JPH055495B2 - - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Field of Industrial Application) The present invention is directed to magnetic resonance (MR).
The present invention relates to a magnetic resonance imaging method for obtaining a non-targeted multi-echo image of a subject using the resonance phenomenon, and particularly relates to a magnetic resonance imaging method that can shorten data collection time.

(Prior art) Magnetic resonance development is a development in which atomic nuclei with non-zero spin and magnetic moment placed in a static magnetic field resonantly absorb and emit only electromagnetic waves of a specific frequency. If H ₀ is the static magnetic field strength, then this nucleus resonates at the angular frequency ω ₀ shown in the following equation.

The method of performing biological diagnosis using the principle of ω ₀ = γH ₀ or more is as follows:
The electromagnetic waves of the same frequency as those induced after the above-mentioned resonance absorption are subjected to signal processing to obtain, for example, a tomographic image of the subject.

In this case, the collection of diagnostic information by magnetic resonance is
Although it is possible to excite and collect signals from all parts of a subject placed in a static magnetic field, due to limitations in the device configuration and clinical requirements for imaging images, in reality it is only possible to excite and collect signals on specific slice planes. I am trying to collect signals.

In this kind of magnetic resonance imaging method,
There is a multi-echo image sequence in which multiple echo images are obtained from the same slice site. This sequence excites a specific slice region of the subject using a 90° radio frequency (RF) pulse and a slicing gradient magnetic field.
After a predetermined period of time, a 180° high-frequency pulse is applied,
At echo time TE, a first echo signal with a high S/N ratio and suitable for obtaining anatomical findings is obtained, and after the same predetermined time, a 180° high-frequency pulse is sequentially applied, and a first echo signal is obtained at each echo time TE. Although the S/N is lower than that of the first echo signal, the 2nd, 3rd, 4th, etc. n-th echo signals suitable for observing the lesion are obtained by having different transverse relaxation times T2, and this procedure is performed, for example, in the matrix of the image to be generated. If the number is 256, the first to nth echo images are generated by repeating 256 times to obtain a set of data in the entire phase direction of Fourier space.

In addition, in an asymmetric multi-echo image sequence, a first echo image is obtained to obtain anatomical findings, and a second echo image (instead of an image using the second echo signal described above, a third echo image is obtained to obtain findings of lesion contrast). ,
4th...Using the nth echo signal) where the echo time TE of the second echo signal is not twice that of the first echo signal, e.g. 30/90 msec
or 20/80msec. FIG. 6 is a diagram showing an example of this type of asymmetric multi-echo image sequence. As shown in FIG. 6, the sixth
Apply the 90° pulse and slicing gradient magnetic field shown in Figures a and b. An encoding gradient magnetic field and a read gradient magnetic field of intensity E ranging from a low frequency region to a high frequency region in the phase direction in Fourier space were applied to the region determined by this excitation procedure, as shown in Fig. 6c and d. Afterwards, a first 180° pulse and a slicing gradient magnetic field are applied to obtain a first echo signal at an echo time TE. Furthermore, a second 180° pulse and a slicing gradient magnetic field are applied to obtain a second echo signal at an echo time of 4TE. By repeating this excitation procedure and data collection procedure a predetermined number of times, it becomes possible to obtain two echo images with asymmetric echo times.

According to such an asymmetric multi-echo image sequence, it is possible to obtain a first echo image for obtaining anatomical findings and a second echo image for obtaining lesion contrast findings by executing the imaging sequence once. The clinical benefits are great.

Here, the data acquisition time (scan time) in the above-mentioned asymmetric multi-echo image sequence will be considered. That is, the scan time is TR
(repetition time) × average number of additions × matrix size, so for example, if TR = 2000 msec and average number of additions = 2256 matrix, then 2000 × 2 ×
256=1024000msec, which is about 17 minutes.

