JPH0531090A - Mr device - Google Patents

Abstract

PURPOSE:To facilitate automatic adjustment with improved reproducibility by providing correcting circuits each having a D/A-converter applied with the control signal from the outside and a differentiating circuit adding the differential output of its analog output to the original input signal and outputting it. CONSTITUTION:Each of correcting circuits 51-54 has a D/A-converter 66, a reverse amplifier 62, a capacitor 63, a resistor 64, and a nonreverse amplifier 65, a differentiating circuit is constituted of the capacitor 63, the resistor 64, and the nonreverse amplifier 65, and the D/A-converter 66 and the reverse amplifier 62 are used as circuits determining the amplification factor of the differentiating circuit when the size of the input signal of the differentiating circuit is adjusted. The input signal from a gradient magnetic field wave-form generator is applied to the reference voltage input terminal of the D/A-converter 66, the data for setting parameters are inputted to the digital input terminal, and the analog output is fed to the input terminal of the reverse amplifier 62.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MR apparatus (nuclear magnetic resonance diagnostic apparatus) for performing imaging and the like using NMR (nuclear magnetic resonance).

[0002]

2. Description of the Related Art In an MR device, it is necessary to generate a gradient magnetic field (also referred to as a gradient magnetic field) in each direction of three orthogonal axes for imaging, and therefore a gradient magnetic field in each direction is generated. And a gradient power supply for supplying a current to each coil. A reference waveform is input from the gradient magnetic field waveform generator to the gradient power supply, and a gradient current (gradient current) having a waveform corresponding to this waveform is supplied to the coil.

In this MR device, it is important to accurately generate various gradient magnetic field pulses in a desired waveform. However, when a gradient magnetic field is applied in pulses, eddy currents are generated in the conductors such as the constant temperature bath of the superconducting magnet for static magnetic fields. Cannot be formed. For example, when a gradient current having a waveform as shown in FIG. 5A is supplied, the actual magnetic flux waveform becomes as shown by B in FIG.

Therefore, conventionally, a correction circuit as shown in FIG. 6 is inserted between the gradient magnetic field waveform generator and the gradient power source to operate the reference waveform to differentiate the rounded waveform. Is added to perform eddy current compensation. In order to perform high-order correction on this gradient magnetic field waveform, it is necessary to provide a correction circuit for each order. In FIG. 6, the first to fourth correction circuits 5
1 to 54 are provided. These are variable resistors 61,
It is composed of an inverting amplifier 62, a capacitor 63, a resistor 64 and a non-inverting amplifier 65. Capacitor 63, resistor 6
4. The non-inverting amplifier 65 constitutes a differentiating circuit and adjusts the time constant by adjusting the resistance value. Variable resistor 6
By adjusting 1, it is possible to change the magnitude of the input signal to this differentiating circuit, so the amplification factor of the differentiating circuit is adjusted.

Conventionally, these parameters are adjusted by manually adjusting a variable resistor or the like. That is, since the optimum value of each parameter is different for each MR device, it is necessary to adjust them for each device.

[0006]

However, as can be seen from FIG. 6, there are many parameters to be adjusted, and the work of changing the time constant is delicate, and the reproducibility is not so great, which is troublesome.

In view of the above, the present invention makes it possible to easily perform the adjustment work of the parameters for appropriately correcting the influence of the eddy current, the reproducibility is good, and the automatic adjustment by computer control is also possible. M with magnetic field correction circuit
It is intended to provide an R device.

[0008]

