JPH03218734A - Tuning method for receiving coil in nuclear magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To suppress the fluctuation of a resonance point even if an L value is varied, and to always measure the signal with high quality by executing the control so that a tuning voltage applied in order to take tuning is varied in accordance with a respiratory motion, with respect to a fluctuation of the resonance point which follows a fluctuation of the L value caused by a shape variation of a flexible type receiving coil. CONSTITUTION:A tuning voltage generating part is contained in a sequencer 12, and its tuning voltage generating part 1 generates a prescribed voltage as usual. A respiration sensor 3 generates a waveform corresponding to a motion executed by respiration of a body 6 to be examined. An obtained respiratory waveform 5 is amplified by an amplifier 4 and added to a voltage from the tuning voltage generating part 1 by an adder 2, and applied to a receiving coil 20b. As for a relation of an applied voltage of a variable capacity element of a tuning part in the receiving coil 20b and the capacity, the higher the voltage is, the smaller the capacity becomes. As a result, when an L value becomes large by inspiration and the voltage rises, the capacity decreases, and on the contrary, in the case of exhalation, the L value becomes small, the capacity increases, and a variation of a tuning point can be suppressed, comparing with the time when the tuning voltage is constant.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a nuclear magnetic resonance imaging apparatus (hereinafter referred to as MHI
(abbreviated as "device"), in particular, an MRI device with a function to always keep the tuning of the flexible high-frequency receiving coil in the best condition.
Regarding equipment.

[Conventional technology]

