JPH06114031A - Mr device - Google Patents

Abstract

PURPOSE:To accurately perform control even when the flip angle of an exciting nuclear spin is small. CONSTITUTION:A waveform of a full scale is generated in a D/A converter 82 and the carrier wave from a signal generator 7 is modulated in amplitude on the basis of the waveform by a multiplier 81 and subsequently attenuated by a digital attenuator 83 to be set to a predetermined amplitude value. An attenuation quantity setting register 84 stores the attenuation quantity of the digital attenuator 83 so that the amplitude value of an RF pulse corresponds to the flip angle of an excited nuclear spin and controls the digital attenuator 83 corresponding to the stored attenuation quantity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MR device for performing imaging, spectroscopy, etc. using the NMR (nuclear magnetic resonance) phenomenon, and more particularly to improvement of its transmission system.

[0002]

2. Description of the Related Art In an MR device, a nuclear spin is excited by irradiating a subject with a plurality of excitation RF pulses having different flip angles, as represented by a spin echo method, and receiving an echo signal generated thereafter. And collect data. In order to execute such a pulse sequence, it is necessary to generate a high-frequency signal as an RF pulse whose amplitude is modulated so that the waveform and the amplitude of the envelope correspond to the flip angle.

Conventionally, a transmission system for transmitting this RF pulse is usually constructed as shown in FIG. That is, information about the excitation frequency and phase is given to the signal generator 7 to generate a sinusoidal high frequency signal of that frequency / phase,
This is sent to the transmission circuit 8. On the other hand, the waveform information having an amplitude value is also sent to the D / A converter 82 of the transmission circuit 8, and the D / A converter 82 generates a waveform whose amplitude is controlled. The amplitude value of this waveform mainly corresponds to the flip angle. With the same waveform, when the flip angle increases, only the amplitude increases, and when the flip angle is small, the amplitude decreases. The high frequency signal and the waveform are multiplied by the multiplier 81, whereby the high frequency signal is amplitude-modulated with the waveform. Thus, the amplitude-modulated wave whose envelope has the waveform of the D / A converter is output as an RF pulse.

[0004]

However, conventionally, there is a problem that the control accuracy of the flip angle is poor when the flip angle is small.

That is, in the conventional circuit in which the waveform of the modulated wave is controlled including the amplitude and the carrier wave is amplitude-modulated by the modulated wave, if the amplitude value of the modulated wave is reduced in accordance with a small flip angle, Since the part having a small dynamic range of the modulation circuit (multiplier) is used, the control accuracy of the envelope waveform of the signal after modulation deteriorates.

Therefore, conventionally, the nuclear spin cannot be controlled as designed by the pulse sequence using the RF pulse having the small flip angle, which deteriorates the image quality and the signal-to-noise ratio.

In view of the above, an object of the present invention is to provide an MR device having an improved transmission system so that an RF pulse having an amplitude corresponding to a small flip angle can be generated with high precision.

[0008]

In order to achieve the above object, in the MR apparatus according to the present invention, a means for applying a static magnetic field in the space in which the subject is placed and a gradient of a predetermined waveform in the space. A means for applying a magnetic field, an amplitude modulation means for modulating a carrier wave having a predetermined frequency and phase with a modulation wave having a waveform of a constant amplitude value according to waveform information, and an output of the amplitude modulation means It is characterized in that it comprises an attenuating means for attenuating according to the amplitude information, a means for irradiating the subject with the attenuated signal, and a means for receiving an NMR signal from the subject.

[0009]

[Function] Since the amplitude of the signal after the amplitude modulation is attenuated according to the amplitude information, when the carrier wave is amplitude-modulated by the modulation wave, the waveform of the modulation wave is full scale regardless of the flip angle. Can be a waveform of. Therefore, even when an excitation high frequency signal with a small flip angle is generated, amplitude modulation can be performed in a portion with a large dynamic range, and the controllability of the envelope waveform of the excitation high frequency signal is improved. As a result, even when the nuclear spins of the subject are excited with a small flip angle, accurate flip angle control can be performed, and an MR image with an excellent signal-to-noise ratio and good image quality can be obtained.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows only the transmission system, and the overall configuration of the MR device is as shown in FIG. First, the overall structure of the MR device will be described. In FIG. 3, the main magnet 1 is for generating a static magnetic field, and a gradient magnetic field is applied by the gradient magnetic field coil 2 so as to be superposed on this static magnetic field. The space to which the static magnetic field and the gradient magnetic field are applied is an examination space in which the subject 3 is placed. An excitation RF pulse is applied to the subject 3
An RF coil 4 for irradiating the subject and receiving an NMR signal generated in the subject 3 is attached.

A gradient magnetic field power source 5 is connected to the gradient magnetic field coil 2 and is supplied with gradient magnetic field generating power. An RF pulse subjected to amplitude modulation is supplied from the transmission circuit 8 to the RF coil 4. That is, the carrier wave from the signal generator 7 is amplitude-modulated in the transmitting circuit 8 by a modulating wave having a predetermined waveform and sent to the RF coil 4.

