JPH0222652B2 - - Google Patents

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The present invention relates to nuclear magnetic resonance (nuclear magnetic resonance).
resonance (hereinafter abbreviated as “NMR”)
This invention relates to an improvement of the receiving circuit in an NMR apparatus that utilizes phenomena to determine the distribution of specific atomic nuclei within a subject from outside the subject.

[Prior art]

NMR is a method of obtaining chemical information using the magnetic properties of atomic nuclei. In other words, when an atomic nucleus in a static magnetic field is excited with radio-frequency energy, the density of the atom and its bonding state with its surroundings can be determined from the resonance signal (hereinafter referred to as an NMR signal) generated by the resonance phenomenon. NMR using this principle
The device has recently attracted attention as a diagnostic device that provides anatomical and functional information.

The NMR apparatus irradiates the subject with high frequency energy as described above, but the high frequency energy at this time adversely affects the operation of the receiving circuit that detects the above-mentioned NMR signal. In other words, in the receiving circuit, saturation of the high frequency amplifier and associated response delay,
Furthermore, it causes destruction.

FIG. 1 is a block diagram showing an example of a conventional circuit that addresses this problem. In the NMR receiving circuit 1, 2 is a receiving coil section for detecting NMR signals;
4 is a preamplifier that amplifies the signal from the receiving coil unit 2; 4 is a gate circuit that controls the transmission of the output from the preamplifier 3 to the subsequent stage to prevent saturation of the high-frequency amplifier circuit due to the influence of high-frequency transmission pulses; and 5 is the gate circuit described above. A high frequency amplifier inputs and amplifies the output from the preamplifier 3 via a gate circuit 4, 6 a signal processing circuit that inputs the output from the high frequency amplifier circuit 5 and performs various signal processing, and 7 a sequence for the entire NMR apparatus. A control signal is output to the gate circuit 4 by a pulse programmer that controls the gate circuit 4. In an NMR device, the high-frequency transmission pulses sent out from the transmission coil include pulse modulated waves with different pulse widths (such as 90° and 180° pulses), and transmission waves modulated with complex waveforms such as Gaussian waveforms. is used, and the pulse programmer 7 needs to generate a signal corresponding to the pulse length of such a transmission wave. In addition, the pulse length of the received signal in the receiving circuit depends on the damping characteristics of the transmitting coil and the receiving coil and the value of Q, which changes depending on the size of the object entering the coil, and generally the pulse length of the excitation signal applied to the transmitting coil. becomes longer. Therefore, the pulse length of the control signal from the pulse programmer 7 that is applied to the gate circuit 4 must be constantly adjusted in response to the above-mentioned changes in the transmitting and receiving system, which requires complicated processing. It has its drawbacks.

〔the purpose〕

The present invention has been made to solve the above problem, and has a simple configuration that prevents saturation of the high frequency amplifier circuit in the receiving circuit due to high power transmission pulses, and also responds to changes in the transmitting/receiving system. The purpose is to realize a receiving circuit that can perform the following functions.

〔overview〕

According to the present invention, a transmission pulse is detected by a search coil in a receiving circuit for an NMR device, and a control signal generated by a control signal circuit in response to this detection signal is used to switch the gain of a preamplifier and/or turn on/off a gate circuit. The above objectives can be achieved.

[Description of Examples] The present invention will be described below based on the drawings. Second
The figure is a block diagram showing one embodiment of a receiving circuit for an NMR apparatus according to the present invention. The same parts as in FIG. 1 are given the same reference numerals. 2 is a signal coil section that detects the NMR signal, 13 is a preamplifier that amplifies the signal from this receiving coil section 2, 14 is a gate circuit that controls the transmission of the output from this preamplifier 13 to the subsequent stage, and 5 is the preamplifier 13. 6 is a signal processing circuit that inputs the output from the high frequency amplifier circuit 5 and performs various signal processing, and 8 is a search circuit that detects high frequency transmission pulses. A coil 9 inputs the detection signal obtained from the search coil 8 and generates a control signal pulse having a length corresponding to the length of this detection signal (on the time axis) to control the preamplifier 13 and gate circuit 14. This is a control signal generation circuit.

FIG. 3 is an electric circuit diagram showing a specific example of the control signal generation circuit 9 and gate circuit 14 shown in FIG.

In the control signal generation circuit 9, D ₁ , D ₂ , and D ₃ are series-connected diodes to which the output of the search coil 8 is applied, and C ₁ has one end connected to the anode of this diode D ₃ and the other end connected to the common. a capacitor, r ₁ is a resistor whose one end is connected to the one end of this capacitor, and L is a resistor whose one end is connected to this resistor r ₁
A high-frequency inductance whose other end is connected to the power supply voltage Vcc, Q is a transistor whose base terminal is connected to the one end of the capacitor _C1 , and _r2 whose one end is connected to the collector terminal of this transistor Q. D4 is a collector current limiting resistor whose other end is connected to the one end of the inductance L, and _D4 is a forward bias prevention protective diode whose cathode terminal and anode terminal are connected to the base terminal and emitter terminal of the transistor Q, respectively. be. The gate circuit 14 constitutes a current control attenuator based on a ring modulator, and 141
is an input terminal to which the output from the preamplifier 13 is added;
One end of T ₁ on its primary side is connected to this input terminal,
An input side transformer whose other end is connected to a common and whose secondary side is connected to the emitter terminal of the transistor Q of the control signal generation circuit 9, _D5 to _D8 each have an anode terminal and a cathode terminal connected to each other. A diode that is connected to form a ring.
One end and the other end of the secondary side of the transformer T ₁ are connected to the anode terminals of D ₅ and diode D ₇ , respectively. _T2 is an output side transformer, 14, in which the cathode terminals of the diode _D7 and the diode _D5 are connected to one end and the other end of the primary side, respectively, and the intermediate terminal of the primary side is connected to the common.
2 is an output terminal to which one end of the secondary side of the transformer T ₂ is connected. The capacitor C ₂ and the resistor r ₃ constitute a series circuit connected between the middle point of the secondary side of the transformer T ₁ and the common, and have the effect of terminating the gate circuit 14 at this point in terms of high frequency.

