JPH0618640A - Nucleus magnetic resonance detector - Google Patents

Abstract

PURPOSE:To perform tuning at a position away from a sample coil without reducing Q and at the same time to reduce electric field loss at the sample coil for preventing generation of discharge. CONSTITUTION:In the title detector where a sample coil 30 is connected to a tuning matching circuit for resonating at a specified frequency by the tuning matching circuit, the sample circuit 30 is connected to the tuning matching circuit with a coaxial line and a resonance circuit is formed by the sample coil 30 and the coaxial line. Also, the tuning matching circuit is connected to the sample circuit 30 for high and low frequencies with the coaxial line, thus forming the resonance circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear magnetic resonance (NMR) detector using a coaxial line.

[0002]

2. Description of the Related Art FIG. 7 is a diagram showing a conventional NMR detector. In FIG. 7A, the sample coil 1 is connected to a tuning variable capacitor 3 and a matching variable capacitor 4 by a lead wire 2 so as to resonate at a target frequency. The temperature of the sample placed in the sample coil 1 is adjusted. If the lead wire 2 is shortened so that the electrical parts are close to the sample, the heat insulation for temperature adjustment is affected, and the magnetic field is distorted. The performance of the resonator is degraded. Therefore, it is necessary to separate the variable capacitor from the sample coil 1 in order to prevent the influence on the heat insulation for adjusting the temperature of the sample and the distortion of the magnetic field, and therefore the lead wire 2 cannot be omitted. However, the lead wire 2 has a stray capacitance, which causes a factor that the resonance frequency cannot be increased, and the resistance of the lead wire 2 lowers the Q of the detector. Therefore, conventionally, as shown in FIG. 7B, the capacitor 5 is provided in a region where the temperature is adjusted and the sample coil 1 is provided.
The required resonance was generated between the and, and the frequency was finely adjusted by the tuning variable capacitor 3.

A double tuning circuit using a coaxial resonator as shown in FIG. 8 has also been proposed. This circuit uses coaxial resonators 12 and 13 having a λ / 4 length for a high frequency (HF), and an open coaxial resonator 12 is connected to one end of the sample coil 11 and a short-circuit coaxial resonator 13 is connected to the other end. , HF input / output side and low frequency (LF) input / output side are connected with tuning capacitors 14 and 16 and matching capacitors 15 and 17, respectively. Short-circuited coaxial resonator 13 with respect to HF
The electric field is maximized at the input end (point P) of, and the electric field is minimized at the input end (point Q) of the open coaxial resonator 12, and the frequency is adjusted by the tuning variable capacitor 14. At this time, since the electric field becomes minimum at the point Q, there is no loss of HF power flowing to the LF side. Further, for the LF, the coaxial resonator 12 with an open end has nothing to do with it, and since the coaxial resonator 13 acts as an inductance that is grounded, it is connected in parallel to the sample coil 1 and the coaxial resonator 13. The frequency is adjusted by the tuning variable condenser 16 that is present. In this way, the frequency can be adjusted independently for HF and LF.

[0004]

However, even in the circuit shown in FIG. 7B, it is difficult to obtain a high resonance frequency due to the stray capacitance of the lead wire, and when the temperature is changed, the capacitor is changed. There is a problem that the resonance frequency changes due to the influence of the dielectric substance of 5 and the magnetic field is distorted. Further, in FIGS. 7A and 7B, since a high RF electric field is generated in the sample coil in both cases, the sample is dielectrically heated and discharge is likely to occur in the sample coil.

Further, in the double tuning circuit shown in FIG. 8, each electric component (coaxial resonators 12 and 13, variable capacitors 14 to 17) is connected to the sample coil 1 as shown in FIG.
It is necessary to dispose the device immediately next to the installation position. Therefore, if the temperature of the sample is changed, the temperature of each electric component also changes and the circuit constant changes. Also,
As shown in FIG. 10, when the gradient magnetic field coil 18 for NMR-CT is mounted on the detector, the sample coil 1 needs to be arranged in the central portion of the coil 18, so that the sample coil and each electric component are different from each other. The Q is lowered as they are separated. As described above, there are problems that the sample coil and each electric component cannot be separated from each other, mounting is difficult, electric field loss in the sample coil is large, discharge easily occurs, and it is difficult to obtain a high frequency.

