JPH04158839A - Impedance matching system and apparatus for rf coil tuning circuit - Google Patents

Abstract

PURPOSE:To match an impedance with one adjustment of first and second variable capacitors by obtaining a signal proportional to a resistance component of the impedance of an RF coil tuning circuit and a signal proportional to a reactance. CONSTITUTION:An output of a low-pass filter 11a, namely, an output ER of a phase shifting detection circuit 9a becomes proportional only to a resistance component RL of an impedance ZL. Hence, a CPU5 adjusts a capacity of a variable capacitor C1 based on a size of a signal ER and controls it to meet RL=RO. Then, an output of a low-pass filter 11b, namely, an output EX of a phase detection circuit 9b becomes proportional only to a reactance component XL of the impedance ZL. Thus, the CPU5 adjusts the capacity of a variable capacitor C2 based on the size of the signal EX and controls it to meet XL=0 thereby achieving a matching of the impedance ZL of a tuning circuit 1 to a desired value handily and in a short time.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention matches the impedance of a tuning circuit of a transmitting RF coil used in a magnetic resonance imaging apparatus with the impedance of an RF pulse transmitter. The present invention relates to an impedance matching device, and particularly to reducing the time required for matching.

(Prior Art) Generally, a transmitting RF coil used in a magnetic resonance imaging apparatus forms a tuning circuit with a matching capacitor circuit having a variable capacitor, and adjusts the capacitance of this variable capacitor to control the RF pulse transmitter. Matched with impedance.

FIG. 7 shows the configuration of such a tuning circuit 1, and the matching capacitor circuit 2 is composed of a variable capacitor C2 connected in series with respect to the signal input and a variable capacitor C1 connected in parallel. Ru. Further, the RF coil 3 is composed of a resistor R and a coil L geometrically. and,
Each variable capacitor c, , c2 of matching capacitor circuit 2
By setting the capacitance of HWi, the impedance of the tuning circuit 1 is matched.

Conventionally, as an impedance matching method for the tuning circuit 1, for example, a broadband impedance meter or a λ/8
Detectors and those described in US Pat. No. 4,493,112 are known. The principles of impedance matching by these conventional methods will be explained below.

The impedance Z of the tuning circuit 1 shown in FIG. 7 is expressed by the following equation (1).

Therefore, the resistance component RL and reactance component XL of the tuning circuit 1 are expressed by the following equations (2) and (3).

Normally, the reactance component XL is set to 0 [Ω], so if (XL-0) is set in (3) and the relationship between C1 and C2 is determined, Equation (4) is obtained.

When formula (4) is illustrated, it becomes a curve as shown in FIG. 8, and (Xt = 0) always holds on this curve. Therefore, a point is searched on this curve where the resistance component RL of the tuning circuit 1 takes a desired value (for example, Ro).

First, the capacitance of capacitor C2 is increased to (XL -0) while the capacitance of capacitor C1 is minimized (8th
Point A+) in the figure. Then, measure the resistance component RL at this time, and if (RL #Ro), increase the capacitance of capacitor CI a little (point B+), and in this state, capacitor C
2 is reduced to (XL-0) (point A2).

Then, measure the resistance component R0, and when (Rt#Ro), repeat the above operation and point B2-A3→B3→...
・While changing the capacitance of capacitors c, , c2,
Search for a point P that satisfies (Rt, -Ro).

In this way, so that the capacity at this point P is
By adjusting the variable capacitors cl and c2 shown in FIG.
The impedance Z1° of the tuning circuit 1 can be set to a desired value.

(Problem to be Solved by the Invention) However, in such a conventional impedance matching method, since the matching point is searched for while adjusting the capacitance of the variable capacitor CI+02 many times, it takes a long time to achieve matching. However, the disadvantage is that the operation becomes complicated.

