JPH0355029A - Rf coil for mr apparatus - Google Patents

Abstract

PURPOSE:To get rid of influences of a residual impedance and a stray capacitance at a leading part of a coil by forming a capacitor using a electrode plate and an insulator on a part of the coil and connecting them through the coil to obtain a matching of impedance between the coil and an exterior circuit. CONSTITUTION:An RF coil is constituted of a coil 1, an outer cylinder 2, an inner cylinder 3, an electrode plate 4, an insulator 5 and a terminal 6 and the electrode plate is provided between the outer cylinder and the inner cylinder to form a capacitor thereof through the insulator. An impedance between both ends of the coil can be decreased. An exterior circuit is connected through this electrode plate. As the electrode plate connected with a part of the coil member and the exterior circuit is positioned through the insulator, it functions as a capacitor. Therefore, even when the coil is under a resonant condition and the impedance is high, it is divided by the capacitor and an impedance matching with the exterior circuit can be obtained. At the same time, as the capacitor can change the resonant frequency of the coil, tuning can be performed.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is directed to the ``RF'' in a nuclear magnetic resonance apparatus (MR apparatus).
Concerning coil tuning and matching.

[Conventional technology]

Previously known examples close to the present invention include Journal of Magnetic Resonance 36447-45
1 (1 9 7 9) (JOURNAL OFM
AGNETIC RESONANCE 3 6 4
4 7-451 (1979), An EfficientDeco Coil Design Witch Reduces Heating in Conductive Samples in Superconducting Spectrum Meters (1979)
upler Coil Design which R
Educes Heatingin Conducti
ve Samples in Superconduct
tingSpectrometers).

[Problem to be solved by the invention]

The above conventional technology divides the coil to enable tuning at high frequencies, but does not give any consideration to impedance matching with the external circuit, and does not depend on the size of the coil or the sample to be measured. In some cases, there is a problem in that sufficient impedance matching cannot be achieved due to residual intance or stray capacitance in the lead-out portion of the coil.

The present invention aims to eliminate the effects of residual impedance and stray capacitance in the lead-out portion of the coil, and to provide an RF coil that can achieve sufficient impedance matching with an external circuit. do.

[Means to solve the problem]

In order to achieve the above purpose, a capacitor was formed using a plate and an insulator in a part of the coil, and the coil was connected through this capacitor in order to match the impedance of the external circuit. It is something.

Furthermore, in order to form a capacitor, a conductor plate is used as a member of the coil, and an electrode plate of the same shape is laminated with an insulator interposed therebetween. The capacitance of the capacitor can be adjusted by changing the facing area of the coil and the electrode plate and the interval therebetween (thickness of the insulator).

To form a capacitor using another method, a conductor tube is used as a coil member, and an electrode plate is formed concentrically with the conductor tube to form the capacitor. As in the previous example, the capacitance can be adjusted by adjusting the facing area of the coil and the electrode plate and the interval therebetween (thickness of the insulator).

Furthermore, in order to achieve the above object, a part of the coil member is formed as a capacitor, and a mechanical movable mechanism is provided in this part so that the area or distance between the electrodes can be changed to achieve tuning and This allows for adjustment of consistency. Furthermore, in this case, the purpose can be achieved by adding a variable capacitor instead of using a mechanical movable mechanism.

[Effect]

Since part of the coil member and the plate connected to the external circuit are connected through an insulator, they act as a capacitor.Therefore, even if the coil is in a resonant state and the impedance is high, it is divided by the capacitor and the impedance is matched with the external circuit. I can do it. At the same time, the capacitor also changes the resonant frequency of the coil so that tuning can be achieved.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. No. ↓
The figure shows a one-turn coil that is a modification of the saddle shape coil. The specific size is that the diameter of the entire cylinder is approximately 11
an, the length is approximately 150. The RF coil is coil 1
, outer cylinder 2, inner cylinder 3, electrode plate 4, insulator 5, and terminal 6, and the coil, outer cylinder, cylinder, and electrode have a thickness of 0.25 mm.
The radiation is approximately 2.5Ω with a copper plate, and the thickness of the insulator is 0.1on+
Layer several layers of Teflon tape to ensure the required spacing.

The current flowing through the RF coil passes through the coil from the outer cylinder to the inner cylinder, forming a resonant circuit. On the other hand, matching with external circuits is performed from the terminals through the electrode plates. Further, the space between each cylinder is filled with an insulator. This electrical equivalent circuit is shown in FIG.

