JPH01101450A - Receiving probe for nmr - Google Patents

Abstract

PURPOSE:To make an impedance matching adjustment with small electric power by connecting a diode bridge which can be applied with a bias voltage to a capacitance element which constitutes a resonance circuit. CONSTITUTION:The capacitance element 2 is connected to an inductance element 1 which forms a receiving coil to constitute a resonance circuit. The diode bridge 6 is connected to this resonance circuit through a DC blocking capacitor 9. When a forward bias voltage is applied to a control terminal 7 provided to the bridge 6, the resistance value of the bridge 6 becomes small and the resonance circuit enters a nonresonance state. At this time, the impedance of a transmitting probe (not shown in figure) is adjusted. Further, when an NMR signal is received from an object to be measured, the transmitting probe (not shown in figure) outputs sent pulses during the reverse bias application to apply a reverse bias for resonance, thereby obtaining the NMR signal from an output terminal.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an NMR receiving probe that is used together with a transmitting probe to detect an NMR signal, and particularly relates to an NMR receiving probe that can adjust the impedance matching of the transmitting probe with low power. 4. [Prior Art] FIG. 3 is a circuit diagram showing a conventional NMR receiving probe described, for example, on page 130 of rNMR Video (published by Igaku Shoin Sanders, March 1985).

In the figure, (1) is an inductance element that becomes a receiving coil, and (2) is a capacitance element that is connected to inductance element (1) and becomes a resonant capacitor, and these constitute a resonant circuit.

(3) is a capacitance element (2) that constitutes a resonant circuit.
an impedance matching capacitor connected to one end of the
(4) is an output terminal provided at the other end of the impedance matching capacitor (3) and the capacitance element (2). Although not shown, the output terminal of the receiver 10-b (
4) is connected to a receiving preamplifier via a coaxial cable.

(5) is a pair of diodes connected in parallel to the resonant circuit, and a pair of diodes (5a) and (5) are connected in parallel and inversely.
b), and is designed to prevent magnetic coupling with a transmitting probe (not shown).

Next, the operation of the conventional NMR receiving probe shown in FIG. 3 will be explained.

First, when a transmission pulse consisting of a high frequency magnetic field is applied by the transmission probe, the inductance element (1)
Since a large current is induced in the diode pair (5), the diode pair (5) becomes conductive, and the resonant circuit is short-circuited to become non-resonant.

On the other hand, when receiving the NMR signal from the object to be measured after the application of the transmission pulse is finished, the diode pair (5) becomes non-conductive because the NMR signal has low power. Therefore, the resonant circuit enters a resonant state, and an NMR signal is output from the output terminal (4).

Furthermore, when adjusting the impedance matching of the transmitting probe, it is necessary to bring the receiving probe into a non-resonant state, so a large amount of power is supplied to the transmitting probe as in the case of NMR signal measurement.

[Problems to be solved by the invention] As described above, the conventional NMR receiving glove requires a large amount of power because the receiving probe is made into a non-resonant layer by making the diode pair (5) conductive. However, there was a problem in that impedance matching adjustment could not be easily performed.

The present invention was made in order to solve the above-mentioned problems, and an object of the present invention is to obtain an NMR receiving probe that can perform impedance matching adjustment of a transmitting probe with low power.

[Means for Solving the Problems] The NMR receiving probe according to the present invention includes a diode bridge connected in parallel to a capacitance element constituting a resonant circuit, and a control terminal for applying a bias voltage to the diode bridge. , an impedance element inserted between the control terminal and the diode bridge, and a DC blocking capacitor inserted between the diode bridge and the capacitance element.

In this invention, when magnetic coupling with the transmitting probe is to be canceled by changing the bias voltage from the control terminal, the diode bridge is switched from a non-conducting state to a conductive state and the receiving probe is placed in a non-resonant state. , when receiving an NMR signal, the receiving probe is placed in a resonant state.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings. 1st
The figure is a circuit diagram showing one embodiment of the present invention, and (1) to
(4) is similar to the above.

(6) is a capacitance element (1) that constitutes a resonant circuit.
A pair of bin diodes (6a) and (6b) connected in parallel in series on the anode side, and a pair of bin diodes (6a) and (6b) connected in parallel in parallel with this diode pair and connected in series in reverse on the cathode side. Diode (6
c) and (6d). Therefore, the resistance value of each bin diode (6a) to (6d) is the resistance value of the entire diode bridge (6).

(7) is a pair of control terminals for applying a bias voltage to the diode bridge (6), and the connection point of one diode pair (6b) and (6b) and the connection point of the other diode pair (6C) and (6d). are connected to each connection point.

(8a) and (8b) are resistors for applying bias voltage inserted between each control terminal (7) and the diode bridge (6), and may be other impedance elements such as inductance elements. .

