JPH049413B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an NMR imaging device (nuclear magnetic resonance image display device) that determines the position of a target atomic nucleus in a specimen by a nuclear magnetic resonance phenomenon, and further relates to Specifically, when a gradient magnetic field is applied to a static magnetic field, a magnetic field detection sensor is provided that compensates for the influence of the gradient magnetic field and outputs a signal corresponding only to the intensity of the static magnetic field, and the detection signal by the magnetic field detection sensor is used to The present invention relates to an NMR imaging apparatus that corrects fluctuations in the static magnetic field.

(Prior art) NMR imaging equipment uses a static magnetic field coil that creates a uniform static magnetic field H ₀ and a magnetic field that creates a magnetic field with linear gradients in each of the x, y, and z directions in the same direction as the static magnetic field H ₀ . A magnet section consisting of a magnetic field coil applies high-frequency pulses (high-frequency electromagnetic waves) to a subject placed within the magnetic field formed by the magnet section, and generates a high-frequency pulse (high-frequency electromagnetic wave) from the subject.
It has a transmitting/receiving section that detects an NMR signal, a control/image processing section that controls the operation of the transmitting/receiving section and the magnet section, processes detected data, and displays an image.

In such an NMR imaging device,
Generally, it is difficult to reduce the drift of a static magnetic field generating section (a magnet section including a static magnetic field coil) to zero, and it is known that resonance conditions deviate from predetermined conditions over time. When the resonance frequency shift becomes large, NMR excitation becomes impossible. Also, if the resonance frequency shift is small, NMR excitation is performed, but the resolution of the image decreases,
It is known that artifacts may appear, leading to a deterioration in image quality.

On the other hand, since the magnetic field generated by the magnet section is a composite magnetic field in which a static magnetic field and a gradient magnetic field are superimposed, it is not easy to control the strength of only the static magnetic field to a predetermined value.

In the past, efforts were made to solve these problems.
As an NMR imaging device, for example,
There is one disclosed in No. 60-111141.

The NMR imaging device includes a proton magnetometer that outputs an analog signal based on fluctuations in a static magnetic field, an A/D conversion circuit that converts the output signal from the proton magnetometer into a digital signal, and an output signal of the A/D conversion circuit. A latch circuit operates in response to a signal from the latch control circuit, a D/A conversion circuit converts the output signal of the latch circuit into an analog signal, and a static magnetic field is generated by the signal from the D/A conversion circuit. It has a magnetic field control circuit that operates the generator.

In the above configuration, the latch circuit holds the output of the A/D conversion circuit immediately before the gradient magnetic field is applied, in response to a control signal from the latch control circuit. Then, when a gradient magnetic field is applied, a held signal is output. Further, when no gradient magnetic field is applied, a signal from the A/D conversion circuit is output. Thereby, the magnetic field control circuit can perform a control operation using a signal that is not affected by the gradient magnetic field as a measured value, and the static magnetic field can be stabilized.

(Problem to be Solved by the Invention) However, in the conventional NMR imaging apparatus, the measurement signal of the control calculation when the gradient magnetic field is applied is past data, that is, the A/V immediately before the gradient magnetic field is applied. Since the output of the D converter is held and its value is used as the measurement signal, there is a limit to the ability to control the static magnetic field. Therefore, there was a problem that it was difficult to continuously obtain high quality images.

The present invention has been made in view of this point, and its purpose is to provide an NMR imaging apparatus that can continuously obtain high-quality images.

(Means for Solving the Problems) The NMR imaging apparatus of the present invention that achieves the above object compensates for the influence of the gradient magnetic field when a gradient magnetic field is applied to the static magnetic field, and responds only to the intensity of the static magnetic field. A magnetic field detection sensor that outputs a signal is provided, and a detection signal from the magnetic field detection sensor is used to correct fluctuations in the static magnetic field.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings.

1 to 3 are configuration diagrams showing one embodiment of the present invention, in which FIG. 1 is a configuration diagram of an NMR imaging apparatus, FIG. 2 is an illustration of the installation position of a magnetic field detection sensor, and FIG. The figure is a configuration diagram of the static magnetic field measurement/control section.

