JPH06285040A - Magnetic resonance video system - Google Patents

Abstract

PURPOSE:To obtain uniform signal intensity of a blood vessel picked up by an MRA image technique by constituting the system so that a magnetic field is controlled and applied so as to overlap an end part of an adjacent slab, and pieces of their information are all used effectively. CONSTITUTION:By a static magnetic field magnet 1, a magnetic field uniformity adjustment coil 3 and a gradient magnetic field generation coil 5, a uniform static magnetic field and a gradient magnetic field are applied to an examinee 7, and also, by a probe 9, a high-frequency magnetic field is applied. A system controller 12 obtains a slab by executing a pulse sequence. In order to obtain the slab, a selecting/exciting RF pulse is applied by a considerable number of times by a repeat time TR, and with regard to Ge and Gs, encoding is executed prescribed times. In this case, a parameter of the pulse sequence is controlled so that an end part of the adjacent slab overlaps. This pulse sequence is controlled mainly by the repeat time TR, an echo time TE, a flip angle theta, etc., and emphasizes a signal from a blood flow flowing in an excited area.

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, and more particularly to a method for obtaining a blood vessel image by utilizing a magnetic resonance phenomenon.

[0002]

Pulse sequences used in magnetic resonance imaging can be designed to collect 2D and 3D data sets containing information about blood vessels in a region of interest. One of the most commonly used methods is based on the TOF (Time Of Flight) phenomenon.

This is because the blood flow (spin) flowing into the selected region is any high frequency pulse (RF pulse).
It is a fresh spin that I have not experienced, and utilizes the fact that the signal strength from the blood vessel is large. On the other hand, the region surrounding the blood vessel exhibits a saturation phenomenon after receiving a plurality of high frequency pulses, so that the signal from the region becomes very small. In order to achieve such a situation, it is possible to shorten the repetition time TR in the pulse sequence.

The signal from a blood vessel decreases exponentially as the number of high frequency pulses it experiences increases. Therefore, a blood vessel in a state where it traverses in a thick slab (three-dimensional tomographic image) or bends along a complicated path in the slab has a signal lower than the actual signal in the blood vessel portion near the outflow region in the slab region. Since it is a value, it is displayed as a low brightness value on the image. At this time, if processing is performed by a certain projection method (for example, maximum intensity projection method (MIP)) and the result is displayed, an accurate blood vessel image cannot be obtained.
Particularly in the case of smaller blood vessels, in the extreme case, the signal strength of the blood vessel is reduced to the background strength, and it appears that the blood vessel is occluded. If such a situation occurs, the physician may underestimate the vessel diameter or overestimate the stenotic disorder, which may lead to misdiagnosis. With such a blood vessel image, the value of image diagnosis is inevitably very small.

Therefore, a plurality of narrow excitation regions (hereinafter referred to as "thin
The above problems can be partially solved by arranging "slabs") without gaps to obtain the data. However, when a single slab shape is viewed, it is difficult to obtain an ideal rectangular slice profile due to various restrictions caused by a magnetic resonance imaging apparatus. It dulls and excites other than the target region due to side lobe characteristics. Such a profile adversely affects the image resolution.

Therefore, as one method for overcoming this problem, there is known a method of selectively exciting a plurality of slab regions so as to overlap a portion of the slab. According to this method, the length of the blood vessel running perpendicular to the image plane is shortened, and therefore, among the blood vessels included in the slab as described above, the amount of attenuation of the signal generated from the blood vessel flowing out at the slab end surface is reduced. , A more uniform intensity distribution can be obtained over the entire blood vessel.

Each slab is composed of slices, which can be used to create a 3D data set. When collecting several slab data and imaging them, the slice data located in the middle of the slab is used to create a 3D data set,
If an image is created by the P method, a good blood vessel image of the subject can be obtained. In addition, the multi-slab technology is less affected by phase distortion, and fine blood vessels can be displayed with better resolution than other 2D and 3D imaging technologies.

However, as described above, in the case where there is a complicated traveling blood vessel in the slab region or when the blood flow velocity is slow, even if the actual signal intensity is constant along the blood vessel. , The observed signal strength of blood vessels is not constant but changes. Also, since the signal strength decreases along the slab, the signal strength at the end of one slab and the signal strength of the next slab that is in contact with this slab are different, and in reality they are continuous signal strengths. However, there is a problem that the same image intensity is not obtained when the images are combined. That is, of the data at both ends of the slab, when only the data on one side is used, the images appear as discontinuity when the respective slabs are combined to form the final image. For this reason, there is a possibility of causing deterioration of image quality and causing a misdiagnosis in performing image diagnosis, which is a great hindrance.

[0009]

As described above, due to the shape of the blood vessel, the slow flow velocity, and the difference in the signal strength between the slabs, the signal strength obtained from the blood vessels is not constant and changes. However, there is a problem in that the image quality may be deteriorated and an erroneous diagnosis may occur in performing image diagnosis.
Therefore, the present invention provides a magnetic resonance imaging apparatus for uniformly obtaining the signal intensity of blood vessels imaged by the MRA imaging technique.

[0010]

In order to solve the above-mentioned conventional problems, the present invention provides a magnetic resonance signal generated by applying a high-frequency magnetic field and a gradient magnetic field to an object placed in a static magnetic field. In a magnetic resonance imaging apparatus for collecting and obtaining a magnetic resonance image, means for controlling and applying a high frequency magnetic field and a gradient magnetic field so that the ends of adjacent slabs partially overlap, and the high frequency magnetic field and the gradient magnetic field A means for reconstructing a slab by collecting magnetic resonance signals generated from the subject based on the above, a means for calculating the intensity of the luminance signal of the corresponding voxel among the adjacent slabs, and a calculation by this means Of the intensities of the luminance signal of each voxel that has been selected, a means for selecting the larger luminance signal, and correcting the slab using the voxels selected by this means, 3 on the basis of the corrected slab
A magnetic resonance imaging apparatus is constituted by means for generating a three-dimensional magnetic resonance image.

[0011]

According to the present invention, when the blood vessel signals of two adjacent slabs are attenuated, the signal intensity attenuated at the end of a certain slab can be corrected by the signal value of the next overlapped slab. The discontinuity that occurs in can be reduced. Up to now, the information on one side that is overlapping is discarded, whereas in the present invention, all the information of the slab area that is overlapping is effectively used. Such processing has the effect of reducing discontinuity when images are combined.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing the configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention. In the figure,
The static magnetic field magnet 1, the magnetic field homogeneity adjustment coil 3 and the gradient magnetic field generation coil 5 are driven by an excitation power supply 2, a magnetic field homogeneity adjustment coil power supply 4 and a gradient magnetic field generation coil power supply 6, respectively. As a result, a uniform static magnetic field and a gradient magnetic field having a linear gradient magnetic field distribution in three directions which are orthogonal to each other in the same direction are applied to the subject 7. A high frequency signal is sent from the transmitter 10 to the probe 9, and a high frequency magnetic field is applied to the subject 7. Here, the probe 9 may be used for both transmission and reception, or may be provided separately for transmission and reception. The magnetic resonance signal received by the probe 9 is quadrature-phase detected by the receiver 11 and then transferred to the data collector 13 for A / D conversion.
Sent to 4. As described above, the excitation power supply 2, the magnetic field homogeneity adjustment coil power supply 4, the gradient magnetic field generation coil power supply 6, and the transmission unit 1
0, the receiving unit 11, and the data collecting unit 13 are all controlled by the system controller 12. The system controller 12 and the electronic computer 14 are controlled by a console 15, and the electronic computer 14 performs image reconstruction processing based on the magnetic resonance signals sent from the data acquisition unit 13 to obtain the image data. Get the data. The obtained image is displayed on the image display 16.

In the magnetic resonance imaging apparatus configured as described above, the system controller 12 executes the pulse sequence shown in FIG. Thereby, the three-dimensional tomographic image S (hereinafter referred to as “slab”) shown in FIG. 3 can be obtained. In this embodiment, in order to obtain the slab S composed of 128 × 128 × 12 data, the selective excitation RF pulse is applied 128 times at the repetition time TR, Ge is encoded 128 times, and Gs is encoded 12 times. It is carried out. Therefore, the final three-dimensional angiography is obtained by connecting the slabs in the slice direction, but in the present invention, the pulse sequence parameters are set so that the ends of the adjacent slabs overlap as will be described later. Control. This pulse sequence is controlled mainly by the repetition time TR, the echo time TE, the flip angle θ, etc. to enhance the signal from the blood flow flowing into the excited region, and
The value is chosen to reduce the signal from the background tissue surrounding the blood vessel. That is, the pulse sequence is selected to produce angiography based on the TOF phenomenon.

Now, in order to simplify the explanation, consider two adjacent slabs S1 and S2. FIG. 3 shows a cross section of two overlapped slabs obtained by performing a pulse sequence. The matrix size of each slice is 128x128 voxels. Specifically, the slab consisting of 12 slices has 6 slices overlapping with other adjacent slabs (hatched portion in FIG. 3).

The present embodiment is not limited to this, and the number of slabs, the number of slices of each slab, the number of voxels of one slice, or the overlapping region can be arbitrarily applied. In order to obtain a target area (Field-of-View: FOV), the frequency offset and frequency band of the high frequency magnetic field and the gradient magnetic field strength of these parameters are preset before execution.

