JPH024330A - Automatic impedance adjusting device for mri device - Google Patents

Abstract

PURPOSE:To automatically execute efficient adjustment by dividing the impedance of a probe head into a real part and an imaginary part with a detecting part and sending a control signal, which adjusts the values of circuit elements for real part adjustment and imaginary part adjustment, from a control part to the probe head based on a result to be detected as a voltage. CONSTITUTION:When a real part voltage (Re side voltage) to be detected by a detecting part 7 is positive, the control signal is supplied so as to reduce C3 of a probe head 6 since it can be decided as Zi<Zc. Thus, since the Zi goes toward a direction to be larger than the Zc, when the real part voltage goes to 0(V), the supply of the control signal to C3 is stopped. When an imaginary part voltage to be detected by the detecting part 7 is negative, the control signal is supplied so as io enlarge C4. Thus, in correspondence to the real part voltage and imaginary part voltage which are detected by the detecting part 7, a control part 8 adjusts the values of C3 for real part adjustment and C4 for imaginary part adjustment. Then, the control part is operated so as to supply a negative feedback(NFB) signal to C3 or C4 of the probe head 6 in order to match the impedance Zi to the characteristic impedance Zc.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention 1 (Industrial Application Field) The present invention relates to an automatic impedance adjustment device for an MRI apparatus that adjusts the impedance of a probe head to be equal to the characteristic impedance.

(Prior Art) A probe head is used as an antenna for receiving an MR multiplied signal sent from a subject in an MRI apparatus. Fifth
The figure shows an example of an equivalent circuit of this probe head, and a power supply line (characteristic impedance Zc, 50Ω in most cases) is connected to this probe head and routed according to the diagnosis mode.

Here, C1 and C2 are variable capacitors, coils, and R
is the equivalent parallel resistance of the coil, and Zi is the impedance visible from the input side. By the way, when using this probe head, the impedance seen from the input side (hereinafter simply referred to as impedance) z is due to the following two reasons.
It is necessary to adjust i to be equal to the characteristic impedance Zc. (2) In the case of Z+Zc, MR multiplied signal transmission loss occurs in the feeder line, resulting in deterioration of S/N characteristics. ■Low-noise amplifiers used in MRI equipment are often designed with an input impedance of Zc''''C, so in the case of 71-Zc, that is, when connected to a signal source with a signal source impedance other than ZC, noise Since figure matching is sacrificed, low noise is not necessarily guaranteed.

In the equivalent circuit of Fig. 5, variable capacitors C2 and C1
The relationship between z and impedance zi is expressed by the following equation.

As is clear from the above equations (1) and (2), when zi is increased, C2 becomes smaller and C1 becomes larger.

That is, if R and L are constant, if C2 is made small (of course, C1 is made a little large accordingly), the impedance Zi of the probe head adjusted to be a pure resistance can be made large.

Next, the impedance Zx of the circuit portion within the dotted line is calculated as shown in the following equation.

(However, R and zr are real numbers.) Also, the impedance ZY of the two C2 circuit parts is expressed by the following equation.

...(4) That is, as is clear from the above formulas (3) and (4), Z
i is CI, Z× formed by R, L and two 0s
ZY formed by It turns out that you can cancel it out and make it zero.

Based on the above facts, in order to satisfy the condition of Zi = Zc as described above, if there is a relationship of Zi <7:c, C2 should be adjusted small and the imaginary part generated at that time should be canceled out. By adjusting C1 to a large value, zi may be adjusted to become large. If zi>zc, conversely, zl may be adjusted to be smaller by adjusting C2 larger and C1 smaller.

By the way, the probe head alone is required to have a high Q, and since the subject approaches the probe head during imaging, the equivalent circuit as a first approximation of the probe head during actual imaging is as shown in Figure 5. The result is shown as shown in FIG. Here, (:, s, C'S is the stray capacitance between the human body and the coil, and RP is the equivalent resistance of the human body. Therefore,
The impedance zi also changes to Z'i. Figure 6 can be equivalently transformed as shown in Figure 7,
Furthermore, it can be simplified as shown in Fig. 8. 7th
In the figure, C's is the equivalent capacity when C3, C'S, and Rp are converted in parallel, and R'p is the equivalent capacity after conversion in the same way as C. However, . In such an equivalent circuit of FIG. 8, 1":
From the condition of =lc, the relationship between the variable capacitors C2, C't and the impedance Zc is expressed by the following equation based on equations (1) and (2) above.

(ω=2πf; f(R2) is the MR multiplied signal resonance frequency) Therefore, by adjusting C'1.02 as in the case of FIG. 5, the condition of Zi =Zc can be satisfied.

