JPH07178070A - Magnetic resonance imaging method - Google Patents

Abstract

PURPOSE:To obtain the data necessary for regenerating the image in a short time, and to improve the quality of the image, by dividing a specific integration number, obtaining the data for every divided integration number, and adding the collected the data. CONSTITUTION:This MRI device has a static magnetic field generating system 101, an exciting system 102 to generate a high-frequency magnetic field, a receiver system 103, an inclined magnetic field generating system 104, and the like. A phase encode inclined magnetic field is applied to the excited atomic nucleus spin so as to give a phase variation, and a sequence to measure the encode signal while applying a lead-out inclined magnetic field is repeated so as to obtain a two-dimensional or a three-dimensional measurement data. In this case, the integration number repeated by the same phase encode inclined magnetic field intensity is divided into small integration numbers at random, a two-dimensional or a three-dimensional data is obtained to every divided number, these data are collected and added, and the image regeneration is carried out by composing the measurement data.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tomographic imaging apparatus (hereinafter referred to as an MRI apparatus) utilizing a nuclear magnetic resonance phenomenon, and more particularly to a data integration method at the time of imaging and an imaging method after data acquisition.

[0002]

2. Description of the Related Art Generally, in imaging by an MRI apparatus, first, nuclear spins (hereinafter, simply referred to as spins) of atoms constituting a tissue of a subject are excited by a high frequency pulse, and then a phase encode gradient magnetic field is applied to the excited nuclear spins. Then, a phase change is applied, and a procedure of measuring an echo signal while applying a readout gradient magnetic field is repeated by changing the phase encoding gradient magnetic field. One image can be acquired by repeating this process.

By the way, in imaging by such a magnetic resonance imaging apparatus, respiratory movements and body movements of a subject cause artefacts and deteriorate image quality. There is a method of increasing the number of times of integration as a means for reducing such an artifact, particularly an artifact caused by a periodical exercise such as a breathing exercise, and improving the image quality of an image.
Here, the data integration is performed by repeating the procedure from the spin excitation to the echo signal measurement described above without changing the phase encoding. That is, as shown in FIG.
Assuming that the number of times of integration is N, the data for the integration (N times) is continuously acquired when the gradient magnetic field of the same phase encoding is output. This is called a projection and is indicated by a vertical arrow in the figure. The data obtained by this one projection is added up at any time, and one data about the projection is obtained. After all the integrations for one encoding are completed, the phase shifts to the next phase encoding (indicated by a diagonal dotted line arrow). In this way, the phase encode number is changed from 1 to P (for example, 2 while changing the phase encode number).
The projections up to 56) are repeated to obtain P data.

[0004]

However, in order to obtain the data set necessary for reproducing one image,
Since data must be acquired by the number of phase encodes from P to P, if the shooting is interrupted before acquiring the data required for image reproduction, normal image reproduction cannot be performed. That is, in the above-described conventional imaging method, since the procedure from the spin excitation to the echo measurement is repeatedly integrated without changing the gradient magnetic field of the phase encoding, when the subject moves during imaging, The data at the time of a certain phase encoding in the set of data required for reproduction becomes the data affected by the motion, so that there is a problem that motion artifacts occur in the reproduced image. Further, when the shooting is stopped midway due to some circumstances, the conventional integration method cannot secure a set of data necessary for reproduction, and thus there is a problem that the image cannot be reproduced.

It should be noted that when the subject is moving, it is possible to reduce the artifacts due to the movement by shortening and speeding up the photographing time, but generally shortening the photographing time leads to deterioration of the image quality and improvement of the image quality. For this reason, there is also a problem that if the number of times of integration is increased, an artifact due to extension of the photographing time increases again, which becomes a vicious circle.

The present invention solves the above-mentioned conventional problems, has little influence on the movement of the subject, can acquire a set of data in a short time, and can obtain a high-quality image. An object is to provide a magnetic resonance imaging method. Another object of the present invention is to provide a magnetic resonance imaging method that makes it possible to reproduce an image by eliminating the need for re-imaging even if the imaging is stopped midway.

