JPH021581A - High frequency probe for nmr and adjusting method thereof - Google Patents

Abstract

PURPOSE:To use the title probe as a transmitting and receiving probe, as well by setting a pin diode to a forward bias and nullifying a parallel resonance circuit at the time of transmitting a pulse and shifting a resonance frequency of a high frequency coil, etc., by the parallel resonance circuit at the time of non-transmission of a pulse. CONSTITUTION:When a high frequency coil 1 generates a high frequency magnetic field as a transmitting probe, a pin diode 5 is set to a forward bias an a parallel resonance circuit 4 is nullified. At the time of non-transmission of a pulse, the pin diode 5 is set to a reverse bias or a zero bias, the parallel resonance circuit 4 is coupled to a resonance capacitor 2 through an impedance matching capacitor 3, and a resonance frequency of the high frequency coil 1 and the resonance capacitor 2 is shifted. Accordingly, the probe can be used as a transmitting and receiving probe, as well, and an NMR high frequency probe by which a waveform of a transmitting pulse is not distorted, and also, impedance matching can be executed by small power is obtained.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention provides an NM that can perform impedance matching adjustment with low power and can also be used as a transmitting/receiving probe.
This article relates to a high frequency probe for R and its adjustment method.

[Prior Art] FIG. 5 is a circuit diagram showing a conventional NMR transmitting probe described in, for example, Japanese Patent Application Laid-Open No. 61-124854. In FIG. 9, (1) is a high-frequency coil serving as a transmitting coil; (12) is a resonance capacitor connected to the high frequency coil (1). (13) is the resonance capacitor (12
) is an impedance matching capacitor connected in series, and also functions as a resonance capacitor.

(16a) and (16b) are input terminals provided at both ends of the impedance matching capacitor (13), into which a transmission pulse from a transmission fi (not shown) is 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 (16b), the diode pair (20) becomes conductive and a resonant current flows through the resonant circuit, causing a current to flow from the high-frequency coil (1) toward the object to be measured (not shown). Generates a high frequency magnetic field.

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 higher than when transmitting a pulse, so the magnetic coupling between the receiving probe and the transmitting probe is eliminated, and the receiving probe receives the NMR signal without decreasing its sensitivity. be able to.

In addition, when adjusting the impedance matching of the transmitting probe, in order to make the diode pair (20) conductive, high power is applied to the input terminals (16a) and (16) in the same way as when transmitting pulses.
b) is applied.

[Problems to be Solved by the Invention] As described above, the conventional NMR transmitter 10-b and its adjustment method conduct the diode pair (20) when transmitting a pulse, and turn the diode pair (20) on when receiving an NMR signal. Since it is in a non-conducting state, it cannot be used as a transmitting/receiving probe, and the pin diodes (20a) and (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] A parallel resonant circuit connected to the NMR high frequency probe impedance matching circuit according to the present invention, a pin diode connected to the parallel resonant circuit, and a method for controlling on/off of the pin diode. A control terminal is provided.

Further, the method for preparing a high frequency probe for NMR according to the present invention includes:
Depending on the load condition of the object to be measured inserted into the high frequency coil,
The capacitance IC of the impedance matching capacitor and the capacitance 1i of the capacitance element can be adjusted in conjunction with CC.

In addition, another method for preparing a high-frequency probe for NMR according to the present invention includes a first step in which the capacitance Ce is initially set in a no-load state and then fixed, and impedance matching is performed with the object to be measured inserted in the high-frequency coil. The first step includes a second step.

[Function] In the high-frequency probe for NMR of the present invention, when the high-frequency coil as a transmitting probe generates a high-frequency magnetic field, the pin diode is forward biased to disable the parallel resonant circuit, and when not transmitting pulses, the pin diode is reverse biased or With zero bias, the parallel resonant circuit is coupled to the resonant capacitor via an impedance matching circuit, and the resonant frequencies of the high frequency coil and the resonant capacitor are shifted.

Furthermore, in the NMR high frequency probe preparation method of the present invention, the sum of each capacitance (CC+C3) is kept constant.

In another method of preparing a high frequency probe for NMR according to the present invention, impedance matching is performed by fixing the capacitance JICC constant after initial setting.

