JPH0526148B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a pulse Fourier transform type nuclear magnetic resonance apparatus, and particularly to a nuclear magnetic resonance apparatus suitable for exciting and receiving resonance signals of various nuclides.

[Background of the invention]

As is well known, when the strength of the DC magnetic field of the nuclear magnetic resonance apparatus is determined, the frequency of the nuclear magnetic resonance signal (hereinafter simply referred to as resonance frequency) is determined depending on the nuclide; for example, a DC of 2.1 Tesla. In the magnetic field, as shown in Figure 1, hydrogen nuclei are 90MHz, fluorine nuclei are 84.6MHz, phosphorus nuclei are 36.4MHz, carbon nuclei are 22.6MHz,
The resonance frequency is determined depending on the nuclide, such as 13.8MHz for deuterium nuclei, 9.2MHz for nitrogen nuclei, and 5.6MHz for nitrogen nuclei. When the DC magnetic field changes, the resonance frequency changes linearly according to the relationship ω ₀ =γH ₀ (where ω ₀ is the resonance frequency, γ is the gyromagnetic ratio, and H ₀ is the strength of the DC magnetic field). As can be seen from Figure 1, the frequency band in which the resonance signal appears is roughly divided into two, 5 to 36 MHz and 84.6 to 90 MHz, and this band is extremely wide.

On the other hand, the configuration of a general pulsed Fourier transform type nuclear magnetic resonance apparatus is as shown in FIG. A pulse power amplifier 4 generates a signal at a resonant frequency, and the pulse power amplifier 4 applies this signal as a high-frequency pulse to a probe 5 placed in a uniform DC magnetic field (not shown).

The application of a high-frequency pulse causes a resonance phenomenon in the measurement sample loaded in the probe, and the resonance signal, which decays over time, is detected (received) by the detection coil installed in the probe 5 and amplified by the high-frequency amplifier 6. At the same time, the output signal of the first AC signal source 2 is frequency-converted by the frequency mixer 7 .

Further, this frequency-converted signal is amplified by an intermediate frequency amplifier 8, and this amplified signal is phase-detected by a phase detector 9 using the output signal of the second AC signal source 1 to extract a signal in the audible frequency range. is amplified by an audio frequency amplifier 10 to obtain a free induction decay signal. This free induction attenuation signal is then Fourier transformed by a data processing device (not shown) to obtain a frequency domain spectrum.

Generally, the amplitude of the excitation pulse applied to the probe 5 is about 50 to 100 volts, and the received resonance signal is on the order of microvolts, so even a small amount of high frequency leakage from the pulse power amplifier 4 will interfere with measurement. Therefore, the intermediate frequency amplifier 8 is used for amplification. Therefore, the resonant frequency and the frequency of the intermediate frequency amplifier need to be separated. When performing phase detection with the phase detector 9, for example, if the Quadrature Detection method (hereinafter abbreviated as QD method), which has many advantages, is adopted, phase detection is performed on two signals with a 90° phase shift, so the frequency of the intermediate frequency amplifier The lower the value, the easier it is to handle.

On the other hand, as already mentioned in Fig. 3, the frequency band of the resonant signal is broadly divided into A and B, and generally, the frequencies of the intermediate frequency amplifier are divided into the three frequencies of f ₀ , f ₁ , and f ₂ in the figure.
f ₁ and f ₂ are Quadrature Detection
Not suitable for method.

Also, the other undesired sideband component jumps into f ₀ and f ₁ , deteriorating the S/N ratio (in Figure 3, when the frequency of the intermediate frequency amplifier is selected as f ₂ , for the band A) can be made irrelevant by attenuating the A' band (unwanted band) with a filter, but if the frequency is
That's not the case with f ₀ and f ₁ . ) Therefore, the higher the frequency of the intermediate frequency amplifier, the better.

That is, it is known that the sideband components appear at (ω ₀ ±fi)n, where fi is the frequency of the intermediate frequency amplifier. However, ω ₀ is a resonance frequency, and n is a natural number. Therefore, if fi is f ₂ in Figure 3, then
Sideband components appear in bands higher than _f2 and lower than A, and do not appear in the measurement target bands A and B, so it is easy to remove these sideband components with a filter etc. be.

On the other hand, if fi is set to f ₀ or f ₁ , sideband components will appear in the regions of bands A and B and will be mixed with the resonant frequency ω ₀ to be measured, so it must be removed using a filter etc. Therefore, the S/N ratio deteriorates.

[Purpose of the invention]

The purpose of the present invention is to increase the frequency of the intermediate frequency amplifier.
It can be used at a low frequency suitable for phase detection using the QD method, and when measuring multiple nuclides over a wide frequency band.
It is an object of the present invention to provide a nuclear magnetic resonance apparatus that can prevent deterioration of the S/N ratio of a desired resonance signal by eliminating the influence of sideband components.

[Summary of the invention]

In order to achieve the above object, the present invention mixes the output signals of a first alternating current signal source and a second alternating current signal source with different frequencies to create a signal at the nuclear magnetic resonance frequency, pulse-amplifies the signal, and generates a direct current. A magnetic field is applied to a probe placed in the magnetic field, and the resonance signal is received by a detection coil and amplified by a high frequency amplifier.
This amplified signal is mixed with the output signal of the first AC signal source by a first mixer, and this is amplified by a first intermediate frequency amplifier whose tuning frequency is the frequency of the output signal of the second AC signal source. , Furthermore, phase detection is performed using the QD method using the output signal of the output signal source of the second AC signal source to obtain a free induction attenuation signal of an audio frequency, and this free induction attenuation signal is Fourier-transformed by a data processing device to obtain a spectrum. In the nuclear magnetic resonance apparatus, the frequency of the output signal of the second alternating current signal source is set to a low frequency suitable for phase detection by the QD method, and the first alternating current signal source is set to a low frequency suitable for phase detection using the QD method. The frequency of the output signal is formed to be adjustable over a wide range in accordance with the range, a bandpass filter is inserted on the output side of the high frequency amplifier, and the pass frequency band of the bandpass filter is set to correspond to the range of nuclear magnetic resonance frequencies of the multinuclides. The frequency band of the first AC signal source and the frequency band of the band filter are set to correspond to a plurality of divided bands, and the pass frequency band is switchable. It is characterized by providing a band control section that switches according to the nuclide.

In this way, the wide range of measurement frequency bands involved in the measurement of multiple nuclides is divided into multiple bands, the pass frequency bands of the bandpass filters are set in accordance with this, and the band control unit adjusts the resonant frequency of the nuclides to be measured. Correspondingly, the pass frequency band of the band filter is switched, and this band filter is provided on the output side of the high frequency amplifier (that is, on the input side of the first intermediate frequency amplifier). The probability that waveband components will jump into the intermediate frequency amplifier can be greatly reduced. In other words, the sideband components related to the resonant frequency of the measurement target appear symmetrically with respect to the tuning frequency of the intermediate frequency amplifier as the target axis, so by setting the pass frequency band of the bandpass filter to a narrow range, the sideband components Components can be removed.

As a result, the frequency of the intermediate frequency amplifier can be set to a low frequency suitable for phase detection using the QD method, and when measuring multiple nuclides over a wide frequency band, the influence of sideband components can be eliminated to obtain the desired S/ N
This can prevent deterioration of the ratio.

However, in the above invention, if the resonant frequency of the object to be measured is close to the tuning frequency of the first intermediate frequency amplifier, the tuning frequency will be included in the pass frequency band of the bandpass filter, so that the side waves of the resonant frequency of the object to be measured will be This means that the band component cannot be removed by a band filter.

Therefore, another invention of the present invention provides, in addition to the configuration of the above invention, a third AC signal source having a different frequency from the second AC signal source, and a third AC signal source and the first AC signal source. a second mixer that mixes the output signal of the AC signal source, a third mixer that mixes the output signal of the bandpass filter and the output of the third AC signal source, and a third mixer that mixes the output signal of the third AC signal source; a second intermediate frequency amplifier that amplifies the output of the third mixer using a tuning frequency; and a fourth mixer that mixes the output signal of the second intermediate frequency amplifier and the output signal of the second mixer. and a changeover switch for switching the output signal of the first mixer input to the first intermediate frequency amplifier to the output signal of the fourth mixer, and the band control section is provided with a switch that switches the resonant frequency of the measurement target to the output signal of the fourth mixer. Determine whether or not it belongs to the pass frequency band of the band filter that includes the tuning frequency of the first intermediate frequency amplifier, and when this determination is affirmative, output a command to switch the changeover switch to the fourth mixer side. The invention is characterized in that it has a means for doing so.

That is, the second intermediate frequency amplifier system is configured to amplify at a different tuning frequency, and the band of the sideband component is moved away from the resonant frequency of the measurement target.
This is intended to prevent deterioration of the S/N ratio.
In this case, the output of the second intermediate frequency amplifier deviates from the optimal frequency for phase detection using the QD method, so in another invention of the present invention, the output is further converted through the first intermediate frequency amplifier to obtain the optimal frequency.
The QD method is used for detection.

[Embodiments of the invention]

FIG. 4 shows the configuration of the main parts of the nuclear magnetic resonance apparatus according to the present invention. As shown in FIG. 5, in this example, the resonance frequency band is divided into five bands A ₁ to A ₄ and B.
Assume that the tuning frequency of the intermediate frequency amplifier is set to the band _A2 .