[0007]

In order to achieve the above object, the magnetic resonance imaging method of the present invention excites a nuclear spin of an atom constituting a specific site in the body, and a phase encoding gradient magnetic field is applied to the excited nuclear spin. Is applied to give a phase change, and a sequence for measuring an echo signal while applying a read-out gradient magnetic field is repeated while changing the intensity of the phase-encoding gradient magnetic field.
Obtaining dimension measurement data, a magnetic resonance imaging method for reconstructing an image by processing the measurement data, wherein the sequence is a magnetic resonance imaging method in which the same phase encoding gradient magnetic field strength is repeated a predetermined number of times, The integration count is arbitrarily divided into smaller integration counts, two-dimensional or three-dimensional measurement data is acquired for each divided integration count, these measurement data are weighted and added, the measurement data is synthesized, and an image is reproduced. It is a thing.

Another aspect of the magnetic resonance imaging method of the present invention is that the number of times of integration is arbitrarily divided into smaller numbers of times of integration, and two-dimensional or three-dimensional measurement data is acquired for each divided number of times of integration. The set of measurement data obtained for each number of integration is reconstructed as image data, the reconstructed image data is weighted and added, and the combined image data is acquired.

[0009]

[Operation] Even if the subject is moved during imaging by causing the total number of times to be arbitrarily divided into smaller number of times and causing an artifact such as movement of the subject during imaging, the divided and integrated measurement at that time is performed. By changing the weighting of the data (or the image data reconstructed from it), it is possible to reduce motion artifacts in the finally obtained image.

Further, by arbitrarily dividing the total number of times, it is possible to shoot a data set required for image reproduction in a shorter time. Generally, if the shooting time is short, the artifact due to the motion is also reduced. Therefore, by repeating the measurement divided in this way, it is possible to obtain an image in which the artifact is extremely reduced. Moreover, since the required number of times of integration is substantially secured, the image quality can be improved.

Further, even if the photographing is stopped halfway due to some circumstances, it is possible to reproduce the image by utilizing the divided and accumulated data acquired before the cancellation.

[0012]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of an embodiment of an MRI apparatus to which the magnetic resonance imaging method of the present invention is applied. The MRI apparatus includes a static magnetic field generation system 101 that generates a uniform static magnetic field in a space where a subject is placed,
In order to generate nuclear magnetic resonance in the tissue of the subject, an excitation system 102 that generates a high-frequency magnetic field, a receiving system 103 that receives and detects a signal generated from the subject, and then A / D converts the strength of the magnetic field. Magnetic field generation system 1 for linearly changing X in the X, Y, and Z directions independently of each other
04, an image processing system 10 for performing various calculations necessary for image reproduction based on measurement data from the receiving system 103.
5, a sequence control system 106 for controlling the operation timing of each system in the above configuration, a probe 107 used for high frequency transmission / reception, and a console 108 for performing operations. Further, the image processing system 105 has a memory (not shown) for storing the measurement data or the image data after the measurement data is processed, and has a function of performing a weighting operation described later on these data.

The static magnetic field generating system 101 typically comprises a magnet such as a permanent magnet, a normal conducting magnet, a superconducting magnet or the like. The gradient magnetic field generation system 104 includes gradient magnetic field coils wound in three axial directions for generating gradient magnetic fields in three directions, and the gradient magnetic field coils and the probe 107 are arranged in a static magnetic field generating magnet. For imaging in such a configuration, there are several different methods such as a spin echo method and a gradient echo method depending on how to apply a high frequency magnetic field and a gradient magnetic field (pulse sequence), but typically, first, the excitation system 102 is used. A high-frequency magnetic field pulse is applied to the subject to excite a specific region of the tissue of the subject, and then a gradient magnetic field is applied by the gradient magnetic field generation system 104 to give a phase change to the excited spins. The gradient magnetic field at this time is for giving spatial information to the measured signal and is called a phase encode gradient magnetic field. In some cases, a high-frequency magnetic field pulse is further applied to invert the spin, and then the echo signal is detected by the probe 107 while applying the readout gradient magnetic field and measured by the receiving system 103. The drive of the excitation system 102, the gradient magnetic field generation system 104, and the reception system 103 during this period is controlled by the sequence control system 106, and is measured by a predetermined pulse sequence.

