JPS6263848A - Nmr imaging device - Google Patents

Abstract

PURPOSE:To correct the variation of a static magnetic field by outputting a signal which has compensated the influence caused by a gradient magnetic field at the time when the gradient magnetic field is applied to the static magnetic field, by a magnetic field detecting sensor. CONSTITUTION:By a static magnetic field coil 2, a uniform static magnetic field H0 is formed in the (z) axis direction, and also by a (z) axis coil of a gradient magnetic field coil 4, a linear gradient magnetic field in the (z) axis direction is applied, and superposed to the static magnetic field H0. In such a case, to two pieces of gradient compensating coil 7C, a current of the same waveform as a current for energizing the (z) axis coil flows. Also, in the vicinity of a sensor coil 7B, a magnetic field for cancelling the linear gradient magnetic field in the (z) axis direction, which is superposed to the static magnetic field H0 is formed. By (x) axis and (y) axis coils of the gradient magnetic field coil 4, the gradient magnetic field in the (x) axis and the (y) axis directions is applied successively by a prescribed pattern, and a high frequency pulse based on a spin echo method is applied from an exciting coil 9. In such a case, as for a magnetic field detecting sensor 7, since an (x) gradient magnetic field and a (y) magnetic field are installed at a position of zero, the influence of the (x) gradient magnetic field and the (y) gradient magnetic field does not appear in the sensor coil 7B.

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 based on nuclear magnetic resonance phenomena. More 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 NMR, which is used to correct fluctuations in the static magnetic field.
Relating to an imaging device.

(Conventional technology) The NMR imaging device uses a uniform static magnetic field i-+.

A magnetic part, a magnetic part, consisting of a gradient magnetic field coil that creates a magnetic field with a linear gradient in each of the x, y, and z directions. A high-frequency pulse (high-frequency electromagnetic wave) is applied to a subject placed in a magnetic field formed by the subject, and the operation of the transmitting/receiving unit, the erroneous transmitting/receiving unit, and the magnet unit that detects the NMR signal from the subject is controlled. , a control/image processing unit '8 for processing detected data and displaying an image is shown on the right.

In such an NMR imaging device, it is generally difficult to reduce the trigonometric value of the static magnetic field generating section (static magnetic field = magnet section including the coil) to zero, and the resonance condition changes over time from the predetermined condition. It has been known to wander off. When the resonance frequency shift becomes large, NMR excitation becomes impossible. Also, if the Jt ringing frequency shift is small,
Although NMR excitation is performed, it is known that the resolution of the image is reduced and artifacts appear, leading to a reduction in the quality of the image.

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 field to a predetermined value.

Conventionally, as an NMR image/image system that has been developed to solve these problems, for example,
There is one disclosed in No. 1141.

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 an f digital signal, and an A/D conversion circuit. A latch circuit that inputs an output signal and operates according to a signal from a latch control circuit, and a D/D converter that converts the output signal of the launch circuit into an analog signal.
It has an A conversion circuit and a magnetic field control circuit that operates a static magnetic field generator based on a signal from the D/A conversion circuit.

In the above configuration, the latch circuit holds the output of the Δ/D conversion circuit immediately before the gradient magnetic field is applied in response to the control signal from the latch control circuit. Then, when a gradient magnetic field is applied, a held signal is output. Historically, when no gradient magnetic field is applied, A/D
Outputs the conversion circuit signal. As a result, the magnetic field υ1 subcircuit 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 conventional NMR imaging equipment, when a gradient la field is applied, it... l il
The measured signal of 0 is past data, that is, the slope! The output of the A/D converter immediately before the application of the audible sound is held, and its (1
Since t1 is the same as the measurement signal, there is a limit to the ability to control the static magnetic field.1) Therefore, there is a problem in that it is difficult to continuously obtain high-quality images.

The present invention was made in view of the above points, and an object of the present invention is to provide an NMR imaging apparatus that can continuously generate high-quality images.

