JPH0654831A - Magnetic resonance imaging system - Google Patents

Abstract

(57) [Abstract] [Purpose] Functional imaging that detects the diffusion / perfusion region without using "diffusion enhancement pulse" and "contrast agent" and visualizes how the region changes with time on the order of seconds. Is to realize. [Structure] By utilizing the fact that the magnetic susceptibility changes when hemoglobin in blood binds to oxygen, a method to detect a functional change from the phase distortion caused by the change in the magnetic susceptibility, using high-speed imaging of the second order Take a continuous shot, one specific
The relative phase difference between the images is continuously displayed to visualize the time-varying process.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an in-vivo tomographic imaging apparatus MRI utilizing a nuclear magnetic resonance phenomenon, and more particularly to a method for visualizing temporal changes in magnetic susceptibility of a tissue, which has a function of observing an active site in the brain Used for imaging, etc.

[0002]

2. Description of the Related Art The following is a typical example of a brain functional imaging method using MRI.

(1) The diffusion / intensity of the diffusion enhancement pulse is changed without using a contrast agent,
A method of separating perfusion images.

(2) A method of obtaining a diffusion-weighted image without using a contrast agent.

(3) A method of obtaining a T2-weighted image in a high-speed photographing sequence by using a contrast agent.

[0006] Documents of typical methods are shown below.

(1) D. Le Bihan, E. Breton, D. Lallemand,
MLAubin, J. Vignaud, and M Laval-Jeantet, 'Separatio
n of Diffusion and Perfusion in Intravoxel Incoher
ent Motion MR Imaging ', Radiolgy 168, pp.497-505 (19
88) (2) ME Moseley, Y. Cohen, J. Mintorovitch, J. Kucharc
rzyk, PRWeinstein, andD.Norman, 'Diffusion-weighted
MR Imaging in Experimental Brain Research ', Sylla
bus Special Course: MR 1990, pp.63-67 (1990) (3) BRRosen, JW Belliveau, D.Chien, MSCohen, an
d RMWeisskopf, 'MR Perfusion Imaging', Syllabus Sp
ecial Course: MR 1990, pp.69-84 (1990) The conventional method is to apply a diffusion enhancement pulse (large flow encode pulse) to dephase the signal in the diffusion / perfusion region with local flow. And add contrast to black (1)
Method (2) and injection of contrast agent
There is a method (3) of generating non-uniformity of the magnetic field in the perfusion region and generating a black contrast by the T2 weighted photographing method which emphasizes the effect. Both methods are devised to combine temporally continuous images with turbo flash and ultra-high speed imaging.

However, in the methods (1) and (2), it is not possible to make clear distinction from the contrast on the tissue because the image is blackened on the image. When not combined with the high-speed imaging method, it becomes sensitive to movement because a diffusion enhancement pulse is applied every time measurement is performed.
There was a problem that motion artifacts tend to occur. Further, the method (1) has a problem that the imaging parameter of the diffusion enhancement pulse, which is likely to be influenced by the eddy current, is changed, and then the imaging is performed, and the inter-image calculation is directly affected by the eddy current. On the other hand, the method (3) has a problem in that the use of the contrast agent causes pain to the patient and the non-invasive characteristic of MRI cannot be utilized.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to detect a diffusion / perfusion region without using a "diffusion enhancement pulse" and a "contrast agent", and to display a state of temporal change on the order of seconds. It is to realize functional imaging.

[0010]

[Means for Solving the Problems] The fact that the susceptibility changes when hemoglobin in blood binds with oxygen is used. Since oxygen is a paramagnetic substance, the magnetic susceptibility changes depending on the oxygen content. This manifests itself as a change in magnetic field inhomogeneity.
In MRI, if there is non-uniformity in the magnetic field, density distortion, phase distortion, and spatial distortion occur on the image. The conventional method utilizes the fact that the density decreases due to the dephase effect caused by the non-uniformity in the pixel, but the present invention focuses on the phase distortion to detect the functional change. Conventionally, a method of mapping a change in magnetic susceptibility by phase distortion has been known, but a process of temporal change has not been visualized. The present invention is to combine a phase mapping technique and a high-speed imaging technique to detect a time-varying process, which has been impossible in the past, from a change in phase of continuously captured images. By extracting only the phase change portion and displaying it on an MRI image showing general morphological information, it becomes possible to visually see in which part of the brain the activity is performed.

