JPS612047A - Nuclear magnetic resonator device - Google Patents

Abstract

PURPOSE:To correct the variation of a magnetic field at a rapid response speed by measuring the variation of a static magnetic field and correcting high frequency for observation on the basis of the measured result so that a constant resonance condition is kept. CONSTITUTION:The high frequency generated in an oscillator 5 is supplied to a transmitting/receiving coil 2 as a high frequency pulse through a gate 6 and an amplifier 7 to irradiate an object to be observed as a high frequency pulse magnetic field through the coil 2. A resonance signal induced in the coil 2 is extracted through a gate 8 and an amplifier 9. Since the intensity of a static magnetic field corresponds to the frequency of the high frequency magnetic field at the rate of 1 to 1 in the nuclear magnetic resonance condition, the variation of the frequency of the high frequency magnetic field is found out by detecting the variation of the magnetic field intensity and the resonance condition can be kept always constant by correcting the frequency variation of the observing high frequency. Therefore, a conversion table from the variation of the magnetic field intensity to that of the frequency is previously stored in a correcting circuit 15, and at the variation of the magnetic field intensity, frequency variation corresponding to said variation is fed back to the oscillator 5. Consequently, the oscillation frequency is immediately corrected and the variation of the magnetic field is corrected.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an NMR device that can eliminate the adverse effects of static magnetic field fluctuations, and is particularly suitable for use in an NMR imaging device using a superconducting magnet. It is.

[Prior Art] A static magnetic field used in an NMR apparatus is required to have high magnetic field stability, and a magnetic field stabilizing means is usually provided.

This magnetic field stabilization means causes a reference sample for magnetic field stabilization placed in a static magnetic field to cause nuclear magnetic resonance, detects fluctuations in the static magnetic field based on changes in the resonance signal, and uses the detection signal to detect static magnetic resonance. The so-called NMR locking technique, which controls the magnetic field strength, is widely used.

[Problems to be Solved by the Invention] In such conventional techniques, since the strength of the static magnetic field is controlled, there is a problem in response speed. In addition, NMR devices that use superconducting magnets as a means of generating a static magnetic field have the problem that the static magnetic field strength deviates significantly over time, leaving the range in which the static magnetic field can be corrected, and is not stabilized. be.

[Means for Solving the Problems] The present invention has been made in view of this point, and involves repeatedly irradiating an observation target in a static magnetic field in a pulsed manner with an observation high frequency wave having the resonance frequency of the observation nucleus. In a nuclear magnetic resonance apparatus configured to repeatedly detect a free induction decay signal generated after irradiation, a reference sample containing the observation nucleus is placed in a static magnetic field near the observation object, and a reference sample is placed close to the reference sample. a reference sample transmitting coil; a means for supplying the observation high frequency wave to the reference sample transmitting coil until the high frequency pulse of the free induction attenuation signal detection method is irradiated to the observation target; a reference sample receiving coil for receiving a resonance signal from the reference sample receiving coil, a means for detecting a variation in static magnetic field strength based on the resonance signal obtained from the reference sample receiving coil, and a detection obtained from the detecting means. The above problem is solved by providing means for changing the frequency of the observation high frequency wave based on the signal.

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. In the figure, 1 is a superconducting magnet for generating a static magnetic field. Inside the static magnetic field is an observation transmitter/receiver coil 2. Reference sample enclosed in a small sample tube 3. A correction transmitting/receiving coil 4 wound around the reference sample is arranged.

Reference numeral 5 denotes an oscillator that generates a high frequency wave having a 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 a gate 6 and an observation power amplifier 7, and is then transmitted through the transmitting/receiving coil. A high-frequency pulsed magnetic field is irradiated onto the observation target. After the high-frequency pulse irradiation, the resonance signal induced in the transmitting/receiving coil 2 is extracted through a gate 8 and sent through an amplifier 9 to a receiving circuit (not shown).

Furthermore, the high frequency generated from the oscillator 5 is transmitted to the gate 10 during the period from the end of one observation to the start of the next observation.
The light is supplied to the transmitter/receiver coil 4 via the power amplifier 11, and is irradiated onto the reference sample 3 via the transmitter/receiver coil 4. The resonance signal induced in the transmitter/receiver coil 4 due to the high frequency irradiation is transmitted to the gate 12 . Amplifier 13. It is extracted as a dispersed waveform signal via the demodulation circuit 14 and sent to the correction circuit 15. The correction circuit 15 generates a frequency offset signal based on the resonance signal and sends it to the oscillator 5. 16 is a control circuit that controls ON-OFF of the gates 6.8.10.12.

In the above configuration, the control circuit 16 turns the gate 6 ON-OFF at the timing shown in FIG. 2(a).
Observation pulses with a short pulse width are taken out at intervals of, for example, about 2 seconds, and are irradiated onto the observation target. After the observation pulse is irradiated, the FID signal induced in the transmitting/receiving coil 2 is taken out via the gate 8 which is turned on at the timing shown in FIG. 2(b) and sent to the receiving circuit.