(Means for Solving the Problems) The NMR imaging apparatus of the present invention that achieves the L2 objective compensates for the influence of the gradient a field when a gradient 'v11' is applied to the static magnetic field. A magnetic field detection sensor that outputs a signal corresponding only to the intensity of the static magnetic field is provided, and the 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 explained in detail with reference to the drawings.

1 to 3 are configuration diagrams showing one embodiment of the present invention, FIG. 1 is a configuration diagram of an NMR imaging apparatus, FIG. 2 is a diagram explaining the setting δ position of a magnetic field detection sensor, and FIG. Figure 3 is
FIG. 3 is a configuration diagram of a static magnetic field measurement/control unit.

The magnet section of the NMR imaging apparatus is composed of a static magnetic field coil 2 energized by a static LA field coil driver 1 and a gradient magnetic field coil 4 energized by a gradient magnetic field coil driver 3 . The magnetostatic coil drive unit 1 is operated by a static magnetic field measurement/control unit 5 that manually receives a magnetic field strength signal from a specific location within the magnet unit (described later) and receives a predetermined signal (
The start and stop of driving is done by following the signal from 3. The gradient magnetic field coil 4 includes one coil each for x, y, and z, and each coil energization mode follows the control signal of the driver 6 that operates the gradient magnetic field coil drive unit 3. . The magnetic field detection sensor 7 has a ringing frequency of 11 when the magnetic field 1-
A container 7△ containing a substance that depends on o, for example, F water D20, a coil 7B wound around this container 7A, and two coils placed with the container 7A in between. Z gradient compensation] file 7C.

7 Gradient Compensation 2] The gradient magnetic field coil drive unit 3 is installed in the coil 7C.
A current having the same waveform as the output of is provided via amplifier 8. The second gradient compensation coil 7C is a magnetic field detection sensor.
7 is shown in Figure 2 (4 in Figure 2) and 4Z2 is Z@gradient magnetic field coil). It is designed to cancel the shadow change of the l gradient magnetic field near the position of 7B.

Normally, the gradient magnetic field in an NMR imaging device is applied so that the origin is at the center O of the image area, and the sign (orientation) changes to the left, right, or above F of the center, and the magnitude changes linearly. There is.

The excitation coil 9 and the detection coil 10 are installed in the magnet part while maintaining the positions rotated by 90' around the Z-axis. The excitation coil 9 is energized by an output signal of an RF oscillator 13 which is generated via a gate circuit 11, a power amplifier 12, etc. under the control of a controller 6, and emits high-frequency electromagnetic waves to a subject (not shown). (The frequency of the high-frequency electromagnetic wave is the frequency corresponding to the NMR resonance wave of the atomic nucleus to be measured, for example, 42.6MH2 for protons.
/ has become king). Further, the detection coil 10 detects an NMR signal from a desired part of the subject. The detected NMR signal is sent to a preamplifier 14 and a phase detector 15.
The reference signal is input to the memory 16 via the output signal of the RF oscillator 13 and stored therein. Then, the image is read out to the image display section 17 as appropriate. The image display section 17 is composed of a computer and a CRT'6, and is configured to perform predetermined processing (reconstruction processing) using data read from the memory 6 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,
2.→The resonant circuit 21 from the variable capacitance diode D4.02, the variable capacitor C, the coil 713, etc., and the field effect type l-transistor (FET
) Q, Q,,, feedback] An oscillation circuit 23 is constituted by a circuit 22 consisting of C8, etc.
CI-1 and the inductance L1- (the J-column resonant circuit is a high frequency removal circuit for imaging) 13, the frequency counter 24 and the error detection circuit 25 remove the excavated frequency of the path 23, and the RF amplifier 26 count through
A voltage corresponding to the difference between the counted value and the set value is applied to the variable capacitance diode D2, and the oscillation frequency is maintained at a predetermined value by '.
The oscillation frequency is determined by the magnetic field detection sensor 7
Using MR temperature - 16,356MHz/T
i,: set). The oscillator 27 has a frequency of several tens to several K.
A low frequency signal of H7 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 converts the output of the RF amplifier 26 into AM
The detected signal is used to operate the gates of FET Q2 to keep the oscillation width of the oscillation circuit 23 constant. The baseline correction circuit 29 is connected to the drain side of the FET Q2, and removes a ΔM acid component caused by a change in the oscillation frequency of the oscillation circuit 23. The phase detection circuit 30 uses the output signal of the first 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 amplifying crystal 31. p+
The calculation circuit 32 performs control calculations (proportional/integral calculation Fi) using the signal given from the memory 33 as a set value and the signal from the phase detection circuit 30 as a measurement value, and operates the static magnetic field coil drive section 1. do.

