JP2011227028A - Tomogram image reconstruction method and x-ray ct apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a tomogram image reconstruction method capable of preventing generation of an artifact due to the presence of a part of an object on the outer side of a CT reconstruction area even when such part is present, in a method of constructing the tomogram image of the object by executing a tomogram image reconstructing operation including forward projection processing using projection data obtained by the CT imaging of the object.SOLUTION: In a tomogram image reconstructing operation method including a step of executing forward projection processing on the tomogram image of an object constructed by a certain method, by extending an area to be used in the forward projection processing to an area which is on the outer side of a CT reconstruction area and in which the projection data are projected to the light receiving surface of a radiation detector, an influence of an object being present outside the CT reconstruction area A is included in the calculation and therefore it is possible to prevent occurrence of an artifact due to the presence even if it occurs.

Description

  The present invention relates to a tomographic image reconstruction method for constructing a tomographic image of an object using radiation, such as X-ray CT and SPECT, and an X-ray CT apparatus using the method.

  In X-ray CT, SPECT (Single Photon Emission CT), or PET (Positron Emission Tomography), radiation that has passed through the object at multiple angles is detected by a detector, and radiation projection data at multiple angles is collected. Then, the tomographic image of the object is obtained by reconstructing the projection data.

  Several methods for reconstructing projection data are known. Analytical methods such as FBP (Filtered Back Projection), algebraic methods such as ART (Algebraic Reconstruction), and OS-MLEM (Ordered-Subsets Maximum) Statistical methods such as -Likelihood Experiment-Maximization) are known (for example, see Non-Patent Document 1).

  In addition, a method has been proposed that employs the SA (Simulated Annealing) method and can eliminate FBP in image reconstruction (see, for example, Patent Document 1).

  In these reconstruction methods, processing called forward projection processing is performed except for simple FBP. The forward projection process is a process in which a tomographic image (including a zero image) constructed by some method is numerically projected on the detection surface in a direction connecting the focal point of the radiation and the detection surface of the radiation detector, For each pixel constituting the tomographic image, luminance information of the pixel is assigned to a projection area on the detection surface when radiation is simulated from the focal position of the radiation toward each pixel, and the detector The detection probability by each pixel is calculated.

  By the way, in an industrial CT apparatus using radiation such as X-rays, generally, a sample stage for placing an object is provided between a radiation source and a radiation detector, and the object is irradiated with radiation. However, the radiation source and radiation detector pair and the sample stage are relatively rotated, and the radiation detector output, that is, the radiation projection data of the object W is collected at every minute angle, and the radiation projection data at the multiple angles. A tomographic image of the object is obtained by reconstruction calculation using.

  In an ordinary industrial CT apparatus, a rotary table is provided between the radiation source 51 and the radiation detector 52 as schematically shown in FIG. 6 by the front view (A) and the plan view (B). A sample stage having 53 is provided, and the rotation axis R of the rotary table 53 is provided in a direction orthogonal to the radiation optical axis direction L connecting the radiation source 51 and the radiation detector 52. Further, instead of providing the rotary table 53, there is a configuration in which a pair of the radiation source 51 and the radiation detector 52 is rotated around the object W.

  In the CT apparatus, as the distance between the object and the radiation source is shortened, the projection magnification can be increased, and a CT image in which a minute part is greatly enlarged can be obtained. However, when the object W is a plate-shaped article such as a circuit board as shown in FIG. 7, the object W is rotated or the pair of the radiation source and the radiation detector is rotated. In addition, the object W and the radiation source 51 interfere with each other, and there is a limit to bring them close to each other, which is a limitation for increasing the projection magnification.

  In order to solve such a problem, conventionally, an apparatus called an oblique CT apparatus or an inclined CT apparatus has been put into practical use. A configuration example of the photographing system is schematically shown in FIG. That is, by arranging these in a positional relationship in which the line L connecting the radiation source 51 and the center of the radiation detector 52 and the rotation axis R of the rotary table 53 are not orthogonal to each other, the object W is shaped like a plate such as a circuit board. Even if it is a thing, it becomes possible to approach the radiation source 51 with respect to the target object W, and it becomes possible to expand a projection magnification significantly compared with the apparatus shown in FIG. 7 (for example, patent document 2). reference).

