JPH04354936A - Mri system - Google Patents

Abstract

PURPOSE:To achieve a higher speed with a shortening of photographing time by reducing time integration of a dephasing inclined magnetic field pulse in a direction of a phase encoding while each of 90 deg. and 180 deg. pulses is shortened to make a pseudo echo asymmetric. CONSTITUTION:Pulses of 90 deg. and 180 deg. (respectively pulses 1 and 2) are generated sequentially at such a short time interval as to generate a spin echo before the center of a data sampling window. A slice thickness-wise rephasing inclined magnetic field pulse 3 and a dephasing inclined magnetic field pulse 4 thereof are applied separately together with the pulses 1 and 2. Moreover, after the application of a reading out-wise dephasing inclined magnetic field 8, a rephasing inclined magnetic field pulse 9 thereof is applied sequentially being inverted. Then, after the application of a phase encoding-wise dephasing inclined magnetic field pulse 6 with time integration thereof reduced, a rephasing inclined magnetic field pulse 7 thereof is applied by a specified number.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MRI apparatus (nuclear magnetic resonance tomography apparatus), and more particularly to an MRI apparatus that performs ultrahigh-speed imaging.

0002

[Prior Art] MRI apparatuses that perform ultra-high-speed imaging have been known for some time (Japanese Patent Laid-Open No. 63
-21426). This is to perform a pulse sequence as shown in FIG. First, as shown in FIG. 4A, a 90° pulse 1 is applied, and at the same time, as shown in FIG. 4B, a pulse 3 of a gradient magnetic field Gs for slice selection is applied. This Gs pulse 3 functions as a dephasing gradient magnetic field pulse that disturbs the phase of nuclear spins in a certain region in the slice thickness direction, and can selectively excite only that region. Next, by applying a rephasing Gs pulse 4, the phases of the nuclear spins that have been disturbed in the slice thickness direction are aligned.

Thereafter, as shown in FIG. 4C, a dephasing pulse 6 of a gradient magnetic field Gp in the phase encoding direction is applied. Further, as shown in FIG. 4D, a dephasing pulse 8 of a gradient magnetic field Gr in the readout direction is applied.

Furthermore, after a time interval τ after the 90° pulse 1, a Gs pulse 5 is applied simultaneously with the 180° pulse 2. Since this Gs pulse 5 extends before and after the 180° pulse 2, the first half acts as a dephasing gradient magnetic field pulse and the second half acts as a rephasing gradient magnetic field pulse.

After this 180° pulse 2, a rephasing Gr pulse 9 is added. Then, when the time integral of the rephasing Gr pulse 9 becomes the same as the time integral of the dephasing Gr pulse 8, a gradient echo 10 is generated as shown in E in FIG. When this rephasing Gr pulse 9 is sequentially inverted between positive and negative, gradient echoes 10 are sequentially generated when the time integrals have the same amount with opposite polarity.

[0006] In response to the generation of the gradient echo 10, rephasing Gp pulses 7 are sequentially applied. 18
A spin echo occurs when time τ has elapsed since 0° pulse 2, but the rephasing G applied up to that point
The waveform is determined so that the total time integral of the p-pulse 7 coincides with the time integral of the dephasing Gp pulse 6, and the time point at which a spin echo occurs coincides with the time point at which the nuclear spins that have been disordered in the phase encoding direction are aligned. The peak of the pseudo echo (envelope of the row of gradient echoes 10) 11 is set at a time τ after the 180° pulse 2.

In this case, since the number of gradient echoes 10 corresponding to the number of phase encodings is generated, one 90° pulse 1 and one 18
Data for the required number of phase encodings can be obtained at once with just one excitation using 0° pulse 2,
Ultra-high-speed imaging can be performed.

It is assumed that the time integral of the dephasing Gp pulse 6 corresponds to half of the maximum phase encoding amount, and the rephasing Gp pulse 7 continues until its total time integral corresponds to the median value of the phase encoding amount. At a given time, pseudoecho 1
1 is set as the peak, and therefore symmetry is obtained in the rows of pseudo echoes 11 and gradient echoes 10. Therefore, by setting the period from the rise to the fall of the pseudo echo 11 as a data sampling window, the pseudo echo 11 is placed in the center of the window.
It has the advantage that the peaks of the images can be matched, and because of their symmetry, an image with less truncation artifacts can be obtained.

