JPH01213559A - High-frequency probe for nmr and its adjusting method - Google Patents

Abstract

PURPOSE:To obtain a high-frequency probe for NMR which can be used as a transmission/reception probe and whose impedance matching can be performed easily by shifting the resonance frequency on a coil side by operating a parallel resonance circuit depending upon whether or not the high-frequency coil generates magnetic field pulses. CONSTITUTION:A parallel resonance circuit 4 provided with an inductance element 4a and capacitance element 4b is connected to an impedance matching circuit containing a coupling coil 2 which is magnetically coupled with a high-frequency coil 1. A small-sized coil to be used for the transmission and reception of a spectrum analyzer is magnetically coupled with the element 4a and the capacity of the element 4b is adjusted so that two peaks can be obtained on both sides of a using frequency by finding the receiving spectrum of the circuit by sweeping the frequency of a high-frequency magnetic field to be impressed. When the coil 1 generates high-frequency magnetic field pulses, the circuit 4 is invalidated, but, when no high-frequency magnetic field pulse is transmitted, the circuit 4 is validated and the resonance frequency on the coil 1 side can be shifted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for adjusting a high frequency probe for NMR that can perform impedance matching adjustment with low power and can also be used as a transmitting/receiving probe11).

[Prior Art] FIG. 5 is a diagram showing a conventional NMR transmitting probe as described in, for example, Japanese Unexamined Patent Publication No. 61-124854. The high frequency coil (12) is a resonance capacitor connected to the high frequency coil (1). (13) is an impedance matching capacitor connected in series to the resonance capacitor (12), forming an impedance matching circuit. Also, in this case, an impedance matching capacitor (1
3) also functions as a resonance capacitor.

(16a) and (16b) are input terminals provided at both ends of the impedance matching capacitor (13), into which transmission pulses from a transmitter (not shown) are input.

(20) is a diode pair inserted between the resonance capacitor (12) and the impedance matching capacitor (13), and a pair of pin diodes (
20a) and (20b), and together with the high frequency coil (1), the resonance capacitor (12), and the impedance matching capacitor (13) constitute a resonance circuit.

Next, the operation of the conventional NMR transmission probe shown in FIG. 5 will be explained.

When a transmission pulse is applied to the input terminals (16a) and (1'6b), the diode pair (20) becomes conductive and a resonant current flows through the resonant circuit, causing the object to be measured (
(not shown) generates a high-frequency magnetic field pulse with a working frequency f0.

After the application of the transmission pulse is finished, the N from the object to be measured is
The MR signal is received by a receiver probe (not shown), but since the NMR signal is of low power, a diode pair <2
0) becomes non-conductive and acts as a small capacitor.

Therefore, when receiving an NMR signal, the resonant frequency of the resonant circuit is the frequency f used during pulse transmission.

Because of this, the magnetic coupling between the receiving probe and the transmitting probe is eliminated, and the receiving probe becomes N
MR signals can be received.

In addition, when adjusting the impedance matching of the transmitting probe, in order to conduct the diode pair (20), a large power similar to that used during pulse transmission is applied to the input terminals (16a) and (16).
b).

Note that the probe is a general term that includes the high frequency coil (1), the impedance matching circuit, and their peripheral circuits.

[Problems to be Solved by the Invention] As described above, the conventional NMR transmission probe and its adjustment method make the diode pair (20) conductive when transmitting a pulse, and keep the diode pair (20) in a non-conductive state when receiving an NMR signal. Therefore, it cannot be used as a transmitting/receiving probe, and pin diodes (20 m), (20
There is a problem in that the waveform of the transmitted pulse is distorted due to the nonlinearity of b). Furthermore, since it is necessary to apply a large amount of power to make the diode pair (20) conductive, there is a problem that impedance matching cannot be easily performed.

This invention was made to solve the above-mentioned problems, and provides a high-frequency probe for NMR that can be used as a transmitting and receiving probe, does not distort the waveform of the transmitted pulse, and can perform impedance matching with low power. The purpose of this study is to find out how to adjust it.

