JP2013027520A - Method and device for generating image, program, and x-ray ct apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the spatial resolution of a high contrast region without increasing noises from a soft tissue region while suppressing artifacts, in an image reconstructed by a X-ray CT apparatus.SOLUTION: In a method of regenerating images, in a process for an image construction from projected data, a step of emphasizing an edge region in a data value profile within a projected data space and a step of mitigating the data emphasis to a region having a contrast of data of a constant level or lower are carried out. To projected data, for example, an image is reconstructed through the two steps of emphasizing an edge region in its profile and smoothing a region whose profile contrast is equal to or lower than a constant level, based on the projected data on which the emphasis and smooth processing are performed.

Description

  The present invention relates to a technique for optimizing the image quality of an X-ray CT (Computed Tomography) image.

  One of the main factors that determine the spatial resolution of an X-ray CT image is the arrangement interval of detection elements that constitute the X-ray detector of the X-ray CT apparatus. If an attempt is made to maximize the Nyquist frequency to increase the spatial resolution, the high-frequency component is accompanied by an increase in noise and artifacts. Conversely, if the noise and artifacts are to be suppressed, the spatial resolution is lowered. Thus, the spatial resolution and the degree of noise and artifacts are usually in a trade-off relationship.

  Therefore, the conventional X-ray CT apparatus balances the spatial resolution and noise and artifacts by adjusting the high-frequency component of the reconstructed image so as to be practically used. That is, in many X-ray CT apparatuses, various frequency enhancement reconstruction functions such as a low frequency enhancement reconstruction function for a soft tissue region and a high frequency enhancement reconstruction function for a high contrast (contrast) region. (See, for example, Patent Document 1, paragraph [0005], etc.).

JP 2006-068229 A

  By the way, the observer may want to observe both the soft tissue region and the high contrast region depending on the situation.

  In such a case, for example, an image based on the reconstruction function for the soft tissue region and an image based on the reconstruction function for the high contrast region are reconstructed. Then, these two types of images are displayed on the monitor at the same time, or are switched and displayed.

  However, with this method, the size of one image becomes small and it becomes difficult to see, or the movement of the line of sight increases, making the observation work complicated and the diagnosis efficiency worsening.

  In order to further solve this problem, for example, the image is first reconstructed with a high-frequency enhancement reconstruction function for a high-contrast region, and then noise reduction processing is performed only on the soft tissue region of the image, A method of obtaining one image in which both the soft tissue region and the high contrast region have appropriate image quality is also conceivable.

  However, in this method, side effects such as artifacts in the soft tissue region are emphasized in the obtained image.

  Under such circumstances, it is desired to improve the spatial resolution of the high contrast region in the image obtained by the X-ray CT apparatus while suppressing artifacts and without increasing the noise of the soft tissue region.

  In the invention of the first aspect, in the process of generating an image from projection data (data) to be imaged obtained by X-ray CT imaging, an edge in a data value profile in the projection data space Performing a process of emphasizing an area, and performing a process of reducing the emphasis on an area where the contrast of data values is below a certain level in the projection data space or the image space. An image generation method to be executed is provided.

  According to a second aspect of the invention, there is provided means for performing processing for emphasizing an edge region in a profile of data values in a projection data space in a process of generating an image from projection data of an imaging target obtained by X-ray CT imaging. In the above process, there is provided an image generating apparatus comprising means for performing a process of reducing emphasis on an area where the contrast of data values is below a certain level in the projection data space or the image space.

  According to a third aspect of the invention, an edge region in a profile of data values is emphasized in a projection data space in a process in which a computer generates an image from projection data of an imaging target obtained by X-ray CT imaging. And a program for functioning as a means for performing a process of reducing the emphasis on an area where the contrast of the data value is below a certain level in the projection data space or the image space in the process ( program).

  A fourth aspect of the invention is an X-ray CT apparatus that performs X-ray CT imaging, collects projection data to be imaged, and generates an image from the projection data. In the process of generating the image, the projection data Means for emphasizing an edge region in a profile of data values in space, and a process for relaxing the emphasis on a region where the contrast of data values is below a certain level in the projection data space or image space in the process An X-ray CT apparatus is provided.

  The invention of the fifth aspect includes an emphasizing means for emphasizing an edge area in the profile of the projection data, and a smoothing means for performing a smoothing process for smoothing an area having a contrast of a certain level or less in the profile. An X-ray CT apparatus according to the fourth aspect, comprising: reconstruction means for reconstructing an image based on the projection data subjected to the enhancement processing and the smoothing processing.

  Here, the “smoothing means” is an example of “means for performing a process of reducing emphasis”.

  The invention according to a sixth aspect provides the X-ray CT apparatus according to the fifth aspect, wherein the enhancement process is a process for enhancing only an edge region whose edge sharpness is within a predetermined range.

  According to a seventh aspect of the invention, in the enhancement process, the data value in the profile representing the high frequency component of the profile of the projection data is a region where the data value is not less than a first positive threshold value and not more than a desired upper limit value, and the data value The X-ray CT apparatus according to the sixth aspect, which is a process for emphasizing a region that is less than or equal to a negative second threshold value and greater than or equal to a desired lower limit value.