The present invention is intended to solve the above-mentioned problems, and it is not necessary to attach a coaxial capacitor at a position close to the sample coil, and tuning can be performed at a distant position without lowering Q, and the sample coil can be tuned. It is an object of the present invention to provide an NMR detector capable of reducing the electric field loss in the device and preventing the occurrence of discharge.

[0007]

SUMMARY OF THE INVENTION According to the present invention, in a nuclear magnetic resonance detector in which a sample coil and a tuning matching circuit are connected and the tuning matching circuit resonates at a predetermined frequency, the sample coil and the tuning matching circuit are connected to each other. It is characterized in that a resonance circuit is formed by the coaxial line and the sample coil and the coaxial line. Further, the present invention provides a nuclear magnetic resonance detector in which a sample coil is connected to a high-frequency and low-frequency tuning matching circuit, and double tuning is performed by each tuning matching circuit. The tuning matching circuits for frequencies are connected by coaxial lines, respectively, and a resonance circuit is formed by the sample coil and the coaxial line.

[0008]

According to the present invention, the sample coil and the tuning matching circuit are connected by the coaxial line, the resonance circuit is formed by the coil and the coaxial line to perform the tuning, and the double tuning is performed. It is possible to resonate at a target frequency without decreasing Q at a position away from. Further, even if the temperature of the sample is adjusted, the temperature fluctuation in the variable capacitor does not occur, and the mounting space for the electric component can be widened, which facilitates the mounting. Also, NMR
RF magnetic field generated in the sample coil (RF
It is desirable that the RF current flowing through the sample coil be the maximum and the RF electric field is small. In the present invention, since the electric field in the sample coil can be reduced, the electric field loss can be reduced and the occurrence of discharge can be prevented.

[0009]

FIG. 1 is a diagram showing an embodiment of the present invention.
FIG. 1A is a diagram showing a detector, and FIG. 1B is a diagram showing magnitudes of an RF electric field and an RF current in a coaxial line. In the figure,
Reference numeral 30 is a sample coil, 31 is an inner conductor, 32 is an outer conductor, 33 is a tuning variable capacitor, and 34 is a matching variable capacitor.

The inner conductor 31 and the outer conductor 32 form a coaxial line, and the outer conductor 32 is grounded. One end of the sample coil is connected to the tip of the inner conductor 31, and the other end of the sample coil is connected to the point A of the outer conductor 32 for grounding. In addition, a tuning variable capacitor 3 is attached to the other end of the inner conductor.
3. The matching variable condenser 34 is connected and an RF signal is applied. The inner conductor 31 does not have to be the center of the outer conductor.

The coaxial line operates as a λ / 4 (λ is a wavelength) coaxial resonator as a whole, and since the inner conductor of the coaxial line is grounded through the sample coil, the RF electric field at each position (broken line in the figure). , RF current (solid line in the figure) is
As shown in (b), RF at the position of the sample coil
The electric field is minimum, the RF current is maximum, the RF electric field is maximum and the RF current is minimum at the connection position of the tuning circuit, and the resonance frequency can be adjusted by the variable capacitor 33. In this detector, since the coaxial line plays the role of a lead wire, the stray capacitance does not matter, and the sample coil and the tuning circuit can be separated without lowering the Q, so there is a space available. Easy to mount electrical parts,
The structure can be simplified. Further, since the RF electric field is minimized at the position of the sample coil, it is possible to prevent dielectric heating and discharge. Further, since it is not necessary to attach a capacitor to the sample coil, the frequency does not change due to temperature change.