Conventionally, this adjustment was done by preparing an oscillator with an output impedance of Zc (pure resistance) and inserting a directional coupler between it and the probe head to find the point where the reflected power becomes zero through trial and error. was.

<Problem to be solved by the invention> However, in the conventional adjustment method, when manual adjustment is performed, 2
It is required to adjust two variable capacitors at the same time,
There is a problem in that efficiency is low because adjustment operations sometimes rely on intuition and experience. An automatic adjustment method using a microprocessor or the like has been proposed as an alternative to this manual adjustment method, but it is inefficient because many parts rely on the judgment function of the microprocessor.

The present invention was made in response to the above circumstances.
It is an object of the present invention to provide an automatic impedance adjustment device for an MRI apparatus that can automatically perform efficient adjustment using simple hardware.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides circuit elements for real part adjustment and imaginary part adjustment, such as variable capacitors, which are equivalently arranged in parallel. a detection unit that divides the impedance of the probe head including a real part and an imaginary part and detects it as a voltage;
It has a control section that adjusts the values of each of the circuit elements according to the voltages of the real part and the imaginary part and sends a control signal, for example, a negative feedback signal, to the probe head to match the impedance Zi to the characteristic impedance Zc. .

(Function) According to the present invention having the above configuration, the impedance of the probe head is detected as a voltage by the detection section, divided into a real part and an imaginary part, and based on this detection result, the control section sends the impedance to the probe head. A control signal (for example, a negative feedback signal) is sent to adjust the value of the real part adjustment circuit element and the imaginary part adjustment circuit element (for example, both are variable capacitors), so that the value of the impedance Zi becomes the characteristic impedance Z.
automatically matched to the value of c.

<Example) First, the present invention will be explained in detail.

“RF expo east (Nov, 10-12.
1986. Boston.

A paper introduced in “A Com-plex Impedance Met”
er (Carl G, Lodstroem, Dov
-Key) licrowave Corporation
on 1110 Mark Avenue Carpri
nteria, CA 93013-2918)”, as shown in Figure 4, there is a λ/8
By connecting the cable, the impedance of load 2 becomes the real part (Re: Rea! part) and the imaginary part (Im
: Imaginary part) and can be easily detected in a form corresponding to the voltage. still,
DI.

D2, D3, D4 are diodes, R1, R2 are variable resistors, 3 is a DC voltmeter corresponding to the real part, 4 is a DC voltmeter corresponding to the imaginary part, 5 is an attenuator (accurately adjusts the output impedance of oscillator 1 by Zc ( = 50Ω)
It is.

If the λ/8 cable matches the cable length exactly,
Connect a load with a value of ZL = Zc as load 2 and measure the voltage with a voltmeter 3.
, 4 are both ○(v) by adjusting the variable resistors (trimbot type) R1°R2 in advance to set the initial state. In this state, for example, the real part (
When Re) is smaller than Zc, the real part voltmeter 3 shows a positive voltage, and when larger than Zc, it shows a negative voltage. Also,
When the imaginary part (Im) is inductive, the imaginary part voltmeter 4 shows a positive voltage, and when it is capacitive, it shows a negative voltage. Therefore, by observing the indications of the real part voltmeter 3 and the imaginary part voltmeter 4, it is possible to grasp in which direction, larger or smaller, the value of the load impedance Zc deviates. Note that λ is determined by the frequency of the oscillator 1.

The present invention is based on such a principle and will be explained below with reference to the drawings.

FIG. 1 is a block diagram showing an automatic impedance adjustment device for an MRI apparatus according to an embodiment of the present invention. A control unit 8 is connected between them.

The configuration of the detection unit 7 uses a λ/8 cable as shown in FIG. 4, so that the impedance zi of the probe head 6 can be adjusted.
is divided into a real part (Re) and an imaginary part (Im) and detected as voltage by each voltmeter. Further, the control section 8 includes, for example, a real section 8 shown in FIG.
A, the imaginary part side 8B is configured such that a control signal for adjusting each voltage to zero according to the real part voltage and imaginary part voltage detected by the detection part 7 is sent to the circuit of the probe head 6. The power is supplied to variable capacitors C3 and C4 (hereinafter referred to as rc3J and rc4J) as elements.

Here, with reference to the equivalent circuit of the probe head 6 shown in FIG. 3 <a>, the relationship between C3, C4 and impedance zi will be considered.

In addition, in the same figure, L is the inductance of the probe head 6,
r is the equivalent series resistance of the probe head 6, and the angular frequency is ω.

Usually, the impedance Zi of the probe head 6 is matched to the characteristic impedance Zc (50Ω in most cases).

