JPS6123953A - Nuclear magnetic resonance device - Google Patents

Abstract

PURPOSE:To correct the adverse influence of variation of a magnetic field with good responsibility by comparing the position of a peak in a spectrum signal obtained by Fourier-transforming a detected free induction attenuation signal with the position of a peak obtained by the last measurement, and varying the frequency of a high frequency for observation. CONSTITUTION:A transmitting and receiving coil 2 for observation and coils 3X, 3Y, 4X, and 4Y for magnetic field slope generation are arranged in a static magnetic field established by a magnet 1. A high-frequency signal from an oscillator 5 is supplied to the coil 2 through a gate 6 and a power amplifier 7 to irradiate an object of observation arranged internally. A free induction attenuation signal (FID signal) induced at the coil 2 is led out through a gate 8, amplifier 9, and demodulating circuit 10 and sent to a computer 12 through an A/D converter 11. The compuver 12 transforms the FID signal on Fourier basis and compares the position of the peak in the obtained spectrum signal with the position of the peak in the spectrum signal obtained by the last measurement to vary the oscillation frequency of the oscillator 5. Thus, the evil influence of magnetic field variation is corrected with good responsibility.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a nuclear magnetic resonance (NMR) apparatus that can eliminate the negative effects caused by fluctuations in a static magnetic field, and in particular to NMR imaging apparatus such as NMRCT. It is suitable for use in -f.

[Prior art I] High magnetic field stability is required for the static magnetic field used in an NMR apparatus, and a magnetic field stabilizing means is provided.As this magnetic field stabilizing means, a The so-called NMR locking technology is widely used, in which nuclear magnetic resonance is caused in a reference sample for magnetic field stabilization to be placed, and fluctuations in the static magnetic field are detected by changes in the resonance signal, and the strength of the static magnetic field is controlled based on the detected signal. It is being

[Problems to be Solved by the Invention] Such conventional techniques have difficulty in response speed because the strength of the static magnetic field is controlled. In addition, in NMR systems that use superconducting magnets as a means of generating a static magnetic field, the static magnetic field strength deviates significantly over time and becomes static! There is also the problem that the i-field falls outside the range of variable intensity and is not stabilized.

[Means for Solving the Problems] The present invention has been made in view of this point, and it transmits an observation high frequency having the resonant frequency of the observation nucleus to an observation target in a static magnetic field in a pulsed manner through a transmitting/receiving coil. In a nuclear magnetic resonance apparatus in which a free induction attenuation signal generated in the transmitter/receiver coil is repeatedly irradiated and is repeatedly detected and stored in a storage means, means for Fourier transforming the detected free induction attenuation signal; a comparison means for comparing the position of the peak in the spectrum signal obtained by the previous measurement with the position of the peak in the spectrum signal obtained in the previous measurement, and changing the frequency of the observation high frequency based on the output signal from the comparison means. The invention is characterized in that it is equipped with a means for causing Hereinafter, the present invention will be explained in detail using the drawings.

[Embodiment] FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention, and in the figure, 1 is a magnet for generating a static magnetic field. Inside the static magnetic field is an observation transmitter/receiver coil 2. Magnetic field gradient generating coils 3X, 3Y, 4X, and 4Y are arranged.

Reference numeral 5 denotes an oscillator that generates a high frequency wave having the resonant frequency of the observation nucleus. The high frequency wave from the oscillator 5 is supplied as a high frequency pulse to the transmitting/receiving coil 2 via the gate 6 and the power amplifier 7. A high-frequency pulsed magnetic field is irradiated to the observation target placed inside. After the high-frequency pulse irradiation, the free induction damping signal (FID signal) induced in the transmitter/receiver coil 2 is sent to the gate 8. The signal is extracted through an amplifier 9 and a demodulation circuit 10, converted into a digital signal by an A-D converter 11, and sent to a computer 12.

The computer 12 stores the FID signal in a video memory 13.
or stored in the correction memory 14 and the oscillator 5
A correction signal is generated to vary the oscillation frequency of the oscillation frequency. 15 is a gradient generation circuit for supplying current to the magnetic field gradient generation coil; 16 is the gate 6.7. This is a timing control circuit for controlling the A-D converter 11 and the gradient generation circuit 15.

In the configuration as described above, the control circuit 16 has a gate 6.8.
is 0N-OF at the timing shown in Figure 2 (a) and (b).
F. Therefore, an observation pulse with a short pulse width is taken out at appropriate time intervals through the gate 6 and irradiated onto the observation target, and an F'ID signal is induced in the transmitting/receiving coil 2 after the observation pulse fish body 4. is gate 8. Δ− is taken out via the demodulation circuit 10 and operates simultaneously with the gate 8.
It is sent via converter 11 to computer 12 and stored in memory.

FIG. 2(C) shows the operating state of the gradient generating circuit, and when it is ON, F
A D signal FiDa is obtained, and the FID signal is sequentially stored in the memory 13. After the measurement, an image is created based on the large number of FID signals FIDa stored in the memory 13.