By repeating the sequence from the spin excitation to the echo signal measurement while changing the intensity (phase encoding number) of the phase encoding gradient magnetic field, a two-dimensional echo signal set for obtaining one image is obtained. Measurement data is obtained. It is also possible to obtain three-dimensional measurement data by adding another phase-encoding gradient magnetic field.

The measurement data thus obtained by the receiving system 103 is subjected to processing such as two-dimensional Fourier transform, correction count calculation, image reconstruction, etc. in the image processing system 105 to obtain image data, which is a storage device such as an optical disk (not shown). And is displayed as a tomographic image on a display device such as a display. By the way, as for the image obtained by the above-mentioned photographing, the image quality is improved as the number of times the echo signals are integrated in the same number of phase encodes is increased. Therefore, the sequence from spin excitation to echo signal measurement is repeated a plurality of times in the same phase-encoded gradient magnetic field output state. In the magnetic resonance imaging method of the present invention, a predetermined number of times of integration is divided and measured.

FIG. 2 shows such a magnetic field obtained by division and integration.
The division pattern of the sound imaging method is shown. In the figure
1 to P are phase encode numbers, 1 to N₁, 1-N₂... 1
~ N _mRepresents the total number of times of division. P is typical
Is 256 or 512, but more is less
You don't have to. Also, N₁+ N₂+ ... + N_mIs total
The cumulative number of times is N. The cumulative number N is not particularly limited
No, but typically about 2 to several times.

In the method shown in FIG. 2, measurement is first performed with the number of times of integration divided into N ₁ to obtain P measurement data.
That is, the sequence from the excitation of the spins to the acquisition of the echo signal is repeated N ₁ times with the encoding number 1, and the obtained signal data is integrated to obtain one signal data. Next, the encoding step is increased by 1 and the same sequence is repeated N ₁ times with the number of encodes 2 to obtain one signal data. Similarly, while sequentially increasing the encoding steps, the projection is repeated until the number of encodes reaches P, and P pieces of signal data are obtained. The measurement data (a set of P signal data) thus obtained is used as the image processing system 10.
5 is stored in the memory.

Next, the same measurement is performed with the number of times of integration divided into N ₂ , and P measurement data are obtained. After that, all the divided measurements are repeated in the same manner to obtain m (the number of divisions) measurement data. Although the case where the number of phase encodes is sequentially stepped up from 1 to P is described in FIG. 2, the projection may be repeated while randomly selecting the number of phase encodes. Such a pattern is shown in FIG. In this case, for example, the movement of the abdomen of the subject is monitored by a sensor or monitor, and when the movement occurs, a projection with a large phase change is performed, and when the movement is stopped, a projection with a small phase change is performed. As described above, it is possible to select a portion for acquiring an echo signal according to the movement of the subject.

The measurement data D ₁ to
As shown in FIG. 4, D _m is multiplied by weighting functions ω _{1 to} ω _m and then added to obtain combined measurement data D ₀ . The synthetic measurement data D ₀ is subjected to a known image reconstruction process such as Fourier transform to obtain image data I ₀ .
These calculations are performed by the image processing system 105 as described above.
Performed in. Here, the weighting function is all 1 in normal imaging, but when the movement of the subject is monitored by a monitor or the like, among the divided measurements, for example, the measurement in the time zone in which the subject has a large movement is performed. An appropriate function of 1> ω ≧ 0 is used for the obtained measurement data. As a result, it is possible to reduce the contribution of the measurement data that causes the artifacts to the image and reduce the artifacts caused by the movement of the subject.

Although FIG. 4 shows the case where the measurement data is weighted and added to create a new piece of measurement data and the image is reproduced, as shown in FIG. Image data I ₁ ~ using D ₁ ~ D _m
I _m may be created, these image data I _{1 to} I _m may be weighted and added by arbitrary weights ω _{1 to} ω _m , and finally one new image I ₀ may be reproduced.