[Means for Solving the Problems] An impedance matching circuit including a coupling coil magnetically coupled to the NMR high-frequency probe high-frequency coil according to the present invention, and a parallel circuit including an inductance element and a capacitance element connected in parallel to the impedance matching circuit. A resonant circuit, a pin diode connected to one end of this parallel resonant circuit, a control terminal for controlling on/off of this pin diode, a small transmitting and receiving coil magnetically coupled to an inductance element, and these small coils. and a spectrum analyzer connected to the coil and including an oscillator.

Further, the method for preparing a high frequency probe for NMR according to the present invention includes:
The reception spectrum of the parallel resonant circuit is determined by sweeping the frequency of the high-frequency magnetic field applied to the inductance element from a small coil, and the capacitance of the capacitance element is adjusted so that two peak frequencies of this reception spectrum are obtained on both sides of the frequency used. is adjusted.

[Function] In this invention, impedance matching of the high frequency coil is performed in advance, and the capacitance of the capacitance element is adjusted to match the resonant frequency of the high frequency coil side and the resonant frequency of the parallel resonant circuit side. When generating a high-frequency magnetic field pulse, the pin diode is turned on to disable the parallel resonant circuit, and when not transmitting a high-frequency magnetic field pulse, the pin diode is turned off to enable the parallel resonant circuit and the resonant frequency of the high-frequency coil side is adjusted. shift.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings. 1st
The figure is a circuit diagram showing an embodiment of the high frequency probe for NMR according to the present invention, and (1) and (12) are the same as those described above.

(2) is a coupling coil magnetically coupled to the high frequency coil (1), and (3) is an impedance matching capacitor connected to the coupling coil (2), which constitute an impedance matching circuit.

(4) is a parallel resonant circuit connected to the coupling coil (2) and the impedance matching capacitor (3), and includes an inductance element (4a) and a capacitance element (4b) connected in parallel.

(5) is a pin diode connected to one end of the parallel resonant circuit (4), (6) is an input/output terminal connected to the pin diode (5) and the other end of the parallel resonant circuit (4), and (7) is an input/output terminal connected to the other end of the pin diode (5) and the parallel resonant circuit (4). A coaxial cable connected to the input/output terminal (6), <8) is a control terminal for applying a bias voltage to the pin diode (5) via the coaxial cable (7), and (9) is a control terminal (8). and a coaxial cable (7), (10) is a DC blocking capacitor connected to the connection point of the high frequency blocking inductance (9) and the coaxial cable (7), (11) is a transmitter connected to a DC blocking capacitor (10) and a coaxial cable (7).

Figure 2 is a block diagram showing a part of the high frequency probe in Figure 1 in detail.6 In the case of Figure 2, the high frequency coil (1) is a slotted tube type coil, and the coupling coil (2) is a high frequency coil 1) is located close to the outside.

(21) and (22) are small transmitting and receiving coils magnetically coupled to the inductance element (4a); (23)
) is a spectrum analyzer with a built-in oscillator that can sweep the frequency. (24) and (25) are coaxial cables that connect the spectrum analyzer (23) and the small coils (21) and (22), respectively, and the frequency signal from the oscillator in the spectrum analyzer (23) is transmitted through the coaxial cable (24) and (25). ) is applied to the small transmitting coil (21), and the received signal from the small receiving coil (22) is input to the spectrum analyzer (23) via the coaxial cable (25). There is.

Next, the operation of the embodiment of the present invention shown in FIGS. 1 and 2 will be explained with reference to the frequency characteristic diagrams shown in FIGS. 3 and 4.

First, with the pin diode (5) turned on (conducting) and the parallel resonant circuit (4) disabled, the resonant capacitor (
12) and the capacitance C1 of the impedance matching capacitor (3) are adjusted to match the impedance of the arrow at the frequency f0 used for NMR signal measurement.

Next, with the pin diode (5) turned off (non-conducting) and the parallel resonant circuit (4) enabled, the frequency of the oscillator in the spectrum analyzer (23) is swept while a small power and constant amplitude A frequency signal is supplied to a small coil (21). As a result, a high-frequency magnetic field whose frequency has been swept is generated from the small coil (21) and applied to the inductance element (4a), and a reception spectrum is acquired from the small coil (22) magnetically coupled to the inductance element (4a). Ru. The received spectrum observed by the spectrum analyzer (23) consists of a coupling coil (2), an impedance matching capacitor (3), and a parallel resonant circuit (4).
For example, Figure 3 (
It will be like a).

