WO2015115141A1

WO2015115141A1 - Magnetic resonance imaging device

Info

Publication number: WO2015115141A1
Application number: PCT/JP2015/050306
Authority: WO
Inventors: 鈴木　伸一郎; 敦士太田
Original assignee: 株式会社日立メディコ
Priority date: 2014-01-28
Filing date: 2015-01-08
Publication date: 2015-08-06
Also published as: JPWO2015115141A1

Abstract

This magnetic resonance imaging device comprises: a static magnetic field generating unit which applies a static magnetic field to an imaging space; a high frequency magnetic field generating unit which is disposed more to the imaging space side than the static magnetic field generating unit; a bore wall which is disposed between the high frequency magnetic field generating unit and imaging space, and which forms a space in which a subject is placed; and a temperature determination unit which determines whether the temperature of the bore wall exceeds a predetermined temperature. The temperature determination unit includes a thermistor, and a detection circuit which is connected to the thermistor and acquires a voltage signal corresponding to electrical resistivity of the thermistor to make a determination on the voltage signal. The detection circuit comprises a determination circuit which has hysteresis properties, wherein the determination circuit determines an abnormal condition exceeding the predetermined temperature when the temperature of the bore wall rises and the voltage signal reaches a voltage value of a higher temperature than the voltage value corresponding to a first temperature, and determines a normal condition at or below the predetermined temperature when the temperature of the bore wall decreases thereafter and the voltage signal reaches a voltage value of a lower temperature than the voltage value corresponding to a second temperature which is lower than the first temperature.

Description

Magnetic resonance imaging system

The present invention relates to a magnetic resonance imaging (hereinafter referred to as “MRI”) apparatus, and more particularly to a bore wall temperature monitoring function.

An MRI apparatus irradiates a subject with a high-frequency magnetic field pulse (hereinafter referred to as an RF pulse), measures an NMR signal generated by a nuclear spin constituting the subject, and forms the head, abdomen, limbs, etc. It is a device for imaging functions in two dimensions or three dimensions. In imaging, in order to give position information to NMR signals, different phase encodings are given depending on the gradient magnetic field, frequency encoding is performed, and time-series data is measured. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

The object is irradiated with an RF pulse from an RF transmission coil. The frequency of the RF pulse is set to a nuclear magnetic resonance frequency determined according to the static magnetic field strength and the excitation target nuclide. Therefore, the RF transmission coil is designed or adjusted (tuned) so that the resonance frequency matches the nuclear magnetic resonance frequency.

In recent years, MRI apparatuses have been developed to have wide bores that widen the diameter of a space into which a subject is inserted (hereinafter referred to as “bore”), and apparatuses having a diameter exceeding 70 cm have been developed. Along with this, there is a demand for an increase in the diameter of RF transmission coils, gradient magnetic field coils, and superconducting magnets.

Patent Document 1 discloses an MRI apparatus having a function of measuring the temperature of an imaging region and blowing air from an air duct in order to prevent the imaging region in the gantry from becoming locally hot.

JP2009-5759

A birdcage (hereinafter referred to as “BC”) coil is known as an RF transmission coil. However, as the bore diameter of the BC coil increases, the generation efficiency of a high-frequency magnetic field that can be generated in the imaging region decreases. One of the causes is that the coil pattern moves away from the limited imaging area inside the coil, and the other is that the coil pattern extends as the bore diameter increases, resulting in the conductor loss and gradient magnetic field of the pattern conductor itself. The conductor loss due to the induction current flowing through the RF shield embedded in the coil surface is increased.

However, increasing the diameter of the BC coil has less effect on the MRI apparatus price than increasing the diameter of the superconducting magnet or gradient magnetic field coil. For this reason, in order to reduce the device price, there is a tendency to increase the diameter of the BC coil while suppressing the expansion of the diameter of the superconducting magnet and the gradient magnetic field coil. For this reason, the distance between the BC coil and the gradient magnetic field coil (= shield gap) tends to become narrower as the wide bore becomes wider. The popular machine with a bore diameter of 60cm has a shield gap of about 3cm, but the wide bore machine with a diameter exceeding 70cm. Then, it may cut 2cm. When the shield gap is reduced, the loss due to the induction current flowing through the RF shield increases, so that the generation efficiency of the high-frequency magnetic field that the BC coil can generate in the imaging region further decreases.

Recent wide-bore MRI apparatuses tend to supply a large amount of power to the RF transmission coil by using a higher-power RF power amplifier to compensate for such a decrease in efficiency of the RF transmission coil. In the popular machine, the output of the RF power amplifier was about 15 to 20kW, but the wide bore MRI equipment is about 35 to 40kW. Even if the output supplied to the RF transmitter coil is approximately doubled, the high-frequency magnetic field strength that can be generated in the imaging area is almost the same for the popular and wide-bore machines, and the reduction in transmission efficiency due to wide-bore is about 50%. . The remaining 50% is released as thermal energy. That is, the heat generation of elements such as capacitors and diodes inserted in the BC coil antenna pattern is approximately doubled, and the element temperature can be close to 100 ° C. without any cooling. Therefore, as in Patent Document 1, the BC coil is cooled by forced air cooling.