The present invention was made to solve such conventional problems, and its purpose is to provide an impedance matching method and matching device for an RF coil tuning circuit that can easily and quickly match impedance. Our goal is to provide the following.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the method of the present invention includes a first variable capacitor connected in parallel to the supply current, and a second variable capacitor connected in series. In an impedance matching method that matches the impedance of an RF coil tuning circuit consisting of a matching capacitor circuit having a capacitor of A signal EX having a magnitude proportional to the component is obtained, and the first variable capacitor is first adjusted based on the signal ER, and then the second variable capacitor is adjusted based on the signal Ex. The feature is that the impedance of the RF coil tuning circuit is matched.

The device of the present invention also provides an RF coil tuning circuit comprising a matching capacitor circuit having a first variable capacitor connected in parallel with the supply current and a second capacitor connected in series, and an RF coil. An impedance matching device for matching the impedance of a constant current source that supplies a constant current to the tuned circuit, a means for extracting a voltage 1 proportional to the output current of the constant current source, and a phase of the voltage Vl. phase shifting means for obtaining a voltage v2 shifted by 90°; means for determining an adjustment amount of the first variable capacitor based on the voltage vI and the input terminal voltage v3 of the tuning circuit; and means for determining the adjustment amount of the second variable capacitor from v3.

(Function) With the configuration as described above, the output current from the constant current source is supplied to the tuned circuit, and the input terminal voltage v3 of the synchronized circuit is determined. Further, a voltage vI proportional to the output current of the constant current source is extracted, and this voltage vI is multiplied by the input terminal voltage v3.

Then, by removing the high frequency component of the multiplied signal, a signal ER having a magnitude proportional to the resistance component of the impedance of the tuning circuit is obtained.

Further, a voltage v2 is obtained by shifting the phase of the voltage v1 by 90'' by the phase shifting means, and this voltage V2 is multiplied by the input terminal voltage v3.Then, by removing the high frequency component of the multiplied signal, , a signal EX whose magnitude is proportional to the impedance and actance components of the tuned circuit is obtained.

After that, by adjusting the capacitance of the first variable capacitor and the second variable capacitor that constitute the matching capacitor circuit based on the obtained signal ER1Ex, the impedance can be matched easily and in a short time. Become.

(Example) Hereinafter, an example of the present invention will be described in detail based on the drawings. First, the operating principle of the device of the present invention will be explained.

Now, the equations (2) and (3) mentioned above are shown below again.

ωIL4 R R3,-□...(2) R2(1-ωl LCI)"+&ltsuLl...(
3) As is clear from equation (2), the impedance of the tuned circuit 1 2. The resistance component RL of is a function only of variable capacitor C1 and is unrelated to variable capacitor C2. Furthermore, from equation (3), it can be seen that the actance component X is a function of both variable capacitors c, , c2.

Therefore, first adjust the variable capacitor CI to reduce the resistance component R.
5 to be equal to the desired size R8, and then adjust only the variable capacitor C2 to make the reactance component XL zero, the impedance Z of the tuned circuit 1 can be adjusted with one adjustment.
L can be matched. This can be explained with a diagram as shown in FIG. 2. First, the capacitance of the variable capacitor C1 is increased and fixed at a position (point A+) where it becomes (R+, -Ro). Next, the capacitance of the variable capacitor C2 is increased and fixed at a position (point P) at (XL-0).

If the impedance is matched in this way, the eighth
The operation is simpler than the method shown in the figure.

An impedance matching device that can implement the above matching method will be described below.

FIG. 3 is a schematic configuration diagram of an impedance matching device to which the present invention is applied. In the figure, a tuning circuit 1 composed of a matching capacitor circuit 2 and an RF coil 3 is connected to an impedance detector 4.

The impedance detector 4 detects the impedance ZL (=Rt + jXL) of the tuned circuit 1, and outputs a signal E R+ proportional to this resistance component RL and a signal Ex proportional to the reactance component XL to the CPU 5.

Based on the signals ER and Ex, the CPU 5 adjusts the variable capacitor C1 so that the resistance component RL becomes (RL - Ro), and then adjusts the reactance component XL to (Xt=O
) The variable capacitor C2 is adjusted so that

FIG. 1 is a diagram showing in detail the internal configuration of an impedance detector 4, which is the main part of the present invention.

As shown in the figure, the impedance detector 4 includes an AC constant current source 6, a resistor r, a differential amplifier 7, a 90'' phase shifter 8, and phase detection circuits 9g and 9b. .

