the rate of condensation of helium gas, In order to increase the cooling capacity of the machine 105, the frequency of the electric power that the drive power supply unit 118 outputs to the refrigerator 105 is increased.

The drive power supply unit 118 includes an inverter circuit 118a and a delay circuit 118b. The inverter circuit 118a can modulate the frequency of the AC power to be output within a predetermined range under the control of the computer 112 and the sequence controller 116. Can be adjusted. On the other hand, the delay circuit 118b delays the phase of the modulated frequency AC power output from the inverter circuit 118a to a predetermined value under the control of the computer 112 and the sequence controller 116.

Here, the magnetic field error of the superconducting magnet 103 caused by the vibration of the refrigerator 105 will be described. FIG. 3 shows the result of the inventor measuring the error magnetic field at the center of the examination space 102 with high accuracy.

As shown in Fig. 3, the error magnetic field is almost a sine wave, the amplitude is about ± 6 nanotesla, and the frequency is about 60 hertz. Such a magnetic field error is caused by mechanical vibration accompanying the reciprocating motion of the displacer of the refrigerator 105, or the ferromagnetic regenerator of the second cooling unit 211 of the refrigerator 105 moves up and down with respect to the superconducting coil 202. This is due to the disturbance of the magnetic field.

Therefore, in this embodiment, at least during the execution of the pulse sequence, the movable part of the refrigerator is operated in a cycle synchronized with the repetition time (TR) of the pulse sequence. In other words, during imaging of the subject 101, the display of the refrigerator 105 is synchronized with either the gradient magnetic field in the imaging pulse sequence or the application timing of the high-frequency magnetic field, regardless of the signals of the various sensors of the superconducting magnet 103. Operate the sir. As a result, the influence of magnetic field errors can be reduced, and NMR signals can be detected in substantially the same magnetic field, so that a highly accurate image can be reconstructed.

During the time when the subject 101 is not imaged, the operation of the refrigerator 105 is controlled based on the values of various sensors of the superconducting magnet 103 as described above. For example, the frequency of the AC power supplied to the refrigerator 105 is controlled so that the pressure in the helium vessel 204 is constant. Further, when the computer 112 and the sequence controller 116 are stopped, such as at night, the operation of the refrigerator 105 is appropriately controlled by the self-control function of the magnet monitoring unit 117.

Hereinafter, the refrigerator 105 of the MRI apparatus of the present embodiment will be described with reference to the flow of FIG. 4 and the imaging pulse sequence diagram of FIG. The computer 112 operates as follows when the built-in CPU reads and executes the program stored in the built-in memory.

First, the power of the MRI apparatus is turned on and the apparatus is activated (step 601). Since the computer 112 and the sequence controller 116 are stopped before the start-up, the operation of the refrigerator 105 is appropriately controlled by the self-control function of the magnet monitoring unit 117 (self-mode). When the MRI apparatus is activated, the computer 112 switches the operation of the refrigerator 105 from the self mode to the sequence mode in which the drive power supply unit 118 is operated by the control signal of the sequence controller 116 (step 602). Until the inspection (imaging) starts in step 605, the computer 112 measures the value of the pressure sensor 215 of the helium vessel 204 indicating the cooling state of the superconducting magnet 103 (step 603), and the inverter circuit 118a The frequency is set (step 604).
For example, when it is determined that the pressure tends to increase, the frequency of the AC power supplied from the inverter circuit 118a to the motor drive unit 212 is increased, and the frequency of the reciprocating movement of the displacer is increased to increase the cooling capacity. In addition, when it is determined that the pressure tends to decrease, the frequency of the AC power to be supplied is lowered to reduce the cooling capacity.

Next, the computer 112 determines whether or not the operator has instructed the start of inspection (imaging) (step 605). If the start of the inspection has not been instructed, the computer 112 returns to step 603, The control of 604 is repeated. If the start of inspection is instructed, the process proceeds to step 606, where the frequency of the inverter circuit 118a and the delay amount of the delay circuit 118b are set according to the parameters of the imaging pulse sequence (step 606). Hereinafter, the operation in which the computer 112 sets the frequency and the delay amount in step 606 will be described by taking the case where the imaging pulse sequence is a gradient echo sequence as an example.