The magnet section of the NMR imaging apparatus includes a static magnetic field coil 2 that is energized by a static magnetic field coil drive section 1.
and a gradient magnetic field coil 4 energized by a gradient magnetic field coil drive section 3. The static magnetic field coil drive unit 1 is operated by a static magnetic field measurement/control unit 5 that inputs a magnetic field strength signal from a specific location (described later) within the magnet unit and performs predetermined processing (start, stop, etc. of the drive). follows the signal from controller 6). The gradient magnetic field coil 4 includes coils for each axis of x, y, and z, and the energization mode of each coil follows a control signal from a controller 6 that operates the gradient magnetic field coil drive unit 3. The magnetic field detection sensor 7 consists of a container 7A containing a substance whose resonance frequency depends on the magnetic field H ₀ , for example, heavy water D ₂ O, a sensor coil 7B wound around the container 7A, and a sensor coil 7B placed between the container 7A and the container 7A. It is composed of a z gradient compensation coil 7C consisting of two coils arranged. Z gradient compensation coil 7C
A current having the same waveform as the output of the gradient magnetic field coil driver 3 is applied to the amplifier 8 via the amplifier 8. and,
The z gradient compensation coil 7C moves the magnetic field detection sensor 7 to the position shown in FIG. 2 (4Z ₁ and 4Z ₂ in FIG. 2 are z-axis gradient magnetic field coils), that is, on the z-axis and close to the image area. When installed, it is configured to cancel the influence of the z-gradient magnetic field near the position of the sensor coil 7B.

Note that the gradient magnetic field in the NMR imaging apparatus is normally applied so that the origin is the center O of the image area, and the sign (direction) changes left and right or above and below, and the magnitude changes linearly.

The excitation coil 9 and the detection coil 10 are installed in the magnet part while maintaining the positions rotated by 90 degrees around the z-axis. The excitation coil 9 is energized by the output signal of the RF oscillator 13 obtained via the gate circuit 11, power amplifier 12, etc. under the control of the controller 6, and provides high-frequency electromagnetic waves to the subject (not shown). (The frequency of the high-frequency electromagnetic wave is a frequency corresponding to the NMR resonance condition of the atomic nucleus to be measured, for example, 42.6 MHz/T for protons). Further, the detection coil 10 detects an NMR signal from a desired part of the subject. The detected NMR signal is sent to the preamplifier 14 and the phase detector 1.
5 (the reference signal is the output signal of the RF oscillator 13), etc., and is input to the memory 16 and stored therein. Then, the image is read out to the image display section 17 as appropriate. The image display unit 17 is composed of a computer, a CRT, etc.
Data read from the memory 16 is used to perform predetermined processing (reconstruction processing) and display an image.

Next, the static magnetic field measurement/control section 5 will be explained with reference to FIG.

In the static magnetic field measurement/control unit 5, a capacitor
A resonant circuit 21 consisting of C ₁ , C ₂ , variable capacitance diodes D ₁ , D ₂ , variable capacitor C _V , coil 7B, etc., and a field effect transistor (FET) Q ₁ ,
An oscillation circuit 23 is constituted by a circuit 22 consisting of Q ₂ , feedback capacitor C ₀ and the like (the series resonant circuit of capacitor C _H and inductance L _H is a high frequency removal circuit for imaging). The frequency counter 24 and the error detection circuit 25 are connected to the oscillation circuit 23
The oscillation frequency of is counted via the RF amplifier 26,
A voltage corresponding to the difference between the counted value and the set value is applied to the variable capacitance diode D ₂ to maintain the oscillation frequency at a predetermined value. ,6.356M
Hz/T). The LF oscillator 27 has a number of 10 to
A low frequency signal of several KHz is output, and the signal is applied to the variable capacitance diode _D1 to frequency modulate the oscillation frequency of the oscillation circuit 23. The detection circuit 28 performs AM detection on the output of the RF amplifier 26, and uses the detected signal to
The oscillation width of the oscillation circuit 23 is made constant by operating the gate of FETQ ₂ . Baseline correction circuit 29
is connected to the drain side of FETQ ₂ and removes the AM component caused by the change in the oscillation frequency of the oscillation circuit 23. The phase detection circuit 30 uses the output signal of the LF oscillator 27 as a reference signal, and outputs a signal corresponding to the low frequency component of the oscillation circuit 23 obtained via the LF amplifier 31. The PI calculation circuit 32 converts the signal given from the memory 33 into a set value, and also converts the signal given from the memory 33 into a set value.
Control calculations (proportional/integral calculations) are performed using the signals from the as measured values, and the static magnetic field coil drive unit 1 is operated.

