JPH0591986A - Mr imaging system - Google Patents

Abstract

PURPOSE:To prevent an artifact caused by movement of the testee from generation by adding successively a dephase pulse and a rephase pulse based on an inclined magnetic filed in an arbitrary unidirection and generating a resonance signal wherein only a positional information is encoded. CONSTITUTION:For example, in a field echo method, a pulse of an inclined magnetic field Gy which can deny the influence of the inclined magnetic field Gy for a phase encode is added and the phase for magnetization is arranged again and the second echo signal is generated by making a Gx pulse turn back. This pulse sequence is repeated while the magnitude of the inclined magnetic field Gy for the phase encode is changed. As in the data collected from the second echo signal, only frequency is encoded and only a positional information in the X direction of a selected slice face is encoded, if this data is Fourier- transformed, a one dimensional image obtd. by projecting the selected slice face on one axis in the X direction is obtd. and movement of the testee can be found thereby.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention relates to nuclear magnetic resonance (NM).
The present invention relates to an MR imaging apparatus that performs imaging using R).

[0002]

2. Description of the Related Art In an MR imaging apparatus, a specific slice plane of a subject is selectively excited to generate an NMR signal (echo signal) only from the slice plane, and position information in the uniaxial direction in the slice plane is obtained. Data is collected by encoding the received echo signal by encoding the echo signal frequency and other axial position information into the echo signal phase, respectively. The raw data is two-dimensionally Fourier-transformed to decode the position information in the biaxial directions to obtain a tomographic image on the slice plane. For phase encoding, the sequence of selective excitation / reception (data collection) is repeated while changing the magnitude of the gradient magnetic field for phase encoding.

[0003]

However, the MR imaging apparatus requires a relatively long imaging time because the excitation / reception sequence is repeated as described above, and therefore, for example, the subject sneezes during that period. There is an unavoidable occurrence of body movements (body movements) such as coughing and coughing, and as a result, there is a problem that artifacts are likely to occur in the reconstructed image.

In view of the above, the present invention has been improved to prevent the occurrence of artifacts caused by the movement of the subject.
An object is to provide an R imaging device.

[0005]

In order to achieve the above object, in the MR imaging apparatus according to the present invention,
First encoded position information in at least two axis directions
In each of the repetition periods of the pulse sequence for obtaining the resonance signal of, the dephasing pulse and the rephasing pulse due to the gradient magnetic field in the arbitrary 1-axis direction are sequentially added, and the second positional information in the 1-axis direction is encoded. The feature is that a sequence for generating a resonance signal is added.
Since the second resonance signal only encodes the position information in the one-axis direction described above, the data obtained from this resonance signal is Fourier-transformed to obtain an image projected on the one-axis (one-dimensional image). ) Is obtained. Therefore, if the change in the image is captured, the movement of the subject can be detected. Since this one-dimensional image is obtained from one second resonance signal generated in one repetition period, it is possible to capture the motion of the subject in time units of the repetition period of the sequence. That is, when the motion is detected by the one-dimensional image obtained from the second resonance signal in the certain repetition period, the subject has moved in the repetition period. for that reason,
It can be seen that during the repetition period, the data collected from the first resonance signal is also motion-sensitive. Therefore, if the data collected during the repetition period in which the motion is detected is discarded, or the sequence with the same phase encoding amount at that time is performed again, only the data in the non-moving state can be collected. Thus, by performing image reconstruction by excluding the data when it was determined that there was body movement, or by performing image reconstruction processing using the data collected again while the subject is not moving,
It is possible to obtain a reconstructed image free from artifacts due to body movement.

[0006]

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 shows an MR imaging apparatus according to an embodiment of the present invention. In this figure, a gantry 10 is formed in a cylindrical shape, in which a static magnetic field such as a superconducting magnet that generates a strong static magnetic field is formed. A magnetic field magnet 11 and a gradient magnetic field coil 12 are housed. The gradient coil 12 has three orthogonal directions (X
Direction, Y direction, Z direction), three sets are provided so as to generate a magnetic field whose magnetic field strength is inclined. A subject (not shown) is housed in the hollow portion of the gantry 10, and the high frequency coil 13 is attached to the subject.