FIG. 5 is a diagram showing the imaging pulse sequence and the voltage waveform of the AC power supplied to the motor drive unit 212 of the refrigerator 105, and the horizontal axis is time. In the period A of the imaging pulse sequence, in order to specify the cross section of the examination site of the subject 101, a high frequency magnetic field 402 having a frequency corresponding to the magnitude of the magnetic field at the position of the cross section in a state where a gradient magnetic field 401 orthogonal to the cross section is applied. Apply. In FIG. 5, the body axis of the subject 101 coincides with the y axis, and a gradient magnetic field in the y direction is applied as the gradient magnetic field 401 for slicing in order to set a cross section perpendicular to the y direction as the slice plane. For example, the slicing gradient magnetic field 401 is applied in a pulse shape with an intensity of 10 millitesla / meter.

Application of this slicing gradient magnetic field 401 changes the magnetic field strength along the body axis (y-axis) of the subject 101, resulting in a frequency deviation in the NMR phenomenon of 4.28 kHz per centimeter. For example, when inspecting a cross section thickness of 5 millimeters at a location 3 cm away from the center of the magnetic field (in the + y-axis direction), the high frequency power amplifier 109 is 51 MHz + 4.28 × 3 = 12.84 kHz according to the control signal of the sequence control 116. A high frequency power modulated in a Gaussian shape so as to have a band of 2.14 kilohertz at the center frequency is supplied to the high frequency transmitter coil 108. As a result, the high frequency magnetic field 402 of the center frequency and band is applied together with the slicing gradient magnetic field 401.

Next, in period B and period C of the imaging sequence, the cross-section is excited using gradient

magnetic field pulses

403 and 404 in two axes orthogonal to the body axis, here in the front-rear direction (z-axis) and in the left-right direction (x-axis). Encode the position of two spins in the precession of the nuclear spin. Specifically, in period B, for example, a phase encoding gradient magnetic field 403 having an intensity of 0.06 millitesla / meter is applied, and frequency modulation is applied to the precession of nuclear spins along the z-axis.

In the next period C, the high-frequency receiver coil 110 receives the NMR signal 405 in a state where, for example, a gradient magnetic field 404 for frequency encoding of 30 millitesla / meter is applied. The signal processing unit 111 sets a sampling window of 1.6 milliseconds and samples 512 points of NMR signals output from the high-frequency receiver coil 110 at intervals of 1.3 microseconds. This 1.3 microsecond interval samples the maximum frequency (for example, 160.5 kHz) of the NMR signal determined by the gradient magnetic field strength G and FOV magnitude F of the frequency encoding gradient magnetic field 404 and the number of data points P according to the Nyquist theorem. This is the interval set for the purpose.

If a predetermined repetition time (TR) has elapsed from the first slice gradient magnetic field 401, again, with the pulse applied to the slice gradient magnetic field 401, applying a high frequency magnetic field 402 having the same center frequency and band, Excites the nuclear spin of the same cross section of the examination site. Then, a z-axis phase encoding gradient magnetic field 406 is applied. The gradient magnetic field 406 for phase encoding has a strength of, for example, 0.12 millitesla / meter, and applies frequency modulation corresponding to twice the encoded amount along the z axis to the precession of the nuclear spin. Next, for example, the NMR signal 407 is sampled in a state where a frequency encoding gradient magnetic field 404 of 30 millitesla / meter is applied.

For each subsequent repetition time (TR), the sequence from period A to period C is repeated 512 times while changing the amount of change in the z-axis phase encoding gradient magnetic field by, for example, 0.06 millitesla / meter. By performing two-dimensional Fourier analysis on the 512 × 512 matrix data obtained in this way, an image showing the distribution of the NMR signal intensity on the zx plane of the examination site can be obtained.

In the shooting sequence in Fig. 5, the repetition time (TR) is set to 100 milliseconds as an example. The repetition time (TR) is one of parameters that influence the tissue contrast of the examination result image, and is a value determined by the operator's selection. In accordance with this selection, the sequence controller 116 operates the gradient magnetic field power source 107, the high frequency power amplifier 109, and the signal processing unit 111 to realize the imaging pulse sequence of FIG.

In step 606, the computer 112 controls the inverter circuit 118a of the drive power supply unit 118 via the sequence controller 116, and AC power having a period synchronized with the repetition time (TR) is transferred from the inverter circuit 118a to the motor drive unit 212. To be supplied to. Specifically, AC power having a period of n times TR or 1 / n times (where n is an integer) is generated by the inverter circuit 118 a and supplied to the motor driving unit 212. For example, an AC voltage 408 having a cycle (for example, one cycle of 20 milliseconds) synchronized with a repetition time (TR) of the imaging pulse sequence (for example, 100 milliseconds) is supplied to the motor driving unit 212. As a result, the displacer of the refrigerator 105 reciprocates at a period synchronized with the repetition time (TR), so the period of the error magnetic field generated due to the mechanical vibration of the refrigerator 105 is also the repetition time (TR). Synchronize.