Note that the memory 33 stores the output signal value E ₀ of the phase detection circuit 30 when the oscillation frequency of the oscillation circuit 23 is a predetermined value.

Next, the operation of the above configuration will be explained.

The static magnetic field coil drive unit 1, the gradient magnetic field coil drive unit 3, and the gate circuit 10 are driven by operation signals from the controller 6, and each coil 2, 4, and 9 is energized in a predetermined sequence. That is, the static magnetic field coil 2 generates a static magnetic field H ₀ that is uniform in the z-axis direction.
is formed. Further, a linear gradient magnetic field in the z-axis direction is provided by the z-axis coil of the gradient magnetic field coil 4,
Superimposed on the static magnetic field H ₀ (the slice plane is determined). At this time, a current having the same waveform as the current that energizes the z-axis coil flows through the two z-gradient compensation coils 7C. In the vicinity of the sensor coil 7B, a magnetic field is formed that cancels the linear gradient magnetic field in the z-axis direction superimposed on the static magnetic field H ₀ . In this state, the gradient magnetic fields in the x-axis and y-axis directions are sequentially generated in a predetermined pattern (the pattern is determined in relation to the two-dimensional Fourier transform method) by the x-axis coil and y-axis coil of the gradient magnetic field coil 4. At the same time, a high frequency pulse based on the spin echo method is applied from the excitation coil 9 in synchronization with this sequence. At this time, since the magnetic field detection sensor 7 is installed at a position where the x gradient magnetic field and the y gradient magnetic field are zero, the influence of the x gradient magnetic field and the y gradient magnetic field does not appear on the sensor coil 7B. The excitation of the radio frequency pulse generates an NMR signal.
The NMR signal is detected by a detection coil 10, amplified by a preamplifier 14, and then amplified by a phase detector 15.
The signal from the RF oscillator 13 is detected as a reference signal and stored in the memory 16.

On the other hand, in the static magnetic field measurement/control unit 5, the oscillation circuit ₂₃ operates at a _frequency f=1/
It oscillates at 2π(L _c C _c ) ^1/2 (the oscillation frequency is set to 6.356 MHz/T by adjusting the variable capacitor C _V ). A voltage corresponding to the difference between the oscillation frequency and the set value is output from the error detection circuit 25 and applied to the variable capacitance diode _D2 . As a result, the oscillation frequency of the oscillation circuit 23 matches the set value.

By the way, the oscillation frequency of the oscillation circuit 23 is frequency-modulated by the output signal of the LF oscillator 27, and when the oscillation frequency matches the absorption frequency of the deuterium nucleus, the electromagnetic wave is absorbed by the deuterium nucleus. Therefore, the Q of the resonance circuit 21 of the oscillation circuit 23 changes. Therefore, FETQ ₁ of the oscillation circuit 23
A deuterium nuclear NMR phenomenon (change in Q of the resonant circuit 21) appears on the drain side of the resonant circuit 21. At this time, z
Since the z gradient magnetic field near the deuterium nucleus is canceled by the gradient compensation coil 7C,
The magnetic field related to the NMR phenomenon is generated by the static magnetic field coil 2.
(not influenced by the z-gradient field).

The phase detection circuit 30 uses the signal from the LF oscillator 27 as a reference signal, detects and outputs the signal on the drain side of FETQ ₁ . This output signal has the characteristics shown in Fig. 4 (the vertical axis in Fig. 4 is the phase detection circuit 3
0 output signal, the horizontal axis shows the static magnetic field strength). Fourth
The signal value E ₀ in the figure is the output signal of the phase detection circuit 30 when the oscillation frequency of the oscillation circuit 23 is matched to a predetermined value without applying a static magnetic field (state before starting the imaging operation). are stored in the memory 33 in advance and used as set values for control calculations.
It is given to the PI calculation circuit 32.

The PI calculation circuit 32 makes the signal from the phase detection circuit 30 match the set value E ₀ while operating the static magnetic field coil drive section 1 . As a result, the static magnetic field strength becomes a predetermined strength H _S (see FIG. 4).

As described above, the PI calculation circuit 32 controls the static magnetic field strength by continuously inputting a signal from a magnetic field unrelated to the gradient magnetic field (a magnetic field strength signal obtained by canceling the z gradient magnetic field). Therefore, the controllability of the static magnetic field can be improved, and high-quality images can be continuously obtained.

