JP3685551B2 - Difference image imaging method and X-ray CT apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a differential image imaging method and an X-ray CT (Computed Tomography) apparatus, and more particularly to a differential image imaging method and an X-ray CT apparatus for imaging a blood vessel or the like using, for example, a contrast agent.
[0002]
[Prior art]
For example, when an image of a blood vessel or the like of a subject is to be captured using an X-ray CT apparatus, the tomographic image (plane image) is first scanned by scanning the subject before injecting the contrast agent, and then the contrast agent After a predetermined time (several tens of seconds), the same part is scanned to obtain a tomographic image (contrast image), and the difference between the two images (subtraction) is obtained. As a result, the portion common to both images is erased, and only the portion where the contrast medium is spread, that is, the blood vessel image is drawn.
[0003]
[Problems to be solved by the invention]
In this case, since there is a time between the two scans, it is inevitable that the tissue in the subject moves due to a breathing operation or the like between the two scans. For this reason, when the difference is taken, a difference image due to a change in the position of the tissue also occurs. This difference image is an image irrelevant to the contrast agent and hinders correct interpretation of the image.
[0004]
The present invention has been made to solve the above-described problems, and an object thereof is to realize a differential image imaging method and an X-ray CT apparatus that obtain an accurate contrast image.
[0005]
[Means for Solving the Problems]
[1] A first invention for solving the problem is an X-ray CT apparatus for generating an image based on data obtained by helical scanning of a subject with an X-ray irradiation source and an X-ray detector facing the X-ray irradiation source. The difference image imaging method according to claim 1, wherein the difference image is generated based on a substantial difference between each time of data obtained by continuously scanning the same part of the subject a plurality of times. This is an imaging method.
[0006]
According to the first invention for solving the problem, a difference image is generated based on a substantial difference between each time of data obtained by continuously scanning the same portion of the subject a plurality of times. Therefore, the time difference between the data for which the difference is obtained can be shortened, thereby realizing a difference image imaging method for obtaining an accurate contrast image.
[0007]
In the first invention for solving the problem, the “substantial difference” means at least a difference between data of each time or a difference of images generated based on the data of each time. And
[0008]
[2] In a second invention for solving the problem, an X-ray irradiation source and a detector array having a large number of X-ray detectors arranged in a row facing the X-ray irradiation source are arranged in N (≧ 2 ) A multi-row detector array arranged in a row, and a subject is relatively linearly moved along the parallel direction of the multi-row detector array, and the X-ray irradiation source is continuously moved around the subject. An X-ray CT apparatus, comprising: data collection means for collecting data through the multi-row detector array while rotating; and image generation means for generating an image based on the data collected by the data collection means, Differential image generating means for generating a differential image based on a substantial difference between the data of each group obtained through the detector array of each row of the multi-row detector array for the same part of the specimen. This is a featured X-ray CT apparatus.
[0009]
According to the second invention for solving the problem, based on the substantial difference between the data of each group obtained through the detector array of each column of the multi-row detector array for the same part of the subject. Since the difference image is generated, the time difference between the data for which the difference is obtained can be shortened, thereby realizing an X-ray CT apparatus that obtains an accurate contrast image.
[0010]
In the second invention for solving the problem, it is preferable that the difference image generation unit generates the difference image based on the difference between the data of each group in terms of speeding up the generation of the difference image.
[0011]
In the second invention for solving the problem, the difference image generation means may generate a difference image based on a difference between images generated based on the data of each time. This is preferable in that a more precise difference image is obtained by adjustment.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the embodiment.
[0013]
〔overall structure〕
FIG. 1 shows a block diagram of an X-ray CT apparatus. This apparatus is an example of an embodiment of the present invention. In addition, an example of embodiment regarding the apparatus of this invention is shown by the structure of this apparatus. An example of an embodiment related to the method of the present invention is shown by the operation of the present apparatus.
[0014]
In FIG. 1, the X-ray CT apparatus 100 includes an operation console 1, an imaging table 10, and a scanning gantry 20.
The operation console 1 includes an input device 2 for inputting an operator's instructions and information, a central processing device 3 for executing a helical scan process, an image reconstruction (image generation) process, a control signal, and the like. A control interface 4 for outputting to the imaging table 10 and the scanning gantry 20, a data collection buffer (buffer) 5 for collecting data acquired by the scanning gantry 20, and a CRT (cathod-ray tube) 6 for displaying images and the like And a storage device 7 for storing various data and programs.