In Fig. 4, the output signals of a first AC signal source 2 (frequency F ₁ ) whose output signal frequency can be varied over a wide range and a second AC signal source (frequency F ₂ ) 1 with a fixed frequency are mixed by a frequency mixer 3. to generate a signal at the resonant frequency (F ₁ −F ₂ ), which is amplified by the pulse power amplifier 4 via the bandpass filter 12 and sent to the probe 5 placed in a uniform DC magnetic field (not shown). Apply. Then, a resonance phenomenon is caused in the sample loaded in the probe 5, and the resonance signal (F ₁ −
_F2 ) is amplified by the high frequency amplifier 6, and frequency-converted by the frequency mixer 7 via the bandpass filter 13 (the frequency of the frequency-converted signal is _F2 ). Furthermore, this frequency-converted signal is amplified by a first intermediate frequency amplifier (tuned frequency is F ₂ ), and this amplified signal is amplified by a second intermediate frequency amplifier (tuned frequency is F 2 ).
A phase detector 9 performs phase detection using the output signal of the AC signal source 1 to extract a signal in the audible frequency range.
This phase detection output is amplified by an audio amplifier 10 to obtain a free induction attenuation signal. In addition, each of the bandpass filters 12 and 13 has a passing frequency band of
The range of nuclear magnetic resonance frequencies of multiple nuclides is divided into multiple bands, as shown in Figure 5.
It is divided into a plurality of bands A ₁ to _{A 4} and B, and the pass frequency band is switchable.

Here, if the band of the resonance frequency (F ₁ -F ₂ ) and the tuning frequency of the first intermediate frequency amplifier 8 match, the resonance signal (F ₁ -F ₂ ) is amplified by the high frequency amplifier 6 and then passed through the band filter. Frequency mixer 1 through 13
6, the output signal of the third AC signal source 14 (frequency
F ₃ ) (the frequency of the frequency-converted signal is F ₁ −F ₂ +F ₃ ).

This frequency-converted signal is then amplified by a second intermediate frequency amplifier (tuned frequency is F ₁ −F ₂ +F ₃ ) 17,
Further, the frequency mixer 18 performs frequency conversion using a signal obtained by mixing the output signals of the first AC signal source 2 (frequency F ₁ ) and the third AC signal source (F ₃ ) (frequency is F ₁ +F ₃ ). . This frequency converted signal is then transferred to a changeover switch 19 controlled by the band control section 11.
The signal is input to the intermediate frequency amplifier 8 via the phase detector 9 and the audio frequency amplifier 10 to obtain a free induction attenuation signal as described above.

The band control unit 11 includes a band filter 1 corresponding to the resonance frequency obtained by determining the measurement nuclide of the sample.
The frequency bands 2 and 13 and the frequency of the first AC signal source 2 are set, and when the resonance frequency band and the tuning frequency of the first intermediate frequency amplifier 8 match, the changeover switch 19 is set to the frequency mixer 18 side. control to switch to

〔Effect of the invention〕

As explained above, according to the present invention, the following effects can be obtained.

First, a plurality of pass frequency bands of the band filter are set in the range of the resonance frequency of the measurement target, and the band control unit switches the pass frequency band of the band filter in accordance with the resonance frequency of the nuclide to be measured. Since it is provided on the input side of the first intermediate frequency amplifier, it is possible to significantly reduce the probability that sideband components related to the resonant frequency of the measurement object will jump into the intermediate frequency amplifier. As a result, the frequency of the intermediate frequency amplifier can be set to a low frequency suitable for phase detection using the QD method, and when measuring multiple nuclides over a wide frequency band, the influence of sideband components can be eliminated to obtain the desired S/ Deterioration of the N ratio can be prevented.

In addition, in the above, if the resonance frequency of the measurement target is close to the tuning frequency of the first intermediate frequency amplifier, the tuning frequency will be included in the pass frequency band of the bandpass filter, so the sideband of the resonance frequency of the measurement target will be Components may not be removed by a bandpass filter. In this case, as in other inventions of the present invention, a second intermediate frequency amplifier system is provided to amplify at a different tuning frequency, and the sideband component band is moved away from the resonant frequency of the measurement target. /N ratio can be prevented from deteriorating.

Furthermore, since the frequencies of signals handled by all AC signal sources, band filters, high frequency amplifiers, intermediate frequency amplifiers, etc. are approximately the same, the circuit configuration is simplified. In particular, it is possible to easily change the circuit configuration for measuring other nuclides without significantly changing the configuration of the conventional device.

