JPH0341929A - Transmission/reception switching circuit for nuclear magnetic resonance device - Google Patents

Abstract

PURPOSE:To obtain the transmission/reception switching circuit which is excellent in its high speed switching performance, and also, miniaturized by forming the transmission/reception switching circuit as a series body of two lambda/4 circuits formed by combining an impedance inversion circuit and a pin diode, and providing an intrusion obstructing circuit for an RF pulse on the bias circuit of the pin diode, as well. CONSTITUTION:From a bias power source 35, a positive bias current is supplied to a series pin diode 26 and pin diodes 25A, 25B through an intrusion obstructing circuit 30 by the resistance 33 of a low impedance circuit. In this case, as for each pin diode, its resistance value is lowered, the input resistance of impedance inversion circuits 21A, 21B comes near infinity, and when an RF pulse is transmitted from a transmission circuit 12, it is supplied to an RF coil 4 and an RF magnetic field is generated, and simultaneously, the leakage of the RF pulse to a reception circuit 13 side is obstructed by the impedance inversion circuits 21A, 21B, and also, an impedance circuit 31 obstructs the leakage of the RF pulse to the bias power source 35 side, as well by the low impedance circuit 32.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a transmitting circuit that supplies an RF pulse to an RF coil disposed in a magnet section of a nuclear magnetic resonance apparatus, and a receiving circuit that receives a detection signal from the RF coil. The present invention relates to a transmitting/receiving switching circuit that is disposed between the two and switches an RF' coil between transmitting and receiving.

[Conventional technology]

FIG. 5 is a block diagram showing the structure of a magnet section and a signal detection system of a nuclear magnetic resonance imaging device (hereinafter abbreviated as MR device). In the figure, a magnet section 1 includes a static magnetic field coil 2. Gradient magnetic field coil 3. and RP coil 4
, with the static magnetic field coil 2 generating a highly uniform static magnetic field of 100 t in the axial direction in the hollow part accommodating the subject,
RP pulse current is supplied to the RF coil 4 to excite the nuclear spins of the subject, and a signal 2 R'lit, to which position information is added to the gradient magnetic field generated by the gradient magnetic field coil 3, is transferred to the receiving coil. It is detected as follows.

The detection system 10 includes the RF coil 4. A transmission/reception switching circuit 11 that uses the RF coil 4 for transmission and reception, a transmission circuit 12 that supplies RF pulse current f to the RF' coil 4 via the transmission/reception switching circuit, and a weak reception signal received via the transmission/reception switching circuit 11. Receiving circuit 1 that amplifies and phase detection amplifies
3, and a computer system 15 that receives the detected signals via an A/D converter 14, collects nuclear magnetic resonance signals, performs correction 1 image reconstruction, etc., and displays a tomographic image on a display 16. At the same time, the entire detection system is controlled based on the pulse sequence issued by the computer system 15. There is also a known device that is equipped with two sets of body coil and head coil as RF coils and corresponding transmitting/receiving switching circuits 11, and imaging is usually performed using one of the RP coils. However,
Imaging is also performed using two sets of coils simultaneously, one as a transmitting coil and the other as a receiving coil.

By the way, R supplied from the transmitting circuit 12 to the RF coil 4
Since there is a difference of 10 to the 6th power between the power of the F pulse and that of the signal received by the receiving circuit 16, the transmit/receive switching circuit 11 is required to switch the RF pulse to the R pulse when transmitting the RF pulse.
At the same time as supplying to the F coil side, RE to the receiving circuit 13
``There is a need for a function to prevent pulses from entering the preamplifier and prevent damage caused by excessive RF pulses entering the preamplifier.

Additionally, during reception, a function is required to switch the RF coil to the receiving circuit side and transmit the weak detection signal to the receiving circuit without attenuating it. As a conventional circuit having such a function, a transmitting/receiving switching circuit called a duplexer is known. This circuit is connected to a transmitter circuit 11 and a receiver circuit 13 by two high-frequency cables (referred to as λ/4 cables) with a length corresponding to K of the wavelength λ of the RF pulse.
The receiving circuit side end (outlet side) of each λ/4 cable is connected to the ground via a forward resistance of a diode. Therefore, during transmission, the diode becomes conductive and the exit side of the λ/4 cable becomes short-circuited, while the inlet side becomes highly resistive, preventing the RF pulse from entering the λ/4 cable. RF pulses are supplied to the RF coil branched on the side. Further, during reception, the diode is turned off, and the series body of two λ/4 cables functions as a λ/2 cable to transmit the detection signal to the receiving circuit. Therefore, Rr coil, λ/
By matching the characteristic impedances of the four cables and the receiving circuit inlet side to, for example, 50Ω, the detection signal can be transmitted to the receiving circuit without distortion.