  Even in the above-described oblique CT apparatus, a tomographic image can be obtained by reconstruction calculation using a method equivalent to that of the normal cone beam CT apparatus shown in FIG. When back projection processing such as FBP is performed in such an oblique CT apparatus, the radiation source 51 and the radiation detection are schematically shown as a front view in FIG. 9A and a plan view in FIG. Based on the positional relationship between the device 52 and the rotation axis R, the region indicated by A in the figure is the CT reconstruction region. Further, in a normal cone beam CT apparatus, as schematically shown in a front view in FIG. 10 (A) and a plan view in FIG. 10 (B), the radiation source 51, the radiation detector 52, and the rotation axis R Based on the positional relationship, the area indicated by A in the figure becomes the CT reconstruction area.

  As a method for constructing a tomographic image by reconstructing calculation of projection data in a CT apparatus, a method by back projection processing such as the FBP method is general, but as a method for improving the image quality, Sequential asymptotic techniques such as the ART method are being adopted for medical CT apparatuses, that is, CT apparatuses targeting the human body (see, for example, Patent Document 3).

  In a sequential asymptotic technique such as ART, for example, projection data is calculated by forward-projecting a tomographic image constructed by backprojection processing, and the calculation result coincides with the projection data actually obtained by photographing. This is a technique for correcting tomographic images.

  Here, it is known that the data collected by the above-described oblique CT apparatus does not contain sufficient information for strictly reconstructing the three-dimensional structure of the object. For this reason, in a medical CT apparatus, information on the body axis direction of the subject is often lost. In order to improve this, research is being conducted to include an evaluation function for restoring information in the body axis direction in the sequential asymptotic method including the above-mentioned ART method.

International Publication WO2008 / 059982 Pamphlet JP 2005-127886 A JP 2007-117740 A

Tetsuya Kobayashi and three others, “Development of an integrated simulation system for tomographic image reconstruction” [online], [November 12, 2009 search], Internet <URL: http: // www. is. tsukuba ac. jp / lecture / syspro / h20report1 / 04_. pdf

  By the way, unlike a medical CT apparatus, an industrial oblique CT apparatus often photographs a plate-shaped article such as a circuit board as described above, and there are articles outside the CT reconstruction area. There are many. Further, even in an industrial cone beam CT apparatus, in order to increase the magnification of imaging, imaging may be performed in a state where an article exists outside the CT imaging area.

  That is, the CT reconstruction area in the oblique CT apparatus is as shown in FIG. 9, but as shown in FIG. 11, there is a large area B reflected in the projection data outside the CT reconstruction area A. . In sequential asymptotic techniques such as ART and forward projection processing used in the process of reconstruction of tomographic images such as SA, processing is usually performed on the premise that there is nothing in a region other than the reconstruction region. Therefore, if a reconstruction method using a conventional sequential asymptotic method or forward projection processing such as SA is applied to an industrial oblique CT apparatus and a plate-like article such as a circuit board is used as an object, the region B Since an object exists, it causes a remarkable artifact (virtual image) to appear. This is also true when CT imaging is performed in a state where an article is present in an area other than the CT reconstruction imaging area in a normal cone beam CT apparatus, and forward projection processing is performed.

  The present invention has been made in view of such circumstances, and in a reconstruction calculation method that includes forward projection processing in the process of tomographic reconstruction calculation, even if an article exists outside the CT reconstruction area, An object of the present invention is to provide a tomographic image reconstruction method capable of suppressing the generation of artifacts associated therewith and a radiation CT apparatus using the method.

  In order to solve the above problems, the reconstruction calculation method of the present invention collects projection data obtained by detecting radiation transmitted through an object with a detector at a plurality of projection angles, and the collected projection data is collected. In the tomographic image reconstruction method including a forward projection process in which each pixel constituting the tomographic image is projected onto the light receiving surface of the detector in the reconstruction process, while constructing a tomographic image of the object by reconstructing, The area to be subjected to the forward projection process includes a CT reconstruction area and is a preset area among all areas in which projection data may be reflected on the light receiving surface of the detector. (Claim 1).

  Here, in the present invention, among the processing areas used for the forward projection process, the area outside the CT reconstruction area is the size of each pixel used in the forward projection process as the distance from the CT reconstruction area increases. A method of increasing the thickness (Claim 2) can be preferably employed.

  In the radiation CT apparatus of the present invention, a sample stage for mounting an object is provided between the radiation source and the radiation detector, and the pair of the radiation source and the radiation detection device and the sample stage are relative to each other. The rotation mechanism is rotated, the rotation mechanism is driven while irradiating the object on the radiation sample stage from the radiation source, and the tomographic image of the object is obtained using the output of the radiation detector collected at every predetermined angle. In the radiation CT apparatus provided with the reconstruction calculation means to be constructed, the reconstruction calculation process by the reconstruction calculation means has a forward projection process for projecting each pixel constituting the tomogram onto the light receiving surface of the radiation detector, The processing area used for the forward projection process includes a CT reconstruction area, and a preset area among all areas in which projection data may be reflected on the light receiving surface of the detector It characterized by being (claim 3).