Note that the memory 33 stores the output signal value Eo 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.

Static magnetic field coil drive unit 1, gradient! The lift station-oil drive section 3 and gate circuit 10 are driven by operating signals from the controller 1-roller 6, and each coil 2.4 and 9 is energized in a predetermined sequence. Immediately, by the static magnetic field coil 2, Z
@ Uniform force direction static 1a lift] (.

is formed. Also, the Z-axis of the gradient magnetic field coil 4 - the linear gradient position in the Z-axis direction by the coil! A place is given and silence! 4!
The ossicles are cut into fields 1-16 (the slicing plane is determined).

At this time, a current having the same waveform as the current that energizes the biaxial coil flows through the two 7-gradient compensation coils 7C. and,
In the vicinity of the sensor coil 7B, the static magnetic field H61,
: Linear gradient in two axes that folds into Φ! A magnetic field is created that cancels the 4i field. In this state, the gradient la field]x@2]ile and y-axis coil of the gradient la field]
The gradient magnetic field in the axial direction is sequentially formed into a predetermined pattern (the pattern is determined in conjunction with the two-dimensional Fourier transform method)'C
: is given, and a high frequency pulse based on the spin echo method is given from the excitation coil 9 in synchronization with this sequence. At this time, the magnetic field detection sensor 7
Since the X gradient magnetic field JΩ and the yll field are installed at the zero position, the sensor coil 7B has the X gradient magnetic field and the y gradient la
There are no field effects. Due to the excitation of the high frequency pulse, N
An MR signal is generated.

The NMR signal is detected by the detection coil 10, amplified by the preamplifier 14, and then amplified by the RF oscillator 13 by the phase detector 15.
The signal is detected as a reference signal and stored in the memory 16.

On the other hand, in the static magnetic field measurement/control section 5, the inter-oscillation circuit 23
is the frequency f = 1/2π (L C) determined by the inductance LC of the coil 7B and the combined capacitance Cc of the resonance resonator 21.
(The oscillation frequency C is adjusted to 6°35 by adjusting the variable capacitor Cv.
6Ml-1/T). Error detection circuit 25
A voltage corresponding to the difference between the oscillation frequency and the set value is outputted from 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 the same as that of the LF oscillator 27.
When the oscillation frequency matches the absorption frequency of the deuterium nucleus, the mBfl wave is absorbed by the deuterium nucleus. For this reason,
The tt open circuit 21Q of the oscillation circuit 23 changes. Therefore, on the drain side of FETQ of the oscillation circuit 23,
NMR behavior of deuterium nucleus (change in Q of resonant circuit 21)
appears. At this time, since the two-gradient magnetic field near the deuterium nucleus is canceled by the two-gradient compensation coil 7C, the magnetic field related to the NMR phenomenon is determined only by the static magnetic field coil 2 (not affected by the two-gradient magnetic field). ).

The phase detection circuit 30 uses the signal from the l-1 oscillator 27 as a reference signal, detects and outputs the drain side signal of the FET Ql. This output signal has the characteristics shown in FIG. 4 (one axis in FIG. 4 shows the output signal of the phase detection circuit 30, and the horizontal axis shows the static magnetic field strength).

Signal value E in FIG. The oscillation circuit 2 is in a state where no static magnetic field is applied (state 1 before starting the imaging operation)
This signal is an output signal of the phase detection circuit 30 when the oscillation frequency of 3 is made to match a predetermined value, is stored in the memory 33 in advance, and is given to the PI calculation circuit 32 as a set value of the control equation.