Since the phase change is caused not only by the magnetic susceptibility but also by the imperfections of the device (eddy current etc.) and the movement itself, it is necessary to distinguish it from the change of the magnetic susceptibility. For this reason, it is necessary to capture images continuously in time, and perform the correction processing of the phase of another image in real time using the phase at a certain time as a reference. This correction processing can be performed by simply taking the complex conjugate of the reference image which is a complex number and multiplying it by the image to be corrected. If there is no change in the image, the phase of the corrected image becomes zero at all positions, but if there is a change, only the phase at that position changes and appears. The doctor may judge the result by displaying the result as it is, but by extracting only the part where the phase has changed and coloring it on the absolute value information of the image and superimposing it, which part of the brain becomes active. It will be possible to directly observe whether you are active.

[0012]

[Function] When the magnetic susceptibility is χ, the magnetic flux density B and the magnetic field strength H
And have the relationship of the following formula.

B = (1 + χ) H (1) The magnetic susceptibility of water is approximately 10 ppm. Since it is said that the magnetic susceptibility changes by 10% due to the coupling of oxygen,
A change in magnetic field inhomogeneity of about 1 ppm will be detected.

On the other hand, the phase change is represented by Δθ = γ · ΔB · t (2) where γ is the nuclear magnetic rotation ratio, where ΔB is the magnetic field inhomogeneity and t is the time from excitation to measurement. For example, in the case of 1.5T, Δθ = 23 ° · t (3) per 1 ppm. Echo time TE of gradient echo is 4
The phase change in the case of msec is 92 °. When the phase change exceeds 180 °, a phase wrap occurs, so TE
Is set to an appropriate value around 4 msec, the change in oxygen content can be measured with appropriate sensitivity, and functional imaging is achieved. Even if the change in magnetic susceptibility is smaller, it can be dealt with by appropriately extending TE. The above calculation is for 1.5T, but since the magnetic field inhomogeneity is proportional to the magnetic field strength, the phase sensitivity is low even for 0.5T. Therefore, TE can be extended in proportion to it. it can.

Instead of the gradient echo, an echo planer which is so-called ultra high speed imaging may be used. In this case, when the 180-degree pulse is used together, the difference between the 90-180 degree interval and the 180-echo center interval is shifted by about 4 msec as described above.

[0016]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 3 is a block diagram showing an outline of the configuration of the MRI apparatus which is an embodiment of the present invention. In the figure,
Reference numeral 301 is a static magnetic field generation system for generating a uniform static magnetic field, 302 is an excitation system for generating a high-frequency magnetic field for exciting spins, and 303 is a magnetic field strength independently in x, y, and z directions. A gradient magnetic field generation system that can be changed linearly, 304 is a measurement system that receives and detects the electromagnetic waves generated from the subject, and then A / D converts, and 305 is input from the measurement data from the measurement system and the keyboard. A processing device that performs various calculations necessary for image reproduction based on the captured condition, 306 is a CRT that displays the reproduction result and the captured condition,
Reference numeral 307 is a pulse sequence file that stores the control procedure of the operation of each system in the above configuration, and 308 is the operation timing of each device based on the pulse sequence loaded from the pulse sequence file according to the imaging conditions input from the keyboard. It is a sequence control unit that controls.

FIG. 1 shows an example of a processing flow for carrying out the present invention, and FIG. 2 shows an example of a pulse sequence.