(Problems to be Solved by the Invention) As described above, the asymmetric multi-echo image sequence in the conventional magnetic resonance imaging method has the effect of generating one image from a low frequency region to a high frequency region in the phase direction in Fourier space. Since a data set is used, it takes a long scan time to obtain a plurality of asymmetric multi-echo images, which is a problem.

Therefore, an object of the present invention is to provide a magnetic resonance imaging method that can obtain a plurality of asymmetric multi-echo images in a short scan time.

[Structure of the Invention] (Means for Solving the Problems) The present invention has a structure in which the following measures are taken to solve the above problems and achieve the object. That is, the present invention performs multiple echo signal groups having different echo times in Fourier space in order to reconstruct at least two images for the number of times of phase encoding that requires a pulse sequence related to the multi-echo method. In a magnetic resonance imaging method that obtains images from a specific part of a subject, some of the echo signals used to reconstruct one image are placed in a low-frequency region in Fourier space related to that one image, and the other portions are placed in a high-frequency region. Then, a part of the echo signal group for reconstructing the other image is applied to the low frequency region in the Fourier space related to the other image, and the others are applied to the high frequency region in the same Fourier space. reconstructing the echo signals assigned to the Fourier space corresponding to the one image to obtain one image, and the echo signals assigned to the Fourier space corresponding to the other image are reconstructed. Another image is obtained by reconstructing the group, and more specifically, after executing an excitation procedure that causes excitation by magnetic resonance to occur in a specific slice part of the subject, an image with a matrix number N is generated. In a magnetic resonance imaging method, a data collection procedure is performed multiple times at appropriate time intervals to obtain a group of echo data by generating data using a magnetic field mainly consisting of a gradient magnetic field for phase encoding. A first procedure for obtaining first echo data is performed using a magnetic field mainly consisting of a first phase encoding gradient magnetic field having a variable strength over a portion corresponding to half of N, and After the execution, a magnetic field mainly composed of second, third, fourth to n-th phase encoding gradient magnetic fields having the same sign, a different sign, or zero as the first phase encoding gradient magnetic field and a fixed strength is used to generate a magnetic field. 2nd, 3rd, 4th to nth echo data are obtained;
The fourth to nth steps are executed sequentially, and the first to nth steps are repeated for a number smaller than the number of matrices N while varying the intensity of the phase encoding gradient magnetic field. The present invention is characterized in that a plurality of images having a matrix number N and less than n images are generated by combining the echo data groups.

(Function) According to such a configuration, for example, when n=4, it is possible to obtain the first to fourth echo data groups by half the number of repetitions for encoding, that is, N/2, for an image with a matrix number N. As a result, n/2, that is, two echo images can be generated, and these two echo images are an asymmetric multi-echo image.

(Example) An example of the magnetic resonance imaging method according to the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing the general configuration of a magnetic resonance diagnostic apparatus capable of implementing the method of this embodiment.

As shown in FIG. 1, the magnet assembly MA that accommodates the subject P therein generates a high-intensity static magnetic field that acts on the subject P, such as a superconducting type or normal conducting type static magnetic field. Coil 1 and
It includes three gradient magnetic field coils 2 that generate gradient magnetic fields along the X, Y, and Z axis directions, and a transmitting/receiving coil 3 that serves both as transmitting and receiving, for example, for excitation and signal collection.

It also has a static magnetic field control system 4 that performs excitation control of the static magnetic field coil 1 and coolant supply control. X, Y,
The Z-axis gradient magnetic field coil 2 is excitation-controlled by X, Y, and Z-axis gradient magnetic field power supplies 5, 6, and 7. The transmitter/receiver coil 3 is driven by a transmitter 8 during transmission (excitation), and is driven by a transmitter 8 during reception (signal collection).
In this case, it is driven by the receiver 9.

In addition, the generation sequence of gradient magnetic fields and transmission/reception signals by the X, Y, and Z-axis gradient magnetic field power supplies 5, 6, and 7, the transmitter, and the receiver 9, for example, the asymmetric multi-echo image capturing sequence of this embodiment shown in FIG. A computer system 11 that performs overall control and signal processing of the entire system including the sequencer 10 and attached equipment such as a bed, and a display system 12 that displays generated images.