In order to achieve the above object, in the MR device according to the present invention, the correction circuit has a D
A / A converter is provided, an analog output is obtained by inputting a signal from the waveform generator to its reference voltage terminal and an external control signal to its digital input terminal, and sending this analog output to a differentiating circuit, The feature is that the differential output obtained there is sent to the gradient power source after being added to the original signal, and the size of the signal input to the differential circuit can be adjusted by the control signal input to the D / A converter. Adjustment of the amplification factor becomes easy. Further, in the MR device according to the present invention, a CdS output type photocoupler may be used as a resistor constituting the time constant of the differentiating circuit, and a control signal may be externally applied to the input terminal of the photocoupler. The time constant of the differentiating circuit can be adjusted by a control signal from the outside, and the time constant can be easily adjusted.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram showing an MR apparatus according to an embodiment of the present invention, in which a transmission coil 12 and a reception coil 13 are attached to a subject 11 and these are superposed on a static magnetic field formed by a main magnet 15 and superimposed on it. It is arranged in the gradient magnetic field by the gradient coil 14 formed as described above. The gradient coil 14 is configured to be able to independently generate a gradient magnetic field whose magnetic field strength is inclined in each of the three orthogonal axes. The gradient magnetic fields of the three orthogonal axes are the slice selection gradient magnetic field whose magnetic field strength is inclined in the slice thickness direction, and 1 in the slice plane.
The gradient magnetic field for phase encoding has a magnetic field strength inclined in a direction, and the gradient magnetic field for reading (frequency encoding) has a magnetic field strength inclined in another direction in the slice plane. Current is supplied to the gradient coil 14 from the gradient power sources 21, 22, and 23 corresponding to the gradient magnetic field in each direction, and the gradient magnetic field in each direction is formed. A reference waveform from the gradient magnetic field waveform generator 24 is sent to the gradient power supplies 21 to 23 via the correction circuits 25 to 27, and a gradient current having a waveform determined based on the reference waveform is supplied to the gradient coil 14. Supplied.

On the other hand, the transmitting coil 12 has a high frequency power source 3
The RF excitation pulse sent from 3 is supplied. This excitation pulse is transmitted to the synthesizer 3 in the frequency converter 32.
The RF sine wave signal from 4 is used as a carrier signal, which is AM-modulated with the sinc waveform from the RF waveform generator 31 and amplified by the high frequency power supply 33.

NMR generated after the subject 11 is irradiated with RF pulse from the transmission coil 12 to excite its nuclear spin.
The signal is received by the receiving coil 13. The received NMR signal is amplified by the preamplifier 35 and then detected by the quadrature phase detector (complex detector) 36. As a reference signal at that time, a signal generated from the synthesizer 34 is input to the quadrature detector 36. The detected output is then converted into digital data by the A / D converter 37 and taken into the host computer 41. The quadrature detector 36 outputs the difference between the frequencies of the two signals by mixing the reference signal and the received signal.

Under the control of the host computer 41, the sequence controller 42 gives waveform information and generation timing information of each gradient magnetic field pulse to the gradient magnetic field waveform generator 24, and the sine waveform information and generation timing of the RF pulse to the RF waveform generator 31. While giving information, it sends information about the frequency of the carrier signal (corresponding to the resonance frequency) to the synthesizer 34, and further controls the sample timing of the A / D converter 37 and the like.

The host computer 41 has a console 4 having a display device and an input device such as a keyboard device.
3 is connected. The data taken into the host computer 41 is converted into two
The three-dimensional image is reconstructed and the image is displayed on the display device of the console 43.

Now, the correction circuits 25 to 27 for correcting the influence of the eddy current on the gradient magnetic field will be described. All of them have the same structure and are as shown in FIG. That is, here, the first to
Fourth-order correction circuits 51 to 54 are provided. These are for differentiating an input signal and adding the differentiated output to the original input signal. Each of the correction circuits 51 to 54 has a D /
A converter 66, inverting amplifier 62, and capacitor 6
3, a resistor 64, and a non-inverting amplifier 65.
A differentiating circuit is configured by the capacitor 63, the resistor 64 and the non-inverting amplifier 65, and the D / A converter 66 and the inverting amplifier 62 are provided as a circuit that determines the amplification factor of the differentiating circuit by adjusting the magnitude of the signal input to the differentiating circuit. Is used.