The gradient magnetic field coil 2 includes a gradient magnetic field power source 2
A current is supplied from 3. The current waveform is given from the waveform generator 22 under the control of the control computer 21. As a result, three gradient magnetic fields having different directions are generated in a pulsed manner as a slice selection gradient magnetic field, a frequency encoding gradient magnetic field, and a phase encoding gradient magnetic field. The frequency encoding gradient magnetic field is also used to generate a resonance signal by aligning the magnetization phases, and is therefore also called a readout gradient magnetic field. In this embodiment, the gradient magnetic field Gx in the X direction is used as a gradient magnetic field for frequency encoding (reading) and the gradient magnetic field G in the Y direction is used.
It is assumed that y is used as the phase encoding gradient magnetic field and the Z direction gradient magnetic field Gz is used as the slice selection gradient magnetic field.

A high frequency pulse from a high frequency power amplifier 33 is supplied to the high frequency coil 13 via a switch 34. The high frequency sine wave signal from the high frequency generator 31 is sent to the amplitude modulator 32, and it is amplitude-modulated by the waveform signal from the waveform generator 22 as a carrier signal. The output of the amplitude modulator 32 is amplified by the high frequency power amplifier 33 and sent to the switch 34.

As a result, the high frequency coil 1 is applied to the subject.
A high frequency pulse is emitted from 3 and the nuclear spins thereof are excited. The NMR signal generated thereafter is received by the high frequency coil 13. The received NMR signal is sent to the amplifier 41 via the switch 34, amplified by the amplifier 41, detected by the phase detector 42, and then converted into digital data by the A / D converter 43 to be processed by the data processing device. It is taken in by 44. The phase detector 42 outputs the difference between the frequencies of the two signals by mixing the reference signal sent from the high frequency generator 31 and the received signal.

The control computer 21 includes the waveform generator 2
The waveform information of each gradient magnetic field pulse and the high frequency pulse and the generation timing information are given to 2, and the frequency of the carrier signal of the high frequency pulse to the high frequency generator 31 (corresponding to the resonance frequency).
Information regarding the sampling timing of the A / D converter 43, etc. is controlled.

As a pulse sequence for imaging, a usual field echo method, spin echo method,
Although the saturation recovery method and the inversion recovery method can be used, the field echo method is used here. In FIG. 2, a radio frequency pulse (RF) is applied, a slice selection gradient magnetic field Gz is applied, and only a specific slice plane in the Z direction is selectively excited. Next, the phase-encoding gradient magnetic field Gy and the frequency-encoding gradient magnetic field Gx are added, and the position information in the Y direction and the position information in the X direction are encoded into the frequency and phase of the echo signal generated later. By inverting the frequency-encoding gradient magnetic field Gx, the phases of magnetization are aligned, and the first echo signal is generated. Up to this point, it is the same as the normal field echo method.

According to the present invention, thereafter, a gradient magnetic field Gy that cancels the influence of the phase encoding gradient magnetic field Gy.
Pulse is applied. This second Gy pulse is the first
The Gy pulse may have the same magnitude as that of the Gy pulse of FIG. Then, by further inverting the Gx pulse, the phase of the magnetization is aligned again to generate the second echo signal.

Such a pulse sequence is repeated while changing the magnitude of the gradient magnetic field Gy for phase encoding, and data is collected from the first and second echo signals obtained in each repetition period. Since the influence of the phase-encoding gradient magnetic field Gy is removed from the second echo signal, the data collected from this second echo signal is only frequency-encoded, and the position information of the selected slice plane in the X direction is obtained. Only encoded. By Fourier transforming this data, an image (one-dimensional image) obtained by projecting the selected slice plane on one axis in the X direction can be obtained. This one
The three-dimensional image is a projected image of the subject, from which the movement of the subject can be seen.