Then, during the period TL from when the gate 8 is turned OFF and the observation period TS ends to when the next observation starts, the fluctuation of the magnetic field strength is measured as follows.

That is, during the period TL, the gate 10 is turned OFF in a short cycle at the timing shown in FIG. 2(C), and on the other hand, the gate 12 is turned ON at the timing shown in FIG. 2(d).
-0'FF F is applied. Therefore, during the period when the gate 10 is ON, a high frequency pulse is irradiated to the reference sample 3, and the gate 1
2 is ON, a resonance signal induced in the transmitting/receiving coil 4 is extracted through the gate 12, and high frequency irradiation and detection of the resonance signal are performed by time sharing.

The resonance signal detected in a time-sharing manner in this way is a DC signal that indicates where the current resonance point is on the dispersion waveform shown in Figure 3, and it is determined that the strength of the static magnetic field is due to some reason. When the magnetic field strength fluctuates, the signal strength changes depending on the magnetic field strength, so it is possible to detect fluctuations in the magnetic field strength.

By the way, the conditions for nuclear magnetic resonance are expressed as 2πf-γH, where the static magnetic field strength is 1'', the frequency of the high-frequency magnetic field is f, and the gyromagnetic ratio of the observed nucleus is γ, which is the relationship between the static magnetic field strength and the high-frequency magnetic field. There is a one-to-one correspondence with the frequency. Therefore, if the fluctuation ΔH of the magnetic field strength can be detected in this way, 2π(f
Δf that establishes -γ (to H+8) is Δf=γ
It can be immediately obtained as ΔH/2π, and by correcting the frequency of the high frequency for observation by 8f, the resonance condition will always be maintained constant.

Therefore, the correction circuit 15 stores a predetermined conversion table from ΔH (actually resonance signal intensity) to Δf, and the correction circuit 15 stores the resonance signal at the last point detected in a time-sharing manner. Focusing on the intensity, the correction amount Δf corresponding to the resonance signal intensity is determined from the conversion table, and the signal specifying the determined Δf is used as the second signal immediately before the next observation.
The signal is sent to the oscillator 5 at the timing shown in FIG. Since the oscillator 5 immediately changes the oscillation frequency by 8 f based on this, even if the magnetic field strength fluctuates, the next observation will be performed using the corrected high frequency.

Incidentally, in the above-mentioned embodiment, the irradiation of the reference sample with high frequency and the detection of the resonance signal were repeated in a time-sharing manner, but this is not necessarily necessary; for example, as shown in FIG. 2(f),
The gates 10 and 12 may be turned ON-OFF at the timing shown in (Q). In this case, the reference sample is irradiated with the high-frequency pulse once, and immediately after that, the resonance signal is detected once. Since the resonance signal obtained at this time becomes a free induction attenuation signal, it is necessary to perform Fourier transformation in the correction circuit 15 to convert it into a frequency domain signal.

[Effects] As mentioned above, according to the present invention, fluctuations in the static magnetic field are measured using a reference sample during periods other than the observation period, and based on the results, the frequency of the observation high frequency is corrected to adjust the resonance condition. Since it is kept constant, it is possible to correct the adverse effects of magnetic field fluctuations with an extremely fast response speed.

Further, although the adjustable range of the static magnetic field is extremely narrow, the variable range of the frequency can be set wide, so even if the static magnetic field fluctuates significantly, it is possible to perform correction over a wide range.

[Brief explanation of the drawing]

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 dispersion waveforms. 1: Magnet, 2, 4: Transmitting/receiving coil, 3: Reference sample, 5: High frequency oscillator, 6.8, 10.12: Gate, 15: Correction circuit, 16:
control circuit.

Claims (2)

[Claims] (1) In a nuclear magnetic resonance apparatus that repeatedly irradiates an observation target in a static magnetic field in a pulsed manner with an observation radio frequency having a resonance frequency of the observation nucleus, and repeatedly detects a free induction decay signal generated after irradiation, the observation A reference sample including the observation nucleus placed in a static magnetic field near the target, a reference sample transmitting coil placed close to the reference sample, and the next high-frequency pulse sent to the observation target after the free induction decay signal is detected. means for supplying the observation high frequency wave to the reference sample transmitting coil until it is irradiated; a reference sample receiving coil for receiving a resonance signal from the reference sample; and the reference sample receiving coil. and means for changing the frequency of the observation high frequency wave based on the detection signal obtained from the detection means. Features of nuclear magnetic resonance equipment. (2) The nuclear magnetic resonance apparatus according to claim 1, wherein one coil serves as the reference sample transmitting coil and the receiving coil.