The PI calculation circuit 32 converts the signal from the phase detection circuit 30 to a set value E while operating the static magnetic field coil drive unit 1. match. As a result, the static li field intensity becomes a predetermined intensity) - (
, (see Figure 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 two gradient magnetic fields). Therefore, the υ controllability of the static magnetic field can be improved, and high-quality images can be continuously obtained.

Note that the present invention is not limited to the above-mentioned embodiments, and instead of the gradient compensation coil, the sensor coil may be housed and installed in a box made of a low resistance member, such as copper, as shown in FIG. .

In such a configuration, when a gradient magnetic field is applied, the components of the gradient magnetic field are canceled by eddy currents generated on the side walls of the box. Therefore, the magnetic field strength at the position of the sensor coil can be kept constant. This method is effective when two gradient magnetic fields are not applied for a long time.

Further, 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 eliminate the influence of the gradient magnetic field near the magnetic field detection sensor. 1-Toshicell may be used (
If we install a probe coil at the position where x=y=o and Z≠0 and integrate the induced voltage of this coil, we get two gradient magnetic fields? J! 4 degrees), and the cancellation by the gradient magnetic field compensation coil may be the y-gradient, the x-gradient, or a combination thereof.

Furthermore, 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 oscillation frequency of A may be controlled by operating the RF oscillator 13. Furthermore, the reference signal of the phase detector 15 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 reconstruction processing,
Changes in data due to fluctuations in the static magnetic field, for example, positional shifts in 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. A sensor is provided to detect the magnetic field to be output, and fluctuations in the static magnetic field are corrected using the detection signal from the magnetic field detection sensor, so high-quality images can be continuously obtained.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing one embodiment of the present invention, and FIG.
An explanatory diagram of the installation position of the magnetic field detection sensor, Fig. 3 is a configuration diagram of the static magnetic field measurement/control unit, Fig. 4 is an explanatory diagram of the operation of the static magnetic field measurement/control unit, and Fig. 5 is an illustration of the configuration of the static magnetic field measurement/control unit. FIG. 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... ...P ■ Arithmetic circuit.

Claims (10)

[Claims] (1) An NMR imaging device that installs a subject in a static magnetic field, applies a gradient magnetic field and high-frequency electromagnetic waves to the subject according to a predetermined sequence, detects signals based on nuclear magnetic resonance phenomena, and displays images. When the gradient magnetic field is applied, a magnetic field detection sensor 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 a magnetic field detection sensor that outputs a signal corresponding only to the intensity of the static magnetic field; An NMR imaging apparatus comprising: control means for correcting fluctuations. (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 NMR imaging apparatus according to claim 2, wherein 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. (4) The magnetic field detection sensor includes a compensation coil that is energized with the same current waveform as the current that forms the gradient magnetic field, and that cancels the influence of the gradient magnetic field in the vicinity of the magnetic field detection sensor. An NMR imaging apparatus according to claim 1. (5) means for the magnetic field detection sensor to detect a gradient magnetic field in the vicinity of the magnetic field detection sensor; and a compensation coil energized by the output signal of the means to cancel the influence of the gradient magnetic field in the vicinity of the magnetic field detection sensor. An NMR imaging apparatus according to claim 1, comprising: (6) The NMR imaging apparatus according to claim 1, wherein the magnetic field detection sensor is installed in a box made of a low-resistance member. (7) The NMR imaging apparatus according to claim 1, wherein the control means controls the intensity of the static magnetic field. (8) The NMR imaging apparatus according to claim 1, wherein the control means controls the frequency of the high-frequency electromagnetic wave. (9) The NMR imaging apparatus according to claim 1, wherein the control means controls the frequency of a reference signal when detecting a signal based on the nuclear magnetic resonance phenomenon. (10) Claim 1, characterized in that the control means corrects changes in data when displaying the image.
NMR imaging device.