  According to an eighth aspect of the present invention, there is provided the method according to the fifth to seventh aspect, wherein the enhancement processing is performed in at least one profile of a channel direction, a column direction, and a view direction of the projection data. An X-ray CT apparatus according to any one aspect is provided.

  According to a ninth aspect of the invention, the smoothing process smoothes an area in which a data value in a profile representing a high frequency component of the profile of the projection data is equal to or smaller than a positive third threshold and equal to or greater than a negative fourth threshold. An X-ray CT apparatus according to any one of the fifth to eighth aspects is provided.

  According to a tenth aspect of the present invention, in any one of the seventh to eleventh aspects, the smoothing process is performed in at least one profile of a channel direction, a column direction, and a view direction of the projection data. An X-ray CT apparatus is provided.

  According to an eleventh aspect of the present invention, there is provided enhancement means for performing enhancement processing for enhancing edge regions in the profile of the projection data, and first reconstruction of a first image based on the projection data on which the enhancement processing has been performed. In the reconstructing means, the second reconstructing means for reconstructing the second image based on the projection data not subjected to the enhancement processing, and the first image or the second image, the contrast is An image of a corresponding area of the first image is filled in a first area exceeding a certain level, and an image of the corresponding area of the second image is filled in a second area having a contrast equal to or less than the certain level. There is provided an X-ray CT apparatus according to the fourth aspect, comprising generation means for generating an image.

  Here, the “first reconstruction unit”, the “second reconstruction unit”, and the “generation unit” are examples of “a unit that performs processing for relaxing the emphasis”.

  In the invention according to a twelfth aspect, the enhancement processing is performed when the data value in the profile representing the high frequency component of the projection data is greater than or equal to the first positive threshold value and the data value is equal to or smaller than the second negative threshold value. The X-ray CT apparatus according to the eleventh aspect, which is processing for emphasizing a certain region, is provided.

  The invention according to a thirteenth aspect provides the X-ray CT apparatus according to the eleventh aspect or the twelfth aspect, wherein the enhancement processing is performed in at least one profile of a channel direction, a column direction, and a view direction of the projection data. To do.

  In a fourteenth aspect of the invention, the first area is an area where the degree of pixel value variation in the first image or the second image exceeds a certain level, and the second area An X-ray CT apparatus according to any one of the eleventh to thirteenth aspects, wherein the degree of variation is an area that is equal to or less than the predetermined level.

  In a fifteenth aspect of the invention, the first area is an area in which a data value in a profile representing a high-frequency component in a desired direction of the first image or the second image exceeds a certain level. The X-ray CT apparatus according to any one of the eleventh to thirteenth aspects is provided, wherein the area is an area where the data value is equal to or less than the certain level.

  The invention according to a sixteenth aspect is the seventh aspect or the twelfth aspect, wherein the first and second threshold values change according to a type of a reconstruction function used by the first reconstruction unit for image reconstruction. The X-ray CT apparatus of the viewpoint is provided.

  The invention of the seventeenth aspect provides the X-ray CT apparatus according to any one of the fifth to sixteenth aspects, wherein the enhancement process is a sharpening process.

  According to the above aspect of the invention, the enhancement of the edge region is not performed in the reconstruction function or the image space in which the overcorrection that is the cause of the artifact is difficult to adjust, but in the projection data space in which the overcorrection is easily adjusted. Therefore, unnecessary enhancement of non-high-contrast areas on the image and processing to suppress noise that is inherently generated can be performed in the projection data space or the image space, while suppressing artifacts in the reconstructed image. The spatial resolution of the high contrast region can be improved without increasing the noise of the soft tissue region.

1 is a diagram schematically showing a configuration of an X-ray CT apparatus according to a first embodiment. It is a functional block diagram of the central processing unit in the first embodiment. It is a flow figure showing the flow of processing in the X-ray CT apparatus concerning a first embodiment. It is a figure which shows notionally the principal part of the process in the X-ray CT apparatus which concerns on 1st embodiment. It is a figure which shows an example of the edge area | region emphasis process in projection data space. It is a figure which shows an example of the non-high contrast area | region smoothing process in projection data space. It is a figure which shows the comparison result of the MTF (modulation transfer function) curve in the image by 1st embodiment, and another image. It is a figure which shows the comparison result of the MTF value in the image by 1st embodiment, and another image. It is a figure which shows the comparison result of the noise level in the image by 1st embodiment, and another image. It is a figure which shows the comparison result of the aliasing artifact (aliasing artifact) in the image by 1st embodiment, and another image. It is a functional block diagram of the central processing unit in the second embodiment. It is a flowchart which shows the flow of a process in the X-ray CT apparatus which concerns on 1st embodiment. It is a figure which shows notionally the principal part of the process in the X-ray CT apparatus which concerns on 2nd embodiment. It is a figure which shows the example of a synthetic | combination process with a 1st image and a 2nd image. It is a figure which shows the comparison result of the MTF curve in the image by 2nd embodiment, and another image. It is a figure which shows the comparison result of the MTF value in the image by 2nd embodiment, and another image. It is a figure which shows the comparison result of the noise level in the image by 2nd embodiment, and another image. It is a figure which shows the comparison result of the aliasing artifact in the image by 2nd embodiment, and another image.