Usually, a coil is used in parallel resonance, and if it resonates as it is, the impedance at both ends of the coil will be several kilohms.
become. On the other hand, in the external circuit, a transmission line having a characteristic impedance of 50Ω is used. For this reason, 50
Impedance matching with Ω is required.

Therefore, we installed a pole plate between the outer cylinder and the inner case, and these! @ Form a capacitor through the edge. This allows the impedance at both ends of the coil to be lowered. External circuits are connected through this plate.

Impedance matching is determined by the ratio of the spacing between the outer cylinder and the electrode plate, and between the inner cylinder and the electrode plate. Furthermore, since the tuning frequency of the coil changes at the same time, the spacing between each plate is determined by both tuning and matching conditions. In this example, the distance between the outer cylinder and the electrode plate is 0.
4mm. The distance between the inner cylinder and the electrode plate is 0.1mm, the matching impedance is 50Ω, and the tuning frequency is approximately 23 MHz.
It is.

However, the tuning frequency and matching impedance of the entire RF coil actually undergo some variation depending on the sample under test that enters the RF coil. This is mainly due to the formation of stray capacitance between the coil and the sample and the dielectric loss of the sample.

Therefore, in practice, it is desirable to be able to adjust the tuning frequency and matching impedance to some extent.

For this purpose, it is possible to change the capacitance of the capacitor by using a movable structure with the electrode plate, but this would complicate the structure.
In this embodiment, fine adjustment of the tuning frequency and matching impedance is achieved by adding a variable capacitor with a smaller capacitance between the respective electrode plates than the capacitance of the capacitor formed by the coil and the electrode plates. Specifically, a tuning capacitor is loaded between the outer tube and the inner tube in section A in FIG. 1, and a matching capacitor is loaded between the electrode plate and the inner tube in section B. The maximum capacity is 10
It is about pF. However, each of these variable capacitors cannot be adjusted completely independently, and since they influence each other, it is necessary to adjust them alternately. Further, unless the capacitance of these capacitors is sufficiently smaller than the capacitance of the capacitor formed by the coil and the electrode plate, the purpose of the present invention is deviated from and the effects of the present invention cannot be obtained.

Next, FIG. 3 shows the impedance frequency characteristics of the RF coil of this example. The one of this embodiment is shown by a solid line, and the conventional one is shown by a broken line.

According to this embodiment, the frequency characteristic of impedance is steeper than that of the conventional one.

This is an effect due to the reduction of residual inductance and stray capacitance in the coil lead-out section, and shows that a highly sensitive RF coil can be realized and that tuning and matching can be achieved over a wide range even by the influence of the sample to be measured.

The Q value, which indicates the quality of an RF coil, is conventionally about 200 to 300 for a single RF coil, but in this example, it is about 400 to 300.
About 500 can be obtained.

〔Effect of the invention〕

According to the present invention, the residual inductance and stray capacitance of the lead-out portion of the coil can be reduced, so that impedance matching with the external circuit can be achieved over a wide range even when the sample to be measured changes.

Furthermore, since the Q of the coil can be increased by about 1.5 times compared to the conventional type, the sensitivity of the RF coil can be increased.

[Brief Description of the Drawings] Fig. 1 is an external view of an RF coil according to an embodiment of the present invention;
The figure is the electrical equivalent circuit diagram of Figure 1, and Figure 3 is the RF of Figure 1.
The impedance frequency characteristic diagram of the coil is shown.

Claims (4)

[Claims] 1. An RF coil for a nuclear magnetic resonance apparatus (MR apparatus) consisting of a coil that converts high frequency power into a high frequency magnetic field (or a high frequency magnetic field into high frequency power), a tuning/matching capacitor, and their supporting members, A capacitor is formed by incorporating an insulator into a part of the coil member,
M characterized by being able to tune and match
RF coil for R equipment. 2. 2. The RF coil for an MR device according to claim 1, wherein a conductor plate is used as a part of the coil member and a capacitor is formed by forming a laminated structure of the conductor plate. 3. 2. The RF coil for an MR apparatus according to claim 1, wherein a condenser is formed by using a conductor tube as a part of the coil member and forming a concentric cylindrical structure. 4. 4. The RF coil for an MR apparatus according to claim 1, wherein the member forming the capacitor has a movable structure so that tuning and matching can be varied.