In FIG. 3(a), the first peak frequency is observed by the resonance current flowing through the high-frequency coil (1), and coincides with the usage frequency ". Also, the second peak frequency f, is
The resonant current flowing through the parallel resonant circuit (4) is observed.

That is, the first peak frequency f0 is the high frequency coil (1)
and a resonance capacitor (12), and the second peak frequency 14 represents the resonance frequency of the circuit consisting of the coupling coil (12).
2) represents the resonant frequency of the circuit consisting of the impedance matching capacitor (3) and the parallel resonant circuit (4).

Here, when the capacitance cb of the capacitance element (4b) is adjusted, the second peak frequency approaches the operating frequency as shown at 14' in Fig. 3(b), and further as shown in Fig. 3(c). Usage frequency f. becomes consistent with

At this time, the resonant frequency on the parallel resonant circuit (4) side matches the resonant frequency (used frequency) on the high frequency coil (1) side, and the resonant state of the high frequency coil (1) changes.
The resonance current at the usage frequency f0 of the reception spectrum becomes zero, and the reception spectrum has two peak frequencies (ro-
Δ) and (f, +Δ) (Figure 3(e)
), at this time, the frequency characteristics of the resonant current of the high-frequency coil (1) are similarly separated as shown by the broken line in FIG. still,
The value of Δ is about 1 to 5 MHz.

-a, since the shape of the high-frequency coil (1) is large, it is not easy to directly measure the resonant current flowing through the high-frequency coil (1), but as shown in Figure 2, the inductance can be constructed from a solenoid with a diameter of about 1 cm. By using the element (4a), it is possible to easily obtain a reception spectrum corresponding to the resonant current as shown in FIG. 3 via the small coils (21) and (22).

Thus, the adjustment of the probe prior to the measurement of the NMR signal is completed. At the zeroth order, a high frequency magnetic field pulse is transmitted from the transmitting probe of FIG. 1 and an NMR signal is acquired using another receiving probe as follows.

Apply a forward bias voltage to the control terminal (8), flow a forward bias current of about 10 (la A) to the pin diode (5), and turn on the pin diode (5) (conducting). state. At this time, the forward bias current is the coaxial cable (7),
It flows to ground via a loop consisting of a pin diode (5), an inductance element (4a) and a coaxial cable (7).

For example, as a pin diode (5), a unitrode (U
When using 11M7204[1 manufactured by Nitrode, Inc., the on-resistance value is approximately 0.1a due to the forward bias current of 1001IIA.

Here, since the impedance looking from the input/output terminal (6) to the transmitter (11) side is 50Ω, the Q value of the parallel resonant circuit (4) is as low as about 1. In addition, parallel resonant circuit (
Since the resonant frequency on the side 4) almost matches the resonant frequency on the high frequency coil (1) side, the parallel resonant circuit (4) has a high impedance and its existence can be ignored, making it ineffective. Therefore, the resonance frequency on the high-frequency coil (1) side matches the operating frequency f0, as shown by the solid line in FIG.

At this time, since the pin diode (5) is not included in the resonant circuit on the high frequency coil (1) side, no distortion occurs in the transmitted waveform of the high frequency magnetic field pulse, and impedance matching adjustment can be performed with low power. .

Also, since a high frequency blocking inductance (9) is inserted, the transmission pulse from the transmitter (11) is transmitted to the control terminal (
8), and since the DC blocking capacitor (10) is inserted, the bias voltage from the control terminal (8) is not applied to the transmitter (11). Furthermore, since a bias voltage is applied to the pin diode (5) via the coaxial cable (7), the number of transmission lines does not increase even if the control terminal (8) is provided.

When a reverse bias voltage or zero bias voltage of about 30V is applied to the control terminal (8) with NMR to turn the pin diode (5) off (non-conducting), a resonant circuit including the high frequency coil (1) and The parallel resonant circuit (4) is magnetically coupled via the coupling coil (2), and the parallel resonant circuit (4) becomes effective.