  Embodiments of the invention will be described below. However, this does not limit the invention.

(First embodiment)
FIG. 1 is a diagram schematically showing the configuration of the X-ray CT apparatus according to the first embodiment.

  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 that receives input from an operator, a central processing unit 3 that performs control of each unit for imaging a subject, data processing for generating an image, and the like, and a scanning gantry 20. A data collection buffer (buffer) 5 that collects acquired data, a monitor 6 that displays images, and a storage device 7 that stores programs, data, and the like are provided.

  The imaging table 10 includes a cradle 12 on which the subject 40 is placed and put into and out of the opening B of the scanning gantry 20. The cradle 12 is moved up and down and horizontally moved by a motor built in the imaging table 10. Here, the body axis direction of the subject 40, that is, the horizontal linear movement direction of the cradle 12 is the z direction, the vertical direction is the y direction, and the horizontal direction perpendicular to the z direction and the y direction is the x direction.

The scanning gantry 20 includes a rotating unit 15 and a main body 20a that rotatably supports the rotating unit 15. An X-ray tube 21, an X-ray controller 22 that controls the X-ray tube 21, and an X-ray 81 generated from the X-ray tube 21 are supplied to the rotating unit 15 by a fan beam (fan).
an aperture 23 for shaping into a beam or a cone beam, an X-ray detector 24 for detecting X-rays 81 transmitted through the subject 40, and an output of the X-ray detector 24 as X-ray projection data A DAS (Data Acquisition System) 25 (which is also referred to as a data acquisition device) that converts and collects data and an X-ray controller 22, an aperture 23, and a rotating unit controller 26 that controls the DAS 25 are mounted. The main body 20 a includes a control controller 29 that communicates control signals and the like with the operation console 1 and the imaging table 10. The rotating part 15 and the main body part 20 a are electrically connected via a slip ring 30.

  The X-ray tube 21 and the X-ray detector 24 are arranged to face each other with the imaging space in which the subject 40 is placed, that is, the cavity B of the scanning gantry 20 interposed therebetween. When the rotating unit 15 rotates, the X-ray tube 21 and the X-ray detector 24 rotate around the subject 40 while maintaining the positional relationship. A fan beam or cone beam X-ray 81 emitted from the X-ray tube 21 and shaped by the aperture 23 passes through the subject 40 and is irradiated onto the detection surface of the X-ray detector 24. The spreading direction of the X-ray 81 of the fan beam or cone beam in the xy plane is called a channel direction, and the spreading direction in the z direction or the z direction itself is called a column direction.

  The X-ray detector 24 has a plurality of detection elements arranged in the channel direction and the column direction. The DAS 25 converts detection signals obtained by individual detection elements into individual channel data and collects these as X-ray projection data.

  FIG. 2 shows a functional block diagram of the central processing unit in the first embodiment. As shown in FIG. 2, the central processing unit 3 in the first embodiment functionally includes an edge region enhancement unit 301, a non-high contrast region smoothing unit 302, and an image reconstruction unit 303.

  The edge region enhancement unit 301 performs edge region enhancement processing for enhancing the edge region in the profile for the X-ray projection data P1.

  The non-high contrast region smoothing unit 302 performs non-high contrast region smoothing processing for smoothing the non-high contrast region in the profile on the X-ray projection data P1.

  The image reconstruction unit 303 reconstructs the image SG1 based on the processed X-ray projection data P2 that has been subjected to the edge region enhancement processing and the non-high contrast region smoothing processing.

  The edge region enhancement unit 301 is an example of enhancement means in the invention, the non-high contrast region smoothing unit 302 is an example of smoothing unit in the invention, and the image reconstruction unit 303 is reconstruction means in the invention. It is an example.

  Hereafter, the flow of processing in the X-ray CT apparatus according to the first embodiment will be described.

  FIG. 3 is a flowchart showing the flow of processing in the X-ray CT apparatus according to the first embodiment. FIG. 4 is a diagram conceptually showing edge region enhancement processing and non-high contrast region smoothing processing in the projection data space, which are the main parts of the processing.

  In step S1, the subject is scanned to collect a plurality of views of X-ray projection data P0. The X-ray projection data P0 of each view is constituted by channel data obtained by detection elements constituting the X-ray detector 24, that is, individual detection elements arranged in the channel direction and the column direction. The collected X-ray projection data P0 is usually subjected to preprocessing including beam hardening correction, detector channel-to-channel characteristic correction, X-ray intensity correction, etc., but description thereof is omitted here.

  In step S2, the edge region enhancement unit 301 performs edge region enhancement processing in the projection data space as shown in FIG. 4A on the collected X-ray projection data of each view. The edge region enhancement processing is performed, for example, in at least one profile in the view direction, channel direction, and column direction of the X-ray projection data.

  FIG. 5 shows an example of edge region enhancement processing in the projection data space.

  In the example of FIG. 5, first, a profile as shown in FIG. 5B is applied by applying a highpass filter to the profile f0 of the X-ray projection data as shown in FIG. A high frequency component profile f1 representing the high frequency component of f0 is obtained.