[0009]

[Problems to be Solved by the Invention] However, in the conventional ultrahigh-speed imaging described above, 90° pulse 1 and 180° pulse
° Since the center of the sampling window must be located at twice the time interval τ from pulse 2, and half the number of phase encodings 10 must be obtained by that time, it is necessary to obtain τ There was a problem in that it was not possible to shorten the length, and it was difficult to further increase the speed.

An object of the present invention is to provide an MRI apparatus that improves the conventional MRI apparatus that performs ultrahigh-speed imaging and further increases the speed of imaging.

[0011]

[Means for Solving the Problems] In order to achieve the above object, in the MRI apparatus according to the present invention, the time integral of the dephasing gradient magnetic field pulse in the phase encoding direction is made smaller than half of the maximum phase encoding amount. The nuclear spins that had been disturbed in this phase encoding direction were aligned before applying rephasing gradient magnetic field pulses up to half the number of encodings, and the 90° pulse and 180° pulse were
By shortening the time interval between the 90° pulse and the 180° pulse, pseudo echoes can be made asymmetrical, and the speed can be increased by the amount that the time interval between the 90° pulse and the 180° pulse can be shortened. The pseudo echo becomes asymmetrical, and the peak of the pseudo echo is located before the center of the data sampling window from the rise to the fall of the pseudo echo, so the symmetry is broken, but the advantage is that the imaging time can be shortened. Another advantage is that the S/N ratio of the signal can be improved and an image close to a proton density-weighted image can be obtained.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. In FIG. 1, a subject (human body) 11 is placed in a static magnetic field generated by a static magnetic field magnet 12, and a gradient coil 13 and an RF coil 21 are placed in the static magnetic field space. A pulse current of a predetermined waveform is passed through the gradient coil 13 from the gradient power source 14,
A gradient magnetic field is generated that is superimposed on the static magnetic field. Waveform data read from the RAM of the sequencer 32 is sent to the gradient power supply 14 under the control of the computer 31, and a current having a waveform according to the data is output to the gradient coil 13 at a desired timing. The gradient coils 13 are provided in three orthogonal axes, and the gradient power supply 14 outputs current for gradient magnetic fields in each direction correspondingly. The gradient magnetic fields in the three orthogonal axes directions are respectively a gradient magnetic field Gs in the slice thickness direction, a gradient magnetic field Gp in the phase encoding direction, and a gradient magnetic field Gr in the readout direction for slice selection.

The sequencer 32 sends the RF pulse waveform to the modulator 23 , and the modulator 23 amplitude-modulates the high-frequency signal from the high-frequency oscillator 24 according to the pulse waveform, and the modulated high-frequency output is sent to the RF amplifier 22 . It is amplified and sent to the RF coil 21. In this way, RF coil 2
1 , an RF signal is irradiated toward the subject 11 .

The nuclear magnetic resonance signal generated in the subject 11 is R
It is received by the F coil 21. This received signal is amplified by a preamplifier 25 and then sent to a quadrature phase detection circuit 26, where it is subjected to quadrature phase detection using the high frequency signal from the high frequency oscillator 24 as a reference signal. The detection output is sent to the A/D converter 27, converted to digital data, taken into the computer 31 via the sequencer 32, and imaged by image reconstruction processing such as two-dimensional Fourier transformation.

The above-mentioned static magnetic field is set at approximately 0.1 Tesla to 5 Tesla, and a pulse sequence as shown in FIG. 2 is performed. The pulse sequence in FIG. 2 is almost the same as the pulse sequence in FIG. 4, except that the time interval τ between 90° pulse 1 and 180° pulse 2 is shorter than that in FIG.
The difference is that the time integral of the dephasing Gp pulse 6 is smaller than that in FIG. 4, and that the pseudo echo 11 is asymmetrical due to these.