[0015]
The imaging table 10 can move in the body axis direction with a subject (not shown) placed thereon.
The scanning gantry 20 includes an X-ray tube 30, a collimator 50 that forms an X-ray beam, a two-row detector array 60, and an X-ray controller that adjusts X-ray irradiation timing and intensity. (controller) 21, a collimator controller 22 that adjusts the X-ray passage opening of the collimator 50, a data collection unit 23 that collects data detected by the two-row detector array 60, and an X-ray around the body axis of the subject And a rotation controller 24 for rotating the tube 30, the two-row detector array 60, and the like.
[0016]
Here, the X-ray tube 30 is an example of an embodiment of an X-ray irradiation source in the present invention. The double-row detector array 60 is an example of an embodiment of an X-ray detector and a multi-row detector array in the present invention. Note that the detector array is not limited to two rows, and may be a multi-row having three or more rows.
[0017]
The scanning gantry 20 is an example of an embodiment of data collection means in the present invention. The central processing unit 3 is an example of an embodiment of an image generation unit and a difference image generation unit in the present invention.
[0018]
FIG. 2 shows a conceptual configuration of the two-row detector array 60.
This two-row detector array 60 is a combination of a first-row detector array 61 and a second-row detector array 62. The first-row detector array 61 forms a multi-channel detector array in which a large number (for example, 1000) of X-ray detectors 61 (i) are arranged in an arc shape.
[0019]
Similarly, the detector array 62 in the second row also forms a multi-channel detector array in which a large number of X-ray detectors 62 (i) are arranged in an arc shape. Here, i is a channel number and i = 1 to I.
[0020]
The X-ray detectors 61 (i) and 62 (i) are constituted by solid detectors such as a scintillation X-ray detector and a semiconductor X-ray detector.
FIG. 3 is an explanatory diagram of the interrelationship between the X-ray tube 30 and the collimator 50 and the two-row detector array 60. 3A is a front view, and FIG. 3B is a side view.
[0021]
The X-rays radiated from the X-ray tube 30 are formed into a flat fan-shaped X-ray beam Xr by the collimator 50, and the first-row detector array 61 and the second-row detector array of the two-row detector array 60 are used. 62 is evenly irradiated.
[0022]
Here, the straight line L connecting the focal point of the X-ray tube 30 and the center of the two-row detector array 60 is referred to as an angle reference axis. The angle formed by the straight line connecting the focal point of the X-ray tube 30 and each X-ray detector 61 (i), 62 (i) with respect to the angle reference axis L is referred to as a channel angle γ.
[0023]
In the X-ray detectors 61 (I / 2) and 62 (I / 2) at the center of the two-row detector array 60, the channel angle is γ = 0. In the X-ray detectors 61 (1) and 62 (1) at the left end of the two-row detector array 60 in the drawing, the channel angle is γ = −γm. In the X-ray detectors 61 (I) and 62 (I) at the right end of the two-row detector array 60 in the drawing, the channel angle is γ = + γm.
[0024]
Since the channel number i and the channel angle γ have a one-to-one correspondence, the X-ray detectors 61 (i) and 62 (i) are hereinafter referred to as the X-ray detectors 61 (γ) and 62 for convenience of explanation. Expressed as (γ).
[0025]
The subject is carried in with the body axis orthogonal to the fan surface of the X-ray beam Xr. This state is shown in FIG. As shown in the figure, the projection image of the subject sliced by the fan-shaped X-ray beam Xr is projected onto the two-row detector array 60, and the data is measured.
[0026]
The thickness of the X-ray beam Xr at the ISO center of the subject OB gives the slice thickness T. The X-ray beam Xr irradiates the detector arrays 61 and 62 with the slice thickness T divided into two equal parts. That is, the slice thickness per detector array is th = T / 2.
[0027]
The X-ray tube 30, the collimator 50, and the two-row detector array 60 are continuously rotated around the subject OB while maintaining such a relationship, and the imaging table 10 on which the subject OB is placed is in the body axis direction. Helical scan is performed with continuous linear feed. Projection data of the subject is collected at a plurality of (for example, 1000) view angles per rotation of the helical scan.
[0028]
[Collection of view data]
FIG. 5 is an explanatory diagram of the view angle.
An angle β formed by the angle reference axis L and the vertical axis at one angular position where the X-ray tube 30 and the two-row detector array 60 are rotated is referred to as an absolute view angle. When the absolute view angle at a desired image generation position is β = β ₀ , θ = β−β _{0 is referred} to as a relative view angle.
[0029]
Data collected at a relative view angle θ by an X-ray detector having a channel angle γ is represented by D (γ, θ). For example, D (0, θ) if γ = 0, D (−γm, θ) if γ = −γm, and D (+ γm, θ) if γ = + γm.
[0030]
FIG. 6 is an explanatory diagram of data of the opposite view.
In FIG. 6, when focusing on the data D (−γm, θ) shown in (a), the data of the opposite view closest to this on the linear movement axis of the helical scan is the image generation position as shown in (b). Data D (+ γm, θ ′) obtained at the front side or data D (+ γm, θ ″) obtained at the rear side from the image generation position as shown in (c). Here, θ ′ = θ−π−2 (−γm,) and θ ″ = θ + π−2 (−γm).
[0031]
In general, the data of the opposite view of the data D (γ, θ) is D (−γ, θ−π−2γ) on the front side from the image generation position, and D (−γ, θ + π−2γ) on the rear side.
FIG. 7 is a diagram for explaining the position of the relative view angle θ on the linear movement axis z of the helical scan.
[0032]
As shown in FIG. 7B, a position on the z-axis corresponding to the middle between the detector array 61 and the detector array 62 at the absolute view angle β ₀ is used as a reference, and this is set to z = 0.
[0033]
Further, the thickness (slice thickness) of the fan-shaped X-ray beam Xr incident on each detector array 61 and 62 is set to th, the linear movement distance for each rotation of the helical scan is set to d, and p = d / th is set to helical. When the pitch is set, the helical pitch is set to p = 1 in FIG.
[0034]
As shown in FIG. 7A, at the relative view angle θ = −π, the data D1 (γ, −π) obtained by the first detector array 61 is the slice plane data at z = 0, Data D2 (γ, −π) obtained by the second detector array 62 is data on the slice plane of z = −th.
[0035]
However, the z position of each slice plane is represented by the z position at the center of the thickness of each slice at the ISO center of the subject OB.
Further, as shown in FIG. 7B, at the relative view angle θ = 0, the data D1 (γ, 0) obtained by the first detector array 61 is the slice plane data of z = th / 2. The data D2 (γ, 0) obtained by the second detector array 62 is the slice plane data of z = −th / 2.
[0036]
Further, as shown in FIG. 7C, at the relative view angle θ = π, the data D1 (γ, π) obtained by the first detector array 61 is the slice plane data of z = th, Data D2 (γ, π) obtained by the second detector array 62 is data on the slice plane with z = 0.
[0037]
When the state of FIG. 7C is reached, the slice plane of the data obtained by the second detector array 62 matches the slice plane of the data obtained by the first detector array 61 in FIG. 7A. To do. Thereafter, the second detector array 62 moves helically along the same trajectory that the first detector array 61 followed, and measures data.
[0038]
This is shown in FIG. That is, the second detector array 62 measures data at the same position as the data detected by the first detector array 61 with a delay of one scan.
[0039]
[Contrast imaging]
FIG. 9 is a block diagram showing the concept of contrast imaging by this apparatus.
A contrast agent is injected into, for example, a blood vessel of the subject OB, and contrast imaging by a helical scan is started. The helical scan is started after an appropriate time after the contrast medium is injected. This time is determined based on the prediction of the time for the contrast medium to reach the imaging site.
[0040]
X-ray transmission data of the subject OB is collected through the detector arrays 61 and 62 of the two-row detector array 60 by the helical scan and stored in the storage device 7.
[0041]
The image reconstruction device 31 reconstructs a tomographic image of the subject using the measurement data stored in the storage device 7. The image reconstruction device 31 is included in the central processing unit 3.
[0042]
Reconstruction parameters such as the position on the z-axis of the slice to be reconstructed (image generation position), reconstruction function, image reconstruction interval (reconstruction pitch), slice thickness of the reconstruction image, etc. It is set by the operator through the input device 2, and image reconstruction is performed based on the setting. Image reconstruction is performed by, for example, a filtered back-projection method.
[0043]
Prior to image reconstruction, data for a plurality of views necessary for generating an image is calculated. The view data is calculated by selecting, from the data collected by the data collection means, two pieces of data that are the same view data as the desired view or opposite view data, respectively, and that are closest to the z-axis image generation position. This is done by weighting and adding data according to the distance from the image generation position. In addition, the measurement data set by the same detector array is used for calculation of view data.