  Next, as shown in FIG. 5C, in the high-frequency component profile f1, a region where the data value is equal to or greater than a desired first threshold Th1 (> 0) and a desired second threshold Th2 (< 0) An area that is equal to or less than that is detected as an edge area EP to be subjected to enhancement processing.

  However, here, the region where the data value in the high-frequency component profile f1 is not less than the desired upper limit value Lt1 (> Th1> 0) and the region where the data value is not more than the desired lower limit value Lt2 (<Th2 <0) are emphasized. Excluded from the edge region EP to be processed. The reason for this is that if further enhancement processing is applied to a sharp edge that already has a steep slope, overcorrection such as overshoot and undershoot of the CT value in the reconstructed image is performed. This is because it becomes a factor that causes the generated artifacts. Note that the appropriate values of the upper limit value Lt1 and the lower limit value Lt2 are obtained from, for example, a simulation result or an actual trial result.

  When the edge region EP is detected, as shown in FIG. 5D, enhancement processing is performed to emphasize the detected edge region EP in the profile f0 of the X-ray projection data. As this enhancement processing, for example, sharpening processing using a Laplacian filter or unsharp mask, processing for multiplying the channel data value of the edge region EP by a predetermined coefficient, etc. Can be considered.

  As described above, when the edge region EP is enhanced in the projection data space, the high-contrast region is further enhanced in the reconstructed image, and the spatial resolution can be improved. In addition, it is easy to adjust overcorrection such as overshoot and undershoot of CT values in the reconstructed image, as compared with the case where the enhancement of the edge region EP is performed in the reconstruction function or the image space. The occurrence of artifacts can be suppressed. Further, in the profile f0 of the X-ray projection data, since the enhancement process is not performed on the edge region that already represents a sharp edge, overcorrection of the CT value in the reconstructed image can be prevented, and the accompanying artifacts are generated. It can be further suppressed.

  In this example, it is assumed that such edge region EP detection and enhancement processing is performed for each profile in the view direction, channel direction, and column direction of X-ray projection data.

  By the way, when the edge region EP is emphasized in the projection data space instead of the image space as described above, there is the above-described advantage. However, due to the characteristics of the image reconstruction method such as back projection, the reconstructed image is not inherently high contrast. Even the area is emphasized unnecessarily. That is, noise increases even in a non-high contrast region, that is, a soft tissue region in the reconstructed image.

  As can be seen from FIG. 4A, when the X-ray projection data P1 in which the edge region EP is emphasized is backprojected, the data of the emphasized edge region EP is backprojected. This is because it is reflected in all the image areas overlapping with (), the contrast of the image area is increased, and noise is increased.

  Therefore, in the next step, in order to suppress this noise increase, as shown in FIG. 4B, in the profile f0 of the X-ray projection data, non-high contrast region smoothing processing is performed in addition to edge region enhancement processing. Do. When the X-ray projection data P2 in which the non-high contrast region NP is smoothed is back-projected, the data of the smoothed non-high contrast region NP is reflected in all image regions that overlap the back-projected path. The contrast of the image area is reduced and noise is reduced. If back projection is performed in the same way from a large number of view directions, the image area in which the data of the edge area EP is back-projected only once or a small number of times is smoothed by the projection data back-projected from other directions. However, since the original edge region in the reconstructed image is back-projected from the data in which the edge region EP is emphasized from many directions, it is emphasized without being canceled by the data of the smoothed non-high contrast region NP. It remains to be done.

  As described above, by combining the enhancement of the edge area EP and the smoothing of the non-high contrast area NP in the projection data space, when the reconstructed image is developed, the spatial resolution in the high contrast area is improved. In addition, an increase in noise in the soft tissue region can be prevented. Incidentally, it has been found that, depending on the adjustment of the enhancement of the edge area EP and the smoothing of the non-high contrast area NP in the projection data space, it is possible to reduce the noise.

  In step S3, the non-high contrast region smoothing unit 302 performs non-high contrast region smoothing processing in the projection data space on the X-ray projection data P1 of each view on which the edge region enhancement processing has been performed. The non-high contrast region smoothing process is performed, for example, on at least one profile in the view direction, the channel direction, and the column direction of the X-ray projection data P0.

  FIG. 6 shows an example of the non-high contrast region smoothing process in the projection data space.

  In the example of FIG. 6, first, in the profile f0 of the X-ray projection data as shown in FIG. 6 (a), as shown in FIG. 6 (b), a variation index value σ1, which represents the degree of variation in channel data values, For example, the standard deviation of the channel data value per unit width in the profile direction is calculated. Then, as shown in FIG. 6C, a region where the variation index value σ1 is equal to or smaller than a desired third threshold Th3 (> 0) is detected as a non-high contrast region NP.

  When the non-high contrast region NP is detected, as shown in FIG. 6D, a smoothing process for smoothing the detected non-high contrast region NP is performed. As the smoothing process, for example, a smoothing process using a moving average filter (averaging filter) or a Gaussian filter can be considered.

  Thus, by performing a combination of edge region enhancement processing and non-high contrast smoothing processing in the projection data space, it is possible to prevent an increase in noise in the soft tissue region when developed into a reconstructed image, and Depending on the adjustment, the noise can be reduced.