FIG. 2 is a diagram showing an example of an asymmetric multi-echo image sequence of this embodiment executed by the sequencer 10. As shown in Figure 2, in order to determine the excitation site for the subject,
Apply pulse and slicing gradient magnetic fields.

For the region determined by the above excitation procedure, as shown in FIG. A first encoding gradient magnetic field and a read gradient magnetic field are applied, and then a first 180° pulse and a slicing gradient magnetic field are applied to obtain a first echo signal at an echo time TE.

Next, as shown in Fig. 2c and d, a second encoding gradient magnetic field and a read gradient magnetic field are applied, which correspond to a high frequency region in the phase direction in Fourier space, have the same sign as the first encode, and have a fixed strength. After that, a second 180° pulse and a slicing gradient magnetic field are applied to obtain a second echo signal at an echo time of 2TE.
Further, without the third encode, a third 180° pulse and a slicing gradient magnetic field are applied to obtain a third echo signal at an echo time of 3TE.

Next, as shown in FIG. 2 c and d, a fourth encode is generated which corresponds to a low frequency region in the phase direction in Fourier space, has a different sign from the first encode, and has the same strength and fixed strength as the gradient magnetic field for the second encode. A gradient magnetic field for reading and a gradient magnetic field for reading are applied, and then a fourth gradient magnetic field is applied.
A 180° pulse and a slicing gradient magnetic field are applied to obtain a fourth echo signal at an echo time of 4TE.

By repeating the above excitation procedure and the first, second, third, and fourth data collection procedures 256/2 = 128 times, the phase direction in Fourier space of the image is shifted to the low frequency region as shown in Figure 3. As shown in FIG. 4a, a first echo signal group in the low frequency region in the phase direction in Fourier space and a second echo signal group in the high frequency region in the phase direction in Fourier space are obtained. By combining these, a 256 matrix first echo image having mainly the contrast of the first echo signal is formed.

Furthermore, as shown in FIG. 4b, a third echo signal group in the high frequency region in the phase direction in Fourier space and a fourth echo signal group in the low frequency region in the phase direction in Fourier space are obtained, and these are synthesized. By doing so, the contrast of the fourth echo signal was mainly obtained256
A second echo image of the matrix is formed.

As described above, according to the asymmetric multi-echo image sequence of this embodiment, it is possible to obtain the first echo image (using the first and second echo signals) at the echo time TE suitable for obtaining anatomical findings. In addition, it becomes possible to obtain a second echo image (using the third and fourth echo signals) with an echo time of 4TE, which is suitable for obtaining findings of lesion contrast. In this case, the number of repetitions required to form two echo images is half the conventional one (256/2 = 128), so if you calculate the scan time using the same algorithm as the conventional example, it will be half (8 minutes 32 seconds). becomes.

The present invention is not limited to the above embodiments,
The embodiments of each case number in FIG. 5 can be implemented.

The example in Figure 2 is case No. 2, and in case No. 1, the first encode is low frequency and intensity variable (LV).
Assuming that the second, third, and fourth encodings have the same sign as the first encoding and fixed intensity (SF), the first echo image (using the first and second echo signals) at echo time TE and the echo time 3TE A second echo image (using the third and fourth echo signals) can be obtained.

In case No. 3, the first encode is high frequency and variable intensity (HV), the second and third encodes have a different sign from the first encode and fixed intensity (OF), and the fourth encode has the same sign as the first encode. In addition, as intensity fixed (SF), the first echo image (using the first and second echo signals) with an echo time of 2TE and the second echo image (using the third and fourth echo signals) with an echo time of 4TE are used. You will be able to obtain

In case No. 4, the first encode is high frequency and variable intensity (HV), the second encode has a different sign from the first encode and has fixed intensity (OF), there is no third encode (zero), and the fourth encode is The first echo image (first, second
This makes it possible to obtain a second echo image (using the echo signal) and a second echo image (using the third and fourth echo signals) with an echo time of 3TE.