The output end of the constant current source 6 is connected to the tuned circuit 1 via a resistor r, so that a constant current is supplied to the tuned circuit 1. Further, both ends of the resistor r are positive signal input terminals of a differential amplifier 7 having a gain of G.

and the negative signal input terminal, and the differential amplifier 7
The output terminal of is branched into two systems, one of which is connected to the balanced modulator 10a of the phase detection circuit 9a, and the other connected to the balanced modulator 10b of the phase detection circuit 9b via the 90'' phase shifter 8.90 °The phase shifter 8 shifts the phase of the input voltage signal to 9
It is delayed by 0°.

Moreover, the balanced modulators 10a, 1. Ob is also connected to the input end of the tuning circuit 1-, multiplies the two supplied signals, and outputs this to the low-pass filters 11a and 1.1b.

The low-pass filters lla and llb remove high frequency components of the multiplied signal.

Next, the operation of this embodiment will be explained.

Now, if the output current i of the constant current source 6 is i-1+m5Lnωt (5), then the voltage V generated across the resistor r is v, w
ar I, sin ωt − (6), so the output voltage v1 of the differential amplifier 7 is vl
=GV, -rGlm sin
ω t −(73.

Also, the output voltage v2 of 90@phase shifter 8 is v2- rG
1. sin ((Llt +π/2)−r
GI wa cos ωt ・= (
[1] becomes.

On the other hand, the voltage v3 at the input terminal of the tuned circuit 1 is V3-iZt
-1s IZL 1sin (ωt+ψ)...(9) However, 1ZL 1-(RL2+XL')"'.

It is represented by ψ-tan-' (Xt/Rt, ). In the balanced modulator 10a, (VI XV3 ) is obtained. That is, vI XV3 ”” (rGl, sinωt)X
(Is IZL I S in (ωt+ψ))=
(1/2) rG 1. ' I Zt 1 (C06
φ−cos (2ω = 1ψ)) ...(phantom). Then, if the signal obtained as a result of the multiplication is supplied to the low-pass filter lla, high frequency components are removed, so the output ER of the low-pass filter l1g is as follows: ER= (1/2)rGl,' l ZL l (:
c) gψ...(11) Here, since IZLIcO8ψ-Rt -@, the formula 01) is ER-(1/2) rG 1. , 2・R+,
-Q''J, and the output of the low-pass filter lla, that is, the output ER of the phase shift detection circuit 9a, is proportional only to the resistance component RL of the impedance ZI.

Therefore, the CPU 5 shown in FIG. 3 adjusts the capacitance of the variable capacitor C+ based on the magnitude of the signal ER.
Control is performed to satisfy (R+, -Ro).

In addition, in the balanced modulator 11b, the output v of the 90' phase shifter 8
2 and the input terminal voltage v3 of the tuning circuit 1 is calculated. That is, v2 Xv3- (rGl, CO8ωt)x (Is
12 H, 1 sin (ωt+ψ)) = (1,/
2) r GI-21Zt, 1(sin(2ω
t+ψ)+sinψ) (14) Then, if the signal obtained as a result of the multiplication is supplied to the low-pass filter] 1a, high frequency components are removed, so the output E8 of the low-pass filter 11a is Ex = (1/2) rGl, 2 l
ZL I sin ψ...(I5) becomes. Here, since +21,1 sinψ−X +−−QJ3, (9
The formula is Ex −(1,/2)rG I, 2 ・X+,
-Q7), and the output phase of the low-pass filter 10b, that is, the output EX of the phase detection circuit 9b is equal to the impedance Z.
1. It is proportional only to the glue actance component XL. Therefore, the CPU 5 controls the capacitance of the variable capacitor C2 to satisfy (XL-0) based on the magnitude of the signal Ex.

In this way, the impedance ZL of the tuning circuit 1 is matched to a desired value.