The sampling window in which the signal processing unit 111 detects the NMR signal is set at the same cycle as the repetition time (TR). For this reason, the period of the error magnetic field generated due to the mechanical vibration of the refrigerator 105 is synchronized with TR, so that the sampling window (1.6 milliseconds) that is a period for detecting the NMR signal is synchronized with TR. Synchronizes with the period of the error magnetic field. Therefore, the magnitude of the error magnetic field at the time when the sampling window is set for each TR becomes constant, and the NMR signal can be detected without being affected by the change in the error magnetic field.

Although the description has been given of supplying the AC voltage 408 synchronized with the repetition time (TR), since the frequency encoding gradient magnetic field 404 is applied when detecting the NMR signal, the frequency encoding gradient magnetic field 404 is applied. The same effect can be obtained by causing the inverter circuit 118a to generate the AC voltage 408 synchronized with

Thus, by determining the frequency of the driving power of the refrigerator 105 in synchronization with the repetition time (TR) of the imaging pulse sequence or the frequency encoding gradient magnetic field 404, the NMR signals (405, 407,...) Projected at 512 points are determined. ..) Is similarly superimposed with the magnetic field change due to the operation of the refrigerator 105. Therefore, no phase error occurs between the matrix data, and a high-resolution tomographic image of the examination site can be obtained by performing a Fourier analysis on the matrix data.

If it is determined that the inspection (imaging pulse sequence) is completed (step 607), it is determined whether to wait until another imaging pulse sequence is executed or to stop the MRI apparatus (step 608). In the case of standby, the cooling state of the superconducting magnet is checked, and the process returns to step 603 to enter a loop for operating the refrigerator 105 with an appropriate cooling capacity according to the output of the pressure sensor 215.

On the other hand, when the MRI apparatus is stopped, the operation of the refrigerator 105 is switched to the self mode (step 609), and then the MRI apparatus is turned off and finished (step 610). In the self mode, the magnet monitoring unit 117 outputs a control signal to the inverter circuit 118a based on the signal from the pressure sensor 215 to control the frequency of the AC power, and cools the superconducting magnet 103 by continuous operation. As a result, the superconducting magnet 103 can be stably cooled even during the time when the computer 112 and the sequence controller 116 are stopped, such as at night.

Note that the period of the AC voltage 408 supplied to the motor drive unit 212 of the refrigerator 105 is desirably set to a sufficiently large value with respect to the sampling window (1.6 milliseconds). For example, it is preferably 4 times or more of the sampling window, and particularly preferably 10 times or more. This is because if the period of the AC voltage 408 is not sufficiently large with respect to the value of the sampling window, the error magnetic field changes during sampling in the sampling window, so that the NMR signal is affected by the error magnetic field. . Considering the gradient magnetic field strength G, the FOV size F, and the number of data points P, the sampling window is set between 1 ms and 6 ms in a general FOV. The voltage period can be easily set to a value sufficiently larger than the sampling window.

In FIG. 5, the gradient echo sequence is described as an example, but the other imaging sequences are similarly operated by operating the movable part of the refrigerator in synchronization with the selected repetition time (TR). A high-definition tomographic image can be obtained.

In step 606 described above, the period of the AC voltage 408 supplied to the refrigerator 105 is synchronized with the repetition time (TR), and the error magnetic field having the same magnitude is applied at the timing of the sampling window of the NMR signal. However, by delaying the phase of the AC voltage 408 by the delay circuit 118b, the influence of the error magnetic field is further suppressed, and a high-definition tomographic image can be obtained. As shown in FIG. 3, since the change in the error magnetic field is almost a sine wave, the computer 112 controls the delay value set in the delay circuit 118b, so that the sampling window always has a peak of the sine wave of the error magnetic field. The position (maximum value) 31 is always coincident with the valley position (minimum value) 32, or the peak position 31 or the valley position 32 is included.

Since the rate of change of the peak or valley (extreme value) of the sine wave is the smallest, by supplying the AC voltage 408 so that the sampling window coincides with this point, the NMR signal can be detected with a more uniform magnetic field strength. Shooting can be performed.

