X-ray CT apparatus and tomographic image generation method

Abstract

To provide an X-ray CT apparatus and a tomographic image generation method capable of suppressing cone beam artifacts and noise without acquiring projection data by a plurality of energy bins.SOLUTION: An X-ray CT apparatus includes: a data acquisition unit for acquiring first projection data corresponding to a first projection angle range, and second projection data corresponding to a second projection angle range, which is a projection angle range wider than the first projection angle range; an image correction unit for reducing a high frequency component of a first tomographic image reconstructed from the first projection data to generate a first correction image, and reducing a low frequency component of a second tomographic image reconstructed from the second projection data to generate a second correction image; and an image composition unit for compositing the first correction image and the second correction image.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a technique for handling tomographic images obtained by an X-ray CT (Computed Tomography) apparatus, and more particularly to a technique for suppressing artifacts in tomographic images.

Background Art

[0002] An X-ray CT apparatus is a device that reconstructs tomographic images using projection data acquired at a plurality of projection angles by rotating an X-ray source that irradiates a subject with X-rays and a detector that detects the X-rays transmitted through the subject around the subject. The reconstructed tomographic images are used for image diagnosis as medical images. In a cone beam CT apparatus with X-ray irradiation at a wide cone angle and multi-columnation of the detector in the body axis direction of the subject, although the time for acquiring projection data can be shortened, cone beam artifacts that interfere with image diagnosis may occur.

[0003] Patent Document 1 discloses a method and system for suppressing cone beam artifacts in a wide cone angle spectral CT apparatus without increasing noise. Specifically, it discloses decomposing projection data acquired at a plurality of energy bins in the projection domain and unevenly dispersing noise and inconsistencies leading to cone beam artifacts for a plurality of sinograms obtained by the decomposition.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, Patent Document 1 does not consider an X-ray CT apparatus that cannot acquire projection data with a plurality of energy bins. To acquire projection data with a plurality of energy bins, a dual-source CT apparatus, a photon-counting CT apparatus, or the like is required.

[0006] Therefore, an object of the present invention is to provide an X-ray CT apparatus and a tomographic image generation method capable of suppressing cone-beam artifacts and noise without acquiring projection data with a plurality of energy bins.

Means for Solving the Problems

[0007] To achieve the above object, the present invention provides a data acquisition unit that acquires first projection data corresponding to a first projection angle range and second projection data corresponding to a second projection angle range that is wider than the first projection angle range; an image correction unit that reduces high-frequency components of a first tomographic image reconstructed from the first projection data to generate a first corrected image, and reduces low-frequency components of a second tomographic image reconstructed from the second projection data to generate a second corrected image; and an image synthesis unit that synthesizes the first corrected image and the second corrected image. The X-ray CT apparatus is characterized by comprising these components.

[0008] The present invention also provides a data acquisition unit that acquires first projection data corresponding to a first projection angle range and second projection data corresponding to a second projection angle range that is wider than the first projection angle range; a data correction unit that reduces high-frequency components of the first projection data to generate first corrected data, and reduces low-frequency components of the second projection data to generate second corrected data; and a reconstruction unit that reconstructs a tomographic image from combined data obtained by combining the first corrected data and the second corrected data. The X-ray CT apparatus is characterized by comprising these components.

[0009] The present invention also provides a tomographic image generation method, comprising: a data acquisition step of acquiring first projection data corresponding to a first projection angle range and second projection data corresponding to a second projection angle range that is wider than the first projection angle range; an image correction step of reducing high-frequency components of a first tomographic image reconstructed from the first projection data to generate a first corrected image, and reducing low-frequency components of a second tomographic image reconstructed from the second projection data to generate a second corrected image; and an image synthesis step of synthesizing the first corrected image and the second corrected image.

Advantages of the Invention

[0010] According to the present invention, it is possible to provide an X-ray CT apparatus and a tomographic image generation method capable of suppressing cone beam artifacts and noise without acquiring projection data using a plurality of energy bins.

Brief Description of the Drawings

[0011]

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Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the X-ray CT apparatus and the tomographic image generation method according to the present invention will be described with reference to the accompanying drawings. In the following description and the accompanying drawings, components having the same functional configuration will be denoted by the same reference numerals, and redundant description will be omitted.

