JP3184849B2 - Nuclear magnetic resonance detector - Google Patents

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 shows a conventional NMR detector. FIG. 7A shows a configuration in which the sample coil 1 is connected to the tuning variable condenser 3 and the matching variable condenser 4 by a lead wire 2 so as to resonate with a target frequency. The temperature of the sample placed in the sample coil 1 is adjusted. If the lead wire 2 is shortened and the electrical components are arranged close to the sample, the heat insulation for temperature adjustment is affected, and the magnetic field is distorted. The performance of the resonator decreases. Therefore, it is necessary to separate the variable condenser 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, so that the lead wire 2 cannot be omitted. However, the lead wire 2 has a stray capacitance, which makes it impossible to increase the resonance frequency, 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 is caused to occur between the two and the tuning variable condenser 3 finely adjusts the frequency.

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 with respect to high frequency (HF), and connects open coaxial resonator 12 to one end of sample coil 11 and short-circuit coaxial resonator 13 to the other end. , HF input / output side and low frequency (LF) input / output side are connected with tuning variable condensers 14 and 16 and matching variable condensers 15 and 17, respectively. For HF, short-circuited coaxial resonator 13
The electric field is maximized at the input end (point P) of FIG. 2 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 condenser 14. At this time, since the electric field is minimized at the point Q, there is no loss of HF power flowing to the LF side. The open-ended coaxial resonator 12 has nothing to do with LF, and the coaxial resonator 13 acts as a grounded inductance. Therefore, the sample coil 1 and the coaxial resonator 13 are connected in parallel to the LF. The frequency is adjusted by the tuning variable condenser 16. Thus, the frequency can be independently adjusted for HF and LF.

[0004]

However, in the circuit shown in FIG. 7 (b), it is difficult to obtain a high resonance frequency due to the stray capacitance of the lead wire. There is a problem that the resonance frequency changes due to the influence of the dielectric material 5 and the magnetic field is distorted. 7A and 7B, since a high RF electric field is generated in the sample coil, the sample is dielectrically heated and discharge is easily generated in the sample coil.

In the double tuning circuit shown in FIG. 8, each electric component (coaxial resonators 12, 13 and variable condensers 14 to 17) is connected to a sample coil 1 as shown in FIG.
Therefore, it is necessary to dispose the sample immediately beside the installation position. For this reason, when the temperature of the sample is changed, there is a problem that 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 at the center of the coil 18. The Q will drop away. As described above, there is a problem that the sample coil is not separated from each electric component, and mounting is difficult, an electric field loss in the sample coil increases, electric discharge easily occurs, and it is difficult to obtain a high frequency.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and it is not necessary to attach a coaxial capacitor at a position close to a sample coil, and it is possible to tune a remote position without lowering Q, and to provide a sample coil. An object of the present invention is to provide an NMR detector capable of reducing the electric field loss in the above and preventing the occurrence of electric discharge.

[0007]

According to the present invention, there is provided a nuclear magnetic resonance detector in which a sample coil is connected to an adjustment matching circuit, and a resonance is made at a predetermined frequency by the adjustment matching circuit. The sample coil and the same adjustment matching circuit are configured so that the external conductor is at ground potential and surrounds the sample coil, and one end of the sample coil is connected by a coaxial line connected to the external conductor to form a resonance circuit. The present invention also provides a nuclear magnetic resonance detector in which a sample coil is connected to the same adjustment matching circuit for high frequency and low frequency, and a double tuning is performed by each of the adjustment circuits. Place the sample coil and the same adjustment matching circuit for high frequency and low frequency. It surrounds one end of the sample coil is connected by coaxial line which is connected to the outer conductor, characterized in that the sample coil and the coaxial line so as to form a resonant circuit.

[0008]

According to the present invention, the sample coil and the adjustment matching circuit are connected by a coaxial line, a resonance circuit is formed by the coil and the coaxial line, tuning is performed, and double tuning is performed. At a position distant from the target frequency without lowering Q. Further, even if the temperature of the sample is adjusted, no temperature fluctuation occurs in the variable condenser, and the mounting space for the electric components can be widened, so that the mounting becomes easy. Also, NMR
All that is required is an RF magnetic field (RF
It is desirable that the RF electric field flowing through the sample coil reduce the RF electric field at the maximum. In the present invention, the electric field in the sample coil can be reduced, so that the electric field loss can be reduced and the occurrence of discharge can be prevented.

[0009]

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

A coaxial line is formed by the inner conductor 31 and the outer conductor 32, and the outer conductor 32 is grounded. One end of the sample coil is connected to the tip of the internal conductor 31, and the other end of the sample coil is connected to point A of the external conductor 32 to be grounded. A tuning variable condenser 3 is connected to the other end of the inner conductor.
3. The matching variable condenser 34 is connected to apply an RF signal. Note that the inner conductor 31 may not 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 the inner conductor of the coaxial line is grounded through the sample coil, so that the RF electric field at each position (broken line in the figure) , RF current (solid line in the figure) is shown in FIG.
(B), RF at the position of the sample coil
The electric field is minimum and 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 condenser 33. In this detector, the stray capacitance is not a problem because the coaxial line plays the role of a lead wire, and the sample coil and the tuning circuit can be separated without lowering the Q, thereby allowing more space. Easy mounting of electrical components,
The structure can be simplified. In addition, 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. Further, since it is not necessary to attach a capacitor to the sample coil, the frequency does not change due to a temperature change.