When an imaging pulse sequence having a repetition time (TR) faster than the operation cycle that can be set in the refrigerator 105 is executed in step 606, the computer 112 controls the delay circuit 118b so that the error magnetic field is the error magnetic field. It is desirable that the sampling window is located at a point passing through the center of intensity change (zero error magnetic field), that is, at a point 33 where the static magnetic field matches a predetermined static magnetic field intensity. This makes it possible to measure twice the timing, that is, faster than the operating cycle of the refrigerator, compared to the case where the sampling window is always located at the peak or trough position of the sine wave of the error magnetic field. Can do. Therefore, the operation of the refrigerator 105 can be synchronized with the period of the error magnetic field even in the high-speed imaging sequence.

Note that the point where the error magnetic field passes through the center of the error magnetic field intensity change (the point where the static magnetic field matches the predetermined static magnetic field intensity) 33 is the point where the error magnetic field change rate is the largest, but the sampling window has an error. By setting a sufficiently short time with respect to the change of the magnetic field, it is possible to avoid the deterioration of the image quality.

Here, an allowable time error of the AC voltage 408 of the drive power supply unit 118 synchronized with the imaging pulse sequence will be described. The degree of influence of the error magnetic field on the image quality varies depending on the type of imaging sequence, but from experience, in the case of an error magnetic field of 1 nanotesla or less, blurring of the outline of the captured image cannot be visually recognized. Therefore, from the characteristics of the error magnetic field in FIG. 3, the operation of the refrigerator 105 and the repetition time (TR) of the imaging pulse sequence may be synchronized with an accuracy of 10 milliseconds or less. In general, a time accuracy of 1 millisecond can be easily achieved with the inverter circuit 118a and the delay circuit 118b configured by general-purpose electric elements that do not take special measures for improving the time accuracy such as temperature control. Therefore, also in this embodiment, signal measurement can be performed under a stable magnetic field strength with an error magnetic field of 1 nanotesla or less.

In the embodiment described above, in

steps

603 and 604, the cooling capacity is adjusted by controlling the frequency of the AC voltage supplied to the refrigerator 105. However, the present embodiment is not limited to this, and the refrigerator is not limited to this. The cooling output 105 can be compensated by output control of a heater (not shown) arranged in the helium vessel 204 or the like.

That is, in order to balance the amount of heat entering the helium container 204 and the cooling of the refrigerator 105, the refrigerator 105 having a cooling capacity larger than the amount of heat entering this is used, and the second cooling unit 211 of the helium container 204 or the refrigerator 105 is used. A heater 218 may be attached to the heater, and the heat balance may be secured by the amount of heat generated by the heater. A current is applied to the heater 218 from the magnet monitoring unit 117. With this configuration, it is possible to ensure a heat balance not only during

steps

603 and 604 but also during shooting in step 606.

That is, in this embodiment, during imaging, the cooling capacity of the refrigerator changes according to the repetition period (TR) of the imaging pulse sequence regardless of the heat penetration amount of the helium vessel 204, but the heating value of the heater 218 By controlling the above, it is possible to secure a heat balance, and it is possible to maintain the pressure of the helium vessel 204 at a constant pressure very stably even during imaging. As a result, it is possible to improve the quality and stability of the image.

Further, even after the photographing is finished, the heater 218 can be used to accurately control the pressure of the helium vessel 204 within an allowable range, so that the magnetic field stability can be further improved. That is, the alternating magnetic field of the gradient magnetic field used for imaging induces an eddy current inside the conductive member constituting the helium vessel 204 and the radiation shield plate 208, and the dielectric heating phenomenon caused by this eddy current causes approximately 200 milliwatts in the helium vessel 204. Heat increase occurs.

Therefore, before and after photographing, heat intrusion into the helium container 204 occurs due to a dielectric heating phenomenon, and the pressure of the helium container 204 changes sharply. The cooling capacity of the refrigerator 105 cannot be changed rapidly even if the supplied frequency is changed, and it takes more than ten minutes until the cooling capacity changes. For this reason, it is difficult to compensate for a steep pressure change with the cooling capacity of the refrigerator 105, but when the heater 218 is used in combination, the amount of heat generated by the heater 218 changes instantaneously by controlling the supply current. Therefore, the allowable pressure range of the helium vessel 204 can be accurately controlled. Therefore, it is possible to further improve the magnetic field stability.