[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) is the same high frequency coil as described above.

(2) is a resonance capacitor, and the high frequency coil (1)
A variable capacitor for resonant frequency adjustment connected in parallel with (
2a), and a pair of series capacitors (2b) and (2c) connected in parallel to this variable capacitor (2a).

(3) is an impedance matching capacitor connected to the resonance capacitor (2). (4) is a parallel resonant circuit connected to the impedance matching capacitor (3) and the connection point of series capacitors (2b) and (2c), and is connected to an inductance element and a variable capacitor connected in parallel to this inductance element. It is composed of a capacitance element C. Also, this parallel resonant circuit (4
) is set to substantially match the resonance frequency of the high frequency coil (1) and the resonance capacitor (2).

(5) is a pin diode connected to one end of the parallel resonant circuit (4), and (6) is an input terminal connected to the other end of the pin diode (5) and the parallel resonant circuit (4).

(7) is the coaxial cable connected to the input terminal (6), (
8) is connected to the pin diode (5) via the coaxial cable (7).
), (9) is a high frequency blocking inductance inserted between the control terminal (8) and the coaxial cable (7), (10) is a high frequency blocking inductance (9) and a DC blocking capacitor connected to the connection point of the coaxial cable (7), and (11) a transmitter connected to the DC blocking capacitor (10) and the coaxial cable (7).

Moreover, FIG. 3 is a perspective view schematically showing the capacitance adjustment structure of the capacitance element C and the impedance matching capacitor (3), where M is a step motor and G) 4 is provided on the rotating shaft of the step motor M. It's a gear. Gc is a gear provided on the capacitance adjustment shaft of the capacitance element C, and meshes with the gear GM.

G is a gear provided on the capacitance adjustment shaft of the impedance matching capacitor (3), and meshes with the gear Gc.

The ratio of the number of teeth between these gears Gc and G is 1:1, and each capacity adjustment shaft rotates at the same rate in opposite directions as the gear CM rotates, as shown by the arrow. The meshing of each gear (e and G) may be appropriately released by, for example, moving the capacitance XU axis of the impedance matching capacitor (3) in the axial direction.

Next, the operation of the embodiment of the present invention shown in FIGS. 1 and 3 will be described with reference to the frequency characteristic diagram of the resonant current shown in FIG.

First, before transmitting and receiving pulses, the capacitance C2 of the resonance capacitor (2) and the capacitance C1 of the impedance matching capacitor (3) are adjusted by the step motor M, and the high frequency probe impedance matching for NMR (approximately 50Ω
).

Then, when the inductance LL of the inductance element of the parallel resonant circuit (4) is fixed and the frequency of the high-frequency pulse used is ro, the capacitance C of the capacitance element C
Adjust C so that (2πfo)''LL(CC+C3)=1-■.At this time, gears Gc and G.

If the meshing of is released, the capacity C will be increased by the step motor M.
Even if C is adjusted, the capacitance C5 is not changed.

Next, the operation when acquiring an NMR signal using a high frequency pulse will be explained.

When transmitting pulses: First, apply a forward bias voltage to the control terminal (8), and apply a forward bias current of approximately 100 mA to the pin diode (5), turning the pin diode (5) on (conducting). When the forward bias current is the coaxial cable (7)
, through a loop consisting of a pin diode (5), an inductance element and a coaxial cable (7) to ground.

For example, as a pin diode (5), a unitrode (U
Nitrode) 084001 is used, 10
With a forward bias current of 0mA, the on-resistance value is 0.1Ω
It will be about.

Here, since the impedance looking from the input terminal (6) to the transmission fi (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 of 4) almost matches the resonant frequency of the high frequency coil (1) and the resonant capacitor (2), the parallel resonant circuit (4) has a high impedance and its existence can be ignored.

Therefore, the high frequency probe shown in FIG. 1 can be used as a normal transmitting probe, and the resonant frequency of the high frequency coil (1) and the resonant capacitor (2) becomes fo as shown by the solid line in FIG.

At this time, since the pin diode (5) is not included in the resonant circuit including the high frequency coil (1), no distortion occurs in the transmitted waveform of the high frequency magnetic field, 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 transmission a (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.