The pulse sequence of FIG. 2 is the well known G
This is an FE (gradient field echo) sequence, which will be briefly described. First, from the excitation system 302, α degrees (<90
Excitation pulse 202 is generated to excite the hydrogen spin. At this time, at the same time, the slice selection gradient magnetic field 202 is set in order to select a specific slice.
(Hereinafter, all gradient magnetic fields are generated at 303). Continuing from this, in order to cancel the influence of this gradient magnetic field disturbing the phase in the slice direction,
An inversion gradient magnetic field 203 is applied. Phase encode magnetic field 2
Reference numeral 04 is for separating positions in the uniaxial direction (y direction in this example) in the slice, and the intensity is sequentially changed each time the sequence is repeated. Reversal magnetic field of read magnetic field 20
Reference numeral 5 is a gradient magnetic field that shifts the peak of the measurement signal in the read gradient magnetic field 206 so as to come to the center. Following this, a reading gradient magnetic field 206 is applied, and at the same time, the measurement system 304 is used to measure the MRI signal 207.

A method for carrying out the present invention will be described below with reference to FIGS.

<Step 101> An imaging condition such as an imaging sequence, an imaging parameter, and an imaging position is input from a keyboard connected to the processing device 305, and a corresponding pulse sequence is loaded from the pulse sequence file 307 based on the input. Here, the sequence will be described as the GFE sequence in FIG.

<Step 102> Figure 2 loaded
The excitation system 302, the gradient magnetic field generation system 303, and the measurement system 304 are controlled based on the pulse sequence of 1., the sequence for the number of phase encodes is repeated to measure the data for the reference image, and the processing device 305 reproduces the reference image based on the measured data. To do. Usually, image reproduction is performed in parallel with signal measurement. The measurement time is determined by the product of the number of phase encodes and the repetition time TR. For example, 128 encoded T
If R is 10 msec, the measurement time is about 1 second.

<Step 103> Data for phase correction is calculated using the reference image. Specifically, the absolute value of the reference image, which is a complex number, is normalized to 1, and then the data obtained by performing the complex conjugate operation (sign inversion of the imaginary part) is calculated.

<Step 104> Similar to <Step 102>, an image for performing functional measurement is obtained by measuring and reproducing based on the GFE pulse sequence of FIG. The measurement process and the reproduction process are also performed simultaneously.

<Step 105> The correction data of <Step 102> is complex-multiplied by the reproduced image of <Step 104> to correct the phase of the image. As a result, the phase component of the unchanged portion can be made zero and the phase component of only the changed portion can be obtained.

<Step 106> The obtained phase is converted into density and displayed. At this time, only a portion of the phase component exceeding a predetermined value may be extracted and displayed by superimposing it on the absolute value image of the amplitude value of the reproduced image. When further overlapping, it is possible to emphasize the changed portion and display it by combining colors.

In the above steps, the following alternative processing can be considered.

In <Step 102>, a stable image can be obtained if the reference image is photographed after performing the color striking for a while immediately before the photographing is started and the steady state is obtained.

<Step 102> and <Step 104>
The GFE sequence used for the image measurement of is an ultra-high speed sequence (for example, to obtain a measurement signal using a continuous gradient echo instead of the TR shortening sequence of FIG. 2).
Echo planar) may be used. In this case, if the 180-degree pulse is also used, the 90-180 degree interval and 18
0 degree-difference in echo center spacing is 4 ms as previously described
It corresponds by shifting about ec.

It is also possible to calculate the phase of the reference image in <Step 103> and apply the opposite phase in <Step 105>. In that case, it should be noted that the phase unwrap processing is newly required.

In <Step 105>, since γ and t are known from the phase change amount based on the equation (2), ΔB can be calculated, and from the amount of oxygen bound to hemoglobin, the activity level of the function in the brain is obtained. It is also possible to estimate

In the above example, the first image is selected as the reference and the difference is detected, but next, a user such as a doctor or a radiologist looks at the image on the CRT to determine an appropriate part or timing. An example of another processing procedure in which actual function observation is performed from the point of time of instruction will be described.

<Step 401> An imaging condition such as an imaging sequence, an imaging parameter, and an imaging position is input from a keyboard connected to the processing device 305, and the corresponding pulse sequence is loaded from the pulse sequence file 307 based on the input. At this time, if it is desired to designate the image while actually observing the image, the automatic movement mode of the photographing position is designated. The imaging position will be described as the GFE sequence in FIG. 2 as in the previous case.

<Step 402> FIG. 2 loaded
The excitation system 302, the gradient magnetic field generation system 303, and the measurement system 304 are controlled based on the pulse sequence of 1., the sequence for the number of phase encodes is repeated to measure the data for the reference image, and the processing device 305 reproduces the reference image based on the measured data. To do.

<Step 403> The absolute value and phase of the image (complex number) obtained in the previous step are calculated.

<Step 404> The absolute value and the phase are converted into the density, and the images are displayed in parallel, for example. afterwards,
Returning to <Step 402>, the process is repeated. At this time, if the position automatic movement mode is set in the imaging condition of <Step 401>, the slice position is moved according to the parameter at that time, and the process returns to <Step 402>. The movement can be easily performed by changing the modulation frequency of the excitation pulse.

<Step 405> When the user sees the image displayed in <Step 404> and determines that it is an appropriate image (timing and position), the user interrupts the system.

<Step 406> The phase correction data is calculated using the reference image displayed at the time of the interruption, and is stored in the memory. Specifically, after the absolute value of the reference image is normalized to 1, the complex conjugate operation (sign inversion of the imaginary part) data is calculated and stored in the memory. At this point, the slice position is fixed.

<Step 407> Similar to <Step 402>, an image for performing functional measurement is measured based on the GFE pulse sequence of FIG. 2 and reproduced.

<Step 408> The correction data stored in <Step 406> is complex-multiplied by the reproduced image in <Step 407> to correct the phase of the image. As a result, the phase component of the unchanged portion can be made zero and the phase component of only the changed portion can be obtained.

<Step 409> The obtained phase is converted into density and displayed. At this time, only the portion of the phase component that exceeds a predetermined value may be extracted and displayed by superimposing it on the absolute value image of the amplitude value of the reproduced image. When further overlapping, it is possible to emphasize the changed portion and display it by combining colors.

According to the above-mentioned embodiment, since the state at an arbitrary time point can be used as a reference and the change from that time point can be detected, it is possible to easily observe the result of externally stimulating the brain with light or the like. Become.

Further, it is also possible to carry out imaging at a site having periodic movement by imaging by breathing or gating the heart.

[0044]

As described above in detail, according to the present invention, the diffusion / perfusion region is detected as a change in magnetic susceptibility without using the "diffusion enhancement pulse" and the "contrast agent", and the second order is detected. It becomes possible to realize functional imaging that visualizes the state that changes with time.

[Brief description of drawings]

FIG. 1 is a flowchart showing an operation of a first exemplary embodiment of the present invention.

FIG. 2 is an example of a pulse sequence implementing the present invention.

FIG. 3 is an example of a block configuration diagram for implementing the present invention.

FIG. 4 is a flowchart showing the operation of the second exemplary embodiment of the present invention.

[Explanation of symbols]

301: static magnetic field generation system, 302: excitation system, 303: gradient magnetic field generation system, 304: measurement system, 305: processing device, 30
6: CRT, 307: pulse sequence file, 30
8: Sequence control unit.

Claims (7)

[Claims] 1. A magnetic resonance imaging apparatus having a static magnetic field, a gradient magnetic field, a high-frequency magnetic field generator, a detector for extracting a nuclear magnetic resonance signal from an inspection object, and a processor for performing various calculations including image reproduction, A unit for continuously capturing images, a unit for calculating the phase of the image, a unit for calculating a phase difference with another image using the phase of a specific one image as a reference, and the phase difference are displayed. A magnetic resonance imaging apparatus having means. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the means for continuously capturing images includes means for capturing images of the same region and means for continuously moving the position. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the means for continuously capturing images is a GFE (gradient field echo) sequence having a short excitation interval (TR). 4. The magnetic resonance imaging apparatus according to claim 1, wherein the means for calculating the phase of the image is performed in parallel with the means for capturing the image. 5. The magnetic resonance imaging apparatus according to claim 1, further comprising means for externally designating a specific one image to be used for reference. 6. The method according to claim 1, further comprising means for converting the displayed phase difference into a rate of change of the magnetic susceptibility that changes with time and displaying the converted rate.
Magnetic resonance imaging device. 7. The magnetic resonance imaging apparatus according to claim 1, further comprising means for displaying the phase difference to be displayed by superimposing it on an MRI tomographic plane representing normal morphological information.