[Brief explanation of drawings]

Figure 1 is a diagram showing the resonance frequencies of various atomic nuclei,
Figure 2 is a block diagram showing the configuration of the main parts of a conventional nuclear magnetic resonance apparatus, Figure 3 is a diagram showing the relationship between the resonance signal reception system band and the tuning frequency of the intermediate frequency amplifier, and Figure 4 is the main part of the present invention. FIG. 5 is a block diagram showing the configuration of a main part of an embodiment of the nuclear magnetic resonance apparatus according to the invention, and FIG. 5 is a diagram showing an example of band division of the resonance frequency applied to the invention. 1, 2, 14... AC signal source, 3, 7, 15,
16, 18... Frequency mixer, 4... Pulse power amplifier, 5... Probe, 6... High frequency amplifier,
8, 17... Intermediate frequency amplifier, 9... Phase detector, 10... Audio frequency amplifier, 11... Bandwidth control section, 12, 13... Bandwidth film, 19... Changeover switch.

Claims (1)

[Claims] 1. Mixing the output signals of a first AC signal source and a second AC signal source with different frequencies to create a signal at the nuclear magnetic resonance frequency, which is pulse amplified and placed in a DC magnetic field. The resonant signal is received by a detection coil and amplified by a high frequency amplifier, and this amplified signal is mixed with the output signal of the first AC signal source, and this is applied to the second AC signal source. The frequency of the output signal of the second AC signal source is amplified by an intermediate frequency amplifier with a tuning frequency, and the output signal of the output signal source of the second AC signal source is phase-detected by the QD method to obtain a free induction attenuation signal of an audio frequency. In a nuclear magnetic resonance apparatus in which a spectrum is obtained by Fourier transforming a free induction decay signal using a data processing device, the frequency of the output signal of the second AC signal source is set to a low frequency suitable for phase detection by the QD method, The first alternating current signal source is made to correspond to the range of nuclear magnetic resonance frequencies of multiple nuclides so that the frequency of the output signal can be adjusted over a wide range, and a bandpass filter is inserted on the output side of the high frequency amplifier, and the bandpass filter A plurality of pass frequency bands are set corresponding to a plurality of bands obtained by dividing the nuclear magnetic resonance frequency range of the multi-nuclide into a plurality of bands, and the pass frequency bands are configured to be switchable, and the first alternating current signal 1. A nuclear magnetic resonance apparatus, comprising: a band controller for switching a source frequency and a pass frequency band of the band filter in accordance with a nuclide to be measured. 2. Mix the output signals of the first AC signal source and the second AC signal source with different frequencies to create a signal at the nuclear magnetic resonance frequency, pulse amplify this signal, and apply it to a probe placed in a DC magnetic field. , the resonance signal is received by a detection coil and amplified by a high frequency amplifier, and this amplified signal is mixed with the output signal of the first AC signal source by a first mixer, and the output signal of this first mixer is The frequency of the output signal of the second AC signal source is amplified by a first intermediate frequency amplifier that has a tuning frequency, and the output signal of the output signal source of the second AC signal source is further phase-detected by the QD method to obtain an audible frequency. In a nuclear magnetic resonance apparatus in which a free induction attenuation signal of the QD is obtained and a spectrum is obtained by Fourier transforming the free induction attenuation signal in a data processing device, the frequency of the output signal of the second AC signal source is
Set to a low frequency suitable for phase detection using
The first alternating current signal source is made to correspond to the range of nuclear magnetic resonance frequencies of multiple nuclides so that the frequency of the output signal can be adjusted over a wide range, and a bandpass filter is inserted on the output side of the high frequency amplifier, and the bandpass filter A plurality of pass frequency bands are set corresponding to a plurality of bands obtained by subdividing the nuclear magnetic resonance frequency range of the multi-nuclide species, and the pass frequency bands are switchable, and the first AC signal A band control unit is provided for switching the frequency of the source and the pass frequency band of the bandpass filter in accordance with the nuclide to be measured, and further includes a third AC signal source having a frequency different from that of the second AC signal source, and the second AC signal source. a second mixer that mixes the output signal of the third AC signal source and the first AC signal source; and a third mixer that mixes the output signal of the bandpass filter and the output of the third AC signal source. , a second intermediate frequency amplifier that uses the frequency of the output signal of the third mixer as a tuning frequency and amplifies the output of the third mixer; and a second intermediate frequency amplifier that amplifies the output of the third mixer; a fourth mixer for mixing the output signal and a changeover switch for switching the output signal of the first mixer input to the first intermediate frequency amplifier to the output signal of the fourth mixer; In the step, it is determined whether or not the resonant frequency of the measurement object belongs to the pass frequency band of the bandpass filter that includes the tuning frequency of the first intermediate frequency amplifier, and when this determination is affirmative, the changeover switch is A nuclear magnetic resonance apparatus comprising means for outputting a switching command to a fourth mixer side.