JPH06254064A - Magnetic resonance imaging system - Google Patents

Abstract

PURPOSE:To improve an S/N while shortening a photographing time by varying the intensity of a read-out gradient magnetic field in accordance with the phase encoding quantity in a magnetic resonance imaging system which collects an echo signal while varying the phase encoding. CONSTITUTION:In this system which is provided with a static magnetic field magnet 1, an X-axis-Z-axis gradient magnetic field coil 2 and a transmitting/ receiving coil 3 in a gantry 20, and collects an echo signal while varying the phase encoding quantity along the phase encoding direction, a high frequency component in the phase encoding direction utilizes a multi-echo method, and a data collection is executed in a prescribed data collection time, and with a prescribed band width by a read-out gradient magnetic field having prescribed intensity. On the other hand, a low frequency component in the phase encoding direction executes the data collection in a data collection time being longer than the data collection time and with comparatively narrow band width by the read-out gradient magnetic field of comparatively low intensity. In such a way, the band width for the data collection is narrowed, so that the S/N is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus which collects echo signals while changing the amount of phase encode along the phase encode direction.

[0002]

2. Description of the Related Art In recent years, a multi-echo method has been developed which collects data for one slice at high speed. In this multi-echo method, as shown in a pulse sequence in FIG. 8, a 180 ° pulse is repeatedly applied after excitation by one RF pulse (90 ° pulse), and phase encoding is performed for each echo. This is a method of collecting a plurality of echo signals (two echo signals in the figure) by changing the amount step by step. Therefore, the multi-echo method can significantly reduce the imaging time as compared with the data acquisition method in which the encoding amount is changed by one step for each excitation by the RF pulse. The multi-echo method uses T2 which emphasizes the lateral relaxation time.
The effect is great if it is applied to shooting of an emphasized image.

However, this multi-echo method has the following problems. That is, if the TE time (the time interval from the application of the RF pulse to the start of the collection of echo signals) is not so different between the echo signals without changing the number of multi-echoes, the data collection of each echo signal is inevitably made. The time (hatched portion in FIG. 8) is shortened. When the data acquisition time is shortened, the bandwidth of the echo signal is widened. Normally, the S / N of an echo signal is inversely proportional to the square root of the bandwidth of the echo signal, so that if the bandwidth of the echo signal is widened, the S / N will be lowered.

[0004]

SUMMARY OF THE INVENTION The present invention has been made to address the above-mentioned circumstances, and an object thereof is to provide a magnetic resonance imaging apparatus capable of improving S / N while shortening imaging time. is there.

[0005]

The magnetic resonance imaging apparatus according to the present invention changes the data acquisition time and the data acquisition bandwidth by changing the intensity of the read gradient magnetic field according to the amount of phase encoding. It is characterized by

[0006]

According to the magnetic resonance imaging apparatus of the present invention, the bandwidth of data acquisition is widened by increasing the intensity of the read gradient magnetic field for a predetermined phase encoding amount which may be low S / N. Although the acquisition time is shortened and the intensity of the read gradient magnetic field is reduced only for a predetermined amount of phase encoding required to have a high S / N, the data acquisition time is lengthened, but the data acquisition bandwidth is increased. The band is narrowed.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment for a magnetic resonance imaging apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of this embodiment.

In the gantry 20, the static magnetic field magnet 1, the X-axis,
A Y-axis / Z-axis gradient magnetic field coil 2 and a transmission / reception coil 3 are provided. The transmission / reception coil 3 may be directly attached to the subject instead of being embedded in the gantry.
It may be placed on the bed 13. The static magnetic field magnet 1 as the static magnetic field generator is configured by using, for example, a superconducting coil, a normal conducting coil or a permanent magnet. The X-axis / Y-axis / Z-axis gradient magnetic field coil 2 has an X-axis gradient magnetic field Gx and a Y-axis gradient magnetic field G.
It is a coil for generating the y and Z axis gradient magnetic fields Gz.
The transmitter / receiver coil 3 has a high frequency (RF).
e) A coil for both transmission and reception for generating a pulse to excite a target portion and detecting a magnetic resonance signal (MR signal) generated by magnetic resonance. The subject P placed on the bed 13 is an imageable region in the gantry 20 (a spherical region where an imaging magnetic field is formed,
It is possible to collect data only in this area).

The static magnetic field magnet 1 is driven by the static magnetic field controller 4. The transmission / reception coil 3 is connected to the transmitter 5 during excitation (transmission) and to the receiver 6 during detection of magnetic resonance signals (reception). The X-axis / Y-axis / Z-axis gradient magnetic field coil 2 includes an X-axis gradient magnetic field power source 7, a Y-axis gradient magnetic field power source 8 and a Z axis.
It is driven by the axial gradient magnetic field power supply 9.