NMR = " = ) First, apply a reverse bias voltage or zero bias voltage of about 30 V to the control terminal (8) to turn the pin diode (5) off (non-conducting). This causes the high frequency coil (1 ) and a parallel resonant circuit (4) are coupled via an impedance matching capacitor (3).

At this time, the above equation 0, ωo'LL(CC+C*)=
1 However, ω. (=2πf0): If the capacitance i CC is adjusted to satisfy the angular frequency, it is unavoidable from experimental and theoretical calculations that the resonance state of the high frequency coil (1) and resonance capacitor (2) will change. There is.

That is, as shown by the broken line in Fig. 2, the resonance frequency is fo - Δ
and f. Separate to +Δ. According to experiments, the resonant frequency 1
When 0 was 25 MHz, Δ was about IMHz. Therefore, the resonant current at the resonant frequency f0 during pulse transmission becomes almost zero.

Thus, the resonant frequency is f. By separating the layers from each other, it is possible to prevent magnetic coupling between a high frequency probe as a transmitting probe and a receiving probe (not shown). Therefore, the receiving probe is a device that can receive NMR signals without decreasing sensitivity, and the load varies depending on the part of the human body inserted into the high-frequency coil (1). Impedance matching capacitor (
3) It is desirable to appropriately fine-tune the capacitance C1. In this case, each capacitance MCC and C1 are adjusted in opposite directions to each other as shown in Figure 3, so the impedance matching capacitor (3) and capacitance If element C is composed of variable capacitors of the same type and the capacitance changes per unit rotation angle are equal, the capacitance jt is the sum of CC and C1 (CC + C3)
does not change, and can satisfy the adjusted equation (2).

In the above embodiment, an impedance matching capacitor (3) is used as the impedance matching circuit, but a coupling coil (not shown) magnetically coupled to the high frequency coil (1) and an impedance matching capacitor (not shown) connected to this coupling coil are used as the impedance matching circuit. A capacitor may also be used.

In addition, although we have explained the case where the high frequency probe is used as a transmitting probe and a receiving probe is provided separately, if a forward bias voltage is always applied to the control terminal (8) and the pin diode (5) is turned on, transmitting and receiving can be performed. It can also be used as a probe.

Furthermore, in the above adjustment method, the impedance matching capacitor (
3) and the capacitance element C are adjusted in conjunction with each other, but the adjustment method shown in Flowchart 1 of 4I7I may also be used.

In this case, first, the capacitance l CC of the capacitance element C is determined in an unloaded state (first step SL).

That is, in a state where the inductance LL of the inductance element is fixed as described above, the capacitance CC is determined so as to satisfy the formula (2), and thereafter this capacitance CC is fixed. At this time, for example, another receiving coil (not shown) may be arranged and the capacity ffi CC may be determined using a known method based on the characteristics of the reflected signal.

Next, insert the object to be measured as a load into the high-frequency coil (1), and while adjusting the capacitance c2 of the resonance capacitor (2) and the capacitance tcz of the impedance matching capacitor (3),
Impedance matching of the high frequency probe at the operating frequency r0 is performed (second step S2).

According to experiments, when the receiving coil used with a high-frequency probe is a surface coil, even if the capacity Mk CC is the value adjusted in the first step S1, the sensitivity of the surface coil only decreases by about 5%, which is sufficient. I found out that it can be used.

Note that the initial setting of the capacitance CC (first step Sl) must be performed in a no-load state with as little load as possible. This is because, for example, if the volume and jicc are initially set to N with a load with a large loss such as a human body inserted, the sensitivity of the surface coil will drop significantly when measuring water, etc. with almost no load, making it impractical. This is because

After the impedance matching of the high frequency probe is completed in this manner, an NMR signal is obtained in the same manner as described above (third step S3).

