JPH0442936B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a method for extracting image data in magnetic resonance imaging, and particularly relates to an improvement in a method for shortening scan time.

(Prior art) Conventionally, when collecting a spin echo image, which is a T ₂ (transverse magnetic field relaxation time) weighted image, using a magnetic resonance phenomenon, a two-dimensional Fourier transform method (2DFT method) is used.
In this case, a set of data for the entire Fourier space was obtained by attaching phase information to echoes that occur at the same echo time (TE time), for example, the first echo (see Figure 5).

Therefore, if the scan time is, for example, the time width (TR) of one period of the pulse sequence: 2000 msec, the number of average additions: 2 times, and the matrix size: 256 matrices, the scan time will be the value multiplied by these, so it will be approximately 17
Reach minutes.

Therefore, various proposals have been made to shorten the scan time, such as a half-encoding method, a half-matrix method, a 128×128 scan method, and a hybrid method.

(Problems to be Solved by the Invention) However, while the half-encoding method and the half-matrix method can obtain the same resolution as the 2DFT method, the S/N decreases to 1/√2. In addition, the resolution of the 128 x 128 scan method is
It is 1/2 that of the 2DFT method, and artifacts due to Gibbs ringing are likely to occur. Another problem with the hybrid method was that it was technically difficult.

The present invention has been made in view of the above problems, and its objectives are to improve resolution, S/N,
The objective is to shorten scan time without reducing intertissue contrast.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention converts the encoding gradient magnetic field into a static magnetic field before the first echo and second echo of the echo signal are respectively generated. The encoding gradient magnetic field applied before the first echo is applied in a superimposed manner to the first echo is applied in a range in which phase modulation is performed to give the first echo a high frequency component of image data in the entire Fourier space. The magnetic field strength is changed for each selective excitation of spins in the planned slice cross section, and the encoding gradient magnetic field applied before generating the second echo is phase modulated to give low frequency components of the image data. The gist of this method is to extract image data in the entire Fourier space based on the first echo and the second echo using a pulse sequence that keeps the magnetic field strength constant within a range.

(Function) According to the pulse sequence of the present invention, high frequency data is obtained from the first echo with less signal level attenuation, so images reconstructed by the 2DFT method maintain good resolution and are free from data censoring. This prevents Gibbs ringing.

Furthermore, since low frequency data is obtained with the second echo having a long echo time, T ₂ difference enhancement is sufficient and the S/N and inter-tissue contrast of the reconstructed image are maintained satisfactorily.

Moreover, in the case of the pulse sequence of this invention,
The two echoes, the first echo and the second echo, are 1
Since data can be collected by multiple excitations, the time required to collect image data is halved compared to the case of a pulse sequence in which image data of the entire Fourier plane is obtained using only the first echo.

(Example) FIG. 2 is a block diagram schematically showing a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) to which the present invention is applied.

This MRI apparatus includes a main magnet 1 for generating a static magnetic field,
A probe 2 that transmits high-frequency excitation pulses (hereinafter referred to as RF pulses) and receives magnetic resonance signals (hereinafter referred to as MR signals) such as echo signals under the static magnetic field obtained by the main magnet 1, and a static magnetic field generated by the main magnet 1. The probe 2 and each gradient magnetic field coil 3 are controlled according to a pulse sequence, and MR signal processing is performed. It is equipped with a control computer 4 and a monitor 5 such as a CPT.

The pulse sequence used by the control computer 4 is set in a memory (not shown), and the set state is schematically shown in FIGS. 1A and 1B.

However, FIG. 1A shows the case where the encoding gradient magnetic field is changed in the positive phase direction, and FIG. 1B shows the case where the encoding gradient magnetic field is changed in the negative phase direction.

In addition, in Figure 1 A and B, RF/MR: RF
Pulse and MR signal, G _S : Gradient magnetic field for slicing,
G _E : Encoding gradient magnetic field, G _R : Reading gradient magnetic field.

In the pulse sequence of this embodiment, each time the slicing gradient magnetic field G _S is superimposed on the static magnetic field,
First, 90° pulse, then first 180° pulse, then second 180° pulse, and so on, RF pulses are superimposed in time series, and the 90° pulse of RE pulse and the first
During the 180° pulse, an encoding gradient magnetic field R _E for phase encoding is applied in a superimposed manner as shown by code A.
In addition, in order to extract the first echo caused by the first 180° pulse, a read gradient magnetic field G _R is applied in a superimposed manner,
Furthermore, after the first echo is extracted, an encoding gradient magnetic field R _E for phase encoding as shown in code B is applied in a superimposed manner, and a second 180° pulse is applied after applying the encoding gradient magnetic field shown in code B. A read gradient magnetic field is applied in a superimposed manner to extract echoes.

Therefore, in the pulse sequence of Fig. 1A,
Regarding the encoding gradient magnetic field G _E denoted by symbol A, each time the spins in the intended slice section of the subject are selectively excited with an RF pulse in the range of the maximum magnetic field strength from half the predetermined maximum magnetic field strength, When changed, phase modulation is performed to give a high frequency component to the first echo. Therefore, image data of high frequency components in the phase encoding direction can be extracted from the first echo.