In this way, in this embodiment, the phase detection circuits 9a, 9
b, impedance Z1. A signal ER1 with a magnitude proportional to the resistance component RL and a signal EX with a magnitude proportional to the reactance component XL are obtained, and after adjusting the variable capacitor CI based on the signal ER, the variable capacitor C2 is adjusted based on the signal Ex. Thus, the impedance 21 of the tuning circuit 1 is matched to a desired value.

Therefore, it becomes possible to match impedance easily and in a short time.

FIG. 4 is a diagram showing an example in which an operational amplifier is used as the constant current source 6. The constant current source 6 is a well-known one, and the output current i is expressed as i-Rr.e, /R5-R3=.■. In addition, in this example, the resistor R3 included in the constant current source 6 is
It is used as the resistor r shown in FIG.

Further, as shown in FIG. 5, a constant current source can be configured with a circuit using transistors. In this case, by making resistors R4 and R5 high resistance, i = e, /RE
...(19).

FIG. 6 is a configuration diagram showing a modification of the present invention.

In this modification, the differential amplifier 7 is not used, and the output voltage of the power source e included in the constant current source 6 is taken out as the voltage v1, and the voltage v2 is obtained by shifting the phase by 90 degrees. Now, e, −Es s inωt −(2
0), so vl−Ess i nωt −(21)
-EScosωt (22), which has the same form as the above-mentioned equations (7) and (8). Therefore, the output ER of the phase detection circuit 9a is the resistance component RI of the impedance ZL.

The output Ex of the phase detection circuit 9b is proportional to the actance component xL.

In this way, in this modification, the signals ER and Ex can be obtained without providing the differential amplifier 7.

[Effects of the Invention] As explained above, in the present invention, a signal proportional to the resistance component of the impedance of the RF coil tuning circuit and a signal proportional to the reactance component are obtained, and thereby a matching capacitor circuit is configured. a first variable capacitor;
and adjusting the capacitance of the second variable capacitor.

Therefore, it is not necessary to adjust each variable capacitor many times, and it is possible to achieve the effect that impedance can be matched by adjusting by -degrees.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing the procedure for adjusting the capacitance of variable capacitors c, c2 according to this embodiment, and FIG. 3 is a schematic diagram of the structure of this embodiment. Figure, 4th
The figure is a diagram specifically showing the configuration of the constant current source of this embodiment.
This figure shows another example of a constant current source, and FIG. 6 is a configuration diagram showing a modified example. Further, FIG. 7 is a diagram showing a tuning circuit, and FIG. 8 is a diagram showing a conventional procedure for adjusting the capacitance of variable capacitors c, c2. 1... Tuning circuit 2... Matching capacitor circuit 3.
...RF coil (equivalent circuit) 4...Impedance detector 6...Constant current source 7...Differential amplifier 8...90°
Phase shifters 9a, 9b...phase detection circuits 10a, 1. Ob...balanced modulator

Claims (2)

[Claims] (1) R consisting of a matching capacitor circuit having a first variable capacitor connected in parallel with the supply current and a second capacitor connected in series, and an RF coil.
In the impedance matching method of matching the impedance of the F coil tuning circuit, a signal E_R with a magnitude proportional to the resistance component of the impedance of the RF coil tuning circuit and a signal E_X with a magnitude proportional to the reactance component are obtained, and first RF coil tuning characterized in that the first variable capacitor is adjusted based on the signal E_R, and then the second variable capacitor is adjusted based on the signal E_X to match the impedance of the RF coil tuning circuit. Circuit impedance matching method. (2) R consisting of a matching capacitor circuit having a first variable capacitor connected in parallel with the supply current and a second capacitor connected in series, and an RF coil.
An impedance matching device that matches the impedance of an F-coil tuned circuit, comprising: a constant current source that supplies a constant current to the tuned circuit; a means for taking out a voltage v_1 proportional to the output current of the constant current source; phase shifting means for obtaining a voltage v_2 by shifting the phase of v_1 by 90 degrees; means for determining an adjustment amount of the first variable capacitor based on the voltage v_1 and the input terminal voltage v_3 of the tuning circuit; and the voltage An impedance matching device for an RF coil tuning circuit, comprising: means for determining the adjustment amount of the second variable capacitor from v_2 and voltage v_3.