This one-dimensional image data is sent to the control computer 21, and the control computer 21 monitors it to determine whether or not the subject has moved. If it is determined that there is motion in a certain repetition period, the data collected from the first echo signal in that repetition period should also be affected by the motion, so this motion is collected in the detected repetition period. The data that was saved is discarded. Then, the control computer 21 controls so that the sequence of the same phase encoding amount at that time is performed again, and collects the data in the non-moving state again. Thus, data is collected only when the subject is not moving.
By performing a two-dimensional Fourier transform in the data processing device 44 using this data, the image on the slice plane can be reconstructed and an image that is not affected by body movement can be obtained. Although the image quality is poor, it is also possible to reconstruct the image by excluding the data when it is determined that there is a body movement.

In the above, the gradient magnetic field G for frequency encoding is used.
The second half of x is used as a dephasing pulse, and a second echo signal is generated by adding a rephasing pulse having the opposite polarity. The gradient magnetic field that can be used as a dephasing pulse or rephasing pulse is Not limited to this, a gradient magnetic field Gy for phase encoding or a gradient magnetic field Gz for slice selection can be used. Furthermore, these gradient magnetic fields Gx, Gy, and Gz can be combined to create a gradient magnetic field in an arbitrary direction, and this can be used as a dephase pulse or a rephase pulse. In this way, it is possible to obtain the one-dimensional image in the direction by encoding the position information in the arbitrary direction and detect the motion in the arbitrary direction. In addition, FIG.
After the last rephasing pulse Gx is finished, if the dephasing pulse and the rephasing pulse due to the gradient magnetic field in the other direction are added to generate the third echo signal, in addition to the motion detection in the X direction, The movement in the other direction can be detected, and the movement can be detected in the two or three directions at the same time.

Although the field echo method has been described above, the present invention can of course be applied to other pulse sequences such as the spin echo method or pulse sequences such as the three-dimensional imaging method. In the case of the spin echo method, the second echo signal is generated prior to the first echo signal by adding the dephasing pulse and the rephasing pulse due to the gradient magnetic field in an arbitrary direction in the period between the 90-degree pulse and the 180-degree pulse. You can also let it. In this case, if the second echo signal is generated by adding the dephase pulse and the rephase pulse before the phase encoding gradient magnetic field Gy pulse, a gradient magnetic field Gy pulse that cancels the phase encoding gradient magnetic field Gy. Is unnecessary.

[0017]

As described in the above embodiments, according to the MR imaging apparatus of the present invention, the movement of the subject can be detected easily and surely by devising the pulse sequence and there is no body movement artifact. A reconstructed image can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of an MR imaging apparatus according to the present invention.

FIG. 2 is a time chart showing a pulse sequence according to the embodiment.

[Explanation of symbols]

 10 Gantry 11 Static magnetic field magnet 12 Gradient magnetic field coil 13 High frequency coil 21 Control computer 22 Waveform generator 23 Gradient magnetic field power supply 31 High frequency generator 32 Amplitude modulator 33 High frequency power amplifier 34 Switch 41 Amplifier 42 Phase detector 43 A / D conversion Device 44 Data processing device

Claims (1)

[Claims] 1. A means for generating a static magnetic field, a gradient magnetic field generating means for generating a gradient magnetic field in each direction for slice selection, phase encoding and frequency encoding, and a magnetic field placed in the static magnetic field and the gradient magnetic field. Excitation means for irradiating a subject with a high-frequency signal for excitation, data collection means for receiving resonance signals from the subject and collecting data, and controlling the gradient magnetic field generation means, excitation means, and data collection means Then, while irradiating a high frequency signal, a gradient magnetic field for slice selection, phase encoding and frequency encoding is added to generate a first resonance signal in which position information in at least two axial directions is encoded, and A dephasing pulse and a rephasing pulse due to a gradient magnetic field in the uniaxial direction are sequentially added to generate a second resonance signal in which position information in the uniaxial direction is encoded. A control unit that repeats the sequence for changing the gradient magnetic field for phase encoding described above,
M including a data processing unit that performs image reconstruction processing from the data collected from the first resonance signal.
R imaging device.