[Problem to be solved by the invention]

In the case of the conventional transmitting/receiving switching circuit, even if the frequency of the RF' pulse is set to, for example, 20 MHz, which is much higher than normal, the length of the λ/4 cable is nearly 4M, and there is a problem that the storage space thereof becomes large.

In addition, shortening the imaging time is an important issue in MR equipment, and the conventional circuit that uses a diode to bypass the R F' / There is a problem in that it cannot follow switching.

An object of the present invention is to obtain a transmission/reception switching circuit that has excellent high-speed switching performance and is miniaturized.

[Means to solve the problem]

In order to solve the above problems, according to the present invention, an RF
A button is placed between the transmitting circuit that supplies the 'HALS' and the receiving circuit that receives the detection signal of the RF coil, and switches the RP coil between transmitting and receiving. series pin diodes and two sets of impedance inverting circuits connected in series with each other; a pln diode connected to each outlet side of the impedance inverting circuits and having one end grounded; and one end connected to the outlet side of the series pin diode. the RF' coil, a bias power supply circuit that supplies a bias current as a switching signal to each pin diode, and an impedance inverting circuit provided in this bias power supply circuit, the stain source side of which is terminated with a low impedance circuit. and an RF pulse intrusion prevention circuit.

[Effect]

A λ/4 circuit (K wavelength circuit) is an impedance inversion circuit consisting of a π-shaped circuit of an inductance L and a capacitor C.
By using a configuration in which a pin diode is provided on the exit side of the pin diode, a bias t[? By controlling switching between positive bias and zero bias or negative bias using a predetermined pulse sequence from the computer system, RF pulses are almost completely prevented from entering the receiving circuit during 1 transmission, and the received signal is sent to the receiving circuit during reception. Not only can switching control be carried out without distortion, but also the switching circuit can be made compact.Additionally, the bias circuit side is equipped with an RP pulse intrusion prevention circuit consisting of an impedance inversion circuit terminated with a low impedance circuit. In this way, the intrusion prevention circuit functions as a λ/4 circuit, and can prevent RF pulses and received signals from leaking to the bias power supply side. Since it can be made significantly smaller than when using a switching signal, it is possible to reduce the slowing of the rise of the bias current as a switching signal. For example, when transmitting, the bias current supplied to each pin diode can be set to a square wave corresponding to the RF pulse width. It becomes possible to make it into a pulse. Therefore, it is not necessary to increase the bias voltage in order to slow down the rise of the bias current, and the pin diode can be switched at high speed with a low bias voltage.

〔Example〕

The present invention will be explained below based on examples.

Fig. 1 is a configuration diagram of a transmitting/receiving switching circuit according to an embodiment of the present invention, Fig. 2 is an HA diagram of the principle of the impedance inverting circuit in the circuit of Fig. 1, and Fig. 3 is a pin diagram for the circuit of Fig. 1. FIG. 3 is a resistance value versus bias current characteristic diagram of a diode. In the figure, a DC blocking capacitor 1 of a transmitting circuit 12
A series body consisting of a series pin diode 26 and two sets of impedance inversion circuits 21A and 21B is connected between the DC blocking capacitor 13A of the receiving circuit and the inductance 22 and the capacitor 13A of the receiving circuit. An impedance inversion circuit 21A consisting of a π-type circuit of 23.24.

The exit side of 21B is a pin diode 2 with one end grounded.
5A and 25B, respectively.

One end of the RF" coil 4 is connected to the outlet side of the series pin diode 26 in the housing. Furthermore, the series pin diode 26 is connected to an impedance inverting circuit 31, and a low impedance circuit 3 consisting of a resistor 33 and a capacitor 34.
A direct current bias current is supplied to the pin diodes 25A and 25B via the series pin diode 26 connected to the DC bias power supply 35 through the intrusion 1 targeting circuit 30 consisting of be done.