Embodiment

[0013] The overall configuration of the X-ray CT apparatus 100 will be described with reference to FIG. 1. In FIG. 1, the horizontal direction is the X-axis, the vertical direction is the Y-axis, and the direction perpendicular to the paper surface is the Z-axis. The X-ray CT apparatus 100 includes a scanner 200 and an operation unit 250. The scanner 200 includes an X-ray tube 211, a detector 212, a collimator 213, a drive unit 214, a central control unit 215, an X-ray control unit 216, a high voltage generation unit 217, a scanner control unit 218, a bed control unit 219, a collimator control unit 221, a preamplifier 222, an A / D converter 223, a bed 240, and the like.

[0014] The X-ray tube 211 is a device that irradiates an object 210 placed on the bed 240 with X-rays. X-rays are irradiated from the X-ray tube 211 to the object when a high voltage generated by the high voltage generation unit 217 is applied to the X-ray tube 211 according to a control signal transmitted from the X-ray control unit 216.

[0015] The collimator 213 is a device that limits the irradiation range of the X-rays irradiated from the X-ray tube 211. The irradiation range of the X-rays is set according to a control signal transmitted from the collimator control unit 221.

[0016] The detector 212 is a device that measures the spatial distribution of the transmitted X-rays by detecting the X-rays transmitted through the object 210. The detector 212 is disposed opposite to the X-ray tube 211, and a large number of detection elements are two-dimensionally arranged in the plane facing the X-ray tube 211. The signal measured by the detector 212 is amplified by the preamplifier 222 and then converted into a digital signal by the A / D converter 223. Thereafter, various correction processes are performed on the digital signal, and projection data is acquired.

[0017] The drive unit 214 rotates the X-ray tube 211 and the detector 212 around the subject 210 according to the control signal transmitted from the scanner control unit 218. As the X-ray tube 211 and the detector 212 rotate, X-rays are irradiated and detected, thereby acquiring projection data from a plurality of projection angles. The data collection unit for each projection angle is called a view. The arrangement of each detection element of the two-dimensionally arranged detector 212 is such that the rotation direction of the detector 212 is called a channel, and the direction orthogonal to the channel is called a column. As illustrated in FIG. 2, the projection data is identified by a view, a channel, and a column. The opening angle of the X-rays irradiated from the X-ray tube 211 in the channel direction is called a fan angle.

[0018] The bed control unit 219 controls the operation of the bed 240, and keeps the bed 240 stationary or moves it at a constant speed in the Z-axis direction, which is the body axis direction of the subject 210, while X-rays are irradiated and detected. The scan with the bed 240 stationary is called an axial scan, and the scan while moving the bed 240 is called a helical scan, respectively.

[0019] The central control unit 215 is a device that controls the operation of the scanner 200 described above according to an instruction from the operation unit 250, and specifically includes a CPU (Central Processing Unit), an MPU (Micro Processor Unit), etc.

[0020] The operation unit 250 will be described. The operation unit 250 includes an image generation unit 251, an image processing unit 252, a storage unit 254, a display unit 256, an input unit 258, etc.

[0021] The image generation unit 251 is a device that reconstructs a tomographic image using the projection data acquired by the scanner 200, and specifically includes a CPU, a GPU (Graphical Processing Unit), etc. The image processing unit 252 is a device that performs various image processes to make the tomographic image suitable for diagnosis, and specifically includes a CPU, a GPU, etc.

[0022] The memory unit 254 is a device that stores projection data, tomographic images, and images after image processing, specifically, an HDD (Hard Disk Drive), an SSD (Solid State Drive), etc. The display unit 256 is a device that displays tomographic images and images after image processing, specifically, a liquid crystal display, etc. The input unit 258 is a device used when an operator sets acquisition conditions (tube voltage, tube current, scan speed, etc.) of projection data and reconstruction conditions (reconstruction filter, FOV size, etc.) of tomographic images, specifically, a keyboard, a mouse, a touch panel, etc. The mouse may be other pointing devices such as a track pad or a track ball.

[0023] Using FIG. 3, an example of the flow of the process executed in the first embodiment will be described step by step.

[0024] (S301) The image generation unit 251 acquires projection data. The projection data may be calculated based on the detector output output by the detector 212, or may be read from the memory unit 254.

[0025] (S302) The image generation unit 251 reconstructs a tomographic image using the projection data acquired in S301. Note that the tomographic image reconstructed in S302 is corrected so as to suppress cone beam artifacts and noise.

[0026] Using FIG. 4, an example of the flow of the corrected image reconstruction process of S302 will be described step by step.

[0027] (S401) The image generation unit 251 acquires half-scan data and full-scan data from the projection data acquired in S301. The half-scan data is projection data with a projection angle range of 180 degrees or more and less than 360 degrees. For example, when the fan angle of the X-ray CT apparatus 100 is φ, the projection data has a projection angle range of 180 degrees + φ. The full-scan data is projection data with a projection angle range wider than that of the half-scan data. For example, the projection data has a projection angle range wider than 180 degrees + φ. An example of the half-scan data and the full-scan data is shown in FIG. 5. In the half-scan data and the full-scan data illustrated in FIG. 5, the centers of the views coincide.

[0028] (S402) The image generation unit 251 reconstructs a half-scan image as a first tomographic image by back-projecting the half-scan data acquired in S401.

[0029] (S403) The image generation unit 251 generates a first corrected image by reducing the high-frequency components of the half-scan image reconstructed in S402. For reducing the high-frequency components, an arbitrary low-pass filter, for example, a Gaussian filter, is used. Note that the high-frequency components and the low-frequency components are separated, for example, by the Nyquist frequency of the X-ray CT apparatus.

[0030] Although the half-scan image has relatively few cone-beam artifacts but relatively much noise, the first corrected image generated by reducing the high-frequency components is an image with relatively few cone-beam artifacts and suppressed noise.

[0031] (S404) The image generation unit 251 reconstructs a full-scan image as a second tomographic image by back-projecting the full-scan data acquired in S401.

[0032] (S405) The image generation unit 251 generates a second corrected image by reducing the low-frequency components of the full-scan image reconstructed in S404. An arbitrary high-pass filter is used to reduce the low-frequency components. For example, the second corrected image may be generated by subtracting from the full-scan image an image obtained by using a Gaussian filter for the full-scan image. When a Gaussian filter is used to reduce the high-frequency components in S403, the same Gaussian filter is also used to reduce the low-frequency components in S405.

[0033] Although the full-scan image has relatively little noise but has relatively many cone-beam artifacts, the second corrected image generated by reducing the low-frequency components is an image with relatively little noise and the cone-beam artifacts suppressed.

[0034] (S406) The image generation unit 251 synthesizes the first corrected image generated in S403 and the second corrected image generated in S405. For example, a synthesized image is generated by adding the first corrected image and the second corrected image. Note that the synthesized image may also be generated by weighted addition of the first corrected image and the second corrected image. The weighting coefficients used for weighted addition may be preset for each part.

[0035] By the processing flow illustrated in FIG. 4, the first corrected image with relatively few cone-beam artifacts and suppressed noise and the second corrected image with relatively little noise and suppressed cone-beam artifacts are synthesized. As a result, a synthesized image with the cone-beam artifacts and noise suppressed can be generated. Return to the description of FIG. 3.

[0036] (S303) The image generation unit 251 causes the display unit 256 to display the tomographic image generated in S302. The tomographic image displayed on the display unit 256 is used for image diagnosis.

[0037] By the processing flow illustrated in FIG. 3, a tomographic image with cone beam artifacts and noise suppressed can be displayed on the display unit 256, so that image diagnosis can proceed smoothly.

Example

[0038] In the first embodiment, corrected image reconstruction by synthesizing a half-scan image with reduced high-frequency components and a full-scan image with reduced low-frequency components was described. The corrected image reconstruction is not limited to the processing flow illustrated in FIG. 4. In the second embodiment, corrected image reconstruction by reconstructing scan data obtained by synthesizing half-scan data with reduced high-frequency components and full-scan data with reduced low-frequency components will be described. Note that since the processing flow other than the corrected image reconstruction processing flow is the same as that in the first embodiment, the description thereof will be omitted.