[0044]
A tomographic image is reconstructed by filtered back projection using the plurality of view data calculated in this way.
First, it is performed using data calculated from the measurement data set of the detector array 61, and the image data is temporarily stored in the image buffer 32. The image buffer 32 is provided inside the central processing unit 3. Next, a tomographic image at the same slice position is reconstructed using data calculated from the measurement data set of the detector array 62. This image is an image of the same cross section after one scan time.
[0045]
The subtractor 33 performs a difference operation between this image and the stored image in the image buffer 32. The subtracter 33 is provided inside the central processing unit 3. The difference calculation is performed for each corresponding pixel, whereby a difference image is formed. The difference image represents the change within one scan time for the same cross section. This difference image is stored in the storage device 7 through the image processing device 34. The image processing device 34 is provided inside the central processing device 3.
[0046]
Difference images are similarly formed and stored in the storage device 7 at other positions on the z-axis. These difference images are appropriately given to the CRT through the image processing apparatus and displayed as a visible image.
[0047]
For example, the concentration of the contrast medium in the blood changes as time passes, as shown in FIG. That is, a curve is drawn in which the concentration of the contrast agent reaches a peak that increases with time and then gradually decreases. In the case of an artery, both the increase rate and the peak value are high as in curve A, and in the case of a vein, both the increase rate and the peak value are low as in curve B.
[0048]
In the course of such a concentration change, when an image with a contrast agent concentration of a1 (b1) is reconstructed based on the measurement data of the detector array 61, for example, based on the measurement data of the detector array 62 The reconstructed image has a density state of, for example, a2 (b2) after one scan time has elapsed.
[0049]
Therefore, in the difference image, the artery is represented by a CT value corresponding to the concentration difference a2-a1, and the vein is represented by a CT value corresponding to the concentration difference b2-b1. Further, the portion where the contrast agent does not flow is canceled by the difference calculation.
[0050]
Here, the time difference between the two images for which the difference is obtained is a rotation time of one scan, that is, one pitch of the helical scan, and is usually, for example, 1 second, 0.5 seconds when short, and about 2 seconds at long. Become. Accordingly, since it is easy to stop the body movement of the subject OB by holding the breath in the meantime, the position change of the tissue can be eliminated, whereby the image of the portion where the contrast agent does not flow is completely erased and accurate imaging is performed. An image can be obtained.
[0051]
In addition, since it is not necessary to capture a plain image, it is not necessary to image the subject twice with time as in the conventional case, and the efficiency is greatly improved.
In the contrast image captured by this apparatus, the CT value of the blood vessel image corresponds to the change in the concentration of the contrast agent within a certain time, and thus represents the blood flow velocity. That is, this apparatus performs blood flow velocity imaging. This is a feature of the present invention that cannot be realized by the difference between the conventional plain image and contrast image.
[0052]
Due to such features, the arterial phase and the venous phase can be clearly identified in a single contrast image, and for example, for malignant tumors such as cancer, the difference in blood flow velocity between the main body and the peripheral part. Based on this, it is possible to clearly determine the lesion.
[0053]
By the way, for the generation of the difference image, instead of obtaining the difference of the reconstructed image, the difference of the view data may be obtained before the back projection, and the back projection may be performed using this difference data. This is based on the fact that the same result can be obtained whether the data addition or subtraction is performed before or after the back projection due to the linearity of the back projection operation.
[0054]
In this case, for example, as shown in FIG. 11, the view data calculation device 35 first calculates data of a plurality of views at the image generation position using the measurement data set of the detector array 61 collected in the storage device 7. The This view data is stored in the data buffer 36.
[0055]
Next, using the measurement data set of the detector array 62 collected in the storage device 7, data of a plurality of views at the same image generation position is calculated. This view data is subtracted by the subtractor 33 between the same views as the data in the data buffer 36 to form differential view data.
[0056]
The image reconstruction device 31 backprojects this difference view data to reconstruct an image. An image reconstructed when the view data is difference data becomes a difference image. The difference image is stored in the storage device 7 through the image processing device 34, and is read out as appropriate and displayed on the CRT 6.
[0057]
This method is preferable in that the image reconstruction is performed only once and the generation of the difference image is accelerated. On the other hand, the above-described method of subtracting between images is preferable in that a more precise difference image is obtained by fine adjustment of each image.
[0058]
As described above, the example in which the helical scan is performed has been described. However, the imaging table 10 may be stopped and a normal axial scan may be performed, and the imaging table 10 may be sent by the slice thickness th every time one scan is completed. .
[0059]
In this case as well, a measurement data set having a time difference of one scan is obtained by the detector arrays 61 and 62, so that a difference image can be generated in the same manner as described above. This method is preferable in that it does not require view data calculation as in the case of helical scanning.
[0060]
In addition, an example in which the detector array has two columns has been described. However, when the number of detector arrays is three or more, the time difference between the measurement data in the first column and the measurement data in the subsequent columns increases by an integral multiple of the scan time. Therefore, a difference image for a desired time difference can be obtained by selecting a combination of data pairs for which a difference is obtained.
[0061]
【The invention's effect】
As described above in detail, according to the first invention for solving the problem, based on the substantial difference between the data of each time obtained by continuously scanning the same part of the subject a plurality of times. Thus, since the difference image is generated, the time difference between the data for which the difference is obtained can be shortened, thereby realizing a difference image imaging method for obtaining an accurate contrast image.
[0062]
According to the second invention for solving the problem, a substantial difference between the data of each group obtained through the detector array of each column of the multi-row detector array for the same part of the subject is obtained. Since the difference image is generated based on this, the time difference between the data for which the difference is obtained can be shortened, thereby realizing an X-ray CT apparatus that obtains an accurate contrast image.
[Brief description of the drawings]
FIG. 1 is a block diagram of an exemplary apparatus according to an embodiment of the present invention.
FIG. 2 is a conceptual configuration diagram of a two-row detector array in an apparatus according to an example of an embodiment of the present invention.
FIG. 3 is a conceptual configuration diagram of an X-ray irradiation / detection system in an apparatus according to an example of an embodiment of the present invention.
FIG. 4 is a conceptual configuration diagram of an X-ray irradiation / detection system in an apparatus according to an example of an embodiment of the present invention.
FIG. 5 is an explanatory diagram of views and data in an apparatus according to an example of an embodiment of the present invention;
FIG. 6 is an explanatory diagram of views and data in an apparatus according to an example of an embodiment of the present invention.
FIG. 7 is an explanatory diagram of a helical scan in an apparatus according to an example of an embodiment of the present invention.
FIG. 8 is a conceptual explanatory diagram of data collection in an apparatus according to an example of an embodiment of the present invention.
FIG. 9 is a conceptual explanatory diagram of difference image generation in an apparatus according to an example of an embodiment of the present invention.
FIG. 10 is an explanatory diagram of contrast imaging in an apparatus according to an example of an embodiment of the present invention.
FIG. 11 is a conceptual explanatory diagram of difference image generation in the apparatus according to the exemplary embodiment of the present invention.
[Explanation of symbols]
100 X-ray CT apparatus 1 Operation console 2 Input device 3 Central processing unit 4 Control interface 5 Data collection buffer 6 CRT
7 Storage Device 10 Imaging Table 20 Scanning Gantry 21 X-ray Controller 22 Collimator Controller 23 Data Acquisition Unit 24 Rotation Controller 30 X-ray Tube 50 Collimator 60 Two-row Detector Array OB Subject 31 Image Reconstruction Device 32 Image Buffer 33 Subtractor 34 Image processing device 35 View data calculation device 36 Data buffer

Claims (2)

X-ray CT for generating an image based on data obtained by helical scanning of a subject by an X-ray irradiation source and a multi-row detector array having two or more multi-row X-ray detector arrays facing each other A difference image imaging method in an apparatus, comprising:
Helical scan is performed so that the X-ray detector arrays in adjacent rows in the multi-row detector array collect data at the same position in the subject with a shift of one rotation of the X-ray irradiation source, and perform the helical scan. A difference image imaging method, comprising: generating a difference image based on a substantial difference between the data at the same position in the X-ray detector array of each column obtained in this way. An X-ray irradiation source;
A multi-row detector array formed by arranging N (≧ 2) rows of detector arrays in which a number of X-ray detectors facing the X-ray irradiation source are arranged in a row;
Helical scanning is performed so that the detector arrays in adjacent rows in the multi-row detector array collect data at the same position in the subject with a shift of one rotation of the X-ray irradiation source, and the multi-row detector Data collection means for collecting data through the array;
Image generating means for generating an image based on the data collected by the data collecting means;
Differential image generation means for generating a difference image based on a substantial difference between the data at the same position of the subject in the detector array of each column collected by the data collection means. X-ray CT system.

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