In the imaging pulse sequence, since the gradient coil generates a pulsed gradient magnetic field in the static magnetic field space generated by the superconducting magnet 103, the gradient coil assembly 106 vibrates due to magnetic drag. Since a large vibration sound is generated by this vibration during MRI imaging, the vibration sound is perceived as noise for the subject in many imaging modes. On the other hand, the refrigerator 105 also generates operating noise, and depending on the subject, the plosive sound “push, push shoe,...” Associated with the volume expansion of the refrigerant gas is better than the mechanical vibration sound of the vertical motion of the displacer. May feel unpleasant noise.

According to the MRI apparatus of the embodiment of the present invention, since the operation of the refrigerator 105 is synchronized with the imaging pulse sequence, the operation sound of the refrigerator 105 is synchronized with the imaging sound accompanying the vibration of the gradient coil assembly 106, and the mask Is done. For this reason, a plosive sound is not felt by the subject. Noise is more frustrating when it is randomly generated from multiple sound sources, regardless of the intensity of the sound, but the present invention, which is unified into one sound, not only improves image quality, but also the subject. It also has the effect of improving the inspection environment.

In addition, the operator of the MRI apparatus confirms by sound that the operation sound of the refrigerator 105 and the imaging sound of the gradient coil assembly 106 are synchronized, so that the MRI apparatus can be operated without being affected by the error magnetic field. It can be easily confirmed that it is operating. For this reason, when the image captured in this state is blurred, the operator can determine that the cause is not the influence of the error magnetic field but other factors such as the result of the subject moving during the imaging. Therefore, the subject can be alerted and the examination site can be securely fixed, and the examination can be immediately performed again. Thereby, a favorable test result can be obtained. Therefore, there is an effect that not only the quality of inspection can be improved but also the inspection efficiency can be improved.

<Conductive cooling superconducting magnet structure>
In the embodiment described above, the superconducting magnet 103 that is cooled by the combined use of the liquid refrigerant and the refrigerator has been described as shown in FIG. 2, but the present invention is also applied to the conduction-cooled superconducting magnet that directly cools the coil by heat conduction by the refrigerator. It is possible to apply.

FIG. 6 is a diagram showing a structure of the superconducting magnet 103 with conduction cooling. The superconducting coil 202 is wound around a conductive cooling coil bobbin 301 and directly fixed to the vacuum vessel 205 by a plurality of supports 207. The conductive cooling coil bobbin 301 is made of an aluminum alloy having good heat conduction, has resistance to electromagnetic force applied to the superconducting coil 202, and has a mass for securing a heat storage capacity. The coil bobbin 301 is in thermal contact with the second cooling unit 211 of the refrigerator 105 at the thermal contact point 302. The thermal contact point 302 is sandwiched with an indium metal plate having extremely good thermal conductivity. For this reason, the mechanical vibration of the refrigerator 105 is directly applied to the superconducting coil 202. Thereby, in addition to the magnetic field disturbance due to the reciprocating motion of the magnetic regenerator in the second cooling unit 211, an error magnetic field is generated in which the vibration magnetic field of the coil bobbin 301 due to the mechanical vibration of the superconducting coil 202 is combined.

The radiation shield plate 208 has the same structure as the superconducting magnet 103 in FIG. The plurality of supports 207 are provided with a radiation shield plate 208 and a thermal contact point 303 at an intermediate point close to the vacuum vessel 205, and have a structure that reduces conduction heat transmitted to the conductive cooling coil bobbin 301.

As sensors for monitoring the operating state of the conduction cooling superconducting magnet, a cryogenic temperature sensor 304 for measuring the temperature of the coil bobbin 301 for conduction cooling and a temperature sensor 216 for the radiation shield plate 208 are arranged. In

steps

603 and 604 in FIG. 4, the computer 112 measures the value of the temperature sensor 303 of the conductive cooling coil bobbin 301 and sets the frequency of the inverter circuit 118a.

Other configurations are the same as those of the above-described embodiment, and thus description thereof is omitted.