Impedance inversion circuits 2iA, 21B, and 31, as shown in FIG.
zb and the characteristic impedance of the circuit @Zo,
Resistance f on the inlet side and outlet side: Ro, Rt?
In the case of Knee, the circuit operates as Zo=VL1/λ/4 circuit. That is, Zaw±jV'R6R4, Zb=+
For example, when the resistance R1 on the outlet side is zero, the resistance Re on the inlet side becomes infinite, which prevents the intrusion of the high frequency 'IIt flow having the wavelength λ.

Also, pin diodes 25A, 25B, and 26
As shown in Figure 3, which shows the forward resistance value vs. bias current characteristic diagram, when the bias JIIt current is zero or negative, it shows a high resistance of more than Ω, whereas in the case of positive The forward resistance decreases exponentially by flowing a bias current t-, and the resistance value decreases to 1Ω or less by flowing a predetermined positive bias current at.

Therefore, during transmission in the circuit shown in FIG.
Supplied to diodes 25A and 25B. At this time, each p
The resistance value of the in diode decreases significantly, and the input resistance of the impedance inverting circuits 21A and 21B becomes substantially infinite. Therefore, when an RF pulse is transmitted from the transmitting circuit 12, the RF pulse is supplied to the RF' coil 4 through the series pin diode 26t which is in a conductive state, and at the same time an RFa field is generated in the magnet section 1 of the MR equipment, and at the same time, the receiving circuit 13 side The impedance inversion circuits 21A and 21B can almost completely prevent leakage of the RP pulse to the impedance inversion circuits 21A and 21B. As shown, the impedance inversion circuit 31 on the intrusion prevention circuit 30 side is terminated on the bias power supply 35 side by the low impedance circuit 32, so the input resistance of the impedance inversion circuit 31 as seen from the transmission circuit 12 side is high resistance, and the bias voltage is reduced. Leakage of RF pulses to the power source 35 side can also be prevented. Furthermore, when the characteristic impedance Zo t of the impedance inverting circuit is 50Ω, and the frequency of the RP pulse is 22 [ ] MHz, the value of the inductance 22 of each impedance inverting circuit is 398 nf (as it is extremely small, the command signal of the computer system 15 158, it is possible to avoid the slowing of the rise of the positive bias current controlled in the form of a square wave pulse which is somewhat longer than the duration of the RF' pulse, and the pulse hood RF of the bias current in the form of a square wave pulse can be avoided. It can be shortened according to the pulse duration.

Next, switching from the transmitting state to the receiving state is performed for each pi.
This is done by switching the bias current supplied to the n-diode to zero or negative. That is, by switching the bias current, the impedance inversion circuit 21A,
The exit side of 21B is in an open state, and the two impedance inversion circuits 21A and 21B function as a λ/2 circuit,
In addition, since the series pin diode 26 has a high resistance (off state B), the received signal detected by the R1i' coil is divided into two impedance inverting circuits 2 as a λ/2 circuit.
1A, 21Bt passes through with low loss and is transmitted to the receiving circuit 13, and the electrical noise from the transmitting circuit 12 is blocked from propagating to the receiving circuit side by the series pin diode 26, so the weak received signal total noise can be detected without the influence of

If the intrusion blocking circuit 30 on the bias power supply side is formed using a check coil, the inductance required to obtain 40 dBt as an RF/<Rus attenuation amount will be 10 μHa degrees.

On the other hand, when the impedance inverting circuit 31 is used, the inductance is only 398 nH, which is about 1/25th, as described above, and the inductance that affects the rise of the square wave pulsed bias current is small. The impact can be significantly reduced. Therefore, it is possible to perform square-wave pulsed bias current, that is, high-speed imaging, and there is no need to increase the bias voltage to increase the rise time Dt of the bias current, so there is an advantage that the bias power supply itself can be miniaturized. It will be done.

