JPH0222651B2 - - 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 the improvement of a transmitter 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.

In the transmission circuit of a pulsed NMR device, when a high frequency pulse modulated by an input modulation pulse is passed through a power amplifier to supply large power to the transmission coil, the input modulation pulse is turned off due to the damping characteristics of the transmission coil and reception coil. Even after this, a phenomenon occurs in which the high frequency waves have a long tail. Therefore, the weak signal that occurs immediately after the input modulation pulse is
The NMR signal is hidden by the high power pulse, resulting in a delay in the start time of receiving the NMR signal.
This results in a decrease in S/N. In order to prevent such a phenomenon, it has conventionally been required that the high-frequency output pulse has rapid rise and fall characteristics.

As a conventional example, a method has been proposed in which a pulse with a 180° phase inversion is added immediately after the input pulse to cancel residual high frequencies (Hold: Fast
recovery，high sensitivity NMR probe and
preamplifier; Rev.Sci.istrum., Feb.1979 50
(2)). In this method, the pulse length of the phase-inverted pulse is determined in advance through experiments and stored in the pulse programmer.The optimum length of the phase-inverted pulse changes when the pulse length and pulse power are changed, so the pulse length of the phase-inverted pulse is This has the disadvantage that the programmer's control becomes complicated.

The second conventional example was carried out by Apaddin University, and as shown in Fig. 1, the residual high frequency detected by the search coil 1 is passed through the detector 2 and then compared with the modulated pulse input by the comparator amplifier 3 and amplified. , the transmission power output from the transmitting coil 4 is controlled by the output thereof. In this case, there is a drawback that the phase of the high-frequency pulse output cannot be easily changed.

〔the purpose〕

The present invention was made to solve the above problems, and it is possible to obtain a high-frequency pulse output that is faithful to the input pulse modulation waveform, and also to easily change the phase of the high-frequency pulse output.
The purpose is to realize a transmitter circuit for NMR equipment.

〔overview〕

The gist of the present invention for achieving the above object is to provide a splitter circuit which inputs a high frequency carrier wave and outputs a signal having a predetermined phase relationship therewith and its inverted signal; a selection means for selecting and outputting one of the two; a modulator for amplitude modulating the output of the selection means; a transmitter coil to which a signal related to the output of the modulator is applied; and a high-frequency pulse sent from the transmitter coil. A search coil that detects the output, a detector that detects the detection signal output from the search coil,
It has a subtraction circuit that obtains the difference between the output of the detector and the modulated pulse input, and an absolute value circuit that outputs the absolute value of the output of the subtraction circuit. The output of the selection means is switched according to the polarity, and the output from the absolute value circuit is used as the modulation input of the modulator to obtain the high-frequency pulse output with quick rise and fall. The transmitting circuit for an NMR apparatus is characterized in that the phase difference of the high frequency pulse output with respect to the high frequency carrier wave can be selected by an external signal applied to the means.

[Explanation of Examples]

FIG. 2 is a configuration block diagram showing an embodiment of a transmitting circuit for an NMR apparatus according to the present invention. The same parts as in FIG. 1 are designated by the same numbers. Reference numeral 5 denotes a carrier wave input terminal to which a high frequency carrier wave is added, and 6 inputs the high frequency carrier wave from this carrier wave input terminal 5, and converts it into a 0° phase fundamental wave (signal with the same phase as the high frequency carrier input) and a 180° phase shifted wave. (hereinafter referred to as a splitter), 7 is a selection means for selecting and outputting either the fundamental wave or the inversion wave output from this splitter 6, and 8 is this selection means 7. 9 is a third gate that turns on/off passage of the modulated wave output from this modulator 8; 10
4 is a power amplifier that receives the modulated wave output from the gate 9 and performs power amplification; 4 is a transmitter coil that receives the output from the power amplifier 10 and outputs a high-power modulated high frequency wave toward the subject;
1 is a search coil that detects the high frequency pulse output from this transmitting coil 4; 2 is this search coil 1;
This is a wave detector that detects the detection signal from and detects its envelope. 11 is a modulated pulse input terminal to which a modulated pulse is applied; 3 is a subtraction circuit or comparison amplifier circuit which inputs the modulated pulse from this modulated pulse input terminal and the detection output from the detector 2 and obtains the difference between the two; Reference numeral 12 denotes an absolute value circuit which obtains the absolute value of the error output from the comparison amplifier circuit 3 and uses the absolute value output as the modulation input of the modulator 8. 13 is a selection signal input terminal to which an external signal (phase difference selection signal) for selecting the phase difference between the high frequency output from the selection means 7 and the high frequency carrier wave input is applied. In the selection means 7, C1 is a first comparator to which a modulated pulse input from the modulated pulse input terminal is applied and detects its positive value; C2 is a first comparator to which an error output from the comparison amplifier circuit 3 is applied;
A second comparator detects the negative value. G1
is a first gate circuit to which the fundamental wave outputted from the splitter 6 is added and turns on/off its passage;
G2 is a second gate circuit to which the inverted wave output from the splitter 6 is added and turns on/off its passage; K1 is a coupler that outputs the high frequency wave that has passed through either of the gate circuits G1 and G2 to the modulator 8; , S1 is a changeover switch that switches the state of application of comparison outputs from the comparison circuits C1 and C2 to the gate circuits G1 and G2 in accordance with the phase difference selection signal. 14 is an OR circuit that ORs the comparison outputs from the comparison circuits C1 and C2, and its output is the third one.
is added to the gate circuit 9.