  In the radiation CT apparatus of this invention, the structure (Claim 4) provided with the setting means which sets the area | region used for the said forward projection process can be employ | adopted suitably.

  In the present invention, not only the CT reconstruction area A in FIGS. 10 and 11 described above, but also an area outside the CT reconstruction area including an area where projection data is reflected on the radiation detector is subjected to forward projection processing. By making it an area to serve, it is intended to solve the problem.

  That is, for example, in the oblique CT apparatus of FIG. 11, the radiation actually incident on the radiation detector is transmitted through the region B including the region A in FIG. 11, and in the region B other than the region A. If an object exists, the projection data actually detected by CT imaging includes transmission information of the object. When a plate-like article such as a circuit board is used as an object and a part of the area is enlarged and photographed, the object is always photographed in a region B other than the CT reconstruction area A that is an attention area. Become. Therefore, there is an inevitable discrepancy between the projection data obtained by CT imaging and the data obtained by forward projection processing considering only the CT reconstruction area A. For example, in the reconstruction operation using the sequential asymptotic method, the above mismatch is a fatal problem because a tomographic image is constructed by fitting an image obtained by forward projection processing with an image based on actually captured projection data. This causes an artifact in the tomographic image. This is the same for the reconstruction operation based on SA, and the same problem occurs for all methods including the forward projection process in the reconstruction operation process.

  Therefore, in the present invention, in the reconstruction calculation method including the forward projection process in the reconstruction calculation process of the tomographic image, the region to be subjected to the forward projection process is not limited to the CT reconstruction area, but outside the radiation detector. Extend to a pre-set area that may be reflected in the. As a result, the occurrence of artifacts can be prevented by correcting the tomographic image so that the image obtained by the forward projection process matches the image based on the radiation projection data obtained by CT imaging.

  Here, widening the forward projection processing area in this way leads to a long processing time. The invention according to claim 2 solves this problem. That is, in the invention according to claim 2, in the region B set as the processing region of the forward projection processing in the present invention, with respect to the region excluding the CT reconstruction region A, the size of each pixel increases as the distance from the region A increases. That is, the processing is executed with the pixel size (pixel equivalent length) increased. Thereby, the number of calculation processes can be reduced.

  The invention according to claim 3 is a radiation CT apparatus using the above-described reconstruction calculation method of the present invention, and the invention according to claim 4 includes a region setting means for performing forward projection processing. Generation of artifacts by setting the forward projection processing area so as to include the area where the subject is present and the area reflected on the radiation detector according to the setting state at the time of CT imaging It is possible to suppress an increase in the number of calculation processes due to processing of the waste area while suppressing the above.

  According to the present invention, even when a plate-shaped article such as a circuit board is magnified using an industrial oblique CT apparatus or the like, a reconstruction calculation method including a sequential asymptotic method and a forward projection process such as SA is performed. It is possible to obtain a high-quality tomographic image without artifacts.

  Further, as in the invention according to claim 2, calculation for reconstruction is performed by executing processing with the pixel size being increased as the distance from the CT reconstruction area is increased. The above effect can be achieved without making the time much longer than before.

It is a block diagram of embodiment of this invention. It is a flowchart showing the example of the operation | movement procedure of the reconstruction calculation of embodiment of this invention. It is explanatory drawing of the area | region where the back projection and forward projection process in embodiment of this invention are performed, (A) is a front view, (B) is a top view. It is a conceptual diagram showing the pixel size used by the back projection and forward projection process in embodiment of this invention. It is explanatory drawing of the forward projection process area | region in the case of applying this invention to a normal cone beam CT apparatus. It is a schematic diagram which shows the structural example of the general imaging | photography system of industrial CT apparatus, (A) is a front view, (B) is a top view. It is explanatory drawing of the approach limit of the radiation source and target object in the imaging | photography system shown in FIG. It is a schematic diagram which shows the structural example of the imaging system of an oblique CT apparatus. It is explanatory drawing of CT reconstruction area | region in an oblique CT apparatus, (A) is a front view, (B) is a top view. It is explanatory drawing of CT reconstruction area | region in a normal cone beam CT apparatus, (A) is a front view, (B) is a top view. It is explanatory drawing of the area | region reflected in the light-receiving surface of the radiation detector in an oblique CT apparatus.