BC coil is close to the subject and greatly affects the temperature of the bore. According to IEC (International Electrotechnical Commission), it is regulated that the temperature of the surface of the bore wall on the subject side does not always exceed 41 ° C. even during imaging. Therefore, in an MRI apparatus that performs forced air cooling, if the cooling fan stops due to a failure and the surface temperature of the bore exceeds the temperature defined by IEC, imaging must be interrupted.

In the case of forced air cooling, whether or not the cooling fan has failed can be detected by providing a sensor that detects the operation of the cooling fan. However, even if the cooling fan is operating normally, if the duct of the cooling air is broken, the bore cannot be cooled and the bore surface temperature rises, but the bore surface temperature is directly measured. Unless this is done, an increase in the surface temperature of the bore cannot be detected. Even if forced air cooling is operating normally, if an imaging sequence (scan) with high output intensity of both RF and GC is performed continuously beyond the expected range of use, the surface temperature of the bore May exceed a safe temperature for the subject. Therefore, it is desired to directly measure the surface temperature of the bore with a temperature sensor.

It is known that there are two types of temperature sensors: contact type and non-contact type. The former includes thermistors, thermocouples, platinum resistors, and fiber optic thermometers, and the latter includes infrared radiation thermometers. However, in the case of a contact-type thermometer, it is necessary to connect an electric wire to the thermometer other than the optical fiber thermometer and connect it to the detection circuit.

When measuring the surface temperature of the bore and the temperature of the BC coil, it is necessary to place the wire near the bore, and the high-frequency electromagnetic field generated by the BC coil generates high-frequency normal mode noise in the wire. . The intensity (amplitude) of this high-frequency noise has the same order (number of digits) as the intensity of the detection signal of the thermistor, which causes malfunction of the A / D converter of the detection circuit, and stable temperature measurement cannot be performed.

On the other hand, the optical fiber thermometer is noise-free, but on the other hand, since the fiber is easily damaged by bending or impact, wiring is difficult, and there is a concern that it may be damaged during imaging or maintenance of the subject. The price is also high. On the other hand, in the case of a non-contact type thermometer, it is desirable to measure at an angle of about ± 30 ° perpendicular to the measurement point in order to avoid measurement errors.

∗ Since the BC coil is located only in the range of about ± 25cm in the center of the bore, a sensor must be attached at least near the entrance of the magnet bore. However, since the entrance of the bore has a particularly high static magnetic field strength and is also exposed to a high-frequency electromagnetic field, it is difficult to install an infrared radiation thermometer in which the sensor unit and the main body are integrated. Even if it can be installed, the heat generation part may not be visible from the sensor depending on the physique and posture of the subject.

The present invention has been made in view of the above problems, and is to provide an MRI apparatus having a function of monitoring the temperature of the bore surface during imaging in real time.

In order to achieve the above object, according to the present invention, a static magnetic field generator that applies a static magnetic field to an imaging space, a high-frequency magnetic field generator that is disposed closer to the imaging space than the static magnetic field generator, A bore wall that is disposed between the high-frequency magnetic field generation unit and the imaging space and forms a space in which the subject is disposed, and a temperature determination unit that determines whether the temperature of the bore wall exceeds a predetermined temperature An MRI apparatus is provided.

The temperature determination unit includes a thermistor and a detection circuit which is connected to the thermistor and obtains a voltage signal corresponding to the electric resistance value of the thermistor and determines the voltage signal. The detection circuit includes a determination circuit having a hysteresis characteristic, and when the temperature of the bore wall rises and the voltage signal reaches a voltage value higher than the voltage value corresponding to the first temperature, If it is determined that the abnormal condition exceeds the predetermined temperature, and then the temperature of the bore wall decreases, and reaches a voltage value on a lower temperature side than a voltage value corresponding to a second temperature lower than the first temperature. In this case, it is determined that the normal state is not more than the predetermined temperature.

According to the present invention, the temperature of the bore surface during imaging can be monitored in real time.

Sectional drawing of the gantry 200 of the MRI apparatus which concerns on embodiment of this invention. The block diagram which shows the circuit structure of the temperature determination of embodiment of this invention. FIG. 3 is a graph showing output characteristics of the Schmitt trigger circuit of FIG. 3 is a graph showing an example of output characteristics of the thermistor 51 in FIG. The perspective view of an example of a birdcage coil (BC coil). 1 is a block diagram showing the overall configuration of an MRI apparatus according to an embodiment of the present invention. The perspective view of the gantry of the embodiment of the present invention. Sectional drawing of the gantry 200 of an MRI apparatus in case the bore wall 70 also serves as the cylindrical body 85 of the BC coil 58. Sectional drawing of the gantry 200 of an MRI apparatus in case the bore wall 70 also serves as the cylindrical body 85 of the BC coil 58.