The operation of the circuit configuration as described above will be explained next. FIG. 3 is a time chart showing waveforms at various parts of the circuit shown in FIG. When no modulated pulse input is applied to terminal 11, comparator C1,
Since the output of C2 is 0, gates G1, G2, and 9 are all turned off, and the power amplifier 10 does not output high frequency. When a modulated pulse input A is applied to terminal 11, the applied period comparator C
The output of 1 becomes 1, and the gate G1 is turned on. At the same time, the output G of the OR circuit 14 also becomes 1, and the gate 9 is also turned on. As a result, a high frequency wave having a phase difference of 0° with respect to the input high frequency carrier wave is supplied to the transmitting coil 4 after being modulated by a modulation pulse in the modulator 8. This state continues while the modulated pulse input A is 1. Here, in FIG. 3C, the output of the comparison amplifier circuit 3 becomes an excessively large value at the time of rising, and this becomes an amplitude modulation signal of the modulator 8, and a high frequency signal with a large amplitude is outputted from the modulator 8. A strong high-frequency signal in phase with the high-frequency carrier wave is applied to the transmitting coil 4, and as a result, the high-frequency pulse output rises quickly. Modulated pulse input A
When becomes 0, the output E of the comparator C1 immediately becomes 0 and the gate G1 is turned off. However, the high frequency pulse detected by the search coil 1 is transmitted to the transmitter/receiver coil 4.
Due to the damping characteristics of 1, a high frequency wave with a phase difference of 0° persists (albeit attenuated), and the output B of the detector 2 also has a characteristic of delaying the falling edge of the pulse. Therefore, the output C of the comparator amplifier 3 becomes negative, and therefore the output F of the comparator C2 becomes 1, turning on the gate G2. Since the output G of the OR circuit 14 becomes 1, the gate 9 continues to be turned on. The negative output C of the comparator amplifier 3 is inverted by the absolute value circuit 12 to become a positive output D, which is applied to the modulator. As a result, a high frequency wave having a phase difference of 180° from the high frequency carrier input is modulated by the output D of the absolute value circuit 12 in the modulator 8, and is then supplied to the transmitting coil 4 via the gate 9 and the power amplifier 10. Ru. Since this high frequency with a phase difference of 180° and the residual high frequency with a phase difference of 0° are in opposite phases, they cancel each other, and the residual high frequency is rapidly attenuated. Here, the phase of the high frequency output from the selection means 7 is set accurately in the selection means 7, and a high frequency signal having an exactly opposite phase to the phase of the high frequency pulse output (that is, the same phase as the residual high frequency) is sent to the transmitting coil 4.
Since it is added to , there is always a negative feedback. That is, high-frequency signals having attenuated waveform envelopes and having opposite phases cancel each other out, resulting in faster attenuation. When the residual high frequency attenuates to 0, the detector 2
Since the output B of the comparator C2 becomes 0 and the output F of the comparator C2 also becomes 0, the gate circuits G2 and 9 are turned off. The gate circuit 9 functions to increase the on/off ratio of high frequency pulse output. That is, in the above example,
The comparison amplifier circuit 3 and the absolute value circuit 12 detect the deviation of the envelope of the high frequency pulse output with respect to the modulation pulse. Depending on the polarity of the modulation pulse input and comparison amplifier circuit output, a signal with the same phase as the high-frequency pulse output (carrier wave) is used at the rising edge, and a signal with the opposite phase to the high-frequency pulse output phase at the falling edge. By amplitude modulating the pulse with the output of the absolute value circuit 12, it operates to cancel the deviation from the modulated pulse. As a result, the rise and fall of the high-frequency pulse output becomes faster.

In Figure 2, a high-frequency pulse with a 0° phase difference is selected using the phase difference selection signal, but when a high-frequency pulse output with a 180° phase difference is required (this is often necessary depending on the NMR excitation method). ) is a phase difference selection signal that can be used to switch the two contacts of the changeover switch S1 to opposite sides. In this case, the residual high frequency with a 180° phase difference is canceled by the high frequency with a 0° phase difference.