In the present invention, during such video measurements, magnetic field fluctuation detection measurements are performed, for example, once every 10 video measurements. That is, the timing control circuit 16
As shown in FIG. 2(C), the operation of the gradient generating circuit 15 is stopped once every 10 video measurements. Since there is no magnetic field gradient, the FID signal I Db obtained at this time has high intensity from a wide range of the measurement target, and the FID signal I Db is stored in the memory 1 by the computer 12.
4.

Then, the computer 12 immediately performs Fourier transform on the FIDb using the Fourier transform program it has, and obtains an NMR spectrum as shown in FIG. 3(a), for example.

The horizontal axis of the NMR spectrum obtained in this way (
It corresponds to the strength of static magnetic field or the frequency of high-frequency magnetic field,
When the strength of the static magnetic field changes for some reason, the entire spectrum shifts by an amount corresponding to the change, as shown in FIG. 3(b). However, by repeatedly measuring this spectrum and determining the amount of shift in the spectrum, it is possible to detect the amount of change in the static magnetic field strength, including the direction of the change.

By the way, the conditions for nuclear magnetic resonance are expressed as 2πf-γ['', where the static magnetic field strength is the frequency of the H1 high-frequency ta field and γ is the gyromagnetic ratio of the observation nucleus, and the static magnetic field strength and the high-frequency magnetic field are There is a one-to-one correspondence with the frequency. Therefore, if we can detect the variation Δ1 in magnetic field strength in this way, then 2π
Δf that makes (f+Δf)-γ(1''+ΔH) hold is Δ
It can be immediately obtained as f-γ△H/2π, and if the frequency of the high frequency for observation is corrected by that 8f,
The resonance conditions will always be kept constant.

Therefore, the computer 12 stores a predetermined conversion table from ΔH (actually, the 8th phase of the spectrum) to 8f, and the computer 12 stores a conversion table determined in advance from ΔH (actually, the 8th phase of the spectrum) to 8f, and the computer 12 is used to perform measurements for detecting magnetic field changes at appropriate intervals. Each time, the obtained N IV
Compare the IR spectrum with the spectrum obtained in the previous measurement, find the amount of shift of the spectrum, find the correction M8f corresponding to the amount of shift from the conversion table, and calculate the found 8f.
The signal specifying the
It is sent to the energizer 5 at the timing shown in FIG. 5(d).

Based on this, the oscillator 5 instantly changes the oscillation frequency by 8 f, so even if the magnetic field strength fluctuates,
The next video measurement will be performed using the corrected frequency radio frequency. If the Fourier transform takes time, the frequency of the oscillator 5 can be changed at the time when Δf is obtained, not just at the timing shown in FIG. 2(d) (please do not worry).

Note that the computer 12 may detect the amount of shift 1 in the spectrum based on the concept of pattern recognition, or it may focus on a specific peak specified in advance and calculate the amount of shift of that peak. However, various methods can be used.

In addition, [in the embodiment described above, the 14i lift fluctuation detection measurement was performed once every 10 video measurements, but if the fluctuation of the magnetic field strength is small, it may be performed once every 100 times. You can also choose the number of times you want to do it as appropriate.

[Effect] As detailed above, according to the present invention, fluctuations in the static magnetic field can be reduced to N
Measurements are made based on shift 1 of the MR spectrum, and the resonance conditions are kept constant by correcting the frequency of the observation high frequency based on the results, so the negative effects due to magnetic field fluctuations can be corrected with an extremely fast response speed. I can do it. Further, although the variable range of adjustment of the static magnetic field is extremely narrow, the frequency of frequency can be varied, so even if the static magnetic field fluctuates significantly, it is possible to perform correction over a wide range.

Furthermore, in this embodiment, since no magnetic field gradient is generated when performing measurements for detecting magnetic field fluctuations, it is possible to obtain an FID signal that is extremely strong compared to a case where a magnetic field gradient exists. Therefore, the obtained NMR spectrum becomes clear, and it is possible to detect the shift amount with high precision.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a timing diagram for explaining its operation, and FIG. 3 is a diagram for explaining a spectrum shift. 1: Magnet, 2: Transmitting/receiving coil, 3X, 3Y, 4X, 4Y: Magnetic field gradient generation coil, 5:
High frequency oscillator, 6, 8: Gate, 10: Demodulation circuit, 12: Computer, 13.14: Memory, 15: Gradient generation circuit, 16: Timing control circuit.

Claims (1)

[Claims] (1) A high-frequency observation wave having the resonance frequency of the observation nucleus is repeatedly irradiated in a pulsed manner to an observation target in a static magnetic field via a transmitter/receiver coil, and after irradiation, the free induction attenuation signal generated in the transmitter/receiver coil is repeatedly detected and stored. In the nuclear magnetic resonance apparatus, the means for Fourier-transforming the detected free induction decay signal and the means for Fourier-transforming the detected free induction decay signal, and the means for detecting the position of the peak in the spectral signal obtained by the Fourier transformation in the spectral signal obtained in the previous measurement. A nuclear magnetic resonance apparatus characterized by comprising a comparison means for comparing the position of a peak, and a means for changing the frequency of the observation high frequency wave based on an output signal from the comparison means.