The echo signal from the subject 3 received by the RF coil 4 is sent to the receiving circuit 10 via the preamplifier 9 and the received signal is phase-detected using the signal from the signal generator 7 as a reference signal. The detected signal is sampled by the A / D converter 11, converted into digital data, and taken into the computer 6.

The computer 6 controls the waveform and amplitude value of the modulated wave of the excitation RF pulse in the transmission circuit 8, determines the frequency and phase of the signal generator 7, and determines the sampling timing of the A / D converter 11. Further, the gradient magnetic field power supply 5 is controlled to arbitrarily program the timing, waveform, intensity, etc. of the gradient magnetic field pulse. Further, processing for reconstructing an image from the collected digital data is performed. The image display device 12 connected to the computer 6 displays a reconstructed image and the like.

Next, the RF pulse transmission system (signal generator 7
And a portion of the transmission circuit 8) will be described with reference to FIG. Information about the excitation frequency and the phase is sent from the computer 6 to the signal generator 7, and a high frequency sine wave having a frequency and a phase corresponding thereto is generated, and this is transmitted by the transmission circuit 8.
Sent to. The transmission circuit 8 includes a multiplier 81, a D / A converter 82, a digital attenuator 83, an attenuation amount setting register 84, and a control circuit 85.

The D / A converter 82 includes a computer 6
The information of only the waveform of the envelope of the RF pulse, which does not include the information on the amplitude, is sent from the RF pulse envelope, and the waveform of the constant amplitude is generated accordingly. This waveform is sent to the multiplier 81 as a modulation wave, and the carrier wave sent from the signal generator 7 is amplitude-modulated. Since the envelope of the output signal from the multiplier 81 has a waveform of constant amplitude, the amplitude is changed by the digital attenuator 83.

Therefore, the information on the amplitude of the envelope of the RF pulse is supplied from the computer 6 to the control circuit 85.
And the attenuation setting register 84 is controlled accordingly. This attenuation amount setting register 84 is
The attenuation amount of the digital attenuator 83 is stored so that the amplitude value of the F pulse corresponds to the flip angle of the excited nuclear spin. The digital attenuator 83 is stored in accordance with the stored attenuation amount. Control.

As the digital attenuator 83, a so-called programmable attenuator in which a large number of attenuators 21 and high speed switching elements 22 and 23 for switching the attenuators 21 are arranged as shown in FIG. 2 can be used.
The high speed switching elements 22 and 23 are composed of, for example, PIN diodes or GaAs elements. A control signal read from the attenuation amount setting register 84 corresponding to the attenuation amount is applied to the high-speed switching elements 22 and 23 to turn on / off the control signal, thereby varying the attenuation amount at a switching speed of several nanoseconds. can do.

As described above, since the output of the multiplier 81 is separately attenuated by the digital attenuator 83 to obtain a desired amplitude value, the D / A converter 82 always generates a full-scale waveform for multiplication. By always performing the multiplication in the device 81 at the full scale, it becomes possible to perform the multiplication in a portion having a large dynamic range. for that reason,
Even in the case of a small flip angle, an RF pulse whose envelope is controlled with high accuracy can be obtained as in the case of a large flip angle, and accurate flip angle control of nuclear spin in the subject 3 can be performed. It is possible to prevent deterioration of image quality and deterioration of signal-to-noise ratio due to incorrect excitation.

The present invention is not limited to the above embodiment, and it goes without saying that the concrete configuration of the digital attenuator 83, its concrete control system, etc. can be variously constructed.

[0020]

According to the MR device of the present invention, it becomes possible to accurately control the flip angle when exciting the nuclear spins of the subject even when the flip angle is small. It is possible to obtain an MR image having an excellent ratio and good image quality.

[Brief description of drawings]

FIG. 1 is a block diagram of a transmission system according to an embodiment of the present invention.

FIG. 2 is a block diagram of the digital attenuator of the same embodiment.

FIG. 3 is an overall block diagram of the MR device according to the embodiment.

FIG. 4 is a block diagram of a transmission system of a conventional example.

[Explanation of symbols]

 1 Main Magnet 2 Gradient Magnetic Field Coil 3 Subject 4 RF Coil 5 Gradient Magnetic Field Power Supply 6 Computer 7 Signal Generator 8 Transmitter Circuit 9 Preamplifier 10 Receiver Circuit 11 A / D Converter 12 Image Display Device

Claims (1)

[Claims] 1. A means for applying a static magnetic field in a space in which an object is placed, a means for applying a gradient magnetic field having a predetermined waveform in the space, and a carrier wave having a predetermined frequency and phase Amplitude modulating means for modulating with a modulating wave having a waveform of a constant amplitude value according to the information, attenuating means for attenuating the output of the amplitude modulating means according to the amplitude information, and the signal after the attenuation. To irradiate the subject and N from the subject
And a means for receiving an MR signal.
R device.