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

2 to 4 show 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
Numerals 1 and 53 are tuning variable condensers, and 52 and 54 are matching variable condensers.

The RF coil 40 includes a coil 41 and an inner conductor 4.
2. 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 a tuning variable condenser 5 is connected to the other end (point b) of the coaxial resonator 44.
1. The matching variable condenser 52 is connected to be an HF input / output terminal. The coaxial resonator 44 is for adjusting the resonance frequency of the RF coil 40 to HF. At the point a, a coaxial resonator 45 is connected via a fine adjustment L48 to the coaxial resonator 4.
The other end of 5 is connected to a coaxial resonator 46 via fine adjustment L49, 50. The coaxial resonators 45 and 46 are for setting the connection point (point c) of the fine adjustment L49 and 50 to zero impedance at the HF frequency. Further, a tuning variable capacitor 53 and a matching variable capacitor 54 are connected to a point c, which is an LF input / output terminal. The coaxial resonators 45, 4
Numeral 6 has a length of λ / 4 with respect to the wavelength λ of HF, and the outer conductors are each grounded.

The tuning of the HF frequency will be described with reference to FIG. 3. The voltage and current of the standing wave in the RF coil 40 and the coaxial resonator 44 (FIG. 3A) are as follows.
3, the voltage is minimum and the current is maximum at the ground point, and the fine adjustment L and the coaxial resonator 4
4, the voltage becomes maximum and the current becomes minimum at the point b (the position of the tuning variable condenser 51 and the matching variable condenser 52 of HF). At this time, regarding the coaxial resonators 45 and 46 shown in FIG. 3C, the fine adjustment L48, the coaxial resonator 45, and the fine adjustment L4
9 and 50, the voltage and current distribution as shown in FIG.
3, the position of the matching variable condenser 54), the voltage becomes 0, that is, the impedance becomes 0, so that the RF is changed 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. The voltage and current of the standing wave in the RF coil 40 and the coaxial resonator 44 (FIG. 4A) are as follows.
3 and the coaxial resonator 44 is open at the point b. Since the coaxial resonator simply functions as a conductor connected to the fine adjustment L, the voltage is the minimum and the current is the maximum at the ground point of the coil 41. And almost no current flows to the fine adjustment L 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,
Referring to the coaxial resonators 45 and 46 shown in FIG. 4C, the fine adjustment L48, the coaxial resonator 45, and the fine adjustment L4
The voltage and current distributions as shown in FIG. 4D are obtained by the coaxial resonators 9, 50 and 46, and the voltage increases at the point c (the position of the tuning variable condenser 53 and the matching variable condenser 54 of the LF). be able to.

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

As a coaxial resonator, as shown in FIG. 5, a microstrip line 44 ',
45 'and 46' are formed and configured, and these are
Connection may be made at 47 'to 50'. Note that the entire back surface of the dielectric is grounded.

The connection relationship between the fine adjustment L and the coaxial resonator is not limited to that shown in FIG. 6 (a). FIG.
As shown in (c), a variable condenser 63 is connected and fine adjustment L
6 is omitted, a variable condenser 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 a variable condenser 63 at an intermediate position of the coaxial resonator 62 and omit the fine adjustment L61.

[0020]

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

[Brief description of the drawings]

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

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

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

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

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

FIG. 6 is a diagram illustrating the 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 illustrating an arrangement of each electric component.

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

[Explanation of symbols]

Reference numeral 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 to 46: coaxial resonator, 47 to 50: fine adjustment inductance, 51, 53: tuning variable condenser, 52,
54 ... Matching variable condenser.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G01N 24/00-24/14 G01R 33/20-33/64

Claims (2)

(57) [Claims] 1. A nuclear magnetic resonance detector in which a sample coil is connected to an adjustment circuit to resonate at a predetermined frequency by the adjustment circuit.
The sample coil and the adjustment matching circuit are arranged so that the external conductor is at the ground potential and surrounds the sample coil, and one end of the sample coil is connected by a coaxial line connected to the external conductor to form a resonance circuit. A nuclear magnetic resonance detector characterized by the above-mentioned. 2. A sample coil in which a sample is arranged in a coil in a nuclear magnetic resonance detector in which a sample coil is connected to the same adjustment circuit for high frequency and low frequency and double tuning is performed by each of the same adjustment circuit. And the adjustment circuits for high frequency and low frequency, with the outer conductor at the ground potential and surrounding the sample coil.
A nuclear magnetic resonance detector characterized in that one end of a sample coil is connected by a coaxial line connected to an external conductor, and a resonance circuit is formed by the sample coil and the coaxial line.