Furthermore, although the above description shows a case where a rectangular pulse is used as the modulated pulse input, the present invention is not limited to this, and it is possible to use modulated pulse inputs of various waveforms such as a sine wave pulse and a GAUSSIAN pulse.

According to the transmitting circuit for an NMR device configured as described above, damping is always automatically applied regardless of the magnitude or pulse width of the high-frequency pulse output, so it is possible to easily realize modulation that is faithful to the modulated pulse input.

Figure 4A shows a high-frequency pulse waveform with a delay when there is no feedback from the search coil.
FIG. 4B shows a high frequency pulse waveform whose rise and fall are sharpened by the circuit shown in FIG.

It is also often required in pulsed NMR instruments.
The 180° shifted high-frequency pulse can be easily switched using only the phase difference selection signal, and the same damping characteristics can be achieved.

FIG. 5 is a block diagram showing a second embodiment of the present invention, in which the number of selectable phase differences is increased to four. The same parts as in FIG. 2 are designated by the same reference numerals. 61 has a 0° phase difference with respect to the high frequency carrier wave input.
a first splitter that outputs a component with a 90° phase difference;
62 is a second splitter which obtains outputs with a 0° and 180° phase difference in addition to the 0° phase difference output from this splitter 61;
A third splitter 17 obtains outputs with a 0° and 180° phase difference from the 90° phase difference output; Selection means 131 and 132 switch between arbitrary two high frequencies having an inverted phase relationship with each other from among the 90° and 270° phase difference outputs (with respect to the high frequency carrier input) from the splitter 63. This is a selection signal input terminal to which a phase difference selection signal (a 2-bit binary signal) for selecting a combination of the two high frequencies in the selection means 17 is applied. In the selection means 17, G11, G12, G13, and G14 are gate circuits that turn on and off outputs of 0°, 180°, 90°, and 270° phases from the splitters 62 and 63, respectively;
K2 is a coupler which adds the output from the turned-on gate circuit among these gate circuits G11 to G14 to the modulator 8, and S11 and S12 are selected by the phase difference selection signal and have a phase inversion relationship with each other. This is a changeover switch that switches the gate circuits corresponding to the two high frequencies based on the outputs from the comparators C1 and C2.

In FIG. 5, changeover switches S11 and S are selected by phase difference selection signals applied to terminals 131 and 132.
12 selects high frequencies with a phase of 90° and 270°.
That is, the output of the comparator C1 turns on the gate circuit G13, and the output of the comparator C2 turns on the gate circuit G1.
4 is turned on, and the residual high frequency of the 90° phase high frequency pulse is canceled by the 270° phase high frequency. 0° and 180°, 180° and 0°, 90° and
You can select a phase combination of 270° or 270° and 90°. The operation of the other parts (with the same sign) is the second
This is the same as the case shown in the figure.

Using various carrier waves whose phases are shifted in this way is very convenient depending on the NMR excitation method. In the example shown in FIG. 5, the splitter 61 separates the phase into 0° and 90° phases, but other combinations of phases are also possible, and the number of combinations can also be increased.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to realize a transmitter circuit for an NMR apparatus that can obtain a high-frequency pulse output that is faithful to the input pulse modulation waveform and can easily change the phase of the high-frequency pulse output.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an example of a conventional transmitting circuit for an NMR device, Fig. 2 is a block diagram showing an embodiment of the present invention, and Fig. 3 is a time chart showing the operation of each part of the circuit in Fig. 2. , FIG. 4 is an explanatory diagram showing a high frequency pulse output waveform, and FIG. 5 is a block diagram showing a second embodiment of the present invention. 1... Search coil, 2... Detector, 3... Subtraction circuit, 4... Transmission coil, 6, 62, 63...
Splitter circuit, 7, 17... selection means, 8...
Modulator, 12... Absolute value circuit.

Claims (1)

[Claims] 1. A splitter circuit that inputs a high-frequency carrier wave and outputs a signal having a predetermined phase relationship therewith and its inverted signal, a selection means that selects and outputs either the signal or the inverted signal, and this selection means. a modulator for amplitude modulating the output of the means; a transmitting coil to which a signal related to the output of the modulator is applied;
A search coil that detects the high-frequency pulse output sent out from the transmitting coil, a detector that detects the detection signal output from the search coil, and a subtraction circuit that obtains the difference between the detector output and the modulated pulse input. , an absolute value circuit that outputs the absolute value of the output of the subtraction circuit, and switches the output of the selection means according to the modulation pulse input and the polarity of the output of the subtraction circuit, and the absolute value circuit By using the output from the modulator as the modulation input of the modulator, the high-frequency pulse output with quick rise and fall is obtained, and the phase difference of the high-frequency pulse output with respect to the high-frequency carrier wave is determined by an external signal applied to the selection means. A transmitting circuit for an NMR device, characterized in that it is possible to select.