At this time, in order to subtract from the encoding gradient magnetic field _GE with symbol A, as shown by symbol B, if the encoding gradient magnetic field GE is maintained constant at half the predetermined maximum magnetic field strength, the encoding gradient magnetic field _GE is applied. ,
The second echo has a higher phase encoding amount than the first echo.
Since the phase encoding amount of the echo becomes small, the encoding gradient magnetic field G _E indicated by the symbol A is
Each time the intensity of the second echo is sequentially changed, phase modulation is performed to give a low frequency component to the second echo. Therefore, image data of low frequency components in the phase encoding direction can be extracted from the second echo.

In addition, as in the pulse sequence of FIG. 1B, the encoding gradient magnetic field indicated by symbols A and B
By reversing the polarity of G _E , image data of high frequency components in the phase encoding direction can be extracted from the first echo, and image data of low frequency components in the phase encoding direction can be extracted from the second echo. I can do it.

In this way, when the image data of the entire Fourier space is extracted using the first echo and the second echo as schematically shown in Figure 3, the image reconstructed by the 2DFT method uses the high frequency data of the first echo. It has a high resolution, and Gibbs ringing due to data truncation is prevented, and artifacts due to this are not generated.

Furthermore, since the echo time from the 90° pulse of the RF pulse until the second echo is generated is naturally longer than the echo time of the first echo, using the low frequency data of the second echo can sufficiently emphasize the _T2 difference. Therefore, the S/N is high and sufficient contrast between tissues can be obtained.

Next, another embodiment will be described in which image data is extracted according to the pulse sequence shown in FIGS. 4A and 4B.

In the pulse sequence of the other embodiment, instead of the subtractive encoding gradient G _E applied as indicated by the symbol B in the embodiment of FIGS. Apply a gradient magnetic field _GE for subtractive encoding. In this case, since it is after the second 180° pulse, the subtractive encoding gradient magnetic field G _E indicated by symbol C has a polarity reversed from that in the case of symbol B in FIGS. 1A and 1B.

In other embodiments of the pulse sequence shown in FIGS. 4A and 4B, the same effects as in the embodiment according to the pulse sequence shown in FIGS. 1A and 1B can be obtained.

As mentioned above, in each embodiment of the present invention, the pulse sequence is configured such that the first echo takes high frequency component data and the second echo takes low frequency data, so the image reconstructed by the 2DFT method has a high resolution. , S/N, and contrast are maintained well. What's more, the scan time for data collection is halved compared to conventional methods.

In addition, the low frequency component data is taken with the first echo,
If the pulse sequence is configured to obtain high frequency component data in the second echo, it is disadvantageous because the T ₂ difference emphasis will be insufficient.

[Effects of the Invention] As explained above, according to the image data extraction method in magnetic resonance imaging to which the present invention is applied, high frequency component data is obtained using the first echo, and low frequency component data is obtained using the second echo. It is possible to construct a pulse sequence that allows Then, by extracting image data with the pulse sequence obtained according to the present invention,
If the image is reconstructed using the 2DFT method, the resolution,
Good S/N and contrast can be maintained, and scan time can be reduced to 1/2 compared to conventional methods. In addition, for this reason, the present invention enables spin echo images with a long echo time, which are preferred for their high lesion detection ability, to be obtained in half the time of conventional methods.
This has an extremely large effect.

[Brief explanation of drawings]

1A and 1B are time charts schematically showing a pulse sequence of an embodiment to which the present invention is applied, FIG. 2 is a block diagram schematically showing an MRI apparatus to which the present invention is applied, and FIG. 3 4A and 4B are time charts schematically showing pulse sequences of other embodiments to which the present invention is applied, and FIG. 5 is a conventional data extraction process. It is an explanatory view showing an outline typically.

Claims (1)

[Claims] 1. A pulse that extracts image data of the entire Fourier space from an echo signal by changing the magnetic field strength of an encoding gradient magnetic field every time a high-frequency excitation pulse is transmitted to selectively excite spins in a predetermined slice section. a sequence in which the encoding gradient magnetic field is applied superimposed on the static magnetic field before each of the first echo and second echo of the echo signal is generated, and the encoding gradient is applied before the first echo is generated; Regarding the gradient magnetic field, the magnetic field strength is changed for each selective excitation of the spins within a range in which phase modulation is performed to give the first echo a high frequency component of the image data, and before generating the second echo, Regarding the applied encoding gradient magnetic field, a pulse sequence is used to keep the magnetic field strength constant within a range where phase modulation is performed to give low frequency components of the image data, and the magnetic field is converted into a Fourier space based on the first and second echoes. A method for extracting image data in magnetic resonance imaging, characterized by extracting entire image data. 2. The encoding gradient magnetic field applied before the second echo is generated is applied before the application of the high-frequency excitation pulse that selectively excites the second echo. Image data extraction method in magnetic resonance imaging. 3. The magnetic field according to claim 1, wherein the encoding gradient magnetic field applied before the second echo is generated is applied after the high-frequency excitation pulse that selectively excites the second echo is applied. Image data extraction method in resonance imaging.