The X-axis gradient magnetic field power source 7, the Y-axis gradient magnetic field power source 8, the Z-axis gradient magnetic field power source 9 and the transmitter 5 are driven by a sequencer 10 in a predetermined sequence, and an X-axis gradient magnetic field Gx, a Y-axis gradient magnetic field Gy, A Z-axis gradient magnetic field Gz and a radio frequency (RF) pulse are generated in a predetermined pulse sequence described later. In this case, the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis gradient magnetic field Gz are mainly used as the phase encoding gradient magnetic field Ge, the reading gradient magnetic field Gr, and the slice gradient magnetic field Gs, respectively. The sequencer 10 determines the pulse sequence unique to the present invention, which will be described later. The computer system 11 drives and controls the sequencer 10, captures the magnetic resonance signal received by the receiver 6, and applies a two-dimensional Fourier transform process (2DFT) to generate a tomographic image of the subject. , Is displayed on the display unit 12. Next, the operation of the apparatus of this embodiment configured as described above will be described with reference to the pulse sequence shown in FIG.

In this embodiment, the multi-echo method is applied to the high-frequency component in the phase encoding direction (the portion where the absolute value of the encoding amount is large) to shorten the data acquisition time,
On the other hand, the low frequency component in the phase encoding direction (the portion where the absolute value of the encoding amount is small) lengthens the data acquisition time, narrows the bandwidth of the data acquisition, and improves the S / N.

FIG. 2A shows a pulse sequence relating to high frequency components in the phase encode direction. Here, it is assumed that the phase encode amount changes by one step between −256 and +256, and the high frequency component in the phase encode direction is a portion where the absolute value of the encode amount exceeds 16. At the encoding step in which the encoding amount is included in the high frequency component, the multi-echo method is applied as described above. That is, as shown in FIG.
° Multiple times, here 2
180 times pulse is applied. After each application, the read gradient magnetic field Gr having a predetermined intensity is applied for a predetermined time, and the first echo and the second echo are sequentially acquired during the data acquisition period indicated by the diagonal lines. Encoding amount e2 when collecting the second echo
Is 1 for the encoding amount e1 when collecting the first echo.
Is the encoding amount (e1 + 1). With this multi-echo method, the imaging time is significantly shortened as compared with the case of collecting one echo signal with one excitation.
Here, since two echoes are collected by one excitation, the imaging time of the high frequency component is reduced to about 1/2. To enhance this shortening effect, the number of echoes collected by one excitation may be increased.

On the other hand, at the encoding step in which the encoding amount shifts to the low frequency component (the portion where the absolute value of the encoding amount is 16 or less) (depending on the order of movement of the encoding amount, it is normal to go from the minus side to the 0 encoding amount and then to the plus side). 2A), the pulse sequence shown in FIG. 2A is changed to the pulse sequence shown in FIG. That is, 90 °
After the application of the pulse and the 180 ° pulse, the reading gradient magnetic field Gr having a lower intensity than that of the high frequency component is set to prolong the data acquisition period (hatched line). This narrows the bandwidth of data collection and improves S / N.
At this time, the intensity of the read gradient magnetic field Gr can be halved compared to that of the high frequency component, and the data collection time can be doubled to double the band. At this time, if the sampling pitch is doubled for the high frequency component, the sampling number becomes the same as for the high frequency component, and the two-dimensional Fourier transform process can be executed without matching with the data of the high frequency component. When the sampling pitch cannot be changed according to the intensity of the read gradient magnetic field Gr and the sampling numbers do not match between the high frequency component and the low frequency component, the data of either the high frequency component or the low frequency component is used. Interpolation along the time axis direction is required to match the sampling numbers. Thus, for low frequency components, S
Although the data acquisition period is sacrificed instead of improving / N, the total imaging time should be discussed in consideration of the effect of reducing the imaging time of the high frequency component. That is, although the low-frequency component having an absolute value of 16 or less is made long in this case to narrow the band, since the total encoding width is normally −256 to +256, the influence of the narrowing on the total shooting time is small. Yes, the total acquisition time can be reduced to about 56% of collecting one echo signal with one excitation.

By the way, when a low frequency component that narrows the bandwidth of data collection is viewed on the spatial frequency plane defined by the encoding direction and the reading direction, it becomes a portion shown by the diagonal lines in FIG. The width of this narrowing frequency component is
It may be determined for each imaging region and each type of image according to the relationship between the required S / N and the imaging time. For example, if the width of the frequency component for narrowing the band is narrowed, the photographing time is shortened, but the S / N of the low frequency component is lowered. Further, if the width is widened, the S / N is improved, but the effect of shortening the photographing time is reduced.

As described above, according to the present embodiment, for the high frequency component which may have a low S / N, by increasing the intensity of the read gradient magnetic field Gr, the data acquisition bandwidth is widened, but the data acquisition time is increased. Can be shortened and high S
For the low frequency component for which / N is required, the intensity of the read gradient magnetic field Gr is reduced to increase the data acquisition time, but the data acquisition bandwidth is narrowed to S /
N can be improved.

The present embodiment is not limited to setting the frequency component for narrowing the band to the low frequency component centering on the 0 encoding amount, but may be set as follows. That is, it may be set to any part in the encoding direction,
As shown in FIG. 4, you may set to a high frequency component. In the above description, the low frequency component is narrowed, but in this case,
The contrast of low frequency components is more important, eg T2
(Horizontal relaxation time) Effective for emphasized images. Further, as shown in FIG. 4, setting a frequency component that narrows the band to a high frequency component is performed by, for example, a proton density image or T1 (longitudinal relaxation time)
This is effective for emphasized images.

