JP2558834Y2 - Impedance matching device - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an impedance matching device, and is particularly useful as an impedance matching device interposed between an RF coil of an MR device and a transmission line to an amplifier.

[0002]

2. Description of the Related Art The impedance of an RF coil in an MR apparatus varies depending on the size of a patient's body and the like. on the other hand,
The impedance of the transmission line to the amplifier side is generally constant at 50Ω. Therefore, in order to efficiently supply the RF pulse signal to the RF coil and to receive the RF signal efficiently, an impedance matching device is provided between the RF coil and the transmission line to the amplifier side, and the impedance of the RF coil is reduced. It is necessary to perform impedance matching corresponding to the change.

FIG. 3 shows an example of such a conventional impedance matching device. The impedance matching device 300 is a π-type circuit including the variable capacitors 31 and 33 and the variable inductor 32, and is provided between the RF coil 36 and the transmission paths to the amplifiers of the transmission circuit 34 and the reception circuit 35. 37 is a transmission / reception switching circuit. When the impedance of the RF coil 36 changes depending on the patient, the variable capacitors 31 and 33 and the variable inductor 32 are adjusted to perform impedance matching.

[0004]

In the above-described conventional impedance matching circuit 300, the variable capacitors 31,
If the impedance conversion ratio is changed by adjusting the variable inductor 33 and the variable inductor 32, the amount of phase shift also changes at the same time.

However, when the amount of phase shift changes, for example, in a "Quadrature drive" for simultaneously processing 0 ° and 90 ° signals of the RF coil, the signals do not become 0 ° and 90 °, resulting in a reduction in efficiency and the like. There is a point.

It is not impossible to change the impedance conversion ratio while keeping the phase shift constant, but there is a problem that the calculation of the adjustment amounts of the variable capacitors 31, 33 and the variable inductor 32 becomes complicated.

Therefore, an object of the present invention is to provide an impedance matching device capable of easily changing an impedance conversion ratio while maintaining a constant phase shift amount.

[0008]

SUMMARY OF THE INVENTION The impedance matching apparatus of the present invention comprises a plurality of transmission circuits having the same characteristic impedance with the same phase shift amount or a plurality of transmission circuits having at least one different characteristic impedance with the same phase shift amount. And a transmission circuit selecting and interposing means for selecting one or more of the plurality of transmission circuits and interposing between the two circuits having different characteristic impedances. It is assumed that.
In the above-described configuration, it is preferable that the plurality of transmission circuits include a λ / 4 coaxial line.

[0009]

In the impedance matching device of the present invention, a plurality of transmission circuits having the same phase shift amount are prepared in advance. Then, one of the transmission circuits is selected by the transmission circuit selecting and interposing means, and is interposed between two circuits having different characteristic impedances. Alternatively, two or more of the transmission circuits are selected by the transmission circuit selecting and interposing means, and they are arranged in parallel and interposed between two circuits having different characteristic impedances.

Since a plurality of transmission circuits have the same phase shift amount, even if one or two or more transmission circuits are in parallel, the phase shift amounts will also be equal. On the other hand, since the characteristic impedance is different between one case and two or more cases in parallel, the impedance conversion ratio can be changed. That is, it is possible to easily change the impedance conversion ratio while maintaining the phase shift amount constant.

[0011]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. Note that this does not limit the present invention. FIG. 1 is a configuration diagram of an impedance matching device 100 according to a first embodiment of the present invention.

1 has a characteristic impedance of Z01,
The coaxial cable has a line length of λ / 4 (or an odd multiple of λ / 4). No. 2 has a characteristic impedance of Z02 and a line length of λ /
4 (or an odd multiple of λ / 4). No. 3 has a characteristic impedance of Z03 and a line length of λ /
4 (or an odd multiple of λ / 4). Since the coaxial cables 1, 2, and 3 have a line length of λ / 4 (or an odd multiple of λ / 4), the phase shift amount is π / 2 (or an odd multiple of π / 2) in each case.

Reference numeral 4 denotes a switch for intermittently connecting one end of the coaxial cable 1 to the transmission line 10 to the amplifier. Reference numeral 5 denotes a switch for interrupting the other end of the coaxial cable 1 with respect to the RF coil 101. These switches 4 and 5 are linked and turn on / off at the same time.

Reference numeral 6 denotes a switch for interrupting one end of the coaxial cable 2 with respect to the transmission line 10 to the amplifier. Reference numeral 7 denotes a switch for interrupting the other end of the coaxial cable 2 with respect to the RF coil 101. These switches 6 and 7 are linked, and are turned on / off at the same time.

Reference numeral 8 denotes a switch for intermittently connecting one end of the coaxial cable 3 to the transmission line 10 to the amplifier. Reference numeral 9 denotes a switch for interrupting the other end of the coaxial cable 3 with respect to the RF coil 101. These switches 8 and 9 are linked, and are turned on / off at the same time.

The impedance of the transmission line 10 to the amplifier is Zamp, and the impedance of the RF coil 101 is Zco.
il, and the impedance of the impedance matching device 100 is Z0. When the coaxial cables 1 to 3 are selected so that Z0 and Zcoil = Z0 and Z0, and the switches 4 to 6 are turned on and off, Impedance matching can be performed. As described above, the amount of phase shift is always π / 2.

In the example of FIG. 1, since there are three coaxial cables 1 to 3 having different characteristic impedances, Z0 can be changed in seven steps. Actually, a coaxial cable may be provided so as to be changed only in a stage that does not hinder the observation of the MR device. A plurality of coaxial cables having the same characteristic impedance are provided.
0 may be changed.

FIG. 2 is a configuration diagram of an impedance matching device 200 according to a second embodiment of the present invention. This second
In this embodiment, a plurality of lumped constant circuits 12, 13, and 14 having a fixed phase shift amount and different characteristic impedances are prepared in place of the coaxial cable, and one or more of these circuits are connected to the switch 15, 16, 17, 18, 1
9 and 20, which are interposed between the RF coil 101 and the transmission path 10 to the amplifier.

Reference numeral 12 denotes a π-type lumped constant circuit having a characteristic impedance of Z001 and a phase shift of π / 2. Reference numeral 13 denotes a π-type lumped constant circuit having a characteristic impedance of Z002 and a phase shift amount of π / 2. Reference numeral 14 denotes a π-type lumped constant circuit having a characteristic impedance of Z003 and a phase shift amount of π / 2.

[0020]

According to the impedance matching device of the present invention, it is possible to easily change the impedance conversion ratio while keeping the amount of phase shift constant. Therefore, it is particularly useful as an impedance matching device provided between the RF coil of the MR device and the transmission line to the amplifier side.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a first embodiment of the impedance matching device of the present invention.

FIG. 2 is a configuration diagram of a second embodiment of the impedance matching device of the present invention.

FIG. 3 is a configuration diagram of an example of a conventional impedance matching device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Coaxial cable 2 Coaxial cable 3 Coaxial cable 4-9 Switch 10 Transmission line to amplifier 12 Lumped constant circuit 13 Lumped constant circuit 14 Lumped constant circuit 15-20 Switch 100 Impedance matching device 101 RF coil

Claims (2)

(57) [Scope of request for utility model registration] 1. A plurality of transmission circuits having the same characteristic impedance with the same phase shift amount or a plurality of transmission circuits having at least one different characteristic impedance with the same phase shift amount, and one or two of the plurality of transmission circuits. And a transmission circuit selecting and interposing means interposed between two circuits having different characteristic impedances by selecting at least one of them. 2. The impedance matching device according to claim 1, wherein the plurality of transmission circuits are composed of λ / 4 coaxial lines.