As shown in FIG. 1, the MRI apparatus of the present invention includes a static magnetic field generation unit 2 that applies a static magnetic field to the imaging space 61, and a high-frequency magnetic field generation unit 58 that is disposed closer to the imaging space 61 than the static magnetic field generation unit 2. A bore wall 70 that is disposed between the high-frequency magnetic field generator 58 and the imaging space 61 and forms a space in which the subject 1 is disposed, and a temperature that determines whether the temperature of the bore wall 70 exceeds a predetermined temperature And a determination unit 50.

The temperature determination unit 50 includes a thermistor 51 and a detection circuit 59. The detection circuit 59 is connected to the thermistor 51, obtains a voltage signal corresponding to the electrical resistance value of the thermistor 51, and determines the voltage signal.

The detection circuit 59 includes a determination circuit 53 having hysteresis characteristics as shown in FIG.

When the temperature of the bore wall 70 rises and the voltage signal of the thermistor 51 reaches a voltage value on the higher temperature side than the voltage value corresponding to the predetermined first temperature, the determination circuit 53 determines that the abnormality exceeds the predetermined temperature. Judged as a state. Thereafter, if the temperature of the bore wall 70 decreases and reaches a voltage value on a lower temperature side than the voltage value corresponding to the second temperature lower than the first temperature, it is determined that the normal state is equal to or lower than the predetermined temperature. .

As described above, in the present invention, A / D conversion that is normally performed when the thermistor is used as a temperature sensor is not performed, and only a determination is made as to whether an abnormal state exceeds a predetermined temperature (threshold temperature) or a normal state below a predetermined temperature. . At this time, by making a determination using a determination circuit having a hysteresis characteristic (for example, see FIG. 3) between the voltage corresponding to the first temperature and the voltage corresponding to the second temperature, the determination result becomes a high frequency. Less susceptible to noise.

For example, the voltage signal output from the thermistor 51 is superimposed with high frequency noise having the same frequency as the high frequency magnetic field, as shown in FIG. 4, and this high frequency noise is in the order of amplitude intensity (digits). Is the same level as the voltage signal output by the thermistor, so if the judgment is made with only one judgment criterion (voltage), the judgment result is switched between abnormal and normal at high speed when the voltage signal reaches the vicinity of the judgment criterion. However, in the present invention, the determination can be stably performed by using a determination circuit having hysteresis characteristics.

As the determination circuit 53, a Schmitt trigger circuit can be used as shown in FIG.

The detection circuit 59 preferably includes a variable unit (54, 53b) that makes at least one of the voltage value corresponding to the first temperature and the voltage value corresponding to the second temperature variable. This is due to individual differences in the temperature-resistance characteristics of the thermistor 51, errors in the distance between the thermistor 51 and the high-frequency magnetic field generator 58, which is the heat source, and differences in the degree of thermal contact between the thermistor 51 and the bore wall 70. The operating temperature deviation can be adjusted by the variable section (54, 53b). Therefore, it is possible to accurately determine whether the surface of the bore wall 70 has exceeded a predetermined temperature.

In order to accurately determine the temperature of the bore wall 70, the thermistor 51 is preferably disposed at a position where the thermistor 51 is in thermal contact with the bore wall 70 or the high-frequency magnetic field generator 58 that is a heat source. Further, when the space surrounded by the bore wall has an elliptical cross section, it is desirable that the thermistor be disposed on the bore wall in the major axis direction of the ellipse. This is because, in the major axis direction, the high-frequency magnetic field generator is far from the subject, so that a large amount of electric power is supplied to the high-frequency magnetic field generator and the temperature is likely to rise.

It is desirable that the detection circuit 59 is disposed outside the space (bore) in which the subject is disposed in order to reduce the influence of high frequency noise. The thermistor 51 and the detection circuit 59 are connected by a cable 57 made of nonmagnetic material. Desirably, at least a part of the cable 57 is provided with the high-frequency filter 52 at intervals of 1/8 or less of the wavelength of the high-frequency magnetic field generated by the high-frequency magnetic field generator 58. In a general device such as an antenna device, a high-frequency filter 52 is inserted with a quarter wavelength or less in order to reduce high-frequency noise. However, in the case of an MRI device, a mirror image of noise is generated between the cable 57 and the ground. Therefore, it is desirable to arrange the high frequency filter 52 with 1/8 wavelength or less. Thereby, high frequency noise can be reduced effectively.

The high frequency filter 52 may be any filter as long as it does not pass a signal having a frequency higher than the high frequency generated by the high frequency magnetic field generator 58. For example, an inductor showing high impedance at a high frequency generated by the high-frequency magnetic field generator 58, a resonant circuit of an inductor and a capacitor, or a choke coil can be used. Since the choke coil does not affect the magnetic field of the imaging space 61, a choke coil made of a nonmagnetic material is used.