Further, in the above-mentioned description, the present embodiment is 9
The present invention has been described using the spin echo method in which data is collected by applying a 180 ° pulse after applying a 0 ° pulse.
It may be applied to the field echo method. In this case, the pulse sequence of the low frequency component that narrows the band is as shown in FIG. On the other hand, as shown in FIG. 5B, the pulse sequence of the high-frequency component that does not narrow the band collects the second echo by inverting the gradient magnetic field for reading the second echo after collecting the first echo.

Further, as shown in the pulse sequence in FIG. 6, the present invention can be applied to the case of collecting the second echo by inverting the readout gradient magnetic field after collecting the first echo in the spin echo method.

Further, normally, when collecting a T2 weighted image by the multi-echo method, the first echo for the proton density image having a short time interval TE from the application of the RF pulse to the start of collecting the echo signal and the T2 having a long time interval TE. The second echo for the enhanced image is acquired, and the pulse sequence when the present invention is applied in this case takes a sufficient data acquisition time as shown in the upper part of FIG. For frequency components that do not narrow the band, as shown in the lower part of FIG. 7, it is possible to shorten the imaging time while improving the S / N.

Further, in the multi-slice method, usually, echoes of all slices are sequentially acquired with the same encoding amount.
When the frequency components that narrow the band are different in each slice, the encoding amount is changed for each slice so that the data of the frequency components that narrow the band in each slice are collected in the same pulse sequence, and high frequency exposure is temporally performed. If it is kept constant, the SAR value can be lowered, which is effective from the viewpoint of safety. The present invention is not limited to the above-described embodiments, but can be implemented with various modifications.

[0021]

As described above, the present invention can change the data acquisition time and the data acquisition bandwidth by changing the intensity of the read gradient magnetic field according to the phase encoding amount. , For a predetermined amount of phase encoding that requires a low S / N, by increasing the intensity of the read gradient magnetic field and shortening the sampling pitch,
By shortening the data acquisition time to shorten the imaging time, and by reducing the intensity of the read gradient magnetic field and increasing the sampling pitch only for a predetermined phase encoding amount that requires high S / N. The data acquisition time can be lengthened, and the data acquisition bandwidth can be narrowed. Therefore, it is possible to provide a magnetic resonance imaging apparatus capable of improving the S / N while shortening the imaging time.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of a magnetic resonance imaging apparatus according to the present invention.

FIG. 2 is a diagram showing a pulse sequence representing the operation of the present embodiment.

FIG. 3 is a diagram showing frequency components narrowed by the present embodiment on a spatial frequency plane defined by an encoding direction and a reading direction.

FIG. 4 is a diagram showing another frequency component to be narrowed on a spatial frequency plane.

FIG. 5 is a diagram showing a pulse sequence when the present invention is applied to a field echo method.

FIG. 6 is a diagram showing a pulse sequence when the present invention is applied to a multi-echo method in which a read gradient magnetic field of a second echo is inverted.

FIG. 7 is a diagram showing a pulse sequence when the present invention is applied to acquisition of a T 2 -weighted image.

FIG. 8 is a diagram showing a pulse sequence of a conventional multi-echo method.

[Explanation of symbols]

1 ... Static magnetic field magnet, 2 ... X-axis / Y-axis / Z-axis gradient magnetic field coil, 3 ... Transmitting / receiving coil, 4 ... Static magnetic field control device, 5 ... Transmitter, 6 ... Receiver, 7 ... X-axis gradient magnetic field amplifier, 8 ... Y-axis gradient magnetic field amplifier, 9 ... Z-axis gradient magnetic field amplifier, 10 ... Sequencer, 11 ... Computer system, 12 ... Display unit, 1
3 ... Sleeper, 20 ... Gantry.

Claims (3)

[Claims] 1. A magnetic resonance imaging apparatus for collecting an echo signal while changing a phase encode amount along a phase encode direction, wherein a data acquisition time is changed by changing an intensity of a read gradient magnetic field according to the phase encode amount. Is changed to change the bandwidth of data collection. 2. The high frequency component in the phase encode direction is subjected to data collection with a predetermined data collection time and a predetermined bandwidth by a read gradient magnetic field having a predetermined strength, and the low frequency component in the phase encode direction is the predetermined frequency. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the gradient magnetic field for reading having a strength lower than the strength is used to collect data with a data collection time longer than the predetermined data collection time and a bandwidth narrower than the predetermined bandwidth. . 3. The low frequency component in the phase encode direction is subjected to data collection with a predetermined data collection time and a predetermined bandwidth by a read gradient magnetic field having a predetermined intensity, and the high frequency component in the phase encode direction is the predetermined frequency. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the gradient magnetic field for reading having a strength lower than the strength is used to collect data with a data collection time longer than the predetermined data collection time and a bandwidth narrower than the predetermined bandwidth. .