[0039] An example of the processing flow of the corrected image reconstruction in the second embodiment will be described step by step with reference to FIG. 6.

[0040] (S601) Similar to S401, the image generation unit 251 acquires half-scan data and full-scan data from the projection data acquired in S301.

[0041] (S602) The image generation unit 251 generates first correction data by reducing the high-frequency components of the half-scan data acquired in S601. The reduction of the high-frequency components is performed for each view, and an arbitrary low-pass filter is used.

[0042] (S603) The image generation unit 251 generates second correction data by reducing the low-frequency components of the full-scan data acquired in S601. The reduction of the low-frequency components is performed for each view, and an arbitrary high-pass filter is used. However, the high-pass filter and the low-pass filter are set so that their sum becomes 1 when the high-pass filter used in S603 and the low-pass filter used in S602 are added.

[0043] (S604) The image generation unit 251 synthesizes the first correction data generated in S602 and the second correction data generated in S603. For example, the synthetic data is generated by adding the first correction data and the second correction data. Note that the synthetic data may be generated by weighted addition of the first correction data and the second correction data. The weighting factor used for weighted addition may be set in advance for each part.

[0044] (S605) The image generation unit 251 reconstructs a tomographic image by back-projecting the synthetic data generated in S604.

[0045] By the processing flow illustrated in FIG. 6, a tomographic image with cone beam artifacts and noise suppressed can be reconstructed. Note that in the processing flow of FIG. 6, the number of back-projections is one, which is less than two times in FIG. 4, so the calculation time can be shortened. However, in the processing flow of FIG. 4, high-frequency components and low-frequency components are reduced for the tomographic image, and the calculation can be performed with less memory than reducing high-frequency components and low-frequency components for the projection data as in FIG. 6.

Example

[0046] In the first embodiment, the corrected image reconstruction by synthesizing a half-scan image with reduced high-frequency components and a full-scan image with reduced low-frequency components was described. In the third embodiment, a full-scan pseudo-image corresponding to a full-scan image is generated using two half-scan images, and the use of the full-scan pseudo-image with reduced low-frequency components for corrected image reconstruction will be described. Note that since it is the same as the first embodiment except for the processing flow of the corrected image reconstruction, the description is omitted.

[0047] An example of the processing flow of the corrected image reconstruction in the third embodiment will be described step by step with reference to FIG. 7.

[0048] (S701) The image generation unit 251 acquires first half-scan data and second half-scan data from the projection data acquired in S301. An example of the first half-scan data and the second half-scan data will be described with reference to FIG. 8. The first half-scan data is projection data with a projection angle range of 180 degrees or more and less than 360 degrees. The second half-scan data is projection data obtained by removing the first half-scan data from the full-scan data.

[0049] (S702) The image generation unit 251 reconstructs a first half-scan image as a first tomographic image by back-projecting the first half-scan data acquired in S701.

[0050] (S703) The image generation unit 251 generates a first corrected image by reducing the high-frequency components of the first half-scan image reconstructed in S702. For reducing the high-frequency components, an arbitrary low-pass filter, for example, a Gaussian filter, is used.

[0051] Although the first half-scan image has relatively few cone-beam artifacts but relatively much noise, the first corrected image generated by reducing the high-frequency components is an image with relatively few cone-beam artifacts and suppressed noise.

[0052] (S704) The image generation unit 251 reconstructs a second half-scan image as a third tomographic image by back-projecting the second half-scan data acquired in S701.

[0053] (S705) The image generation unit 251 generates a full-scan pseudo-image, which is an image corresponding to the full-scan image, by synthesizing the first half-scan image reconstructed in S702 and the second half-scan image reconstructed in S704. For example, the full-scan pseudo-image is generated by adding the first half-scan image and the second half-scan image.