That is, since the time interval τ between 90° pulse 1 and 180° pulse 2 is shorter than in FIG.
A spin echo will occur before half the number of phase encoding gradient echoes 10 are generated. Therefore, at the time when this spin echo occurs, the phase disturbance of the nuclear spins in the phase encoding direction is adjusted, that is, before the total time integral of the rephasing Gp pulse 7 reaches half of the maximum phase encoding amount. The time integral of the dephasing Gp pulse 6 is made small so that the time integral reaches the time integral of the dephasing Gp pulse 6. As a result, when rephasing Gp pulses 7 are applied less than half the number of phase encodings, the phases of the nuclear spins in the phase encoding direction become aligned, and this point coincides with the point at which spin echoes occur. can be done.

Therefore, as shown in E of FIG. 2, the peak of the pseudo echo 11 occurs after this time τ, and the peak of the pseudo echo 11 occurs at a point before half the number of gradient echoes 10 that occur by the number of phase encodings. A peak appears and the pseudo echo 11 becomes asymmetrical. A data sampling window by the A/D converter 27 (FIG. 1) is set from the rising edge to the falling edge of the pseudo echo 11, and data with different amounts of phase encoding are obtained from each gradient echo 10. An image is reconstructed by subjecting this data to two-dimensional Fourier transform using a computer 31 (FIG. 1) or the like.

By making the pseudo echo 11 asymmetric, the time interval τ between 90° pulse 1 and 180° pulse 2
can be shortened, and the imaging sequence time can be further shortened accordingly.

Since the data sampling window is set from the rising edge to the falling edge of the pseudo echo 11, the peak of the pseudo echo 11 occurs before the half point of the data sampling window, and symmetry is lost; Using a 25mM nickel chloride aqueous solution as a sample, we performed the above pulse sequence to obtain a signal and conducted an experiment to determine the S/N ratio, and the data shown by the △ mark in Figure 3 was obtained. . For reference, the data obtained in the conventional example (sequence in Figure 4) is
Indicated by a mark. It can be seen from FIG. 3 that the S/N ratio of the signal is improved by making the pseudo echo 11 asymmetric. This seems to be because the shorter τ makes it possible to sample data at a time when the intensity of the nuclear magnetic resonance signal is stronger. In addition, since the slope of the aqueous solution of nickel chloride with respect to the concentration is small, the rate of change in signal intensity with respect to T2 is small, and data that is less affected by T2 can be obtained. It can be seen that the contrast characteristics approach that of .

[0020]

[Effects of the Invention] As described above with respect to the embodiments,
According to the MRI apparatus of the present invention, by making the pseudo echoes asymmetric, it is possible to obtain an advantage that the imaging time can be shortened, and also to improve the S/N ratio of the signal and obtain an image close to a proton density weighted image. Another advantage is that you can.

[Brief explanation of drawings]

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

FIG. 2 is a time chart showing the pulse sequence of the same embodiment.

FIG. 3 is a diagram showing data of experimental results.

FIG. 4 is a time chart showing a conventional pulse sequence.

[Explanation of symbols]

11 Subject 12 Static magnetic field magnet 13
Gradient coil 14 Gradient power source 21 RF coil 22 RF amplifier 23 Modulator 24 High frequency oscillator 25
Preamplifier 26 Quadrature phase detector 27
A/D converter 31 Computer 32 Sequencer

Claims (1)

[Claims] [Claim 1] A 90° pulse and a 180° pulse,
Means for sequentially generating spin echoes at short time intervals such that spin echoes are generated before the center of the data sampling window, adding rephasing gradient magnetic field pulses in the slice thickness direction along with the 90° pulse and 180° pulse, and dephasing the same. means for applying a dephasing gradient magnetic field pulse in the readout direction and then sequentially reversing and applying the rephasing gradient magnetic field pulse; means for adding an integral smaller than half of the maximum phase encoding amount, and then sequentially adding the rephasing gradient magnetic field pulses by a number corresponding to the number of phase encodings in response to the reversal of the rephasing gradient magnetic field pulses; An MRI apparatus comprising: