JPH04200445A - Angiogram display system - Google Patents

Abstract

PURPOSE:To correct artifact of a static part by incorporating a matching processing of an amplitude and a phase between images pertaining to a processing which performs a subtraction between the images obtained by reproduction processing from two different kinds of detection signals to extract a blood vessel. CONSTITUTION:Artifact of a static part is corrected utilizing a difference of amplitude to generate an angiogram of high quality. A rephase image R and a dephase image D are inputted. Phases of the phase images R and D inputted are matched. The image with a phase distortion matched is differentiated from the rephase image to determined an angiogram S corrected in phase. This allows the correction of artifact of the static part which is generated from a difference of amplitude at the static part between the rephase image and the dephase image.

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to an in-vivo tomographic imaging device that utilizes magnetic resonance phenomena, and particularly to a method for improving the image quality of blood vessel images (angio images) obtained by the device. [Prior Art] As a publicly known example regarding angiography (blood vessel imaging), IE Transactions on Medical Imaging MI-5, No. 3,
Pages 140 to 151. 1988 (IEEE, Trans, on Medicine
al Imaging+M I-5, Na3. pp,
140-151.1988). In this document,
There is a description below about angiography. Angiography is a method that separates blood vessels (moving parts) from static parts. The methods can be broadly divided into two types. One is a method that suppresses the signal emitted from the stationary part and detects the signal only from the blood vessel part, and the other method is to subtract the image of only the static part from the image showing both the blood vessel part and the static part, and to create a blood vessel image. This is a method (subtraction method) that attempts to obtain . On the other hand, there are two types of blood vessel properties that can be used to separate the blood vessel part and the stationary part. ■ Fluorescence effect A method that separates the vascular part from the stationary part by using the effect of inflow into or outflow from the cross section (imaging range). ■ Phase change effect When the excited spins move under the applied gradient magnetic field,
A method that uses the property of phase change to separate the vascular area and the stationary area. Since the present invention relates to the subtraction method using the phase change effect of the blood vessel shown in (1) above, this method will be explained in detail below. Now, when the excitation time is 0 time and the position of the spin at time is expressed as 5(t), one phase change is given by the following equation. φ(t)=v/lG(τ)S(r)d τ −
(5) V: nuclear gyromagnetic ratio G: applied gradient magnetic field. If we consider the movement of spin up to the first-order speed,
S is 5(t)=X, +v t −(6
) Xo: position at time 0 = velocity. Substituting equation (5) into equation (6) gives φ(t)
=,y X6 f tG(τ)d x+v v f t
G(τ)t d τ= v X:Mo(t)+ v v
M, (tg') = 47) M, (t): 0th moment M, (t): 1st moment. The 0th moment is the area of the gradient magnetic field, and the magnitude that causes a phase change to a stationary spin. Show that. On the other hand, the first moment indicates a proportionality constant that gives a phase change according to the speed of a moving spin whose speed is not O. That is, the presence of a gradient magnetic field generally causes a phase change proportional to the velocity. Blood vessels can be separated using this phase change. Currently, the sub-1-action method, which is most commonly used, obtains blood vessel images in the following manner. Since blood flow in a blood vessel is a laminar flow, it can be thought of as a paraboloid with maximum velocity at the center and O at the periphery. Therefore,
Different phase changes occur depending on the distance from the periphery of the blood vessel. As a result, when viewed from the integrated projection data, the spins cancel each other out, and the signal emitted from the blood vessel decreases. If the first moment is sufficiently large, the signal from the blood vessel can be almost eliminated. On the other hand, if the application of the gradient magnetic field is devised so that the first moment M□ is set to O, it is possible to prevent the phase change from occurring regardless of the speed. In this case, the signal from the blood vessel portion does not decrease and the same signal as the stationary portion is output. A sequence for measuring such a signal is called a phase-insensitive (rephase) sequence. On the other hand, a sequence for measuring the former type of signal in which 1M1 is not O is called a phase sensitive (day phase) sequence. Hereinafter, the image obtained by the rephase sequence will be referred to as a rephase image, and the image obtained by the dayphase sequence will be referred to as a dayphase image. A blood vessel image can be obtained by performing subtraction between the images obtained in these two sequences. As the invention closest to the present invention, Japanese Patent Application No. 17092/1983
There is No. 8. This example is a method for realizing high image quality when the subtraction method described in "1" is applied to an actual image. This invention will be specifically explained below. In fact, in addition to phase changes due to blood flow, phase distortions due to device distortion, eddy current effects, etc. exist on the two measured images. Therefore, if the difference is simply taken between two images, the stationary portion as well as the straight pipe portion will be imaged due to this phase distortion. (Hereafter referred to as stationary part artifact). In order to obtain a high-quality blood vessel image, it is sufficient to image only the blood vessel portion, and as a means of achieving this, it is sufficient to match only the phase distortion between the two images and perform a difference. Now, rephase image R and dayphase image R are defined as follows. R= Aexp(jθ') ・-C
1) p=Bexp(jψ) −=(
2) j: imaginary unit In order to match the phase distortion of the rephase image and the dayphase image, 2 from R-p* (* means complex conjugate)
Find the phase difference between two images. This phase difference includes phase changes due to blood flow and phase distortion.6 In order to match only the phase distortion. These need to be separated. Here, the property that phase distortion changes smoothly while the phase change due to blood flow is rapid is utilized. That is, a low-pass filter is applied to the phase components calculated by R.9 lines, and only phase components that change smoothly are extracted. A high-quality blood vessel image S is obtained by correcting the phase of D using the obtained phase component. The formula for calculating S is shown in the following formula. As is clear from equation (8), since both images are complex numbers, the absolute value of the difference result is taken to obtain S as a real number. S = IRD-Phs(Loti(RD*))l
...(g) Phs: Function for determining phase Low: Low-pass filter function [Problem to be solved by the invention] The above conventional technology is effective when the stationary part artifact is caused by the phase difference between two images. It is. However, a problem arises when the two images differ in both phase and amplitude. Normally, the sequence is designed so that the same signal is output in the still parts of the rephased image and the dayphase image, so if the absolute value is calculated and the difference is calculated between these images, the still parts should be canceled. However, if phase distortion exists in the slice direction, the static portion will not be canceled even if the absolute value difference is performed between the two images. This is in two images. This is because the phase distortion in the slice direction differs slightly depending on the measurement situation and the like. As a result, the detection signals obtained by integrating in the slice direction are different, and the amplitudes in the static parts of the two images are different.In the conventional method, the phase is matched, but the amplitude is not matched. Since it is not taken into account, static part artifacts cannot be corrected. The present invention aims to correct this stationary part artifact. [Means for solving the problem] The above purpose is achieved by the following means. If phase distortion exists in the slice direction, a blood vessel image is obtained by combining both the phase and amplitude in the difference between the rephase image and the dayphase image. Specifically, the following three methods can be considered. For explanation, equations (1) and (2) are shown below again. R=Aexp(jθ)...(1)
D=Bexp(jψ) ・42)■
A method using the difference in amplitude The stationary part artifact existing in the blood vessel image S obtained by equation (8) is corrected using the difference in amplitude, and the blood vessel image S′
get. s' = S - I 1. oti(A-B) l
・-(3)■ Method using amplitude ratio A blood vessel image S# is obtained by using the difference C between the amplitude B of the rephase image @A and the amplitude B of the day phase image and the ratio of B. S' = l RD-Phs (Low (RD book)
)(1+Low(C)/B)1・(4) In the above equation, Phs(Low(R-D*)) is the phase matching + 1 +Low(C)/B is the amplitude matching. There is. Q) Method for simultaneously correcting phase and amplitude A blood vessel image S''' is obtained by applying the following formula to the rephase image R and the day phase image I. S~=IR-D・(Low(R/ D)) l
(9) By combining the phase and amplitude using the three methods described above, it is possible to create a high-quality blood vessel image. [Operation] A method for matching the phase and amplitude in the difference between the rephase image and the dayphase image will be described as an example. It is considered that the stationary part artifact that exists on the phase-corrected blood vessel image is caused not by a phase problem but by a difference in amplitude. Therefore, in the example of equation (3), the static part artifact should be included in the difference A-H between the amplitudes of the two images. Therefore, A-
By extracting the stationary part artifact included in B using a low-pass filter and subtracting the extracted component from the phase-corrected blood vessel image S, it is possible to obtain a blood vessel image with higher image quality than before. In equation (4), the static part artifact is corrected using the ratio of the difference C and B between the amplitude A of the rephase image and the amplitude @B of the day phase image. Namely. By incrementing B to A, the amplitude is matched and stationary artifacts are corrected. Regarding equation (9), the static part artifact is corrected by combining the phase and amplitude at the same time as a complex image instead of processing the phase and amplitude separately. [Examples] Examples according to the present invention will be described below. FIG. 2 is a block configuration diagram of an MHI device implementing the present invention. In order to detect a nuclear magnetic resonance signal from an object to be inspected, a sequence control unit 201 controls each part of the apparatus according to a predetermined procedure, a transmitter 202 of a high-frequency magnetic field pulse that is generated to cause resonance, and a tilt Gradient magnetic field pulse 204 that modulates the magnetic field and magnetic field controller 2 that controls it
03, a receiver 205 that receives and detects nuclear magnetic resonance signals generated from the inspection object, and a processing device [206] that performs various calculations including image reconstruction and phase distortion correction processing, etc.
CRT display 207 for image display and external storage device W2O3 for storing detection signal data, reconstructed image data, etc.
It consists of In the above configuration, a sequence for obtaining a blood vessel image in two-dimensional measurement is shown in FIG. FIG. 3(a) shows a day phase sequence. First, a 90° high-frequency magnetic field pulse 301 is applied simultaneously with a gradient magnetic field pulse 302 in two directions to make the spins in the slice to be imaged resonate. After applying the magnetic field pulse 304, a 180° high frequency magnetic field pulse 305 for generating spin echoes is applied. The generated spin echo signal 306 is then measured while applying a gradient magnetic field pulse 307 in the X direction. This sequence is repeated by changing the intensity of the phase two code pulse: 303. With this sequence, the effects of the magnetic field cancel each other in the stationary part because there is no movement, and the spin phase changes in the moving blood vessel part. FIG. 3(b) is a rephase sequence. This sequence differs from (,) in FIG. 3 only in the gradient magnetic field pulses 308 and 309 in the X direction. 308
indicates that a gradient magnetic field pulse and an inverted gradient magnetic field pulse (a magnetic field whose magnitude does not change but whose sign is reversed) are applied in the X direction. This magnetic field is made to prevent changes in spin phase in both the stationary part and the blood vessel part. Therefore, by subtracting the images from the sequences shown in FIGS. 3(a) and 3(b), it is possible in principle to image only the blood vessel portion whose phase is changing. However, in reality, stationary part artifacts occur due to the effects of various phase distortions due to device distortion, etc., and high-quality blood vessel images may not be obtained. Therefore, a first embodiment will be described in which a high-quality blood vessel image is created by correcting the stationary part artifact of the two levers using the difference in amplitude. An example of the processing flow is shown in FIG. Step 101: This step is a step of inputting the rephased image R and the defezed image. The rephase image R and the dayphase image are as follows. however,
The rephased image R and the dayphased image are complex images. R= Aexp(j e )・
・(1) D=Bexp(jψ)
- (2) Step 102: Here, the phases of the rephased image R and the dayphased image inputted in step 101 are matched. A detailed processing flow is shown in FIG. Step 401: In this step, the R-D book (book means complex conjugate) is calculated, and the phase difference between the rephase image and the dayphase image is determined. R-D*=AB*exp(j (O-ψ))
-(10) Step 402 2 obtained in step 401
A two-dimensional inverse Fourier transform is applied to the phase difference between the two images, converting the real space to the frequency space. That is, F-” (R・DI)
Calculate. Step 403: In this step, F-'(R・DI)
Cut out the low frequency components of. What is the cutout size? For example, when the image size of the rephase image and the dayphase image is 256×256, it is set to about 32×32. Also, it is better to use various filters (Hanning filter, Hamming filter, etc.) for extraction. However, it goes without saying that it may be cut out in a rectangular shape. In step 404, the low frequency component data obtained in step 4.03 is returned to real space again by two-dimensional Fourier transform, and Low (R-p) is calculated. Step 405: In this step, Lotm (R
Find the phase component Ph5(Loty(R-D") of -DI). For example, calculate as follows. Phs(1-ow(R-DI))=Low(R-D books)
)/l Low (R-D book) 1...(11) Step 406: Phs(Lo
D-Phs(Low(R-
D)), only the smoothly changing phase distortion of the day phase image is matched to the rephase image. Steps 402 to 404 are processes for applying a low bass filter. The effect is the same even if smoothing processing such as 3×3 is performed instead of these processings. Step 103 The phase distortion-adjusted image is subtracted from the rephased image by the processing in step 102 to obtain a phase-corrected blood vessel image S. S is a real number image. S = l R-D-Phs (Lotg (R-D book))
'I-' (8) Step 104: In this step, static part artifacts caused by the difference in amplitude in the static parts of the day-phase image and the re-phased image are corrected. FIG. 5 shows a detailed processing flow for correcting static part artifacts present on the blood vessel image S by using the difference in amplitude between the two images. Step 501: Find the difference between the amplitude A of the rephased image and the amplitude B of the dayphased image. This is because it is considered that the static part artifact present on the blood vessel image S is included in the amplitude difference A-B between the two images. The processing from step 502 to step 504 is carried out in Sustub 4.
From 02 to step 404, the processing target is R, and from 9 to A-
Since it is only a substitute for B, detailed explanation will be omitted. However, while R-9 is a complex number, A-B is a real number, so when Fourier transform is performed, it is necessary to perform the transformation with the imaginary part set to 0. In addition, the optimal cutout size of low frequency components for amplitude matching is considered to vary depending on the target image, but in this example it is 16 x 16, and Lo
Calculate w(A-B). Step 505: In this step, a high-quality blood vessel image S' is obtained by taking the difference between the phase-corrected blood vessel image S and Low (AB). S'=5-ILoty(A-B)l
-(3) The reason why the absolute value is not used as the final result in equation (3) is as follows. It is known that normally, the difference result will not be negative in a stationary part. If we take the absolute value of 2 for the difference result, there is a possibility that the negative part will become a stationary part artifact of 1- due to the aliasing effect. In this case, by not taking an absolute value for the difference result, unnecessary stationary part artifacts are prevented from occurring. Step 105: Here, the blood vessel image S' is output to a disk or CRT. Through the processing in steps 101 to 105 described above, it was possible to achieve high image quality of a blood vessel image containing static part artifacts. In this example, the stationary part artifact was corrected by using the difference in amplitude. Next, a second embodiment will be described in which static part artifacts are corrected by using the amplitude ratio. In the MRI apparatus shown in the first embodiment, flowchart 1- for correcting stationary part artifacts included in blood vessel images obtained from the day-phase image and the re-phase image obtained by the sequence shown in FIG. 3 by using the amplitude ratio. is shown in Figure 6. However, this flow omits input/output processing. Step 601: In this step, the phase distortions of the rephase image and the dayphase image are matched. Processing similar to step 102 is performed. The result is D-Phs(Lol
l(RD*)). Step 602: Here, the amplitudes of the two images are matched by using the ratio of the amplitudes of the rephased image and the dayphased image. As shown in FIG. 7, a difference C between the amplitude A of the phase image and the amplitude B of the day phase image is determined. The ratio of C and B is then used to increment B to A. Specifically, the amplitude of the day phase image is adjusted to the amplitude of the rephase image using the following equation. D' = D-Phs(Low(R-D*))(1+L
ow(C)/B)...(12) D' C using the image formula (12) that combines the two phases and amplitudes.
If you do not apply a low-pass filter to 2, you will simply be looking at the phase difference between the two images. Further, the process of applying a low-pass filter is the same process as steps 502 to 504, and in this example, the cutout size of the low frequency component for applying the filter is 16×
It was set at 16. The above processing is based on the amplitude difference C and the amplitude B of the day phase image.
Although the amplitudes were matched using the ratio of A and B, it can be easily inferred that the ratio of A and B may also be used. For example, match it as follows. D"=D-Phs(1, ow(R-D*))B(L
otv(A/B)-1) However, in the example of equation (13), if the value of B is extremely small, the division result will be large, so applying a low-pass filter to the result will have a negative impact on surrounding pixels. Possible (bleeding effect)
. Step 603: In this step, the day phase image whose phase and amplitude are matched is subtracted from the rephase image,
Obtain a high-quality blood vessel image S''. S' = lRD-Phs (Low (R-D')) (1
+Low(C)/B)1...(4) Through the processing in steps 601 to 603 of -F, high image quality of the blood vessel image including static part artifacts was realized. In this example, static part artifacts were corrected by using the amplitude ratio. 1st. The second embodiment was a method in which the phase and amplitude were matched separately, but the third embodiment was a method in which the phase and amplitude were matched simultaneously.
An example is shown below. As shown in the first example

[7] Let D be the day-phase image obtained by the sequence shown in FIG. 3, and R be the re-phase image obtained by the sequence shown in FIG. The following equation is used to simultaneously match the phases and amplitudes of the two images and correct static part artifacts included in the blood vessel image. S"'=lR-D(Loti(R/D))l
...(9) However, in the example of equation (13), equation (11
) If the value of 1) is extremely small, unnecessary stationary part artifacts may occur due to bleeding effects. The above three examples are generalized images obtained by two-dimensional measurement.
Although this is an example of correcting static part artifacts that occur in the second embodiment, it can be easily inferred that the present invention can also be applied to a two-dimensional image that is reproduced by decomposing a three-dimensionally measured signal in the slice direction. Next, a fourth embodiment will be described in which the image sizes and positions of the rephased image R and the dayphased image are somewhat different depending on the photographing conditions and the like. In this case, even if the static part removal method described above is used, since the sizes of the images are different, there is a high possibility that artifacts will occur on the image after subtraction. FIG. 8 shows a processing flow for solving this problem. Here, a correction example will be described when linear (translation, enlargement) distortion exists on the image, but of course a non-linear distortion correction method may also be used. Step 801: In this step, in order to align the two images, the amount of parallel movement and the enlargement factor between the images are determined by chip matching processing. Specifically, 2
Each image is divided into 16 parts, the corresponding divided parts are correlated between the two images, and based on the 16 correlation values,
The optimum amount of parallel movement and enlargement factor for matching the size of R to the size of D are calculated. Step 802: Here, R' is obtained by matching the size of R to the size of D using the amount of parallel movement and the expansion coefficient obtained in Step 801 by an enlargement/reduction process, for example, a cubic convolution process. Step 803: In this step, phase matching processing similar to step 102 is performed. Step 804: Here, subtraction processing similar to step 03 is performed. Effectively, the following occurs. S 2 =lR'-D-Phs(Lo-(R' ・D
*)) l - (14) Step 805: In this step, the amplitude table o) swarming process, which is the same process as step 104, is performed. Through the above processing, even if the two images have different sizes, it is possible to achieve high quality blood vessel images. [Effects of the Invention] According to the present invention, static part artifacts present on blood vessel images can be corrected by matching the phase and amplitude of the re-phase image and the day-phase image, and it is possible to obtain high-quality blood vessel images. can. This correction method is mainly effective for images obtained by two-dimensional measurement, but can also be applied to two-dimensional images reproduced by decomposing three-dimensionally measured signals in the slice direction.

[Brief explanation of the drawing]

FIG. 1 is a flowchart of one embodiment of the present invention.

Claims (1)

[Claims] 1. A means for generating a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field;
means for controlling the generating means according to a prescribed procedure;
In a magnetic resonance imaging apparatus having means for detecting a magnetic resonance signal from a desired inspection region of an object to be inspected, and means for performing various calculations on the detection signal, 2.
A blood vessel image in a magnetic resonance imaging apparatus characterized in that the process of performing subtraction between images obtained by reproduction processing from the different types of detection signals to extract blood vessels includes a process of matching amplitudes and phases between images. Display method. 2. A blood vessel image display method in a magnetic resonance imaging apparatus according to the method described in item 1 above, characterized in that subtraction is performed by correcting the phases of the two images, and then the amplitudes are matched. 3. A blood vessel image display method in a magnetic resonance imaging apparatus, which uses the difference in amplitude between the images when combining the amplitudes of the two images in the method described in item 2 above. 4. In the method of item 3 above, if the two images R and D are defined as follows, R=Aexp(jθ)...(1) D=Bexp(jψ)...(2) j: Blood vessel image S′ whose amplitude and phase are corrected using the difference in imaginary unit amplitude
A blood vessel image display method characterized in that: is expressed by the following formula. S'=S-|Low(A-B)|...(3) S: Blood vessel image with phase correction Low: Low-pass filter function 5, in the method of the second term above, the amplitudes of the two images are matched A blood vessel image display method in a magnetic resonance imaging apparatus, characterized in that the amplitude ratio between images is used. 6. In the method of item 5 above, the two images are
This blood vessel image display method is characterized in that the blood vessel image S'' whose amplitude and phase are corrected using the amplitude ratio is expressed by the following formula: S''=|R− D-Phs(Low(R・D*))(1+
Low (C)/B) | (4) C: A-B Phs: Function that takes phase Low: Low-pass filter function 7, in the method of item 1 above, corrects the amplitude and phase of two images at the same time A blood vessel image display method in a magnetic resonance imaging apparatus, characterized in that subtraction is performed between images after that. 8. A blood vessel image display method in a magnetic resonance imaging apparatus, characterized in that, in the method of item 1 above, a low frequency component of an image signal is utilized. 9. In a magnetic resonance imaging apparatus according to the method of item 1 above, the subtraction is performed after positioning between two types of images obtained by the apparatus to match the amplitude and phase between the images. Blood vessel image display method. 10. In the method described in item 9 above, the horizontal and vertical translation amounts and expansion coefficients are determined between the two types of images, and the alignment between the images is performed by expansion/reduction processing based on the obtained coefficients. A blood vessel image display method in magnetic resonance imaging characterized by: