JPH06133944A - Mr imaging system - Google Patents

Abstract

PURPOSE:To provide a diffusion-stressed image in a short image pickup time without deteriorating picture quality. CONSTITUTION:Echo signals are sequentially generated by application of one 90 deg. pulse and a plurality of subsequent 180 deg. pulses, and MPG of the same magnitude are inserted along the same direction between the 90 deg. pulse and the first 180 deg. pulse and between the first and second 180 deg. pulses while the second and subsequent echo times MTE are made shorter than the first echo time TE.

Description

Detailed Description of the Invention

[0001]

This invention relates to nuclear magnetic resonance (NM).
The present invention relates to an MR imaging apparatus that performs imaging using (R), and in particular, the diffusion (d
The present invention relates to an MR imaging apparatus for obtaining an image enhancement image.

[0002]

2. Description of the Related Art Conventionally, in an MR imaging apparatus, an MPG (M
A pulse sequence of the diffusion-weighted spin echo method is known in which a diffusion-weighted image is obtained by introducing a gradient magnetic field pulse called an "otition Probing Gradient".

[0003]

However, the conventional diffusion-weighted spin echo method has problems such as poor image quality and long imaging time (about 10 minutes). When the imaging time is long, the patient is restrained for a long time, and when the patient moves during that time, an image artifact is generated. This diffusion-weighted spin-echo method is used to image molecular diffusion information and is essentially motion-sensitive, so movement of the patient has a large effect on it, and only images that are almost useless can be obtained. I can't.

In view of the above, the present invention shortens the imaging time to obtain a diffusion-weighted image without deteriorating the image quality, thereby shortening the patient restraint time and reducing the probability of image deterioration due to patient movement. It is an object of the present invention to provide an MR imaging apparatus improved as described above.

[0005]

In order to achieve the above object, the MR imaging apparatus according to the present invention irradiates a subject with a nutation RF pulse and a plurality of subsequent refocusing RF pulses, and sequentially. The echo signal is generated, and the nutation RF pulse and the first refocus R
A gradient magnetic field pulse of the same magnitude and in the same direction is added as an MPG only between the F pulse and between the first refocus RF pulse and the second refocus RF pulse, and the second and subsequent times from the first echo time are added. The feature is that the echo time is short.

[0006]

[Function] Refocusing RF due to imperfections of RF pulse
Even if a pulse is applied, not all are tilted by 180 °, but there is also magnetization that can only be tilted by 90 °. Therefore, the stimulated echo component generated by the first refocus RF pulse is 1 by the second refocus RF pulse.
Since it is inverted by 80 °, the time for which the phases are aligned and signals are generated becomes slow. On the other hand, since the echo time after the second echo is short, it is possible to prevent the signal due to the stimulated echo component from being generated within the time when these echo signals are generated. Therefore, it is possible to eliminate the echo components having different diffusion enhancement factors depending on the echo path, which simplifies the quantitative analysis of the diffusion coefficient. Further, since the MPG is much larger than the other gradient magnetic field pulses, it can be approximated that the diffusion weighting factor for the second and subsequent echo signals is equal to the diffusion weighting factor of the path of the primary echo component of the first echo, which is also quantitative. Analysis becomes easy. In addition, since the echo time after the second time is shortened, it is possible to prevent signal attenuation. As a result, a diffusion-weighted image with high contrast and high quantitativeness can be obtained at high speed.

[0007]

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 is a time chart showing a pulse sequence of a diffusion-weighted fast spin echo method performed by an MR imaging apparatus according to an embodiment of the present invention, and FIG. 2 is a block diagram showing a configuration of the MR imaging apparatus according to the embodiment. is there.

In the pulse sequence of the diffusion-weighted fast spin echo method shown in FIG. 1, when one 90 ° pulse and a plurality of (here, four) 180 ° pulses are sequentially applied, these RF pulses are applied. At the same time, a gradient magnetic field Gs pulse for slice selection is applied. 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.

The MPG may be added only for the gradient magnetic field in one direction, but here, the gradient magnetic fields Gs, G are used.
It is supposed to be given for all p and Gr, and 90
The same direction and the same amount are applied between the 180 ° pulse and the 180 ° pulse, and between the first and second 180 ° pulses.

The gradient magnetic field Gr is MPG after a 90 ° pulse.
Is added for dephasing before, and after the 180 ° pulse is applied, it is added for rephasing one after another near the echo time TE and near MTE. The first echo signal occurs after the echo time TE from the 90 ° pulse, and the second, third, and fourth echo signals occur after each echo time MTE. In this embodiment, the first echo time TE is about 100 ms, and the second, third and fourth echo times MTE are about 10 to 50 ms.

The phase-encoding gradient magnetic field Gp is applied until the first echo signal is generated, and then the same amount is added until the second 180 ° pulse is applied and rewinding.
After the second 180 ° pulse is applied, a gradient magnetic field Gp pulse having a magnitude corresponding to a different phase encoding amount is added, and the same amount of Gp pulse for rewind is added by the time the third 180 ° pulse is applied. After the third 180 ° pulse is applied, a gradient magnetic field Gp pulse having a magnitude corresponding to a different phase encoding amount is further added, and by the time the fourth 180 ° pulse is applied, the same amount of Gp pulse for rewind is added. After applying the fourth 180 ° pulse, a gradient magnetic field Gp pulse having a magnitude corresponding to another phase encoding amount is further applied. Thus, in this embodiment, four nutation RF pulses are followed by four refocusing RF pulses to obtain four echo signals having different phase encoding amounts. That is, data of 4 lines (4 phase encoding amounts) in the raw data space area can be collected with one nutation RF pulse, and the imaging time can be reduced to 1 / echo number (this In the embodiment, it can be shortened to 4).

Considering the behavior of the phase of nuclear spins in such a pulse sequence, the behavior is as shown in the bottom of FIG. That is, the phase shift amount is simply increased by the application of the 90 ° pulse, inverted by the application of the first 180 ° pulse as shown by the solid line, and then changed in the direction in which the phases are aligned, and after the echo time TE, the phase shift amount is changed. 1 echo signal is generated. Since the second 180 ° pulse is applied when the phase shift further proceeds, it is inverted again as shown by the solid line and changes in the direction in which the phases are aligned. After the echo time MTE, the second echo signal Occurs.
Also by the third and fourth 180 ° pulses, as shown by the solid line, they are similarly inverted and changed to the direction in which the phases are aligned, and the third and fourth echo signals are generated after the echo time MTE.

On the other hand, due to the incompleteness of the first 180 ° pulse, 180 ° like the primary echo component above.
A stimulated echo component that does not fall but only 90 ° occurs. This component becomes longitudinal magnetization and maintains its phase shift amount as shown by the dotted line, and the second 180 °
It is tilted an additional 90 ° due to imperfections in the pulse. Eventually, this second 180 ° pulse application causes a 180 ° tilt, and from this point onward, the phases change in the direction in which the phases are aligned. Here, since the echo signal generation by the stimulated echo component is TE> MTE, it is possible to temporally occur after the above-mentioned four echo signals are generated. This allows the four echo signals above to be MP
It is possible to exclude the mixing of the signal component that has been affected by G. This means that the above-mentioned four echo signals do not have echo components having different diffusion weighting factors (b factors), which simplifies the quantitative analysis of the diffusion coefficient.

Generally, the NMR signal intensity S in consideration of diffusion can be expressed by S (x, y, z) ∝exp [-b · D (x, y, z)]. However, D is a diffusion coefficient.
The b factor is b = ∫ | k (t) | ² dt, where k = γ∫G (t) dt. G is the intensity of the gradient magnetic field. Therefore, the G (specifically, MPG) is changed and imaged twice to measure the NMR signal intensities S and So, and the b factors (b, bo) for the two different Gs are calculated to obtain D (x , y, z) = ln [So (x, y, z) / S (x, y, z)] / [b-bo] to obtain the diffusion coefficient D.

Here, since the stimulated echo component does not exist in the first echo signal as described above, the b factor may be calculated for the path of the primary echo of the first echo signal. In this case, since MPG is extremely larger than other gradient magnetic fields, an approximate value can be obtained by calculating the b factor by MPG. Further, the b factors for the second, third, and fourth echo signals are M for gradient magnetic field pulses other than MPG.
Since it is extremely small as compared with PG, it can be approximated to be equal to the b factor obtained for the path of the primary echo of the first echo signal described above.

Next, the structure 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, Gy coil 13 and Gx coil 14 are arranged, and electric currents are made to flow through them as indicated by arrows, whereby gradient magnetic fields Gx, Gy, and Gz in three directions of X, Y, and Z are generated. As the above Gs, Gr, and Gp, one of these gradient magnetic fields Gx, Gy, and Gz is used, or a combination thereof is used to form a gradient magnetic field in a desired direction. The main magnet 11 is for generating a static magnetic field in which the magnetic flux is oriented in the Z direction. These Gz
A current is applied to the coil 12, the Gy coil 13, and the Gx coil 14 from the gradient magnetic field power supply 22. These current waveforms are provided by the 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 supplied to the modulation circuit 25.
In the above, the RF signal from the signal generator 23 is modulated by the signal from the waveform generator 24. The NMR signal received by the reception antenna is sent to the detection circuit 28 through the preamplifier 27, the signal from the signal generator 23 is phase-detected as a reference signal, and further, is sampled by the A / D converter 29 and converted into digital data. Being a computer 2
It is taken into 0.

The computer 20 reconstructs an image by subjecting this data to a two-dimensional Fourier transform process to form an MR image (that is, a pixel value represented by S (x, y, z) above).
To get 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. Then, by matching the frequency of the RF pulse with the resonance frequency, the pulse sequence of FIG. 1 is performed.

When the MPG was changed by the pulse sequence of FIG. 1 and imaging was performed twice and water and acetone were measured, the results shown in FIG. 3 were obtained. The results in FIG. 3 are in good agreement with the past reports, and the quantification was confirmed.

[0020]

As described above, according to the MR imaging apparatus of the present invention, the imaging time can be shortened without deteriorating the image quality as compared with the diffusion weighted image using the conventional spin echo method, and as a result, Since the patient restraint time can be shortened, the probability of image quality deterioration due to patient movement is also reduced. In addition, quantitative measurement can be performed by the diffusion-weighted high-speed spin echo method.

[Brief description of drawings]

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

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

FIG. 3 is a graph showing experimental results according to the same example.

[Explanation of symbols]

 RF high-frequency excitation pulse Gs slice selection gradient magnetic field Gr readout gradient magnetic field Gp phase encoding gradient magnetic field 11 main magnets 12 to 14 gradient magnetic field generating coil 20 computer 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

Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location 9219-2EN

Claims (1)

[Claims] 1. A means for irradiating a subject with one nutation RF pulse and a plurality of subsequent refocusing RF pulses to sequentially generate echo signals, and the nutation RF pulse and the first refocusing RF pulse. The gradient magnetic field pulse of the same magnitude in the same direction is MPG only between the pulse and the first refocus RF pulse and the second refocus RF pulse.
An MR imaging apparatus comprising: an RF pulse generation unit and a control unit that controls the generation of the RF pulse and the gradient magnetic field pulse so that the second and subsequent echo times are shorter than the first echo time.