From Zi = Zc = Z, jωl + r + 2/JωC4 Substitute ■ and from formula ■ 2/C4 = ω2 (L-C3rZc)...
Substituting the 00 formula into the 0 formula, Zc-ω2LC32C +C3Zc・(t)2(L-C3rZc)-r=0Zc
-ω2 LC3Zc +ω2 LC3Zc-ω2 C32rzc 2-r=Q
From C3)O, that is, by setting C3 and C4 to values as shown in formulas 1 and 2, the impedance Zi of the probe head 6 can be made to match the characteristic impedance zC.

By the way, in many cases, Zc is 50 [Ω], and the equivalent series resistance of the probe head 6 also depends on the frequency.
It is several ohms or less. In other words, it can be considered that r < z c. Based on this fact and ■ and ■, we find the approximate formula for 03 and C4.
It can be seen that this mainly contributes to the change in the real part (=RL) of the impedance Zi (=RL+jXL) of the probe head 6.

In addition, equation (2) is derived, which agrees with the resonance condition equation when r=o in FIG. 3(a). That is, it can be seen that the value of C4 mainly contributes to making the imaginary part of the impedance of the blow-ahead 6 (XL > ).

In other words, increasing the value of C3 corresponds to decreasing the characteristic impedance Zc, that is, increasing the real part RL of the impedance Zi of the probe head 6, and also decreasing the value of C3. corresponds to increasing the characteristic impedance Zc, that is, increasing the real part RL.

On the other hand, increasing the value of C4 decreases the value of 2/ωC4, and the imaginary part of the probe head 6 (reactance >
Increasing L means increasing the value of C4, and decreasing the value of C4 means increasing the value of 2/ωC4 and decreasing the imaginary part XL of the probe head 6.

Next, the operation of the control unit 8 will be described in detail.

If the real part voltage (Re side voltage) detected by the detection unit 7 is positive, it can be determined that Zi <Zc according to the principle of the present invention, so a control signal is supplied to reduce C3 of the probe head 6. As a result, zi tends to become larger than Zc, so when the real voltage reaches 0 (v), the supply of the control signal to C3 is stopped.

If the real part voltage detected by the detection unit 7 is negative, it can be determined that zi>Zc, contrary to the above.

A control signal that increases C3 is supplied. As a result, Zi tends to become smaller than Zc, so the real part voltage can be set to O(V).

If the imaginary part voltage (Im side voltage) detected by the detection unit 7 is positive, it can be determined that it is inductive based on the principle of the present invention, so a control signal that reduces C4 is supplied. As a result, the imaginary part is canceled out and goes in the capacitive direction, so when the imaginary part voltage becomes O(V), C4
Stop supplying control signals to.

If the imaginary part voltage detected by the detection unit 7 is negative, it can be determined that the voltage is capacitive, contrary to the above, so a control signal for increasing C4 is supplied. As a result, the imaginary part moves in the inductive direction, so the imaginary part voltage is reduced to O(V)
It can be done.

In this way, the control unit 8 adjusts the values of C3, which has a real part adjustment function, and C4, which has an imaginary part adjustment function, according to the real part voltage and imaginary part voltage detected by the detection part 7, It operates to supply C3 or C4 of the probe head 6 with a control signal that matches the impedance Zi to the characteristic impedance Zc, that is, a negative feedback (NFB) signal.

However, if negative feedback with the same responsiveness is applied to C3 and C4, there is a possibility that Zi will not converge to Zc and will oscillate. Therefore, by setting the negative feedback response speed on either the real part or the imaginary part to be faster than the other,
Since the faster one follows first, Zc (Ω) or j-0 (Ω
), so no vibration occurs. Therefore, zi can be converged to Zc.

Further, when either the real part voltage or the imaginary part voltage becomes 0 (v), the control of C3 or C4 can be automatically stopped by adding an integral operation to these voltages. That is,
By integrating one of the voltages and controlling C3 or C4 in a one-to-one correspondence with this integrated value, the control is automatically performed when either of the detected voltages reaches O(V). It can be stopped.

FIG. 2 is a block diagram showing the specific configuration of the control section 8, where 8A shows the real part side, 8B shows the imaginary part side, 9 is a buffer stage, 10 is an inverting/amplifying stage, and 11 is an inverting/integrating stage. It is. Each output of the inverting/integrating stage 11 is converted and held by an A/D converter after impedance matching, and the real part 8A outputs a control voltage for C2, and the imaginary part 8A outputs a control voltage for C2.
Is there any control over C1 from B? Ii pressure is output. In order to make the response different, the relationship between the time constants C1, R1, C2, and R2 in the inverting/integrating stage 11 is set as ClR1>C2R.
2 (or 02 R2>CIRl). Note that a cable (or equivalent circuit) having an integral multiple of λ/2 length may be connected between the detection unit 7 and the probe head 6.