Of course, when an image is generated by carrying out the embodiment of each case number in FIG. 5, the scan time is half that of the conventional example.

Furthermore, in the above four examples, the case where four echo signals are used is explained, but it can also be applied to the case where five or more echo signals are used. It is possible to obtain n/2 non-target multi-echo images.

In addition, various modifications can be made without departing from the gist of the present invention.

[Effects of the Invention] As described above, in the present invention, a plurality of echo signal groups having different echo times in Fourier space for reconstructing at least two images can be processed using a phase signal that requires a pulse sequence related to a multi-echo method. In a magnetic resonance imaging method that obtains a specific part of a subject by performing encoding for the number of times, a part of a group of echo signals for reconstructing an image is converted to After that, a part of the echo signal group for reconstructing the other image is allocated to the low frequency area in the Fourier space related to the other image, and the other image is allocated to the high frequency area. is applied to a high frequency region on the same Fourier space, the echo signal group allocated on the Fourier space corresponding to the one image is reconstructed to obtain one image, and the echo signal group allocated on the Fourier space corresponding to the one image is obtained, and the echo signal group corresponding to the other image is obtained in the Fourier space. Another image is obtained by reconstructing the echo signal group applied to the above, and more specifically, the first phase encoding gradient has a variable intensity over a portion corresponding to half of the maritx number N. A first procedure for obtaining first echo data using a magnetic field, which is mainly a magnetic field, is performed, and after the first procedure is performed, the first echo data is obtained using a magnetic field.
The second, third, and The second, third, and fourth to nth steps for obtaining the fourth to nth echo data are sequentially executed, and these first to nth steps are performed while changing the intensity of the phase encoding gradient magnetic field while changing the number of matrices. N
By repeating the process over a smaller number of images and combining the resulting echo data groups, it is possible to obtain an asymmetric multi-echo image from a plurality of images with the matrix number N and the number of images being less than n. become.

Therefore, according to the present invention, it is possible to provide a magnetic resonance imaging method that makes it possible to obtain a plurality of asymmetric multi-echo images in a short scan time.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of a general magnetic resonance diagnostic apparatus capable of implementing the magnetic resonance easing method according to the present invention, FIG. 2 is a diagram showing an example of the magnetic resonance easing method of the present invention, and FIG. The figure is a diagram explaining the phase direction frequency band in the Fourier space of an image, FIG. 4 is a diagram explaining the effect of FIG. 2, FIG. 5 is a diagram explaining another embodiment of the present invention, and FIG. It is a figure showing a conventional example. 1... Static magnetic field coil, 2... Gradient magnetic field coil,
3... Transmission/reception coil, 4... Static magnetic field control system, 5,
6, 7... Gradient magnetic field power supply, 8... Transmitter, 9...
Receiver, 10...Sequencer, 11...Computer system, 12...Display system.

Claims (1)

[Claims] 1. A plurality of echo signal groups having different echo times in Fourier space for reconstructing at least two images are executed for the number of times of phase encoding that requires a pulse sequence related to the multi-echo method. In a magnetic resonance imaging method in which images are obtained from a specific part of a subject, some of the echo signals used to reconstruct one image are placed in a low frequency region in Fourier space related to that one image, and the other portions are placed in a high frequency region. Then, a part of the echo signal group for reconstructing the other image is applied to the low frequency area in the Fourier space related to the other image, and the other part is applied to the low frequency area in the Fourier space related to the other image. A group of echo signals assigned to the high frequency region and assigned to the Fourier space corresponding to the one image is reconstructed to obtain one image, and the echo signals assigned to the Fourier space corresponding to the other image are reconstructed. A magnetic resonance imaging method characterized by reconstructing a signal group to obtain another image.