[Effects of the Invention] As described above, according to the present invention, there are provided a parallel resonant circuit connected to an impedance matching circuit, a pin diode connected to the parallel resonant circuit, and a control terminal for controlling on/off of the pin diode. When transmitting pulses, that is, when the high-frequency coil generates a high-frequency magnetic field, the pin diode is biased to IIIT to disable the parallel resonant circuit, and when not transmitting pulses, the pin diode is reverse biased or zero biased to disable the parallel resonant circuit. Since the resonant frequency of the high frequency coil and resonance capacitor is shifted, it can also be used as a transmitting/receiving probe.The high frequency probe for NMR does not distort the waveform of the transmitted pulse and can perform impedance matching with low power. It has the effect of

Further, according to the present invention, each capacity ji CC and C7 are adjusted in conjunction, or the capacity ji initially set in the no-load state is adjusted.
Since CC is fixed at a constant value, there is an effect that a high frequency probe adjustment method for NMR can be obtained which can effectively perform impedance 1λ matching regardless of the load state.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a frequency characteristic diagram for explaining an embodiment of the invention, and Fig. 3 is a circuit diagram showing an embodiment of the invention.
FIG. 4 is a perspective view showing a power transmission mechanism according to an embodiment of the method for preparing a high-circle high-frequency probe for NMR according to the present invention, and FIG. 4 is a flowchart showing a method for preparing a high-circular high-frequency probe for NMR according to another invention - Figure 5 is a circuit diagram showing a conventional NMR transmitting probe. (1... High frequency coil (2)... Resonance capacitor (3... Impedance matching capacitor (4...
Parallel resonant circuit (5)...Pin diode (7...
・Coaxial cable (8)... Control terminal L... Inductance element C... Capacitance element G3, Gc... Gear Sl... First step S2... Second step CC... Capacitance element Capacitance C2... Capacity of resonance capacitor C1... Capacity of impedance matching capacitor.
In the figures, the same reference numerals indicate the same or corresponding parts.

Claims (8)

[Claims] (1) A high-frequency coil, a resonant capacitor connected to this high-frequency coil, an impedance matching circuit connected to this resonant capacitor, a parallel resonant circuit connected to this impedance matching circuit, and a parallel resonant circuit connected to this parallel resonant circuit. A high frequency probe for NMR equipped with a connected pin diode and a control terminal for controlling on/off the pin diode. (2) Claim 1, characterized in that the parallel resonant circuit is composed of an inductance element and a capacitance element connected in parallel to the inductance element.
High-frequency probe for NMR described in Section 1. (3) The high frequency NMR device according to claim 1 or 2, wherein the resonant frequency of the parallel resonant circuit is set to substantially match the resonant frequency of the high frequency coil and the resonant capacitor. probe. (4) The high frequency probe for NMR according to any one of claims 1 to 2, wherein the control terminal is provided via a coaxial cable for high frequency transmission. (5) Capacitance C_2 of the resonant capacitor connected in parallel to the high-frequency coil, capacitance C_3 of the impedance matching capacitor, and capacitance C of the capacitance element in the parallel resonant circuit connected to the impedance matching capacitor.
In the method for adjusting a high frequency probe for NMR, the impedance matching capacitor and the capacitance element are meshed together via a power transmission mechanism, and the sum of the respective capacitances C_3 and C_C is adjusted to be always constant. A method for adjusting a high frequency probe for NMR, characterized by the following. (6) N as set forth in claim 5, wherein the power transmission mechanism is a gear provided on each capacity adjustment shaft.
How to adjust a high frequency probe for MR. (7) The NMR according to claim 5 or 6, characterized in that the meshing of the power transmission mechanism is releasable.
How to adjust the high frequency probe for use. (8) Capacitance C_2 of the resonant capacitor connected in parallel to the high-frequency coil, capacitance C_3 of the impedance matching capacitor, and capacitance C of the capacitance element in the parallel resonant circuit connected to the impedance matching capacitor.
In the method for adjusting the high frequency probe for NMR that adjusts the capacitance C_C, the capacitance C_C is set to ω_o^2・L_L(C_C+C_3)=1, where ω_
o: Angular frequency L_L of the high-frequency pulse to be used: Initially set to be the inductance in the parallel resonant circuit, and thereafter, the inductance L_L and the capacitance C_C are set regardless of the load condition of the object to be measured inserted into the high-frequency coil. a first step of fixing the object to a constant value; and inserting the object to be measured into the high-frequency coil to increase the capacitance C.
A method for adjusting a high frequency probe for NMR, comprising: a second step of performing impedance matching while adjusting _2 and C_3;