At this time, the resonance capacitor (12), the impedance matching capacitor (3), and the capacitance element (4b
) have already been adjusted as described above, so the following equation, ω. 2L Ic 12= 1 ・
・・■(&)o”La cb+ C2/ (1-(J)
o2L2c*)] = 1 -■ However, ω. (=2
πf0): Angular frequency L1: Inductance of high frequency coil (1) L2: Inductance of coupling coil (2) La: Inductance of inductance element (4a). Here, in equation (2), the circuit consisting of the coupling coil (2), the impedance matching capacitor (3), and the parallel resonant circuit (4) has an angular frequency ω. This shows the conditions for resonance.

At this time, the resonance frequency on the high frequency coil (1) side is separated into (f, -Δ) and (f, +Δ) as shown in FIG. 4, so the resonance current at the operating frequency f0 becomes almost zero. According to experiments, the frequency used is f. was 25 MHz, Δ was about IMHz.

In this way, by separating the resonant frequency from fo, it is possible to prevent magnetic coupling between the high frequency probe shown in FIG. NMR signals can be received without any interference.

In the above embodiment, the high frequency probe is used as a transmitting probe and a receiving probe is provided separately. It can also be used as a transmitting/receiving probe.

[Effects of the Invention] As described above, according to the present invention, an impedance matching circuit including a coupling coil magnetically coupled to a high frequency coil;
A parallel resonant circuit including an inductance element and a capacitance element connected to this impedance matching circuit,
A pin diode connected to one end of this parallel resonant circuit, a control terminal for controlling on/off of this pin diode, small transmitting and receiving coils magnetically coupled to an inductance element, and a small coil connected to these small coils. In addition, it is equipped with a spectrum analyzer including an oscillator, and by sweeping the frequency of a high-frequency magnetic field applied from a small coil to an inductance element, the receiving spectrum of the parallel resonant circuit is obtained. The capacitance of the capacitance element is adjusted so that the high-frequency coil generates a high-frequency magnetic field pulse, and the parallel resonant circuit is disabled, and when the high-frequency magnetic field pulse is not transmitted, the parallel resonant circuit is enabled.
Since the resonant frequency on the high frequency coil side is shifted, there is an effect of providing a method for adjusting the high frequency probe for NMR which can be used as a transmitting/receiving probe and can easily perform impedance matching with low power.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a high frequency probe for NMR according to the present invention, FIG. 2 is a perspective view showing the configuration of FIG. 1 in detail, and FIG. 3 is a high frequency probe arrangement for NMR according to the invention. FIG. 4 is a frequency characteristic diagram showing a reception spectrum to explain the method, FIG. 4 is a frequency characteristic diagram showing resonance current to explain the NMR high frequency probe preparation method according to the present invention, and FIG. 5 is a conventional NMR transmitting probe. FIG. (1)...High frequency coil (2)...Coupling coil (3)...Impedance matching capacitor (4)
...Parallel resonant circuit (4a)...Inductance element (4b)...Capacitance element (5)...Pin diode (8)...Control terminal (
21) ... Small coil for transmission (22) ... Small coil for reception (23) ... Spectrum analyzer f0 ... Frequency used fo - Δ, f0 + Δ ... Within the peak frequency, in the figure, The same reference numerals indicate the same or equivalent parts. 11!1 8 Control #P Stumper Fig. 2 22 Reception Coil Fig. 3 Fig. 5

Claims (2)

[Claims] (1) An impedance matching circuit including a coupling coil magnetically coupled to a high frequency coil, a parallel resonant circuit including an inductance element and a capacitance element connected in parallel to this impedance matching circuit, and a parallel resonant circuit connected to one end of this parallel resonant circuit. A pin diode, a control terminal for controlling the pin diode on and off, small coils for transmission and reception magnetically coupled to the inductance element, and a spectrum analyzer connected to these small coils and including an oscillator. High frequency probe for NMR. (2) Sweep the frequency of the high-frequency magnetic field applied to the inductance element from the small transmitting coil under the control of the spectrum analyzer to obtain the received spectrum of the parallel resonant circuit via the small receiving coil; 2. The method of adjusting a high frequency probe for NMR according to claim 1, wherein the capacitance of the capacitance element is adjusted so that two peak frequencies are obtained on both sides of the operating frequency.