JP2616358B2 - MR imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present 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 Various pulse sequences have been devised in an MR imaging apparatus. Among them, a high-speed spin echo method shortens the imaging time without substantially deteriorating the image quality as compared with the conventional spin echo method. Known as one. In this high-speed spin echo method, as shown in FIG. 3, one nutation RF pulse (90 ° pulse) is applied, then applied together with a pulse of a slice selection gradient magnetic field Gs, and then a readout gradient magnetic field Gr. Of the refocus RF pulse (1
An 80 ° pulse) is added together with the Gs pulse, and a pulse of the gradient magnetic field Gp for phase encoding is applied, and a rephase pulse 5 of the gradient magnetic field Gr is applied to generate an echo signal. This refocus RF pulse is referred to as a Gs pulse, G
A plurality of echo signals are generated by giving a plurality of times together with the p pulse and the rephase Gr pulse 5 to collect data of a plurality of lines (a plurality of phase encoding amounts) in the raw data space area. In this manner, data of a plurality of lines can be collected by using one nutation RF pulse and a plurality of refocusing RF pulses, and the imaging time can be reduced to 1 / echo by reducing the number of repetitions of the nutation RF pulse. can do.

[0003]

However, in order to further reduce the imaging time, a certain time (about 30 minutes) is required.
0 ms), it is necessary to generate as many echo signals as possible, so that the echo interval (180 ° pulse interval) must be shortened, and the data sampling interval must be shortened inevitably. S /
There is a problem that the N ratio is deteriorated.

In view of the above, the present invention makes it possible to compensate for the deterioration of the S / N ratio due to the shortening of the data sampling interval by increasing the signal strength of each echo of the multi-echo when performing the high-speed spin echo method. Accordingly, it is an object of the present invention to provide an MR imaging apparatus capable of further reducing an imaging time.

[0005]

In order to achieve the above object, in an MR imaging apparatus according to the present invention, one nutating RF pulse and a plurality of refocusing RF pulses following the nutating RF pulse are sequentially inverted in phase. Means for irradiating the subject such that the time interval between the plurality of refocusing RF pulses is twice as long as the time interval between the first nutation RF pulse and the subsequent first refocusing RF pulse; Means for applying a slice selection gradient magnetic field pulse at the same time as the RF pulse, and only within each interval of the refocusing RF pulse, with respect to the readout gradient magnetic field,
A depolarizing pulse of one polarity, a rephasing pulse of another polarity, and a rephasing pulse of one polarity are sequentially applied so that the waveform becomes symmetrical in the time direction at the center of the interval. A readout gradient magnetic field applying means for generating a signal only once, and applying a phase encoding gradient magnetic field pulse having a different integrated value for each signal before signal generation only within each interval of the refocusing RF pulse. Means for applying a rewind pulse having the same integral value and a different polarity after the signal generation using a gradient magnetic field for phase encoding.

[0006]

When a nutating RF pulse is applied, followed by a refocusing RF pulse, and then a dephasing pulse and a rephasing pulse of a readout gradient magnetic field are sequentially applied, the pulse is inverted by 90 ° and further inverted by 180 °. A signal (spin echo signal) is generated due to the magnetization. Also, the excitation R
Due to the imperfections of F, the signal due to magnetization (gradient echo signal) that fell for 90 ° pulse for the first time and did not fall by 90 ° due to the imperfection of F can also be obtained by sequentially applying the dephase pulse and rephase pulse of the readout gradient magnetic field. Occur. Therefore, these overlap and the signal strength increases.
Therefore, even if the S / N ratio is deteriorated by shortening the data sampling interval, a signal strength sufficient to compensate for the deterioration is obtained, so that higher-speed imaging can be performed by shortening the echo interval. Further, the readout gradient magnetic field includes a depolarization pulse of one polarity, a rephase pulse of another polarity,
Since the waveforms are applied in the order of repolarization pulses of one polarity and the waveform is symmetrical in the time direction, data that is not affected by the movement of the subject in the readout direction is obtained, and an image free from artifacts due to the movement is obtained. Can be obtained.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a time chart showing a pulse sequence of a high-speed spin echo method performed by an MR imaging apparatus according to one embodiment of the present invention, and FIG. 2 is a block diagram showing a configuration of the MR imaging apparatus according to the embodiment.

In the pulse sequence of the fast spin echo method shown in FIG. 1, one 90 ° pulse is followed by a plurality of 1 ° pulses.
When sequentially applying an 80 ° pulse, the phase of the RF pulse is changed to X
The phases are controlled in the order of plus, minus, plus,... (Or vice versa) so that they are opposite to each other. A slice selection gradient magnetic field Gs pulse is applied simultaneously with the application of these RF pulses. The pulse of the gradient magnetic field Gr in the readout direction and the pulse of the phase encoding gradient magnetic field Gp are applied between the 180 ° pulse and the 180 ° pulse. Gp is given a different integral value for each signal before the signal is generated, and after the signal is generated and before the next 180 ° pulse, the same integral value is used to eliminate the influence of phase encoding. Gives a polarity rewind pulse.

The Gr pulse is sequentially applied as a dephase pulse 1, a rephase pulse 2, and a rewind pulse 3 within an interval of 180 ° -180 °. In this case, 18
Since the 0 ° -180 ° interval is considered to be very short, the phase shift due to the inhomogeneity of the static magnetic field can be ignored within this interval. Therefore, the phase shift is caused by the above Gr pulses 1, 2, and 3. That is, 9
Between the time when the 0 ° pulse is applied and the time when the first 180 ° pulse is applied, unlike the conventional case (FIG. 3), the Gr pulse is not applied, so that the phase is not dephased.
The phase shift is caused by the dephase pulse 1 applied after the 0 ° pulse.

The phase shift is as shown at the bottom of FIG. 1, where the solid line indicates the phase change of the magnetization of the spin echo which is tilted by 180 ° by the 180 ° pulse, and the dashed line indicates the phase change of 180 °.
The phase change of the magnetization of the gradient echo falling 90 ° due to imperfections of the ° pulse is shown, respectively. That is,
The magnetization that is tilted 90 ° by the 90 ° pulse is the first 180
When the pulse is tilted 180 ° by the pulse, the spin echo is magnetized. On the other hand, due to the imperfection of the RF pulse, there is a magnetization that remains without falling even when a 90 ° pulse is applied, and when it falls only 90 ° with a 180 ° pulse, it becomes the same as the magnetization of the gradient echo.

Since these are first dephased and changed in phase by the dephasing pulse 1 applied after the 180 ° pulse, as shown in FIG. 1, both the magnetization of the spin echo and the magnetization of the gradient echo change. Thus, when the two coincide, an echo signal is generated together, and these are added to increase the signal strength.

By the way, in the prior art (FIG. 3), the magnetization which has been tilted 90 ° by the 90 ° pulse is dephased by the dephase pulse 4 after the 90 ° pulse, and the phase is as shown by the solid line at the bottom of FIG. Change. This is 180
The pulse is inverted by the pulse and the phase is adjusted by the rephase pulse 5 to generate an echo signal. On the other hand, 1
Since the phase of the magnetization tilted by 90 ° due to the imperfection of the 80 ° pulse is changed by the rephase pulse 5, the phase changes as indicated by the one-dot chain line. As described above, conventionally, the phases of the spin echo and the gradient echo are different from each other, and the gradient echo is merely an artifact of a high frequency component without being added to each other as an echo signal (to eliminate this artifact, a spoil gradient is used). Etc.).

In the pulse sequence shown in FIG. 1, since the rewind pulse 3 is applied following the rephase pulse 2 after the generation of the echo signal, both the magnetization of the spin echo and the magnetization of the gradient echo are restored. So 2
The same phase diagram is obtained after the application of the second 180 ° pulse, and the same operation is repeated.

Since the combination of the gradient magnetic field pulses for generating the echoes is completed only between 180 ° and 180 ° and does not depend on other time domains, the magnetization passing through any path can be obtained. An echo can be generated at the center of the rephase pulse 2, and the signal strength increases.

Further, since the phases of the RF pulses are sequentially made to have the opposite polarities, the phases of the magnetizations passing through any paths can be prevented from having the opposite signs, and the echo signals are added to each other to increase the signal strength. Can be.