It is desirable that the high frequency filter 52 is disposed in the cable 57 disposed in the region where the high frequency magnetic field generator 58 is disposed at intervals of 1/8 wavelength or less of the high frequency. This is because the region where the high-frequency magnetic field generator 58 is disposed has the highest strength of the high-frequency magnetic field and generates high-frequency high-frequency noise in the cable 57.

The cable 57 can be a twisted pair cable or a coaxial cable. By using a twisted pair cable in which two cables are twisted together, magnetic fields generated in the two cables can be canceled and high-frequency noise can be reduced. In the coaxial cable, since the signal line is surrounded by the ground line and electromagnetically shielded, it is possible to reduce high-frequency noise from being superimposed on the signal line.

The cable 57 is preferably disposed closer to the static magnetic field generation unit 2 than the bore wall 70. This is to prevent the cable 57 from projecting toward the space where the subject 1 is placed and to ensure a wide space where the subject is placed. At this time, the thermistor 51 is attached to the surface of the bore wall 70 on the imaging space 51 side so that the temperature of the surface of the bore wall 70 on the imaging space 51 side can be accurately determined, and the cable 57 is provided on the bore wall 70. It is possible to adopt a configuration in which the static magnetic field generating unit 2 is drawn more than the bore wall 70 through the formed through hole 71. Even if the position where the cable 57 is wired is between the bore wall 70 and the high-frequency magnetic field generator 58 as shown in FIG. 1, it is outside the high-frequency magnetic field generator 58 (static magnetic field generator 2 side) as shown in FIG. It may be.

As the high-frequency magnetic field generator 58, for example, a birdcage coil (see FIG. 5) can be used.

When the determination circuit 53 of the detection circuit 59 determines that the abnormal state (exceeds a predetermined temperature), the MRI apparatus desirably stops the execution of imaging. Therefore, as shown in FIG. 6, the control unit (sequencer 4) that controls the generation of high frequency of the high frequency magnetic field generation unit and executes predetermined imaging performs imaging when the determination circuit 53 determines that the abnormal state has occurred. Stop. And when it determines with a normal state again, execution of imaging is enabled.

Hereinafter, embodiments of the MRI apparatus of the present invention will be specifically described with reference to the accompanying drawings.

First, an overall outline of an example of an MRI apparatus according to the present invention will be described with reference to FIGS. 1, 6, and 7. FIG. 1 is a cross-sectional view of a gantry 200 of the MRI apparatus, FIG. 6 is a block diagram showing the overall configuration of the MRI apparatus, and FIG. 7 is a perspective view of the gantry 200. This MRI apparatus obtains a tomographic image of the subject 1 using the NMR phenomenon.

In addition to the static magnetic field generating unit 2 and the high frequency magnetic field generating unit 58, the MRI apparatus includes a gradient magnetic field generating unit 9, a high frequency coil (receiving high frequency coil) 14b, a signal processing unit 7, a sequencer 4, A central processing unit (CPU) 8 is provided.

In the case of the horizontal magnetic field method, the static magnetic field generator 2 generates a uniform static magnetic field in the body axis direction using a tunnel-shaped permanent magnet, normal conducting magnet or superconducting magnet as shown in FIG. In the case of the vertical magnetic field method, a pair of static magnetic field generators 2 are arranged above and below the subject 1.

A high-frequency magnetic field generator (hereinafter referred to as a high-frequency coil) 58 irradiates the subject 1 with a magnetic field (RF pulse) having a frequency that causes nuclear magnetic resonance to occur in the nuclear spins of atoms constituting the biological tissue of the subject 1. A high frequency amplifier 13, a modulator 12, and a high frequency oscillator 11 are sequentially connected to the high frequency coil 58. The RF pulse output from the high frequency oscillator 11 is amplitude-modulated by the modulator 12, amplified by the high frequency amplifier 13, and then supplied to the high frequency coil 58. The sequencer 4 instructs the modulator 12 on the timing of modulation.

In the horizontal magnetic field type MRI apparatus, for example, a birdcage (BC) coil is used as the high-frequency coil 58. As shown in FIG. 5, the BC coil has a plurality of linear conductors 81 arranged in parallel to the central axis of the cylinder and at equal intervals along the circumference of the cylinder, and a pair of annular conductors 82 are arranged at both ends thereof. In this configuration, the plurality of linear conductors 81 are connected. A plurality of resonance capacitors (capacitors) 83 having the same capacity are inserted in series in the annular conductor 82. A PIN diode 84 is inserted in series at the intermediate point of the linear conductor 81 to prevent electromagnetic interference with the receiving high-frequency coil 14b that receives the NMR signal. In the example of FIG. 5, the linear conductor 81 and the annular conductor 82 are formed of a nonmagnetic conductor film formed on an insulating cylinder 85.