As the sample coil, various types such as a solenoid type coil, a saddle type coil and a resonator type coil can be used, and a coaxial resonator composed of an inner conductor, an outer conductor and a sample coil is used. ,
Since the resonance frequency can be easily adjusted in a wide range by adjusting the tuning circuit, the resonance frequency does not have to be λ / 4, and the inner conductor and the outer conductor do not have to be circular in cross section.

2 to 4 are views showing another embodiment of the present invention. In the figure, 40 is an RF coil, 41 is a coil, 42
Is an inner conductor, 43 is an outer conductor, 44 to 46 are coaxial resonators, 47 to 50 are fine adjustment inductances (L), 5
Reference numerals 1 and 53 are tuning capacitors, and 52 and 54 are matching capacitors.

The RF coil 40 includes a coil 41 and an inner conductor 4.
2. The outer conductor 43 is grounded. One end of the coil 41 is connected to the outer conductor 43 and grounded, and the other end of the coil 41 is connected to one end of the inner conductor 42. A coaxial resonator 44 is connected to the other end (point a) of the internal conductor 42 of the RF coil 40 via a fine adjustment L47, and the tuning variable capacitor 5 is connected to the other end (point b) of the coaxial resonator 44.
1. The matching variable capacitor 52 is connected to serve as an HF input / output terminal. The coaxial resonator 44 is for adjusting the resonance frequency of the RF coil 40 to HF. Further, at the point a, the coaxial resonator 45 is connected via the fine adjustment L48 to the coaxial resonator 4
A coaxial resonator 46 is connected to the other end of 5 through L49 and 50 for fine adjustment. The coaxial resonators 45 and 46 are for setting the connection point (point c) of the fine adjustment L49 and 50 to 0 impedance at the HF frequency. A tuning variable capacitor 53 and a matching variable capacitor 54 are connected to point c to serve as an LF input / output terminal. The coaxial resonators 45, 4
Reference numeral 6 has a length of λ / 4 with respect to the wavelength λ of HF, and the outer conductors are grounded.

The tuning of the HF frequency will be described with reference to FIG. 3. For the standing wave voltage and current in the RF coil 40 and the coaxial resonator 44 (FIG. 3A), the coil 41 is the outer conductor 4.
Since it is grounded at 3, the voltage is minimum and the current is maximum at the grounding point, and the L for fine adjustment and the coaxial resonator 4 are used.
4, the voltage becomes maximum and the current becomes minimum at point b (positions of the HF tuning variable capacitor 51 and the matching variable capacitor 52). At this time, looking at the coaxial resonators 45 and 46 shown in FIG. 3C, the fine adjustment L48, the coaxial resonator 45, and the fine adjustment L4 are shown.
9 and 50, the coaxial resonator 46 produces a voltage and current distribution as shown in FIG. 3D, and the point c (LF tuning variable capacitor 5
3, the voltage becomes 0 at the position of the matching variable capacitor 54, that is, the impedance becomes 0. Therefore, RF from the HF side to the LF side
No power flows.

The tuning of the LF frequency will be described with reference to FIG. 4. For the voltage and current of the standing wave in the RF coil 40 and the coaxial resonator 44 (FIG. 4A), the coil 41 is the outer conductor 4
3 is grounded, the coaxial resonator 44 is open at point b, and the coaxial resonator simply functions as a conductor connected to the fine adjustment L. Therefore, at the ground point of the coil 41, the voltage is minimum and the current is maximum. Also, almost no current flows to the L for fine adjustment and the coaxial resonator 44 side, and the voltage is low.
The RF power of the F frequency does not flow to the HF side. At this time,
As for the coaxial resonators 45 and 46 shown in FIG. 4C, the fine adjustment L48, the coaxial resonator 45, and the fine adjustment L4 are provided.
9 and 50, the voltage and current distribution as shown in FIG. 4D is obtained by the coaxial resonator 46, and the voltage becomes large at the point c (positions of the LF tuning variable capacitor 53 and the matching variable capacitor 54), so that LF matching is performed. be able to.