DESCRIPTION OF SYMBOLS 1 Radiation source 2 Radiation detector 3 Rotary table 4 Computer 5 Display 6 Operation part 7 X-ray controller 8 Axis control part R Rotation axis W Object

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a configuration diagram of an embodiment in which the present invention is applied to an oblique CT apparatus using X-rays, and an object W is mounted between an X-ray generator 1 and an X-ray detector 2 so as to be vertical. A turntable 3 that rotates about the rotation axis R is disposed. The line connecting the focal point of the X-ray generator 1 and the center of the X-ray detector 2 is not orthogonal to the rotation axis R, and therefore is not horizontal, and the target W is irradiated with X-rays from obliquely below, The transmitted X-ray enters the X-ray detector 2 obliquely above. The rotary table 3 can be moved in three axial directions including the vertical direction by driving a moving mechanism (not shown), and by this movement, a photographing magnification, a photographing range, and the like can be set. The X-ray generator 1 generates cone beam X-rays, and the X-ray detector 2 is a two-dimensional X-ray detector.

  CT imaging is performed by rotating the rotary table 3 while collecting the X-ray from the X-ray generator 1 toward the object W and collecting the output of the X-ray detector 2 for each minute rotation angle. Is called.

  That is, during CT imaging, the output of the X-ray detector 2 is taken into the computer 4 at every minute rotation angle of the turntable 3 and stored as X-ray projection data at each projection angle in a memory or hard disk.

  The computer 4 performs well-known preprocessing on the X-ray projection data collected at each angle as described above, and constructs a tomographic image of the object W by a sequential asymptotic method according to the procedure described later. . The tomographic image is displayed on the display 5.

  An operation unit 6 including a keyboard, a mouse, and a joystick is connected to the computer 4. By operating the operation unit 6, various commands can be given to the computer 4 or various settings can be made. When the operator sets an area to be used for forward projection processing described later, this can be done by operating the operation unit 6.

  The computer 4 is also under control of an X-ray controller 7 that controls the tube current and tube voltage to be supplied to the X-ray generator 1, and the rotation mechanism and moving mechanism of the turntable 3 are also controlled. Control is performed via the axis control unit 8. Such commands and settings relating to control can also be performed by operating the operation unit 6 described above.

  Next, an example of the reconstruction calculation procedure in the embodiment of the present invention having the above configuration will be described with reference to the flowchart shown in FIG.

  CT imaging is performed under the above-described operation, and projection data at each projection angle obtained thereby is back-projected to construct a first tomographic image. Next, forward projection of the tomographic image is performed to obtain calculation projection data. The result is compared with the projection data actually collected by CT imaging, and the difference is calculated. The tomographic image is obtained by back projecting the difference, and the tomographic image based on the difference is added to the previous tomographic image to correct the previous tomographic image. Then, the projection data obtained by calculation is obtained by forward projecting the corrected tomographic image again, and the difference is calculated by comparing the result with the projection data actually collected by CT imaging in the same manner as described above. The corrected tomographic image is obtained by back projecting the difference and adding it to the immediately preceding tomographic image. This operation is stored as a completed tomographic image when the difference is equal to or smaller than the prescribed difference or the number of repetitions reaches the prescribed number of times.

  The feature of this embodiment is that, firstly, not only the CT reconstruction area but also the X-ray projection data is transferred to the X-ray detector as the target area for the back projection process and the forward projection process in the above operation. This is a point that is set in advance among all the areas to be reflected.

  That is, as shown in the front view in FIG. 3A and the plan view in FIG. 3B, the CT reconstruction region shown by A in FIG. It is a space surrounded by the points P1, P2, P7, and P8. On the other hand, in the embodiment of the present invention, the target area for the back projection process and the forward projection process is the B of all the areas in which the X-ray projection data is reflected in the X-ray detector 2 shown in FIG. This is a preset area. As shown below, this region B is in the shape of a truncated cone having a large diameter on the upper side with the rotation axis R as the center. The upper radius is the distance between the rotation axis R and the following point P3, and the lower radius is the distance between the rotation axis R and the following point P4.

  In the front view (A), the point where the line L1 connecting the focal point 1a of the X-ray generator 1 and the lower edge 2a of the X-ray detector 2 intersects the rotation axis R is P1, and the focal point 1a and the X-ray detector 2 Assuming that the point where the line L2 connecting the edge 2b intersects the rotation axis R is P2, the point P3 is the point where the horizontal line L3 passing through P2 and the line L1 intersect, and the point P4 is the horizontal line L4 passing through P1 and the line L2 Is the point where In the front view (A) and the plan view (B), in addition to the above points P1 to P4, points P5 to P8 are added to clarify the correspondence between the two figures.