Further, as shown in FIG. 8, the bore wall 70 also serves as the cylindrical body 85 of the BC coil 58, and includes a plurality of linear conductors 81, a pair of annular conductors 82, a plurality of resonant capacitive elements 83 and a PIN of the BC coil 58. The diode 84 can also be configured to be disposed on the bore wall 70. In this case, the thermistor 51 can also be arranged in a through hole 71 provided in the bore wall 70 as shown in FIG. The cable 57 is arranged by being drawn out from the region where the linear conductor 81 and the pair of annular conductors 82 are not arranged to the outer peripheral side (static magnetic field generating unit 2 side) of the high-frequency coil 58. Further, as shown in FIGS. 5 and 9, the thermistor 51 may be disposed on the outer peripheral surface of BC coil 58 (the outer peripheral surface of linear conductor 81 or annular conductor 82). Also in this case, the cable 57 is arranged on the outer peripheral side (static magnetic field generating unit 2 side) of the high-frequency coil 58.

A gradient magnetic field generating coil 9 is disposed between the static magnetic field generating unit 2 and the high frequency magnetic field generating unit 58 as shown in FIG. The gradient magnetic field generating coil 9 includes coils that respectively apply gradient magnetic fields in the three-axis directions of X, Y, and Z that are coordinate parts (stationary coordinate parts) of the MRI apparatus. Each coil is connected to a gradient magnetic field power supply 10 for supplying a drive current. The sequencer 4 outputs a command to the gradient magnetic field power supply 10 to supply a drive current to each coil.

This causes the gradient magnetic fields Gx, Gy, and Gz to be applied from the gradient coil 9 in the X, Y, and Z directions. Specifically, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining orthogonal to the slice plane and orthogonal to each other The phase encoding direction gradient magnetic field pulse (Gp) and the frequency encoding direction gradient magnetic field pulse (Gf) are applied in the two directions, and the position information in each direction is encoded in the echo signal, thereby performing imaging.

The receiving high-frequency coil 14b detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1. A signal amplifier 15, a quadrature detector 16, and an A / D converter 17 are sequentially connected to the reception high-frequency coil 14b.

The NMR signal generated by the subject 1 by the RF pulse irradiated from the high frequency coil 58 is detected by the high frequency coil 14b for reception, amplified by the signal amplifier 15, and then at a timing according to a command from the sequencer 4, a quadrature detector 16 is divided into two orthogonal signals. The two systems of signals are each converted into a digital quantity by the A / D converter 17 and sent to the signal processing unit 7.

Sequencer 4 executes a predetermined imaging pulse sequence by controlling the timing of high-frequency magnetic field pulse irradiation, gradient magnetic field application, and quadrature detection, respectively. A CPU 8 is connected to the sequencer 4 and its operation is controlled.

The signal processing unit 7 performs various data processing and display and storage of processing results, and includes a CPU 8, an external storage device such as an optical disk 19 and a magnetic disk 18, and a display 20 including a CRT and the like. When data from the A / D converter 17 is input to the CPU 8, the CPU 8 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result on the display 20, Recording is performed on the magnetic disk 18 or the like of the external storage device.

The operation unit 25 inputs various control information of the MRI apparatus and control information of processing performed by the signal processing unit 7, and includes an input device 21 such as a trackball, a mouse, and a keyboard.

The operation unit 25 is arranged in the vicinity of the display 20, and an operator inputs control information interactively through the operation unit 25 while looking at the display 20.

In addition, the subject 1 is mounted on the top 91 of the bed 90, inserted into a space surrounded by the bore wall 70, and an imaging region is arranged in the imaging space 61.

Next, the temperature determination unit 50 will be further described. The temperature determination unit 50 is a circuit for monitoring the surface temperature of the bore wall 70 during imaging. The temperature determination unit 50 includes a thermistor 51 that is a contact type temperature sensor, and a twisted pair cable 57 connected to the thermistor 51.

The thermistor 51 may be any one as long as the resistance changes depending on the temperature. For example, an NTC thermistor can be used.

The thermistor 51 is installed on the surface of the bore wall 70 or in the vicinity of the high frequency coil 58.

The twisted pair cable 57 that reduces normal mode noise is connected to the electrode of the thermistor 51. In the twisted pair cable 57, RF chokes are inserted as a high frequency filter 52 for reducing common mode noise at intervals of 1/8 or less of the high frequency wavelength irradiated by the high frequency coil 58.

The other end of the twisted pair cable 57 is connected to the detection circuit 59. The detection circuit 59 is disposed at least outside the high-frequency coil 58. Preferably, it is disposed outside the space (bore) in which the subject 1 surrounded by the bore wall 70 is disposed. The output of the detection circuit 59 is input to the sequencer 4. If the detection circuit 59 determines that the surface temperature of the bore wall 70 has exceeded the safe temperature (abnormal state), the sequencer 4 interrupts imaging and the central processing unit (CPU) 8 displays an error message on the display 20 .

The detection circuit 59 includes a high frequency filter (hereinafter referred to as an RF choke) 52, a fixed resistor 55, a power supply 49 that applies a power supply voltage to the thermistor 51 via the fixed resistor 55, and a Schmitt trigger circuit that is a determination circuit 53. And a bypass capacitor 56. The fixed resistor 55 is connected in series with the thermistor 51 via one of the RF choke 52 and the twisted pair cable 57. The other end of the twisted pair cable 57 is connected to the ground.