Note that the present invention provides a means (search coil) for detecting a gradient magnetic field near the magnetic field detection sensor, and uses the output of the means to energize a gradient magnetic field compensation coil to cancel the influence of the gradient magnetic field near the magnetic field detection sensor. (If a search coil is installed at a position where x=y=0 and z≠0 and the induced voltage of this coil is integrated, a signal corresponding to the z gradient magnetic field strength can be obtained). Further, the cancellation by the gradient magnetic field compensation coil may be a y gradient, an x gradient, or a combination thereof.

Further, in the above embodiment, the static magnetic field measurement/control unit 5 operates the static magnetic field coil drive unit 1, but instead of this, the RF oscillator 13 may be operated and its oscillation frequency may be controlled. . Furthermore, a phase detector 15
The reference signal may be changed. Furthermore, the magnetic field detection signal (output signal of the phase detection circuit 30) in the static magnetic field measurement/control unit 5 is input to the computer of the image display unit 17, and during image composition processing, changes in data due to fluctuations in the static magnetic field, for example, The positional deviation of the projection may be corrected.

(Effects of the Invention) As explained above, according to the NMR imaging apparatus of the present invention, when a gradient magnetic field is applied to a static magnetic field, the influence of the gradient magnetic field is compensated for and a signal corresponding only to the intensity of the static magnetic field is generated. Since a magnetic field detection sensor for outputting the magnetic field is provided and a detection signal from the magnetic field detection sensor is used to correct fluctuations in the static magnetic field, high-quality images can be continuously obtained.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is an explanatory diagram of the installation position of the magnetic field detection sensor, Fig. 3 is a block diagram of the static magnetic field measurement/control section, and Fig. 4 is a block diagram showing the installation position of the magnetic field detection sensor. FIG. 2 is an explanatory diagram of the operation of the static magnetic field measurement/control unit. 7... Magnetic field detection sensor, 7A... Container, 7B...
...Coil, 7C...Gradient magnetic field compensation coil, 23...
...Oscillation circuit, 24...Frequency counter, 24...
Error detection circuit, 27...LF oscillator, 30...Phase detection circuit, 32...PI calculation circuit.

Claims (1)

[Claims] 1. A subject is placed in a static magnetic field uniformly formed in the z-axis direction of a space determined by each of the x, y, and z axes, and the subject is subjected to a predetermined sequence. Therefore, by applying x, y, and z gradient magnetic fields and high-frequency electromagnetic waves, a signal based on the nuclear magnetic resonance phenomenon is detected,
In an NMR imaging device that displays images, a magnetic field detection sensor is installed on the z-axis where each of the x and y gradient magnetic fields are zero and near the subject; ,z
A compensation coil is energized with a current having the same waveform as the current supplied to the gradient magnetic field coil, or a current is energized with a current corresponding to the output of a search coil that detects a gradient magnetic field in the z-axis direction near the magnetic field detection sensor. a compensation coil that cancels a linear gradient magnetic field in the z-axis direction that is superimposed on the static magnetic field in the vicinity of the magnetic field detection sensor, compensates for the influence of the z-gradient magnetic field, and outputs a signal corresponding only to the intensity of the static magnetic field. An NMR imaging apparatus comprising: a magnetic field detection sensor; and a control means for correcting fluctuations in the static magnetic field using a detection signal from the magnetic field detection sensor. 2. The NMR imaging apparatus according to claim 1, wherein the magnetic field detection sensor outputs a signal based on a nuclear magnetic resonance phenomenon. 3. The magnetic field detection sensor outputs a signal based on a nuclear magnetic resonance phenomenon of a nuclide different from the target nuclide of the subject.
NMR imaging equipment. 4. The NMR according to claim 1, wherein the control means controls the strength of the static magnetic field.
Imaging equipment. 5. The NMR imaging apparatus according to claim 1, wherein the control means controls the frequency of the high-frequency electromagnetic wave. 6. The control means according to claim 1, wherein the control means controls the frequency of the reference signal when detecting the signal based on the nuclear magnetic resonance phenomenon.
NMR imaging equipment. 7. The NMR imaging apparatus according to claim 1, wherein the control means corrects changes in data when displaying the image.