Next, the configuration of an MR imaging apparatus that performs such a pulse sequence will be described with reference to FIG. Gz coil 1 in main magnet 11
2, a Gy coil 13 and a Gx coil 14 are arranged, and by applying a current to them as indicated by arrows, gradient magnetic fields Gx, Gy and Gz in three directions of X, Y and Z are generated. Gs, Gr, and Gp use any one of these gradient magnetic fields Gx, Gy, and Gz, or use a combination thereof to form a gradient magnetic field in a desired direction. The main magnet 11 generates a static magnetic field whose magnetic flux is directed in the Z direction. These Gz
A current flows from the gradient magnetic field power supply 22 to the coil 12, the Gy coil 13, and the Gx coil 14. Those current waveforms are provided by a waveform generator 21.

A subject (not shown) is inserted into a space to which a static magnetic field and a gradient magnetic field are applied, and a transmitting antenna and a receiving antenna (not shown) are attached to the subject. An excitation RF pulse is supplied from the transmission power amplifier 26 to the transmission antenna. This excitation RF pulse is applied to the modulation circuit 25.
, The RF signal from the signal generator 23 is modulated by the signal from the waveform generator 24. The NMR signal received by the receiving antenna is sent to a detection circuit 28 through a preamplifier 27, phase-detected using the signal from the signal generator 23 as a reference signal, and further sampled by an A / D converter 29 and converted into digital data. Computer 2
It is taken into 0.

The computer 20 processes this data to reconstruct an image to obtain an MR image. The computer 20 controls the waveform of each gradient magnetic field generated from the waveform generator 21 and its timing, controls the RF pulse waveform from the waveform generator 24 and its timing, and further controls the signal generator 23. By making the frequency of the RF pulse coincide with the resonance frequency, the pulse sequence of FIG. 1 is performed.

The data collected by the pulse sequence shown in FIG. 1 is subjected to a two-dimensional Fourier transform by the computer 20 to reconstruct an image. This image is displayed on an appropriate image display device (not shown).

[0020]

As described above, according to the MR imaging apparatus of the present invention, since the signal intensity can be increased, the data sampling interval can be shortened without deteriorating the S / N ratio, the echo interval can be shortened, and the high-speed spin can be achieved. The echo method can be performed at higher speed.

[Brief description of the drawings]

FIG. 1 is a time chart showing a pulse sequence in one embodiment of the present invention.

FIG. 2 is a block diagram showing the configuration of the embodiment.

FIG. 3 is a time chart showing a pulse sequence of a conventional example.

[Explanation of symbols]

 1, 4 Dephase pulse 2, 5 Rephase pulse 3 Rewind pulse RF High frequency excitation pulse Gs Slice selection gradient magnetic field Gr Readout gradient magnetic field Gp Phase encoding gradient magnetic field 11 Main magnet 12-14 Gradient magnetic field generating coil 20 Computer Reference Signs List 21 gradient magnetic field waveform generator 22 gradient magnetic field power supply 23 signal generator 24 RF excitation waveform generator 25 modulation circuit 26 transmission power amplifier 27 preamplifier 28 detection circuit 29 A / D converter

Claims (1)

(57) [Claims] 1. A nutation RF pulse in which one nutation RF pulse and a plurality of subsequent refocus RF pulses are sequentially inverted in phase and the time interval between the plurality of refocus RF pulses is the first nutation RF pulse. Means for irradiating the subject with a pulse so as to be twice as long as the time interval between the refocusing RF pulse and the subsequent refocusing RF pulse, means for applying a slice selection gradient magnetic field pulse simultaneously with these RF pulses, Only within each interval of the RF pulse, with respect to the gradient magnetic field for readout, a depolarization pulse of one polarity, a rephase pulse of another polarity, and one polarity of a polarity so that the waveform becomes symmetrical in the time direction at the center of the interval. Readout gradient magnetic field application means for sequentially applying the rephase pulse of the above, and generating a signal only once at the above-mentioned center point; Only within each of the intervals, a gradient magnetic field pulse for phase encoding having a different integral value is applied to each signal before signal generation, and a rewind pulse having the same integral value and a different polarity for phase encoding is generated after signal generation. Means for applying using a gradient magnetic field.