FIG. 3(b) shows a specific configuration of the probe head 6, and shows an example in which variable capacitance diodes are used as C3 and two C4. C3 composed of these variable capacitance diodes, for Ca, the real part 8 in Fig. 2
Control voltages from A and imaginary side 8B are applied respectively. The higher the applied reverse bias voltage, the smaller the capacitance of a variable capacitance diode (Figure 3).

Next, the operation of this embodiment will be explained.

The real part voltmeter 3 and the imaginary part voltmeter 4 of the detection unit 7 are 0 (V)l.
! : The real part voltmeter 3 is set to the initial state so that
and observe the indication of the imaginary part voltmeter 4.

When the real part voltmeter 3 shows a positive voltage, the voltage is adjusted to O(V) by applying negative feedback to C3 of the probe head 6 by the control unit 8 to reduce this value. When the real part voltmeter 3 indicates a negative voltage, the voltage is adjusted to O(V) by applying negative feedback to increase this value relative to 03.

When the imaginary 0 part voltmeter 4 indicates a positive voltage, the voltage is adjusted to O(V) by applying negative feedback to C4 of the probe head 6 by the control unit 8 to reduce this value. When the imaginary part voltmeter 4 shows a negative voltage, the voltage is adjusted to O(V) by applying negative feedback to C4 to increase this value.

Therefore, even if the majority voltmeter 3 and the imaginary voltmeter 4 indicate a positive voltage or a negative voltage, both will eventually automatically become 0 (v
) is adjusted. This automatically adjusts so that the condition of Zi=Zc is satisfied.

As described above, according to the present embodiment, the impedance Zi of the blower head 6 can be automatically adjusted to be equal to the characteristic impedance Zc without manual operation, so that efficient adjustment can be performed.

Further, since the configuration of the adjustment device is simple, it can be manufactured at low cost, and the throughput of the patient can be improved by automatically adjusting the impedance.

Variable capacitors can be used for C3 and C4 in the probe head 6, and the speed of the variable capacitor actuator can be controlled in proportion to the detected voltage, thereby making it possible to omit the integrating stage. This is because the rotation angle of the variable capacitor is proportional to the integral of the speed, and the detected voltage is O(V).
This is because the variable capacitor automatically stops while maintaining that angle. In this case, it goes without saying that the sign must be adjusted so that the detected voltage and the rotation direction satisfy a negative feedback relationship.

[Effect of the Invention 1] As explained above, according to the present invention, the impedance of the probe head is detected separately into the real part and the imaginary part, and based on the detection results, the circuit elements for real part adjustment and imaginary part adjustment are Therefore, it is possible to provide an impedance adjustment device for an MRI apparatus that can automatically match the impedance of the probe head to the characteristic impedance.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an automatic impedance adjustment device for an MRI apparatus according to an embodiment of the present invention, FIG. FIG. 3(b) is an enlarged circuit diagram of only the probe head shown in the figure. FIG. 3(b) is a circuit diagram showing the specific configuration of the probe head of the device of this embodiment. FIG. 6 is an equivalent circuit of the probe head, FIG. 6 is an equivalent circuit of the probe head during imaging, FIG. 7 is a conversion equivalent circuit of FIG. 6, and FIG. 8 is a conversion equivalent circuit of FIG. 7. 1... Oscillator, 3... Real part voltmeter, 4...
Imaginary part voltmeter, 6... Blow-ahead, 7... Detection part, 8... Control part, 8A... Real part side control part,

Claims (3)

[Claims] (1) For a probe head whose impedance seen from the input side is Zi, and which includes a circuit element for adjusting the real part of this impedance Zi and a circuit element for adjusting the imaginary part in equivalent parallel arrangement, the impedance Zi and , an automatic impedance adjustment device for an MRI apparatus that performs impedance matching with a characteristic impedance Zc, which includes an oscillator whose output impedance is Zc for supplying a signal to the probe head, and an oscillator connected between the probe head and the oscillator to adjust the impedance Zi of the probe head. a detection unit that detects the voltage as a voltage divided into a real part and an imaginary part; and a control signal that adjusts the value of each circuit element according to the voltage of the real part and the imaginary part and matches the impedance Zi to the characteristic impedance Zc. MR characterized by comprising a control section for sending data to a probe head.
Automatic impedance adjustment device for I equipment. (2) The automatic impedance adjustment device for an MRI apparatus according to claim 1, wherein each of the circuit elements is a variable capacitor. (3) The automatic impedance adjustment device for an MRI apparatus according to claim 1, wherein the control signal is a negative feedback signal.