  In step S4, the image reconstruction unit 303 reconstructs the image SG1 based on the processed X-ray projection data P2 that has been subjected to the edge region enhancement processing and the non-high contrast region smoothing processing. For the reconstruction, for example, a three-dimensional filter back projection method is used.

  In step S5, the reconstructed image SG1 is displayed.

  From this, the comparison result of the image obtained by the method of 1st embodiment and the image obtained by the other method is demonstrated.

  FIG. 7 shows a comparison result of the MTF curves. The comparison target is an original image OG that is not subjected to edge region enhancement processing and non-contrast region smoothing processing, and a kernel boost image BG that simply boosts a reconstruction function (recon kernel) and emphasizes high-frequency components. And image SG1 according to the method of the first embodiment in which edge region enhancement processing and non-high contrast region smoothing processing are performed. These images are all generated based on the same X-ray projection data. The subject to be photographed in this comparison result is a phantom for measuring spatial resolution. The horizontal axis of the graph shown in FIG. 7 represents spatial resolution (line-pair / cm). The vertical axis of the graph represents the MTF value, which is a relative value when the MTF value of the original image OG is 100%.

  Similar to the kernel boost image BG, the MTF curve of the image SG1 according to the first embodiment has better frequency characteristics than the MTF curve of the original image OG, and it can be seen that the spatial resolution is improved.

  FIG. 8 shows a comparison result of the MTF values in each of the above images. The subject to be photographed in this comparison result is a phantom for measuring spatial resolution. The vertical axis of the graph shown in FIG. 8 represents the spatial resolution (lp / cm) when the relative values of the MTF values are 5%, 10%, and 50%, respectively, and each of the above spatial resolutions in the original image OG. Is a relative value when 100% is assumed.

  From this result, it can be seen that the MTF curve of the image SG1 according to the first embodiment can improve the MTF value more than the original image OG, similar to the kernel boost image BG.

  FIG. 9 shows a comparison result of the noise level (SD value of CT value (HU)) in each of the above images. The subject to be photographed in this comparison result is a phantom for measuring spatial resolution. The vertical axis of the graph shown in FIG. 9 represents the noise level (SD value of the CT value) for the first to third regions of interest ROI1 to ROI3 that are different from each other, and the above noise levels in the original image OG are 100. It is a relative value in the case of%.

  In general, when the spatial resolution represented by the MTF value or the like is improved, the noise level of the image increases and deteriorates. In the case of the kernel boost image BG boosted with the reconstruction function, this example does not leak, and the noise level rises and deteriorates. On the other hand, in the image SG1 according to the first embodiment, although the MTF value indicating the spatial resolution is improved, the noise level is reduced and improved.

  As described above, according to the first embodiment, there is a very excellent effect that it is possible to achieve both contradictory items such as improvement of spatial resolution and reduction of noise level, which are generally in a trade-off relationship. .

  FIG. 10 shows a comparison result of aliasing artifacts in each of the above images. The imaging target in this comparison result is the human head. In the case of a kernel boost image BG simply boosted with a reconstruction function, the aliasing artifact increases, but in the case of the first embodiment, the aliasing artifact is equivalent to the original image. Depending on the situation, there is a case where it is reduced and improved.

  As described above, according to the first embodiment, the enhancement of the edge region is not performed in the reconstruction function or the image space that is difficult to adjust the overcorrection that is the cause of the artifact, but the projection data space in which the overcorrection can be easily adjusted. As a result of unnecessary enhancement of non-high contrast areas on the image and the process of suppressing inherent noise, the non-high contrast areas can be smoothed in the projection data space. In the image obtained by the X-ray CT apparatus, it is possible to improve the spatial resolution of the high contrast region without increasing the noise in the soft tissue region while suppressing artifacts.

  Further, according to the first embodiment, the upper limit value Lt1 and the lower limit value Lt2 are provided when detecting the edge region to be emphasized. Therefore, the overcorrection due to the enhancement of the edge region and the artifacts that accompany it are eliminated. The suppression effect can be enhanced.

(Second embodiment)
FIG. 11 shows a functional block diagram of the central processing unit in the second embodiment. As shown in FIG. 11, in the second embodiment, the central processing unit 3 functionally includes an edge region enhancement unit 311, a first image reconstruction unit 312, a second image reconstruction unit 313, and an image. A synthesis unit 314 is provided.

  The edge region enhancement unit 311 performs edge region enhancement processing for enhancing the edge region in the profile on the X-ray projection data in substantially the same manner as the edge region enhancement unit 301 in the first embodiment. However, when detecting the edge region, the upper limit value Lt1 and the lower limit value Lt2 are not necessarily provided.

  The first image reconstruction unit 312 reconstructs the first image G1 based on the processed X-ray projection data P1 that has been subjected to the edge region enhancement processing.

  The second image reconstruction unit 313 reconstructs the second image G0 based on the X-ray projection data P0 before the edge region enhancement processing is performed.

  In the image region of the first image G1 or the second image G0, the image composition unit 314 allocates the image of the corresponding region of the first image G1 to the high contrast region and the second region to the non-high contrast region. A composition process for filling the image of the corresponding region of the image G0 is performed to obtain a composite image SG2.