  The preset region of all the regions B in which the X-ray projection data as described above may be reflected on the light receiving surface of the X-ray detector 2 is to be subjected to back projection processing and forward projection processing. Accordingly, it is possible to prevent the occurrence of artifacts due to the presence of an object in a region outside the CT reconstruction region A. In the example shown in FIG. 3, the entire region B that may be reflected on the light receiving surface of the X-ray detector 2 may be the target to be subjected to the back projection process and the forward projection process, A predetermined part of the area (area A must be included) may be the target.

  The second feature of this embodiment is that the size of each pixel used for backprojection processing and forward projection processing, that is, the pixel size (pixel equivalent length), as conceptually shown in the plan view of FIG. Although the CT reconstruction area A is constant, the area B excluding the area A is larger as the distance from the CT reconstruction area A increases. That is, the cells illustrated in FIG. 4 conceptually indicate the pixel equivalent size in the region B, and the pixel size in the region B is set in comparison with the pixel size in the CT reconstruction region A. The size is increased, and is gradually increased further from the CT reconstruction area A.

  By performing back projection processing and forward projection processing using such a pixel equivalent size, the number of calculation processes of each processing can be reduced, and a tomographic image is obtained by a sequential asymptotic method only for the CT reconstruction area A. As compared with the case of reconstructing, a tomographic image free from artifacts can be obtained without lengthening the processing time.

  In the above embodiment, the example in which the rotary table 3 is arranged between the X-ray generator 1 and the X-ray detector 2 has been shown. However, the pair of the X-ray generator 1 and the X-ray detector 2 is rotated. Of course, the present invention can be equally applied to the oblique CT apparatus having the configuration. Needless to say, radiation other than X-rays may be used.

  In the above embodiment, an example using ART as a reconstruction calculation method has been shown. However, the present invention includes a reconstruction calculation including the SA described in Patent Document 1 described above. The method can be applied to all methods including forward projection processing.

  Furthermore, although the example applied to the oblique CT apparatus of the present invention has been described in the above embodiment, the present invention can be equally applied to an ordinary cone beam CT apparatus. That is, for example, as illustrated in FIG. 5, a target W in which an arm-shaped member Wb protrudes from the main body portion Wa is considered. I often shoot in a positional relationship. In this case, the arm-shaped member Wb protrudes from the CT reconstruction area, and the projection data of the arm-shaped member Wb is reflected on the detector 52. When CT imaging is performed in such a setting state and forward projection processing is performed only on the CT reconstruction area A, the projection data of the arm-shaped member Wb is reflected in the detector 52, but there is As a result, the processing is performed assuming that nothing exists in the tomogram, and the effect appears in the tomographic image of the main body portion Wa. Therefore, an area for forward projection processing is set to an area as shown in FIG. With such a setting, the projection data of the arm part is also subjected to the forward projection process, and as a result, it is possible to suppress the influence from affecting the tomographic image of the main body part. The size of the region C can be arbitrarily set, and an appropriate size may be set in advance.

Claims (4)

Projection data obtained by detecting radiation transmitted through the object with a detector is collected at a plurality of projection angles, and a tomographic image of the object is constructed by reconstructing the collected projection data. In the tomographic image reconstruction method including forward projection processing for projecting each pixel constituting the tomographic image onto the light receiving surface of the detector in the construction process,
The area to be subjected to the forward projection process includes a CT reconstruction area, and is a preset area among all areas in which projection data may be reflected on the light receiving surface of the detector. A tomographic reconstruction method.   Among the processing areas used for the forward projection process, the area outside the CT reconstruction area is increased in size as the pixel used in the forward projection process is further away from the CT reconstruction area. The tomographic image reconstruction method according to claim 1. A sample stage for mounting an object is provided between the radiation source and the radiation detector, and includes a rotation mechanism that relatively rotates the pair of the radiation source and the radiation detection device and the sample stage. X-ray equipped with reconstruction calculation means for driving the rotation mechanism while irradiating the object on the radiation sample stage from and using the output of the radiation detector collected at every predetermined angle to construct a tomographic image of the object In CT equipment,
The reconstruction calculation process by the reconstruction calculation means includes a forward projection process for projecting each pixel constituting the tomogram onto the light receiving surface of the radiation detector, and a processing region used for the forward projection process is CT An X-ray CT apparatus characterized by including a reconstruction area and a preset area among all areas in which projection data may be reflected on the light receiving surface of the detector.   The X-ray CT apparatus according to claim 3, further comprising setting means for setting an area to be subjected to the forward projection process.

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