The determination circuit (hereinafter referred to as a Schmitt trigger circuit) 53 includes a comparator 53a and a positive feedback circuit, and a variable resistor 53b is inserted in series in the positive feedback circuit. Of the twisted pair cable 57, the cable to which the fixed resistor 55 is connected is connected to the negative terminal of the comparator 53a, and the voltage signal output from the thermistor 51 is input. The voltage signal of the power supply 49 is input to the positive terminal of the comparator 53a via the variable resistor 54. The output terminal (determination result) of the comparator 53a is input to the sequencer 4.

With such a configuration of the detection circuit 59, the voltage signal of the thermistor 51 as shown in FIG. 4 is input to the negative terminal of the comparator 53a of the Schmitt trigger circuit 53. The voltage signal of the thermistor 51 is superimposed with high frequency noise having a frequency similar to that of the high frequency magnetic field generated by the high frequency coil 58 during imaging. In the case of an NTC thermistor, the voltage signal of the thermistor 51 decreases as the temperature increases, as shown in FIG.

The Schmitt trigger circuit 53 has a hysteresis characteristic as shown in FIG. 3, and indicates that the thermistor 51 is normal when the temperature of the thermistor 51 is lower than the first temperature and the output voltage is about 3 V or more. Outputs high voltage (5V).

When the temperature of the thermistor 51 rises and the output voltage decreases and reaches the first voltage corresponding to the first temperature (about 3V in this case), the output of the Schmitt trigger circuit 53 decreases rapidly and the first voltage If it becomes smaller, a low voltage (1V in this case) indicating an abnormality (error) is output from the output terminal.

When the temperature of the thermistor 51 decreases, the output voltage increases, and when the second voltage corresponding to the second temperature (about 4 V in this case) is reached, the output of the Schmitt trigger circuit 53 increases rapidly and the second voltage If it becomes larger, a high voltage (in this case, 5 V) indicating normal is output.

The resistance value of the fixed resistor 55 determines the temperature-to-voltage conversion sensitivity (dV / dT) of the thermistor 51.

The resistance value of the variable resistor 54 is a resistance value for setting the first voltage. By adjusting the resistance of the variable resistor 54, the first temperature at which the temperature rises and is determined to be in an abnormal state can be adjusted. . On the other hand, the variable resistor 53b is a resistance value that sets the second voltage, and by adjusting the variable resistor 53b, it is possible to adjust the second temperature at which it is determined that the temperature has fallen and returned to the normal state.

<First Embodiment>
The first embodiment will be specifically described below. As shown in FIG. 1, the thermistor 51 is installed on the surface of the bore wall 70 on the imaging space 61 side. The installation position in the in-plane direction is bored relative to the position where the RF power is supplied to the high-frequency magnetic field coil 58 (hereinafter referred to as the BC coil 58), that is, when the RF power is supplied, that is, the position of the capacitor or diode of the BC coil 58 A position facing the wall 70 is desirable.

The position of the detection circuit 59 is preferably as the high frequency magnetic field received decreases as the distance from the BC coil 58 decreases.However, the wiring resistance of the thermistor 51 decreases the temperature responsiveness (dV / dT), so it is in the vicinity of the static magnetic field generation unit 2. It is desirable to arrange. For example, as shown in FIG. 1, the detection circuit 59 can be disposed in a rack 60 disposed in the vicinity of the opening on the back surface of the gantry 200 (the side opposite to the bed 90). As described above, the thermistor 51 and the detection circuit 59 are connected by a twisted pair cable 57, and the twisted pair cable 57 has an interval of 1/8 or less of the wavelength of the high-frequency magnetic field at least in the region where the BC coil 58 is disposed. Insert the RF choke 52 with.

The circuit configuration of the detection circuit 59 is as already described with reference to FIG. By using the resistor 54 that determines the determination level of the comparator 53a as a variable resistor, it is possible to adjust the individual difference of the thermistor, the distance between the thermistor and the heat source, and the deviation of the operating temperature due to the difference in thermal contact. Therefore, the variable resistor 54 is adjusted so that it can be determined that the temperature of the thermistor 51 has reached the limit temperature (here, 41 ° C.).

For example, the resistance 54 may be adjusted while measuring the temperature in the vicinity of the thermistor 51 using an optical fiber thermometer or the like as an adjustment jig. Thus, since it can be accurately determined that the surface of the bore wall 70 has reached the limit temperature (for example, 41 ° C.), it is possible to continue imaging until the limit temperature is reached. Moreover, since the intensity of the high-frequency magnetic field pulse can be increased to the limit of the limit temperature that does not reach the limit temperature, or the frequency of application can be increased, it is easy to execute a high-speed and high-function sequence.

When the resistance value of the variable resistor 54 is changed by adjustment, the feedback amount of the comparator 53a is changed and the hysteresis characteristic is changed. However, by adjusting the feedback amount by the variable resistor 53b, the original hysteresis characteristic is restored. Can do.