  The edge region enhancement unit 311 is an example of enhancement means in the invention, the first image reconstruction unit 312 is an example of first reconstruction means in the invention, and the second image reconstruction unit 313 is The image composition unit 314 is an example of a generation unit according to the invention.

  Hereafter, the flow of processing in the X-ray CT apparatus according to the second embodiment will be described.

  FIG. 12 is a flowchart showing the flow of processing in the X-ray CT apparatus according to the second embodiment. FIG. 13 is a diagram conceptually showing the edge region enhancement processing in the projection data space and the image composition processing (non-high contrast region enhancement relaxation), which are the main parts of the processing.

  In step S11, the subject is scanned to collect a plurality of views of X-ray projection data P0.

  In step S12, the edge region enhancement unit 311 applies to the edge region EP in the profile f0 of the X-ray projection data as shown in FIG. 13A with respect to the collected X-ray projection data P0 of each view. Perform enhancement processing. The edge region enhancement processing here is performed, for example, in at least one profile in the view direction, channel direction, and column direction of the X-ray projection data, as in the first embodiment. However, in the first embodiment, when detecting the edge region EP, an edge having a steep slope is excluded from the target of enhancement processing by providing an upper limit value Lt1 and a lower limit value Lt2, etc. It is not necessary to make such an exclusion.

  As in the first embodiment, when the edge region is enhanced in the projection data space as described above, the high-contrast region in the reconstructed image is further enhanced, and the spatial resolution is improved. Further, compared to the case where edge region enhancement is performed in a reconstruction function or an image space, adjustment of overcorrection is easy, and the occurrence of artifacts associated with the overcorrection can be further suppressed.

  In this example, it is assumed that such edge region detection and enhancement processing is performed for each profile in the view direction, channel direction, and column direction of X-ray projection data.

  By the way, as described in the first embodiment, when the enhancement processing is performed on the edge region EP in the projection data space instead of the image space as described above, there is the above-described advantage, but the characteristics of the image reconstruction method such as back projection are provided. As a result, the soft tissue region developed in the reconstructed image, that is, the non-high contrast region is unnecessarily emphasized, and noise increases.

  As can be seen from FIG. 13A, when the X-ray projection data with the edge region EP enhanced is backprojected, all the data of the emphasized edge region EP overlap with the backprojected path. This is because the image area is reflected, the contrast of the image area is increased, and noise is increased.

  Therefore, in the subsequent steps, in order to suppress this increase in noise, as shown in FIG. 13B, the first reconstructed based on the X-ray projection data P1 subjected to the edge region enhancement processing is performed. In the image G1 or the second image G0 reconstructed based on the X-ray projection data P0 that has not been subjected to the edge region enhancement processing, the high contrast region HM and the non-high contrast region NM are caused by variations in CT values. And ask. The high contrast region HM is filled with the image of the corresponding region of the first image G1 in which the edge region is enhanced, and the edge region enhancement processing is performed on the other non-high contrast regions NM. The image of the corresponding area of the second image G0 that has not been filled is used. As a result, it is possible to prevent an increase in noise in the soft tissue region when developed into a reconstructed image, and it is also possible to reduce the noise depending on the adjustment.

  In step S13, the first image reconstruction unit 312 reconstructs the first image G1 based on the processed X-ray projection data P1 that has been subjected to the edge region enhancement processing. For the reconstruction, for example, a three-dimensional filter back projection method is used.

  In step S14, the second image reconstruction unit 313 reconstructs the second image G0 based on the X-ray projection data P0 before (not performed) the edge region enhancement processing. For the reconstruction, for example, a three-dimensional filter back projection method is used.

  In step S15, the image composition unit 314 determines that the first image G1 and the second image G0 are high-contrast regions or non-high-contrast regions in the image region of the first image G1 or the second image G0. Compositing according to whether it is a region. The high contrast area HM is filled with the image of the corresponding area of the first image G1, and the non-high contrast area NM is filled with the image of the corresponding area of the second image G0. An image SG2 is obtained.

  FIG. 14 shows an example of the synthesis process of the first image and the second image.

  In the example of FIG. 14, first, in the first image G1 or the second image G0, for each local region, an index value σ2 indicating the degree of variation of the CT value, for example, the standard deviation of the CT value per unit area is calculated. To do. Then, a region in which the variation index value σ2 is equal to or less than a desired fourth threshold Th4 (> 0) is detected as a non-high contrast region NM, and the other regions are detected as a high contrast region HM.

  Note that in the CT value profile in one direction on the first image G1 or the second image G0, a standard deviation of the CT value per unit width of a region where the degree of variation of the CT value is not more than a certain level, for example, is desired. Are detected as non-high contrast regions NM and other regions as high contrast regions HM, and such detection is performed in multiple directions on the first image G1 or the second image G0. It is possible to detect the high contrast region and the non-high contrast region two-dimensionally.