In the MHI device, atomic nuclei are irradiated with high-frequency pulses to excite them, and the high-frequency signals emitted from the resonant nuclei (this is called N
MR signal) is detected. This NMR signal is very weak, so in order to obtain a high quality signal, the resonant circuit that makes up the receiving coil has a high Q (quality factor).
is required. The resonance characteristics of the receiving coil vary depending on the subject because there is stray capacitance between the subject and the receiving coil. Therefore, in order to maintain resonance characteristics and achieve good sensitivity, it is necessary to adjust the capacitance of the receiver coil capacitor according to the object to be examined and tune it to the resonance point. This tuning method is described in detail in Japanese Patent Application No. 61-255664. In addition, the receiving coil used to image the abdomen had a fixed shape that matched the average human body size. There is an optimal shape of the coil that gives the best SZN ratio of the image depending on the physique of the subject, so a coil with a fixed shape is too large for a person with a small physique, and the SN ratio of the image obtained may be too large. There was a problem with deterioration. In contrast, patent application No. 15234/1983
By using a receiving coil made of a flexible material as shown in No. 1, the receiving coil can be closely attached to the subject and images with good S/N ratio can be obtained for people of any size. Now you can [Problem to be solved by the invention] However, the above-mentioned flexible receiving coil has the following problems:
The shape changes due to the movement of the abdomen due to the subject's breathing. The inductance (L value) of the receiving coil changes due to changes in the coil shape, and in general, the L value becomes larger during intake.
In the case of exhalation, the L value becomes smaller. Since the control voltage for tuning is constant, there is a problem that the tuning point moves due to changes in the L value due to changes in the shape of the coil, making it impossible to detect high-quality signals and causing image deterioration. The present invention has been made in order to solve the above-mentioned problems, and is a core system that can maintain the receiving coil in a state of constant tuning during signal measurement and obtain high-quality images with a high S/N ratio. The purpose is to provide a magnetic resonance imaging device. [Means for solving the problem] In order to achieve the above object, the capacitance of the variable capacitance element that performs tuning is changed in accordance with the L value of the receiving coil, which changes with the movement of the abdomen due to respiration of the subject. We decided to suppress the fluctuation of the resonance point. That is, when the L value increases, the resonance frequency decreases, so the capacitance is reduced, and conversely, when the L value decreases, the capacitance is increased. Since the capacitance is controlled by the tuning voltage applied to the variable capacitance element, this voltage can be controlled so that the fluctuation of the resonance point is small. [Function] According to the present invention, the tuning voltage applied to the flexible receiving coil is controlled so that the resonance point does not change with respect to the resonance point of the flexible receiving coil that changes in response to the breathing movement of the subject. , the receiving coil is in the best tuned state at any time during signal measurement. As a result, even if the coil shape changes, an optimal reception condition can be obtained, and a high-quality image with a high S/N ratio can be obtained.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail based on the accompanying drawings. FIG. 7 is a block diagram showing the overall configuration of the nuclear magnetic resonance imaging apparatus according to the present invention. This nuclear magnetic resonance imaging apparatus obtains a tomographic image of a subject 6 by using the nuclear magnetic resonance (NMR) phenomenon, and includes a static magnetic field generating magnet 10, a central processing unit (hereinafter referred to as CPU) 11, and a sequencer 12. It consists of a transmission system 13, a gradient magnetic field generation system 14, a reception system 15, and a signal processing system 16. The static magnetic field generating magnet 10 generates a strong and uniform static magnetic field around the subject 6 in a direction perpendicular to the body axis of the subject 6. A normal-conducting or superconducting magnetic field generating means is installed. The sequencer 12 operates under the control of the CPU I, and sends various commands necessary for data collection of tomographic images of the subject 6 to the transmission system 13, the gradient magnetic field generation system 14, and the transmission system 1.
5. The transmission system 13 includes a high-frequency oscillator 17, a modulator 18, a high-frequency amplifier 19, and a high-frequency coil 20a on the transmitting side. The amplitude modulated high frequency pulse is amplified by a high frequency amplifier 19 and then supplied to a high frequency coil 20a placed close to the subject 6, so that the subject 6 is irradiated with electromagnetic waves. It has become. The gradient magnetic field generation system 14 is composed of gradient magnetic field coils 21 wound in three axial directions of X, Y, and Z, and a gradient magnetic field power supply 22 for driving each coil.
By driving the gradient magnetic field power supply 22 of each coil in accordance with the command from 2 ↓, gradient magnetic fields GX, Gy, Gz in the three axis directions of x, y, and z are applied to the subject 6. ．． A slice plane for the subject 6 can be set by applying this gradient magnetic field. The receiving system 15 includes a high frequency coil 20b and an amplifier 23 on the receiving side.
It consists of a quadrature phase detector 24 and an A/D converter 25,
Electromagnetic waves (NMR signals) in response to the subject 6 due to the electromagnetic waves irradiated from the high frequency coil 20a of the transmission system are detected by the high frequency coil 20b placed close to the subject 6, and are detected by the amplifier 23 and the quadrature phase detector 24. The signals are input to the A/D converter 25 via the A/D converter 25 and converted into digital quantities, and further sampled by the quadrature phase detector 24 at the timing according to the command from the sequencer 12, resulting in two systems of collected data, and the signals are subjected to signal processing. It is now sent to system 16. This signal processing system 16 includes a CPU II, a recording device such as a magnetic disk 26 and a magnetic tape 27, and a CRT.
The CPU II performs processing such as Fourier transform, correction coefficient calculation, and image reconstruction, and displays the signal intensity distribution of an arbitrary cross section or the distribution obtained by performing appropriate calculations on multiple signals as an image. The image is displayed on the display 28.

The tuning voltage generating section according to the present invention is the sequencer 12 shown in FIG.
The details are shown in Figure 1. It consists of a conventional tuning voltage generator 1, an adder 2, a respiratory sensor 3, and an amplifier 4. The conventional tuning voltage generator 1 generates a constant voltage as before. The breathing sensor 3 generates a waveform corresponding to the movement of the subject 6 due to breathing. FIG. 2 is a schematic diagram of this waveform, and the L value of the receiving coil is larger during inspiration than during exhalation. The obtained respiratory waveform 5 is amplified by an amplifier 4, added to the voltage from the conventional tuning voltage generator 1 described above by an adder 2, and applied to the receiving coil 20b. Figure 3 shows an outline of the tuning voltage of the present invention, which is obtained by adding the voltage caused by breathing to the conventional tuning voltage. The relationship between the applied voltage and the capacitance of the variable capacitance element of the tuning section in the receiving coil 20b is shown in FIG. 4, as shown in FIG. 4, the higher the voltage, the smaller the capacitance. The change in tuning frequency due to the change in coil shape is approximately 60 kHz.
The change in L value is 0.1μH. The change in capacitance to compensate for this change was actually measured to be 80 pF. Since the capacitance change width of the variable capacitance element is about 120 pF, if tuning is performed at the midpoint of the coil shape change, it is possible to compensate for the change in the L value of the coil by the capacitance change of the capacitor. In other words, when the L value increases during inspiration and the voltage increases, the capacity decreases, and conversely during exhalation the L value decreases and the capacity increases, making it possible to suppress changes in the tuning point compared to when the tuning voltage is constant. can.

Furthermore, the amplifier 4 is capable of adjusting the amplification factor and offset of the respiratory waveform in order to compensate for changes in the L value of the receiving coil and to minimize fluctuations in the resonance point. The above-mentioned respiration sensor has several pairs of infrared transmitting/receiving units 30 and 31 arranged facing each other on the side of the gantry 40 corresponding to both sides of the subject, and receives signals based on the breathing movement of the subject. It is also possible to observe changes in the signal reaching the section 31, or to use a pair of infrared transmitting/receiving sections 30 and 31 installed on a gantry 40 above the subject as shown in FIG. It is also possible to observe changes in the time at which the signal sent from the transmitting section 30 reaches the receiving section 31, or to monitor the respiratory waveform of the subject using a respiratory sensor used in general respiratory gated imaging. It can be anything that can be measured. Note that in the tuning voltage waveform of the present invention shown in FIG.
The fact that is the same as when the subject 6 is exhaling is an example when the conventional tuning voltage 8 is determined by having the subject hold their breath while exhaling.

〔Effect of the invention〕

As described above, the present invention changes the tuning voltage applied for tuning from the conventional constant voltage to the fluctuation of the resonance point due to the fluctuation of the L value due to the change in the shape of the flexible receiving coil. , because it was controlled to change according to respiratory movements, it was possible to suppress fluctuations in the resonance point even if the L value changed,
This has the effect of always being able to perform high-quality signal measurements and improving image quality.

[Brief explanation of drawings]

Fig. 1 is an example diagram of the tuning voltage generation section of the present invention, Fig. 2 is a schematic diagram of a breathing waveform of a subject, Fig. 3 is a schematic diagram of a tuning voltage waveform of the present invention, and Fig. 4 is a diagram of a tuning voltage pair. FIGS. 5 and 6 are schematic diagrams showing the relationship between volumes, FIGS. 5 and 6 are examples of a method of measuring respiratory waveforms, and FIG. 7 is an example of the MRI apparatus of the present invention. 1... Conventional tuning voltage generator, 2... Adder,
3... Breathing sensor, 4... Amplifier, 5... Breathing waveform, 6... Subject, 7... Tuning voltage waveform, 8...
Conventional tuning voltage, 30... Transmitter (for respiratory waveform measurement)
, 31... Receiving part g l Z (2) Kui Tong #3 m Tea 4

Claims (1)

[Claims] 1. A high frequency transmission system having a static magnetic field generating magnet that generates a static magnetic field in the direction of the body axis of the subject or in a direction perpendicular thereto, and irradiating electromagnetic waves to the subject; A nuclear magnetic resonance imaging apparatus comprising a high frequency receiving coil for detecting emitted electromagnetic waves, a high frequency receiving system for amplifying and frequency converting the received signal, and a high frequency control system for controlling the high frequency transmitting system and the high frequency receiving system. A method for tuning a receiving coil in a nuclear magnetic resonance imaging apparatus, characterized in that a tuning voltage generating section of a high-frequency receiving coil of a high-frequency control system generates a control voltage that reduces changes in a tuning point of the receiving coil.