An input signal from the gradient magnetic field waveform generator 24 is applied to a reference voltage input terminal of the D / A converter 66, and data for parameter setting is input to its digital input terminal. The analog output is sent to the input terminal of the inverting amplifier 62.
If the analog output voltage is Vo, then Vo = Vref
(DATA / A). Here, Vref is the voltage applied to the reference input terminal, and DATA is the data applied to the digital input terminal. A is a value specific to the D / A converter 66, and the resolution is determined by this. Now, instead of the variable resistor 61 of FIG.
It will be appreciated that converter 66 may be used. That is, the magnitude of the voltage input to the differentiating circuit can be changed by the data given from the outside. The accuracy and temperature characteristics of the D / A converter are usually higher than those of the fixed resistor or variable resistor. Therefore, the adjustment can be performed with high accuracy and high stability, and the reproducibility of the adjustment is good. It is also possible to automatically adjust a parameter called an amplification factor by supplying data from the host computer 41 or the like.

Further, these correction circuits 25 to 27 are shown in FIG.
It can also be configured as follows. That is, the output terminal of the CdS output type photocoupler 67 is connected instead of the time constant adjusting resistor 64 (FIG. 6) of each of the correction circuits 51 to 54. The photocoupler 67 is a device whose output resistance changes from several tens of Ω to several MΩ depending on the value of the current flowing through the light emitting diode on the input side, and an external control signal may be applied to its input terminal. The digital data, which is a control signal from, is converted into an analog signal by the D / A converter 68 and then added via a resistor, so that control by the digital signal is performed.

In this case, the time constant of the correction term of each order can be changed by the digital signal given to the correction circuits 51 to 54 of each order, and the working efficiency of the adjustment is improved. Also, adjustment with good reproducibility can be performed. Furthermore, it is also possible to automatically adjust a parameter called a time constant by supplying data from the host computer 41 or the like.

The correction circuits 25 to 27 having the above-mentioned configuration
As a result, the differential output is added to the reference waveform output from the gradient magnetic field waveform generator 24 to form a waveform as shown by A in FIG. 4, which is input to the gradient power supplies 21-23. As a result, it is possible to generate a gradient magnetic field pulse having a desired waveform as shown by B in FIG.

In the above description, the correction circuit having the D / A converter 68 for adjusting the amplification factor and the CdS for adjusting the time constant are provided.
Although the correction circuit having the output type photocoupler 67 is configured separately, the correction circuit can be configured to include both of them.

[0020]

According to the MR device of the present invention, the parameters in the gradient magnetic field correction circuit can be adjusted by a control signal from the outside, so that the adjustment work is easy and the reproducibility is also good. Moreover, automatic adjustment by a computer or the like becomes possible.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a block diagram specifically showing the correction circuit of FIG.

FIG. 3 is a block diagram specifically showing a correction circuit according to another embodiment.

FIG. 4 is a waveform diagram for explaining the operation of the embodiment.

FIG. 5 is a waveform chart for explaining the influence of eddy current.

FIG. 6 is a block diagram of a conventional example.

[Explanation of symbols]

11 subject 12 transmitter coil 13 receiver coil 14 gradient coil 15 Main magnet 21 Gradient magnetic field power supply for slice selection 22 Gradient magnetic field power supply for phase encoding 23 Gradient magnetic field power supply for reading 24 Gradient magnetic field waveform generator 25, 26, 27 Gradient magnetic field correction circuit 31 RF waveform generator 32 frequency converter 33 high frequency power supply 34 Synthesizer 35 Preamplifier 36 Quadrature detector 37 A / D converter 41 Host computer 42 Sequence controller 43 console 51 Primary correction circuit 52 Secondary correction circuit 53 Third-order correction circuit 54 Fourth-order correction circuit 62 Inverting amplifier 65 Non-inverting amplifier 66, 68 D / A converter 67 CdS output type photocoupler

Claims (2)

[Claims] 1. A signal inserted from the above-mentioned waveform generator is applied between a reference voltage terminal and a control signal from the outside, which is inserted between a gradient magnetic field waveform generator and a gradient power supply. D / A added
An MR device having a correction circuit including a converter and a differentiating circuit for differentiating an analog output of the D / A converter and adding the differentiated output to an original input signal for output. 2. A differential, which is inserted between a gradient magnetic field waveform generator and a gradient power source, differentiates a signal input from the above waveform generator, and adds the differential output to the original input signal for output. An MR device comprising a correction circuit including a circuit and a CdS output type photocoupler, to which an external control signal is applied to an input terminal, which is inserted as a time constant adjusting resistor of the differentiating circuit.