<Second Embodiment>
Next, a second embodiment of the present invention will be described. In the second embodiment, the thermistor 51 is installed on the outer peripheral surface of the BC coil 58 as shown in FIGS. The installation position is desirably in the vicinity of the capacitor 83 or the diode 84 of the BC coil 58 that becomes high temperature when the BC coil 58 generates a high-frequency magnetic field pulse. In the example of FIG. 5, the thermistors 51 are arranged at two locations on the BC coil 58. Further, a spacer 86 is arranged so that the cable 57 does not directly contact the conductor of the BC coil 58. The bore wall 70 also serves as the cylindrical body 85 of the BC coil 58. Other aspects are the same as those of the first embodiment.

The merit of the configuration of the second embodiment is that the thermistor 51 and the cable 57 are not arranged on the surface of the bore wall 70 on the subject 1 side, so that the surface of the bore wall 70 is smooth when viewed from the subject 1, and the subject There is no fear that 1 contacts the thermistor 51 and it is safe. It is also excellent in design.

Further, since the thermistor 51 monitors the temperature of the BC coil 58 having a temperature higher than the limit temperature (41 ° C.) of the surface of the bore wall 70, a limit temperature (for example, 60 ° C.) higher than the limit temperature (41 ° C.) of the bore wall. ), The variable resistor 54 is adjusted so as to determine that it is abnormal (error) (FIG. 4). The relationship between the limit temperature of the bore wall 70 (41 ° C.) and the temperature of the BC coil 58 can be obtained in advance by measuring the temperature of the BC coil 58 while measuring the temperature of the bore surface with an optical fiber thermometer or the like. Is possible. Further, the variable resistor 54 of the detection circuit 59 may be adjusted so that an abnormality (error) occurs when the temperature of the bore wall 70 reaches the limit temperature (41 ° C.).

<Comparative example>
Here, as another comparative example, consider using another contact type temperature sensor instead of the thermistor.

A thermocouple measures the electromotive force generated by the Seebeck effect using two types of metal conductors. The magnitude of the electromotive force is very small. For example, when the temperature of the non-measurement object is 100 ° C. or less, it is about several μ to several tens μV. On the other hand, the high-frequency noise on the probe wire mounted on the BC coil due to the high-frequency electromagnetic field generated from the BC coil being imaged is limited to several mV to several tens of mV even if it is reduced using a twisted pair and RF choke. It is. Therefore, it is difficult to accurately perform A / D conversion or normal / error binary determination on the voltage output from the thermocouple.

A platinum resistance thermometer measures the change in electrical resistance of a metal with respect to temperature changes. A static magnetic field can be used in a magnetic field, but there is still a problem of high frequency noise in a high frequency magnetic field. However, the platinum resistance thermometer has a feature that it is easily affected by the lead wire resistance of the probe, so it is difficult to put a multistage RF choke in terms of accuracy. There is also a problem that the price is higher than that of a thermocouple or thermistor.

The optical fiber thermometer is noise-free and can measure temperature in real time, but there is a problem that it is expensive and the probe is easily broken. Application to mass-produced products is particularly problematic in terms of cost.

In the embodiment of the present invention described above, the interval at which the RF choke 52 is inserted into the twisted pair cable 57 is preferably 1/8 or less of the wavelength of the high-frequency magnetic field, but if the noise resistance of the detection circuit 59 is high, it is longer than 1/8 wavelength. It may be long. However, in that case, it is necessary to consider that the wiring path of the twisted pair cable 57 is not too close to other cables and the ground conductor.

In the above-described embodiment, the temperature of the bore wall 70 or the high-frequency magnetic field coil 58 is determined by the thermistor 51. However, the present invention is not limited to this, and the temperature of the gradient magnetic field coil 59 is not more than a predetermined temperature. It is also possible to determine whether or not there is. The thermistor 51 is disposed so as to be in thermal contact with the gradient magnetic field coil 59 or disposed in the vicinity of the gradient magnetic field coil 59.

1 subject, 2 static magnetic field generation unit, 3 gradient magnetic field generation unit, 4 sequencer, 5 transmission unit, 6 reception unit, 7 signal processing unit, 8 central processing unit (CPU), 9 gradient magnetic field coil, 10 gradient magnetic field power supply, 11 High-frequency transmitter, 12 modulator, 13 high-frequency amplifier, 14b high-frequency coil (receiving high-frequency coil), 15 signal amplifier, 16 quadrature phase detector, 17 A / D converter, 18 optical disk, 19 magnetic disk, 20 display, 21 input device, 51 thermistor, 52 high frequency filter (RF choke), 53 judgment unit (Schmitt trigger circuit), 53a comparator, 53b variable resistor, 54 variable resistor, 55 fixed resistor, 56 bypass capacitor, 57 cable (twisted pair cable) , 58 High-frequency magnetic field generator (high-frequency coil), 59 detection circuit, 60 racks