[0054] (S706) The image generation unit 251 generates a second corrected image by reducing the low-frequency components of the full-scan pseudo-image generated in S705. An arbitrary high-pass filter is used to reduce the low-frequency components. For example, the second corrected image may be generated by subtracting from the full-scan pseudo-image an image obtained by using a Gaussian filter on the full-scan pseudo-image. If a Gaussian filter was used to reduce the high-frequency components in S703, the same Gaussian filter is also used to reduce the low-frequency components in S706.

[0055] Although the full-scan pseudo-image has relatively little noise but has relatively many cone-beam artifacts, the second corrected image generated by reducing the low-frequency components is an image with relatively little noise and the cone-beam artifacts suppressed.

[0056] (S707) The image generation unit 251 synthesizes the first corrected image generated in S703 and the second corrected image generated in S706. For example, the composite image is generated by adding the first corrected image and the second corrected image. Note that the composite image may also be generated by weighted addition of the first corrected image and the second corrected image. The weighting coefficients used for weighted addition may be preset for each part.

[0057] By the processing flow illustrated in FIG. 7, a first corrected image with relatively few cone beam artifacts and suppressed noise and a second corrected image with relatively little noise and suppressed cone beam artifacts are combined. As a result, a composite image with suppressed cone beam artifacts and noise can be generated. In the processing flow of FIG. 7, although the number of backprojections is two, both are backprojections of half-scan data, so the calculation time can be shortened compared to FIG. 4 that backprojects half-scan data and full-scan data.

[0058] As described above, embodiments of the present invention have been described. Note that the present invention is not limited to the above embodiments, and components can be modified and embodied without departing from the gist of the invention. Also, a plurality of components disclosed in the above embodiments may be appropriately combined. Furthermore, some components may be deleted from all the components shown in the above embodiments.

Explanation of Reference Numerals

[0059] 100: X-ray CT apparatus, 200: Scanner, 210: Subject, 211: X-ray tube, 212: Detector, 213: Collimator, 214: Driving unit, 215: Central control unit, 216: X-ray control unit, 217: High voltage generator, 218: Scanner control unit, 219: Bed control unit, 221: Collimator control unit, 222: Preamplifier, 223: A / D converter, 240: Bed, 250: Operation unit, 251: Image generation unit, 252: Image processing unit, 254: Storage unit, 256: Display unit, 258: Input unit

Claims

1. A data acquisition unit that acquires first projection data corresponding to a first projection angle range and second projection data corresponding to a second projection angle range that is wider than the first projection angle range; An image correction unit that reduces high-frequency components of a first tomographic image reconstructed from the first projection data to generate a first corrected image, and reduces low-frequency components of a second tomographic image reconstructed from the second projection data to generate a second corrected image; An X-ray CT apparatus comprising an image synthesis unit that synthesizes the first corrected image and the second corrected image.

2. A data acquisition unit that acquires first projection data corresponding to a first projection angle range and second projection data corresponding to a second projection angle range that is wider than the first projection angle range; A data correction unit that reduces high-frequency components of the first projection data to generate first corrected data, and reduces low-frequency components of the second projection data to generate second corrected data; An X-ray CT apparatus comprising a reconstruction unit that reconstructs a tomographic image from combined data obtained by combining the first corrected data and the second corrected data.

3. The X-ray CT apparatus according to claim 1, wherein the data acquisition unit further acquires third projection data from which the first projection data has been removed from the second projection data, and the image correction unit generates the second corrected image by synthesizing a third tomographic image reconstructed from the third projection data and the first tomographic image.

4. A data acquisition step of acquiring first projection data corresponding to a first projection angle range and second projection data corresponding to a second projection angle range that is wider than the first projection angle range; Reducing the high-frequency components of the first tomographic image reconstructed from the first projection data to generate a first corrected image, and reducing the low-frequency components of the second tomographic image reconstructed from the second projection data to generate a second corrected image; an image correction step A tomographic image generation method comprising an image synthesis step of synthesizing the first corrected image and the second corrected image.

5. The tomographic image generation method according to claim 4, In the data acquisition step, third projection data from which the first projection data has been removed from the second projection data is further acquired, In the image correction step, the second corrected image is generated by synthesizing the third tomographic image reconstructed from the third projection data and the first tomographic image. A tomographic image generation method characterized by this.

Images