  When the high contrast region HM and the non-high contrast region NM are detected, the high contrast region HM is filled with the image of the corresponding region of the first image G1 subjected to the edge region enhancement processing, and the non-high contrast region NM is filled in. The composite image SG2 of the first image G1 and the second image G0 is generated by applying the image of the corresponding region of the second image G0 that has not been subjected to the edge region enhancement processing. Thereby, the unintentional emphasis which appears with respect to the non-high-contrast area | region NM on an image by an edge area | region emphasis process can be suppressed.

  Thus, combining edge region enhancement processing in the projection data space and replacing an image of a non-high contrast region in the image space with an image based on X-ray projection data that has not been subjected to edge region enhancement processing is combined. As a result, it is possible to prevent an increase in noise in the soft tissue region when developed into a reconstructed image, and to reduce noise depending on the adjustment.

  In step S16, the generated composite image SG2 is displayed.

  Hereafter, a comparison result between an image obtained by the method of the second embodiment and an image obtained by another method will be described.

  FIG. 15 shows a comparison result of the MTF curves. The comparison object is the original image OG, the kernel boost image BG, and the image SG2 obtained by the method of the second embodiment, as in the first embodiment. These images are all generated based on the same X-ray projection data. The subject to be photographed in this comparison result is a phantom for measuring spatial resolution. The horizontal axis of the graph shown in FIG. 15 represents the spatial resolution (lp / cm). The vertical axis of the graph represents the MTF value, which is a relative value when the MTF value of the original image OG is 100%.

  As with the kernel boost image BG, the MTF curve of the image SG2 according to the second embodiment has a better frequency characteristic than the MTF curve of the original image OG, and it can be seen that the spatial resolution is improved.

  FIG. 16 shows a comparison result of the MTF values in the above images. The subject to be photographed in this comparison result is a phantom for measuring spatial resolution. The vertical axis of the graph shown in FIG. 16 represents the spatial resolution (lp / cm) when the relative values of the MTF values are 5%, 10%, and 50%, respectively. Is a relative value when 100% is assumed.

  From this result, it can be seen that the MTF curve of the image SG2 according to the second embodiment can improve the MTF value over the original image OG in the same manner as the kernel boost image BG.

  FIG. 17 shows a comparison result of the noise level (SD value of CT value (HU)) in each of the above images. The subject to be photographed in this comparison result is a phantom for measuring spatial resolution. The vertical axis of the graph shown in FIG. 17 represents the noise level (SD value of the CT value) for the first to third regions of interest ROI1 to ROI3 that are different from each other. It is a relative value in the case of%.

  In the case of the kernel boost image BG, the noise level is increased and deteriorated. On the other hand, in the image SG2 according to the second embodiment, although the MTF value indicating the spatial resolution is improved, the noise level is maintained, and is smaller in comparison with the kernel boost image BG. Has been improved.

  As described above, according to the second embodiment, as in the first embodiment, it is possible to achieve both contradictory items such as improvement in spatial resolution and reduction in noise level, which are generally in a trade-off relationship. .

  FIG. 18 shows a comparison result of aliasing artifacts in each of the above images. The imaging target in this comparison result is the human head. In the case of a kernel boost image BG that is simply boosted with a reconstruction function, the aliasing artifact increases, but in the case of the image SG2 according to the second embodiment, the area is the same as in the first embodiment. Singing artifacts are equivalent to the original image OG.

  As described above, according to the second embodiment, the enhancement of the edge region is not performed in the reconstruction function or the image space that is difficult to adjust the overcorrection that is the cause of the artifact, but the projection data space in which the overcorrection can be easily adjusted. In this case, the image is not subjected to edge enhancement in the non-high-contrast area on the image as an unnecessary enhancement of the non-high-contrast area on the image. Can be processed corresponding to the smoothing in the image space, and as a result, in the image obtained by the X-ray CT apparatus, it is possible to suppress the artifact and increase the noise in the soft tissue region without increasing the noise. The spatial resolution of the contrast region can be improved.

  The invention is not limited to these embodiments, and various modifications can be made without departing from the spirit of the invention.

  For example, in the first embodiment, when detecting an edge region to be enhanced, a reconstruction function (kernel) that uses at least one of the threshold values Th1, Th2, Th3, the upper limit value Lt1, and the lower limit value Lt2 for image reconstruction. ). Accordingly, it is possible to optimize the overall balance of a plurality of factors such as noise reduction, artifact suppression, and spatial resolution improvement in consideration of the frequency components of the image that are emphasized or suppressed by the reconstruction function.

  Further, for example, in the second embodiment, when detecting an edge region to be emphasized, an upper limit value Lt1 and a lower limit value Lt2 may be provided as in the first embodiment. In this case, overcorrection by enhancement of the edge region and further suppression of artifacts that accompany it can be expected. Further, at least one of the threshold values Th1, Th2, Th4 (Th4 ′), the upper limit value Lt1, and the lower limit value Lt2 may be adjusted according to a reconstruction function used for image reconstruction. This makes it possible to optimize the overall balance of multiple factors such as noise reduction, artifact suppression, and spatial resolution improvement, taking into account the frequency components of the image that are enhanced or suppressed by the reconstruction function. it can.