According to this embodiment, since there is no influence of the stray capacitance, a high Q can be obtained even by double tuning at a position away from the RF coil, and even if the temperature of the sample is adjusted, the temperature of the variable capacitor does not change. Further, a space for mounting an electric component such as a variable capacitor can be widened, and it becomes easy to mount the gradient magnetic field coil in the case of NMR-CT in the probe. Further, the distributed constant circuit can reduce the loss of RF power.

As the coaxial resonator, as shown in FIG. 5, a microstrip line 44 'is formed on the dielectric 60.
45 ', 46' are formed and configured, and these are used for fine adjustment L
You may make it connect by 47'-50 '. The entire back surface of the dielectric is grounded.

Further, the connection relationship between the fine adjustment L and the coaxial resonator is not limited to that shown in FIG. 6A, but as shown in FIG. Figure 6
As shown in (c), the variable condenser 63 is connected to the L for fine adjustment.
6 is omitted, or a variable capacitor 63 is connected to an intermediate position of the coaxial resonator 62 as shown in FIG.
As shown in (e), it is also possible to connect the variable capacitor 63 to the intermediate position of the coaxial resonator 62 and omit the fine adjustment L61.

[0020]

As described above, according to the present invention, since the coaxial line plays the role of a lead wire, the stray capacitance does not become a problem, and the sample coil and the tuning circuit can be separated without reducing Q. Since it is a distributed constant circuit, the loss of RF power can be reduced. In addition, since a large space can be taken, the mounting of electric parts can be facilitated, the structure can be simplified, and the gradient magnetic field coil in the case of NMR-CT can be easily mounted in the probe.
Further, since the RF electric field is minimized at the position of the sample coil, it is possible to prevent dielectric heating and discharge from occurring, and since it is not necessary to attach a capacitor to the sample coil, the frequency does not change due to temperature change, Further, even if the temperature of the sample is adjusted, the temperature of the variable capacitor does not change.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of the present invention using a coaxial resonator.

FIG. 2 is a diagram for explaining an embodiment of the present invention using a double tuning circuit.

FIG. 3 is a diagram illustrating a voltage and a current of a standing wave viewed from the HF side.

FIG. 4 is a diagram illustrating a voltage and a current of a standing wave viewed from the LF side.

FIG. 5 is a diagram showing another embodiment of the coaxial resonator.

FIG. 6 is a diagram for explaining the combined use and replacement of a fine adjustment L with a variable condenser.

FIG. 7 is a diagram showing a conventional NMR detector.

FIG. 8 is a diagram illustrating a conventional double tuning circuit.

FIG. 9 is a diagram for explaining the arrangement of each electric component.

FIG. 10 is an explanatory diagram for mounting a gradient magnetic field coil for NMR-CT.

[Explanation of symbols]

30 ... Sample coil, 31 ... Inner conductor, 32 ... Outer conductor, 33 ... Tuning variable capacitor, 34 ... Matching variable capacitor, 40 ...
RF coil, 41 ... Coil, 42 ... Inner conductor, 43 ... Outer conductor, 44-46 ... Coaxial resonator, 47-50 ... Inductance for fine adjustment, 51, 53 ... Tuning variable condenser, 52,
54 ... Matching variable condenser.

Claims (2)

[Claims] 1. A nuclear magnetic resonance detector in which a sample coil and a tuning matching circuit are connected to each other so that the sample coil and the tuning matching circuit are caused to resonate at a predetermined frequency. A nuclear magnetic resonance detector characterized in that a resonance circuit is formed with a coaxial line. 2. A nuclear magnetic resonance detector in which a sample coil is connected to a tuning matching circuit for high frequency and a tuning frequency for low frequency, and double tuning is performed by each tuning matching circuit. A nuclear magnetic resonance detector characterized in that a resonance matching circuit is formed by the sample coil and the coaxial line, each of which is connected to a tuning matching circuit by a coaxial line.