Claims

A static magnetic field generation unit that applies a static magnetic field to the imaging space, a high-frequency magnetic field generation unit that is disposed on the imaging space side of the static magnetic field generation unit, and is disposed between the high-frequency magnetic field generation unit and the imaging space, A bore wall that forms a space in which the subject is disposed, and a temperature determination unit that determines whether the temperature of the bore wall exceeds a predetermined temperature;
The temperature determination unit includes a thermistor and a detection circuit that is connected to the thermistor and obtains a voltage signal corresponding to an electric resistance value of the thermistor and determines the voltage signal.
The detection circuit includes a determination circuit having a hysteresis characteristic, and the determination circuit increases a temperature of the bore wall, and the voltage signal reaches a voltage value higher than a voltage value corresponding to the first temperature. If it is, it is determined that the abnormal state exceeds the predetermined temperature, the temperature of the bore wall is then lowered, the voltage on the lower temperature side than the voltage value corresponding to the second temperature lower than the first temperature A magnetic resonance imaging apparatus characterized in that, when a value is reached, it is determined that the normal state is below the predetermined temperature.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the determination circuit is a Schmitt trigger circuit.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the detection circuit changes at least one of a voltage value corresponding to the first temperature and a voltage value corresponding to the second temperature. A magnetic resonance imaging apparatus comprising:
2. The magnetic resonance imaging apparatus according to claim 1, wherein the thermistor is disposed at a position in contact with the bore wall or the high-frequency magnetic field generation unit, and the detection circuit is disposed outside a space in which the subject is disposed. And
The thermistor and the detection circuit are connected by a cable made of a non-magnetic material, and at least a part of the cable is provided with a high-frequency filter at an interval of 1/8 or less of the wavelength of the high-frequency magnetic field generated by the high-frequency magnetic field generator. A magnetic resonance imaging apparatus.
5. The magnetic resonance imaging apparatus according to claim 4, wherein the thermistor is disposed on an outer peripheral surface of the high-frequency magnetic field generation unit.
5. The magnetic resonance imaging apparatus according to claim 4, wherein the high-frequency filter is an inductor or a resonance circuit of an inductor and a capacitor, and is formed of a nonmagnetic material.
5. The magnetic resonance imaging apparatus according to claim 4, wherein the high-frequency filter is arranged at the interval in the cable in a region where the high-frequency magnetic field generation unit is arranged.
5. The magnetic resonance imaging apparatus according to claim 4, wherein the cable is a twisted pair cable.
5. The magnetic resonance imaging apparatus according to claim 4, wherein the cable is a coaxial cable.
5. The magnetic resonance imaging apparatus according to claim 4, wherein the cable is disposed closer to the static magnetic field generation unit than the bore wall.
5. The magnetic resonance imaging apparatus according to claim 4, wherein the thermistor is attached to a surface of the bore wall on the imaging space side, and a cable connected to the thermistor passes through a through hole provided in the bore wall. The magnetic resonance imaging apparatus is drawn to the static magnetic field generation unit side of the bore wall.
5. The magnetic resonance imaging apparatus according to claim 4, wherein the high-frequency magnetic field generation unit includes a conductor and a cylindrical body that supports the conductor, and the bore wall also serves as the cylindrical body,
The magnetic resonance imaging apparatus according to claim 1, wherein the cable is disposed on an outer peripheral side of the high-frequency magnetic field generation unit.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the high-frequency magnetic field generation unit is a birdcage coil.
2. The magnetic resonance imaging apparatus according to claim 1, wherein the space surrounded by the bore wall has an elliptical cross section, and the thermistor is disposed on the bore wall in the major axis direction of the elliptical shape. A magnetic resonance imaging apparatus.
2. The magnetic resonance imaging apparatus according to claim 1, further comprising a control unit that controls generation of a high frequency of the high-frequency magnetic field generation unit and executes predetermined imaging, wherein the determination circuit includes the determination circuit in the abnormal state. If it is determined, the magnetic resonance imaging apparatus is characterized in that execution of the imaging is stopped.
A static magnetic field generation unit that applies a static magnetic field to the imaging space, a gradient magnetic field generation unit and a high-frequency magnetic field generation unit that are sequentially arranged on the imaging space side of the static magnetic field generation unit, the high-frequency magnetic field generation unit, and the imaging space A bore wall that forms a space in which the subject is placed,
A temperature determination unit that determines whether the temperature of the gradient magnetic field generation unit is equal to or lower than a predetermined temperature;
The temperature determination unit includes a thermistor and a detection circuit that is connected to the thermistor, acquires a resistance change of the thermistor as a voltage signal, and determines the voltage signal;
The detection circuit includes a determination circuit having a hysteresis characteristic, and the determination circuit exceeds the predetermined temperature when the voltage signal reaches a voltage value higher than a voltage value corresponding to a first temperature. And then, if a voltage value lower than the voltage value corresponding to the second temperature lower than the first temperature is reached, it is determined that the temperature is equal to or lower than the predetermined temperature. Magnetic resonance imaging device.