FIG. 4 is a block diagram of a transmission/reception switching circuit showing a different embodiment of the invention. The R1' coil of the MRr device is composed of a Rad coil and a body coil.For example, the body coil is used as a transmitting coil and the head coil is used as a receiving coil for an image pickup tube. For example, the received signal is distorted due to the optical combination, and a problem arises in that the received signal necessary for image reconstruction cannot be obtained. FIG. 4 shows a configuration diagram of a transmitting/receiving switching circuit considering the above-mentioned imaging method. The transmitting/receiving switching circuit 20 on the body coil 4A side is connected to the body coil 4A via a DC blocking capacitor T201 and a series pin diode 56. At the same time, a square wave pulsed bias current is supplied to the body coil 4A via another intrusion prevention circuit 40 button configured similarly to the intrusion prevention circuit 30 and the series pin diode 36. - The ten thousand head coil 4B is a transmission/reception switching circuit 2 configured similarly to the transmission/reception switching circuit 20.
0B! The output signal 208 is input to the receiving circuit 13, and the output side of the transmitting/receiving switching circuit 20 is opened.

When a circuit configured in this manner is transmitting, a bias current in the form of a square wave pulse is supplied from the bias power supply 35 through the intrusion prevention circuits 30 and 40t, and the series PLN diodes 26 and 36 become conductive to generate an RF pulse. The body coil 4A generates an R1' magnetic field. To switch from transmission to reception, the bias current is switched to zero or negative, the body coil 4A is opened by the series pin diode 36, the signal is detected by the head coil 4B, and the signal is received by the reception circuit 13. . With this configuration, magnetic coupling between the coils 4A and 4B during reception is prevented by the body coil 4A being in an open state, and distortion of the received signal from the head coil can be avoided. Furthermore, the intrusion prevention circuit 40 also prevents the rise of the square-wave bias current from slowing down, and the bias current pulse @'jf
Since the HRF pulse can be shortened correspondingly, high-speed imaging and shortening of imaging time based on this becomes possible.

〔Effect of the invention〕

As described above, this invention uses a transmitting/receiving switching circuit of an MR equipment as a series body of two λ/4 circuits including an impedance inverting circuit, a pin diode, and tm, and also uses an impedance inverting circuit as a bias circuit for the pin diode. The configuration is such that an RF' pulse intrusion prevention circuit is provided. As a result, by controlling the bias m current of the pin diode, the RF coil can be switched to the transmitter circuit and receiver (N circuit side), and the inductance of the impedance inverting circuit can be made extremely small.
It is possible to perform high-speed switching due to the steep, low-voltage, square-wave bias current of the pinln diode. Therefore, it is possible to provide a transmitting/receiving switching circuit for a nuclear magnetic resonance apparatus having a high-speed switching function compatible with high-speed imaging. can. The transmitter/receiver switching circuit can be made smaller than conventional circuits using λ/4 cables or check coils, and when the body coil and head coil are used separately as the transmitter/receiver coil, a pin provided on the transmitter coil side can be used. By controlling the diode on and off via an intrusion prevention circuit consisting of an impedance inversion circuit, distortion of the received signal caused by magnetic coupling of both coils can be prevented without impairing high-speed imaging performance of 3C. can do.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the transmitting/receiving switching circuit Mt according to an embodiment of the present invention, Fig. 2 is a diagram explaining the principle of the impedance inverting circuit shown in Fig. 1, and Fig. 3 shows the resistance of the pin diode in Fig. 1. - Bias current characteristic diagram; FIG. 4 is a circuit configuration diagram showing a different embodiment of the present invention; FIG. 5 is a MH
It is a block diagram which shows an example of the magnet part and detection system of I apparatus.

Claims (1)

[Claims] 1) In a device that is disposed between a transmitting circuit that supplies RF pulses to an RF coil disposed in a magnet part of a nuclear magnetic resonance apparatus and a receiving circuit that receives a detection signal of the RF coil, and switches the RF coil between transmitting and receiving. , a series pin diode and two sets of impedance inverting circuits connected in series between the transmitting circuit and the receiving circuit, a pin diode connected to each outlet side of the impedance inverting circuit and having one end grounded; The power supply consists of the RF coil, one end of which is connected to the outlet side of the series pin diode, a bias power supply circuit that supplies a bias current as a switching signal to each pin diode, and an impedance inversion circuit provided in this bias power supply circuit. 1. A transmission/reception switching circuit for a nuclear magnetic resonance apparatus, comprising: an RF pulse intrusion prevention circuit whose side is terminated with a low impedance circuit.