(9) is a DC blocking capacitor inserted between the capacitance element (2) and the diode bridge (6), and the capacitance value is set to 0.01 μF to 0.1 μF. When a bias voltage is applied to the inductance element (1), current is prevented from flowing into the inductance element (1).

(10) is a coupling coil magnetically coupled to the inductance element (1), and an impedance matching capacitor (
3) through which an output terminal (4) is provided.

Note that although the configuration here is such that impedance matching is achieved through the coupling coil (10), the coupling coil (10)
It is also possible to use only the capacitor (3) for matching, as in FIG. 3, without using the capacitor (3). In that case, a diode bridge (6) and a capacitor (3) will be connected in parallel to the capacitance element (2).

FIG. 2 is a configuration diagram showing the shape of the resonant circuit in FIG. 1. As is clear from FIG. 2, the inductance element (1) serving as the receiving coil is loop-shaped, and the impedance matching capacitor (3) is composed of a variable capacitance diode.

Next, the operation of the embodiment of the present invention shown in FIGS. 1 and 2 will be described.

When the transmit pulse is being output from the transmit probe and when adjusting the impedance of the transmit probe,
A forward bias voltage is applied from the control terminal (7) to the diode bridge (6), and the resistance value of the diode bridge (6) is set to about 0.1Ω to 1Ω to bring the resonant circuit of the receiving probe into a non-resonant state.

According to experiments, Oni Lrode's 0M4 was used as the bin diodes (6a) to (6d).
00111 and the resistance value of the resistors (8a) and (8b) is set to 1Ω, the resistance value of the diode bridge (6) can be changed by applying a forward bias voltage of 30V to the control terminal (7). I was able to make it about 1a.

Note that if a current flows through the inductance element (1) when a bias voltage is applied, the static magnetic field in the space will be distorted, and the NMR signal received later will also be distorted.
9) is inserted, the magnetic field of the transmission pulse is generated ideally and the NMR is not distorted.

On the other hand, when receiving an NMR signal, a bias voltage is applied to the control terminal (7) so that the diode bridge (6) becomes reverse biased or zero biased, and the diode bridge (6) is placed in a high impedance state. At this time, the resistance value of the diode bridge (6) is more than 10Ω, so even if the resistors (8a) and (8b) are relatively small resistors of about 1Ω, the sensitivity of the receiving probe will be affected. (Q value) hardly decreases.

For example, when a reverse bias voltage of 30 V was applied using the aforementioned pin diode, the sensitivity decreased by only 5%.

Incidentally, impedance matching adjustment of the receiving probe is performed by a coupling coil (10) and a capacitor (3). In addition, the impedance matching adjustment of the transmitting probe is performed by the impedance matching capacitor inside the transmitting probe, but since the receiving probe can be brought into a non-resonant state by applying a bias voltage of 30V, the power supplied to the transmitting probe is Small power required.

The NMR signal acquired in this way is used to image a desired part of the object to be measured, but if the inductance element (1) is formed in a loop as shown in Fig. 2, the values of each part of the human body such as the eyes and ears can be measured. It can be a surface coil suitable for enlarged imaging.

In this case, the inductance element (1) is 1 to 3 CI11
The sensitivity of the receiving probe can be further increased by forming a copper foil loop with a certain width and placing it with its width perpendicular to the human body.

[Effects of the Invention] As described above, according to the present invention, a diode bridge is connected in parallel to a capacitance element in a resonant circuit, a control terminal for applying a bias voltage to this diode bridge, and a connection between this control terminal and a diode. An impedance element inserted between the bridge and a DC blocking capacitor inserted between the resonant circuit and the diode bridge are provided, and a bias voltage applied from the control terminal to the diode bridge makes the receiving probe non-resonant. Since the state or the resonance state can be easily switched, there is an effect that an NMR receiving probe can be obtained in which impedance matching of the transmitting probe can be easily performed with low power.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of the present invention, FIG. 2 is a configuration diagram showing the shape of the inductance element in FIG. 1, and FIG.
The figure is a circuit diagram showing a conventional NMR receiving probe. (1)...Inductance element (2)...Capacitance element (6)...Diode bridge (6a) to (6d)...Pin diode (7)...
Control terminals (8a), (8b)...Resistors (
9)...DC blocking capacitor In the figures, the same reference numerals indicate the same or corresponding parts. I! ! 1 figure bait 2 figure

Claims (3)

[Claims] (1) A resonant circuit consisting of an inductance element and a capacitance element, a diode bridge connected in parallel to the capacitance element, a control terminal for applying a bias voltage to the diode bridge, and a connection between the control terminal and the diode bridge. An NMR receiving probe comprising an impedance element inserted between the diode bridge and the capacitance element. (2) The NMR receiving probe according to claim 1, wherein the diode bridge is composed of a plurality of pin diodes. (3) The NMR receiving probe according to claim 1, wherein the impedance element is a resistor or an inductance element.