FIG. 5 is a view of the state of FIG. 4 seen from directly above. In the figure, a state in which blood vessels are laid vertically and horizontally in the slab and is positioned in the excitation region corresponding to the slab S1 is displayed. If only this slab is imaged, the blood vessel profile cannot be obtained uniformly, and the blood vessel profile will have a reduced signal strength. In other words, slab S1
In the case of only, for the blood vessel in the part that enters the slab, while the signal intensity is high and a high luminance value is obtained,
This is because the blood vessel at the exiting portion has a low brightness value.

Therefore, in the present invention, as shown in FIG. 4 or 5, the slab S2 is overlapped, and a brighter and more uniform blood vessel is obtained from the luminance value of the voxels in the overlapped region. I'm trying to get a signal. Also, slab S1
It is also possible to enhance the signal in the rest of the vessel by obtaining another slab that overlaps the left part of the vessel. That is, the TOF (Time o
f Flight) -MRA image, the larger one of the two signal intensities (the signal intensity indicating an unsaturated blood flow) is set as the image value.

FIG. 6 illustrates any two overlapping slices of adjacent slabs. That is, the ninth slice (S1, S1,
9) and the third slice (S2, 3) of the slab S2 shown in FIG. The shading of each voxel shall be proportional to the signal strength obtained after applying the pulse sequence shown in FIG. The blood vessel shown in FIG. 5 is orthogonal to the slice, and four voxels are contained in two blood vessels. White voxels represent a strong signal coming from the bloodstream. The remaining voxels represent a saturated background and are displayed in halftone. (S1,
In the lower right blood vessel region (hatched in the figure) of 9), the blood is more saturated than the upper left blood vessel region, and is therefore shown in a darker color.

First, what luminance value is to be assigned to each voxel in the overlapping area is determined. In this embodiment, a larger signal strength is selected between two voxels to be compared. That is, of the two adjacent slabs to be compared, the intensities of the luminance signals for the corresponding voxels are calculated, and the larger intensity signal is selected from the calculated intensities of the luminance signals. It is used as image data.
The brightness value of the voxel thus obtained is as shown in FIG.

It should be noted that in this embodiment, there are two comparisons.
Of the two voxels, the intensities of the corresponding luminance signals are compared, and the larger one is selected.
An average value of two voxels or a weighted addition value of those neighboring voxels may be used.

[0021]

Data composed of overlapped thin slab regions collected by a pulse sequence.
By repeating the above process for voxels in the overlapping regions of the set, the signal from the vessel can be enhanced and the entire data set can be set as the optimal voxel intensity value.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a pulse sequence.

FIG. 3 is a schematic diagram of 3D data generation.

FIG. 4 is a view of two overlapped slabs as viewed from the side.

FIG. 5 is a view of two overwrapped slabs as seen from directly above. .

FIG. 6 is a diagram showing the signal strength of each voxel.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet 2 ... Excitation power supply 3 ... Magnetic field homogeneity adjustment coil 4 ... Magnetic field homogeneity adjustment coil power supply 5 ... Gradient magnetic field generation coil 6 ... Gradient magnetic field generation coil power supply 7 ... Subject 8 ... Bed 9 ... Professional -Bus 10 ... Transmitting unit 11 ... Receiving unit 12 ... System controller 13 ... Data collecting unit 14 ... Electronic computer 15 ... Console 16 ... Image display S1, S2 ... Slab O ... Overlap area 51 Blood flow 52 Blood vessels 61 Unsaturated blood flow 62 Saturated blood flow

Claims (2)

[Claims] 1. A magnetic resonance imaging apparatus for collecting a magnetic resonance signal generated by applying a high frequency magnetic field and a gradient magnetic field to a subject arranged in a static magnetic field to obtain a magnetic resonance image. Means for controlling and applying the high-frequency magnetic field and the gradient magnetic field so that the ends of the slab partially overlap, and collecting the magnetic resonance signals generated from the subject based on the high-frequency magnetic field and the gradient magnetic field to form a slab. The means for reconstructing, the means for calculating the intensity of the luminance signal of the corresponding voxel among the adjacent slabs, and the intensity of the larger luminance signal for the intensity of each voxel calculated by this means The slab is corrected using a signal selecting means and the voxels selected by the means, and a three-dimensional magnetic resonance image is generated based on the corrected slab. Magnetic resonance imaging apparatus characterized by comprising a means for. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the applying unit controls and applies at least one of a frequency offset or a frequency band of a high frequency magnetic field and an intensity of a gradient magnetic field.