  Further, for example, the first and second embodiments are X-ray CT apparatuses, but the invention is not limited to a PET-CT apparatus or a SPECT-CT apparatus in which an X-ray CT apparatus and PET or SPECT are combined, and imaging means. The present invention can also be applied to an image generating apparatus that does not have A program for causing a computer to function as an image generation apparatus having the above-described image generation function, and a storage medium storing the program are also examples of the embodiments of the invention.

DESCRIPTION OF SYMBOLS 1 Operation console 2 Input device 3 Central processing unit 5 Data collection buffer 6 Monitor 7 Memory | storage device 10 Imaging table 12 Cradle 15 Rotating part 20 Scanning gantry 21 X-ray tube 22 X-ray controller 23 Aperture 24 X-ray detector 25 DAS
26 Rotation unit controller 29 Control controller 30 Slip ring 40 Subject 81 X-ray 100 X-ray CT apparatus 301 Edge region enhancement unit 302 Non-high contrast region smoothing unit 303 Image reconstruction unit 311 Edge region enhancement unit 312 First image reconstruction Configuration unit 313 Second image reconstruction unit 314 Image composition unit

Claims (17)

In the process of generating an image from the projection data of the imaging target obtained by X-ray CT imaging,
Performing a process of enhancing an edge region in a profile of data values in a projection data space;
And a step of performing a process of reducing emphasis on an area where the contrast of data values is a certain level or less in the projection data space or the image space. Means for performing processing for emphasizing an edge region in a profile of data values in a projection data space in a process of generating an image from projection data of an imaging target obtained by X-ray CT imaging;
In the process, an image generating apparatus comprising: a unit that performs a process of reducing emphasis on an area in which the contrast of data values is a certain level or less in the projection data space or the image space. Computer
Means for performing processing for emphasizing an edge region in a profile of data values in a projection data space in a process of generating an image from projection data of an imaging target obtained by X-ray CT imaging;
A program for functioning as a means for performing a process of reducing emphasis on an area where the contrast of data values is below a certain level in the projection data space or the image space in the process. An X-ray CT apparatus that performs X-ray CT imaging, collects projection data to be imaged, and generates an image from the projection data,
Means for enhancing an edge region in a profile of data values in a projection data space in the process of generating the image;
In the process, an X-ray CT apparatus comprising: means for performing a process of reducing emphasis on an area where the contrast of data values is below a certain level in the projection data space or the image space. Emphasis means for emphasizing an edge region in the profile of the projection data;
Smoothing means for performing a smoothing process for smoothing an area where the contrast in the profile is a certain level or less;
The X-ray CT apparatus according to claim 4, further comprising: a reconstruction unit that reconstructs an image based on the projection data on which the enhancement process and the smoothing process have been performed.   The X-ray CT apparatus according to claim 5, wherein the enhancement process is a process of enhancing only an edge region whose edge sharpness is within a predetermined range.   The enhancement processing includes a region where a data value in a profile representing a high frequency component of the profile of the projection data is not less than a first positive threshold value and not more than a desired upper limit value, and the data value is not more than a second negative threshold value. The X-ray CT apparatus according to claim 6, wherein the X-ray CT apparatus is a process for emphasizing a region that is greater than a desired lower limit value.   The X-ray CT apparatus according to claim 5, wherein the enhancement process is performed in at least one profile of a channel direction, a column direction, and a view direction of the projection data.   6. The smoothing process is a process of smoothing a region in which a data value in a profile representing a high frequency component of the profile of the projection data is equal to or less than a positive third threshold value and equal to or greater than a negative fourth threshold value. The X-ray CT apparatus according to claim 8.   The X-ray CT apparatus according to claim 7, wherein the smoothing process is performed in at least one profile of a channel direction, a column direction, and a view direction of the projection data. Emphasis means for emphasizing an edge region in the profile of the projection data;
First reconstruction means for reconstructing a first image based on the projection data subjected to the enhancement processing;
Second reconstruction means for reconstructing a second image based on the projection data not subjected to the enhancement processing;
In the first image or the second image, a second region in which the image in the corresponding region of the first image is filled in the first region where the contrast exceeds a certain level, and the contrast is equal to or less than the certain level. The X-ray CT apparatus according to claim 4, further comprising: a generating unit configured to generate an image filled with an image of a corresponding region of the second image.   The enhancement process is a process of emphasizing a region where a data value in a profile representing a high frequency component of the projection data is greater than or equal to a positive first threshold and a region where the data value is equal to or less than a negative second threshold. The X-ray CT apparatus according to claim 11.   The X-ray CT apparatus according to claim 11, wherein the enhancement process is performed in at least one profile of a channel direction, a column direction, and a view direction of the projection data. The first region is a region where the degree of variation in pixel values in the first image or the second image exceeds a certain level,
The X-ray CT apparatus according to claim 11, wherein the second region is a region where the degree of variation is equal to or less than the certain level. The first region is a region where a data value in a profile representing a high-frequency component in a desired direction of the first image or the second image exceeds a certain level,
The X-ray CT apparatus according to claim 11, wherein the second region is a region where the data value is equal to or less than the certain level.   The X-ray CT apparatus according to claim 7 or 12, wherein the first and second threshold values change according to a type of a reconstruction function used by the first reconstruction unit for image reconstruction.   The X-ray CT apparatus according to claim 5, wherein the enhancement process is a sharpening process.