In the above embodiment, the case where the m pieces of measurement data or image data are all stored in the memory and the weighting operation is performed on them is described.
A memory area for each piece of measurement data or image data may be provided and the data weighted sequentially may be integrated. In this case, it is possible to set the number of times of integration and stop the measurement midway.

FIG. 6 shows a method of reproducing an image when the divided integration photographing is stopped midway. In this case, if the divided and integrated photographing is stopped at the (i + 1) th photographing, the i-th data D ₁ acquired up to that point is acquired.
The image is reproduced using the set of ~ D _i . As described above, according to the magnetic imaging method of the present invention, even when the photographing is stopped for some reason, it is possible to reproduce the image using the data acquired so far.

[0023]

As is apparent from the above description, according to the magnetic resonance imaging method of the present invention, a predetermined number of times of integration is divided, data is obtained for each of the divided times of integration, and the data is weighted and added. Therefore, it is possible to acquire the data required for image reproduction in a short time, improve the image quality, and reduce the artifacts in the image due to the movement of the subject. Become. Further, even when the photographing is stopped halfway by such a photographing method, the image can be reproduced as long as the data is acquired at the stage of dividing and integrating by that time.

[Brief description of drawings]

FIG. 1 is an MRI to which an imaging method of the present invention is applied.
The block diagram which shows the structure of a device.

FIG. 2 is a diagram showing an embodiment of an integration method according to the imaging method of the present invention.

FIG. 3 is a diagram showing another embodiment of the integrating method according to the imaging method of the present invention.

FIG. 4 is a diagram showing an embodiment of an image reproducing method according to the imaging method of the present invention.

FIG. 5 is a diagram showing another embodiment of the image reproducing method by the imaging method of the present invention.

FIG. 6 is a diagram showing an embodiment of an image reproducing method according to the present invention when shooting is stopped halfway.

FIG. 7 is a diagram showing a conventional integration method in photographing.

[Explanation of symbols]

 101 ... Static magnetic field generation system 102 ... Excitation system 103 ... Receiving system 104 ... Gradient magnetic field generation system 105 ... Image processing system 106 Sequence control system 107 Probe 108 Operation console

Claims (2)

[Claims] 1. An echo signal which excites nuclear spins of atoms constituting a specific part in the body, applies a phase encoding gradient magnetic field to the excited nuclear spins to change the phase, and further applies a readout gradient magnetic field. A magnetic resonance imaging method in which a sequence for measuring is repeated while changing the intensity of the phase-encoding gradient magnetic field to obtain two-dimensional or three-dimensional measurement data, and the measurement data is processed to reconstruct an image. In the magnetic resonance imaging method in which the sequence is repeated a predetermined number of times with the same phase-encoding gradient magnetic field strength, the number of times of integration is arbitrarily divided into smaller times of integration, and the sequence is repeated for each of the divided times of integration. Acquire two-dimensional or three-dimensional measurement data, add weighting to each of these measurement data, A magnetic resonance imaging method characterized by obtaining synthetic measurement data and reconstructing an image based on the synthetic measurement data. 2. An echo signal is generated by exciting a nuclear spin of an atom constituting a specific part in the body, applying a phase encoding gradient magnetic field to the excited nuclear spin to give a phase change, and further applying a readout gradient magnetic field. A magnetic resonance imaging method in which a sequence for measuring is repeated while changing the intensity of the phase-encoding gradient magnetic field to obtain two-dimensional or three-dimensional measurement data, and the measurement data is processed to reconstruct an image. In the magnetic resonance imaging method, in which the sequence is repeated a predetermined number of times with the same phase-encoding gradient magnetic field strength, the number of times of integration is arbitrarily divided into smaller number of times of integration, and each of the divided number of integration is two-dimensional or three-dimensional. To obtain image data by reconstructing an image based on each of these measurement data. A magnetic resonance imaging method, characterized in that weighted addition of image data is performed to obtain composite image data.