101 subject, 102 test space, 103 superconducting magnet, 104 cryostat, 105 refrigerator, 106 gradient magnetic field coil assembly, 107 gradient magnetic field power supply, 108 high frequency transmitter coil, 109 high frequency power amplifier, 110 high frequency receiver coil, 111 signal processing unit , 112 computer, 116 sequence controller, 117 magnet monitoring unit, 118 drive power supply unit, 201 coil bobbin, 202 superconducting coil, 203 liquid helium, 204 helium vessel, 205 vacuum vessel, 206 vacuum layer, 207 support, 208 radiation shield plate, 210 1st cooling section, 211 2nd cooling section, 213 copper wire, 215 pressure sensor, 216 temperature sensor, 217 sensor, 301 coil cooling coil bobbin, 302 cryogenic thermal contact point, 304 cryogenic temperature sensor, 401 slope for slicing Magnetic field, 402 high frequency magnetic field, 4 03 Gradient magnetic field for phase encoding, 404 Gradient magnetic field for frequency encoding, 405 NMR signal, 408 AC voltage

Claims

A superconducting magnet that generates a static magnetic field in the imaging space, a gradient magnetic field generator that applies a gradient magnetic field pulse to the imaging space, a high-frequency magnetic field generator that irradiates the imaging space with a high-frequency magnetic field pulse, and the imaging space A detection unit that detects a nuclear magnetic resonance signal from the subject, and a control unit that controls operations of the gradient magnetic field generation unit and the high-frequency magnetic field generation unit to execute a predetermined pulse sequence,
The superconducting magnet includes a refrigerator, and the refrigerator includes a movable part for realizing cooling, and a drive part for operating the movable part,
The control unit controls the driving unit to operate the movable unit of the refrigerator at a period synchronized with a repetition time (TR) of the pulse sequence at least during execution of the pulse sequence. Magnetic resonance imaging device.
A superconducting magnet that generates a static magnetic field in the imaging space, a gradient magnetic field generator that applies a gradient magnetic field pulse to the imaging space, a high-frequency magnetic field generator that irradiates the imaging space with a high-frequency magnetic field pulse, and the imaging space A detection unit that detects a nuclear magnetic resonance signal from the subject, and a control unit that controls operations of the gradient magnetic field generation unit and the high-frequency magnetic field generation unit to execute a predetermined pulse sequence,
The superconducting magnet includes a refrigerator, and the refrigerator includes a movable part for realizing cooling, and a drive part for operating the movable part,
The control unit controls the driving unit to operate the movable unit of the refrigerator in synchronization with one of the gradient magnetic field pulses during execution of the pulse sequence. apparatus.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the operation period of the movable part of the refrigerator is n times or 1 / n times (where n is a positive integer) the repetition time (TR). A magnetic resonance imaging apparatus.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit detects the nuclear magnetic resonance signal in the detection unit at a predetermined sampling timing synchronized with any one of the gradient magnetic field pulses in the pulse sequence. Let
The said control part synchronizes the said sampling timing and operation | movement of the said movable part of the said refrigerator, The magnetic resonance imaging apparatus characterized by the above-mentioned.
5. The magnetic resonance imaging apparatus according to claim 4, wherein the control unit matches the sampling timing with a timing at which the fluctuation of the static magnetic field accompanying the operation of the movable unit of the refrigerator becomes an extreme value. And a moving part of the refrigerator is operated.
5. The magnetic resonance imaging apparatus according to claim 4, wherein the control unit is configured such that the sampling timing and the static magnetic field that varies with the operation of the movable unit of the refrigerator coincide with a predetermined static magnetic field intensity. The moving part of the refrigerator is operated so as to match with the magnetic resonance imaging apparatus.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the superconducting magnet includes a superconducting coil, a container for housing the superconducting coil and a refrigerant, a heater disposed in the container, and the heater. A current supply unit for supplying current; and a pressure sensor for detecting the pressure of the space in the container;
The said control part controls the electric current which the said electric current supply part supplies to the said heater according to the pressure which the said pressure sensor detects during execution of the said pulse sequence, The magnetic resonance imaging apparatus characterized by the above-mentioned.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit switches to a mode in which the driving unit operates the movable unit at a predetermined frequency after completion of the pulse sequence. Resonance imaging device.
A superconducting magnet that generates a static magnetic field in the imaging space including the refrigerator, and a controller that executes a predetermined pulse sequence for detecting a nuclear magnetic resonance signal from the subject,
The refrigerator is a method for controlling the operation of the refrigerator in a magnetic resonance imaging apparatus comprising a movable part for realizing cooling and a drive part for operating the movable part,
At least during the execution of the pulse sequence, the step of controlling the drive unit to operate the movable unit of the refrigerator at a period synchronized with a repetition time (TR) of the pulse sequence. Machine operation control method.