FIG. 4 is an electrical circuit diagram showing a specific example of the preamplifier 13 shown in FIG. The resistors R ₁ and R ₂ and the operational amplifier A constitute an inverting amplifier, and a gain control switch S (for example, a high-frequency solid relay) is connected in parallel with the resistor R ₂ .

The operation of the above receiving circuit will be explained below. In Fig. 3, when there is no high-frequency transmission pulse, no electromotive force is generated in the search coil 8, so the diodes _D1 , _D2 , and _D3 are cut off, and the capacitor _C1 is connected to the power supply voltage Vcc via the resistor _r1 . It will be charged. As a result, the base terminal voltage rises, transistor Q turns on, and the power supply voltage Vcc and resistance
A current i ₁ determined by r ₂ is supplied to the gate circuit 14,
Turn on diodes _D5 and _D7 . As a result, the gate circuit 14 is turned on, and the high frequency signal from the preamplifier 13 applied to the input terminal 141 is transmitted to the output terminal 142. When a high frequency transmission pulse is applied, the diode D ₁ when the pulse is negative,
D ₂ and D ₃ become conductive, and the charge stored in the capacitor C ₁ is discharged. With a positive pulse, the diode becomes non-conducting again, but since the time constant of C ₁ r ₁ is sufficiently large for one period of the high-frequency transmission pulse, the potential of the capacitor C ₁ remains constant while the high-frequency transmission pulse is applied. becomes negative. Therefore, during this period, the transistor Q is turned off, and the gate circuit 14
Since no current is supplied to the gate circuit 14, the gate circuit 14 is turned off, and the output signal from the preamplifier 13 is cut off. When the high-frequency signal pulse disappears, the transistor Q is turned on again after a certain delay determined by the time constant r ₁ C ₁ and makes the gate circuit 14 conductive. This gate circuit can maintain an input/output attenuation ratio of 50 dB or more by turning on/off the control current.

The current pulse output from the transistor Q is also used to control the preamplifier 13 shown in FIG. 4, and when the transistor Q is off, the switch S is turned on to set the gain to zero.

In the receiver circuit configured as above, the resistance r ₁
By selecting appropriate values for the capacitor C 1 and the capacitor C ₁ , control can be performed taking into account the damping factor of the transmitting and receiving system. In other words, when a high-power, high-frequency transmission pulse is applied, the gate circuit is automatically turned off and the gain of the preamplifier is set to zero, thereby preventing saturation of the high-frequency amplifier circuit and preventing delays in NMR signal detection due to saturation. Can be done. Note that if a variable trimmer resistor is used as the resistor _r1 , or if a switch is used to switch between a plurality of different resistors, it is possible to respond more precisely to the damping characteristics of the transmitter/receiver system, the influence of the size of the object, etc.

There is also the advantage that saturation and destruction of the high frequency amplifier etc. can be prevented even when high frequency transmission pulses are continuously applied due to runaway of the pulse programmer. (This is not possible with the conventional pulse programmer method.) The above circuit also has the following advantages over the conventional method in which a limiter circuit is provided in the preamplifier section without using a search coil. That is, since the control signal is used to control not only the preamplifier section but also the gate circuit, there is a further effect of suppressing high frequency noise.

FIG. 5 is a block diagram showing a second embodiment of the invention. The difference from the first embodiment is that the output from the control signal generation circuit 9 is also provided to a high frequency amplifier circuit 15 and a signal processing circuit 16 (for example, a phase detector, a filter amplifier, etc.) via a buffer 10. , high frequency noise can be suppressed more completely.

As described above, according to the present invention, it is possible to realize, with a simple configuration, a receiving circuit that can prevent saturation of the receiving circuit due to high-power high-frequency transmission pulses in response to changes in the transmitting/receiving system.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of a conventional NMR receiving circuit, FIG. 2 is a block diagram showing an embodiment of the present invention, and FIGS. 3 and 4 are electrical diagrams showing a specific example of the portion shown in FIG. FIG. 5 is a block diagram showing a second embodiment of the present invention. 2... Receiving coil, 8... Search coil, 9...
...Control signal generation circuit, 13...Preamplifier, 14
...gate circuit.

Claims (1)

[Claims] 1 A receiving circuit for an NMR device that detects the NMR resonance signal generated by irradiating the subject with a high-frequency transmission pulse sent out from the transmitting coil in the receiving coil section, amplifies it with a preamplifier, and controls transmission to the subsequent circuit with a gate circuit. , a search coil that detects a high-frequency transmission pulse and a detection signal output from this search coil are input, and pulse-like control starts immediately with the start of this detection signal and ends at a predetermined timing after the end of the detection signal. and a control signal generation circuit that generates a signal, and the control signal is used to switch the gain of the preamplifier and turn on and off the gate circuit so that the signal based on the high frequency transmission pulse does not reach the subsequent circuit. A receiving circuit for an NMR device characterized by: