X-ray CT apparatus and tomographic image generation method

Abstract

The invention provides an X-ray CT device and a tomographic image generation method which can suppress cone beam artifacts and noise without acquiring projection data by using a plurality of energy bins. This X-ray CT device is characterized by being provided with: 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, which is a projection angle range wider than the first projection angle range; an image correction unit that generates a first corrected image by reducing a high-frequency component of a first tomographic image reconstructed from the first projection data, and generates a second corrected image by reducing a low-frequency component of a second tomographic image reconstructed from the second projection data; and an image synthesis unit that synthesizes the first corrected image and the second corrected image.

Description

Technical Field

[0001] The present invention relates to a technique for processing tomographic images obtained by an X-ray CT (Computed Tomography) apparatus, and also relates 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 that performs X-ray irradiation at a wide cone angle and multi-columnarizes the detector in the body axis direction of the subject, the time for acquiring projection data can be shortened, but cone-beam artifacts that hinder image diagnosis sometimes occur.

[0003] In Patent Document 1, a method and system for suppressing cone-beam artifacts without increasing noise in a wide cone angle spectral CT apparatus are disclosed. Specifically, the following is disclosed: The projection data acquired using a plurality of energy bins is decomposed in the projection domain, and the noise or inconsistency that causes cone-beam artifacts is unevenly dispersed in the plurality of sinograms obtained by the decomposition.

[0004] Patent Document 1: Japanese Patent Application Laid-Open No. 2023-508147

[0005] However, in Patent Document 1, an X-ray CT apparatus that cannot acquire projection data using a plurality of energy bins is not considered. When acquiring projection data using a plurality of energy bins, a dual-source CT apparatus or a photon-counting CT apparatus, etc. is required. Summary of the Invention

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

[0007] To achieve the above object, the present invention provides an X-ray CT apparatus, characterized by comprising: 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 a projection angle range 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.

[0008] Moreover, the present invention relates to an X-ray CT apparatus, characterized by comprising: 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 based on combined data obtained by combining the first corrected data and the second corrected data.

[0009] Furthermore, the present invention relates to a method for generating a tomographic image, characterized by 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.

[0010] Advantages of the Invention

[0011] According to the present invention, it is possible to provide an X-ray CT apparatus and a method for generating a tomographic image that can suppress cone-beam artifacts and noise without using multiple energy bins to acquire projection data. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is an overall structural diagram of the X-ray CT apparatus.

[0013] Figure 2 is a diagram showing the structure of the projection data.

[0014] Figure 3 is a diagram showing an example of the processing flow of Example 1.

[0015] Figure 4 is a diagram showing an example of the processing flow of image reconstruction with correction in Example 1.

[0016] Figure 5 is a diagram showing an example of half-scan data and full-scan data.

[0017] Figure 6 is a diagram showing an example of the processing flow of image reconstruction with correction in Example 2.

[0018] Figure 7 is a diagram showing an example of the processing flow of image reconstruction with correction in Example 3.

[0019] Figure 8 This is a diagram showing an example of first half-scan data and second half-scan data.

[0020] Symbol Explanation

[0021] 100 - X-ray CT apparatus, 200 - Scanner, 210 - Subject, 211 - X-ray tube, 212 - Detector, 213 - Collimator, 214 - Drive unit, 215 - Central control unit, 216 - X-ray control unit, 217 - High voltage generation unit, 218 - Scanner control unit, 219 - Table control unit, 221 - Collimator control unit, 222 - Preamplifier, 223 - A / D converter, 240 - Table, 250 - Operation unit, 251 - Image generation unit, 252 - Image processing unit, 254 - Storage unit, 256 - Display unit, 258 - Input unit. Detailed Embodiment

[0022] 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 drawings, components having the same functional structure are denoted by the same reference numerals, and redundant description is thus omitted.

[0023] [Embodiment 1]

[0024] Use Figure 1 , the overall structure of the X-ray CT apparatus 100 will be described. In Figure 1 , the horizontal direction is set as the X-axis, the vertical direction is set as the Y-axis, and the direction perpendicular to the paper surface is set as 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 table control unit 219, a collimator control unit 221, a preamplifier 222, an A / D converter 223, and a table 240, etc.

[0025] The X-ray tube 211 is a device that irradiates X-rays onto the subject 210 placed on the table 240. The high voltage generated by the high voltage generation unit 217 is applied to the X-ray tube 211 according to a control signal sent from the X-ray control unit 216, and thus X-rays are irradiated from the X-ray tube 211 onto the subject.

[0026] 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 sent from the collimator control unit 221.

[0027] The detector 212 is a device that measures the spatial distribution of the transmitted X-rays by detecting the X-rays that have passed through the subject 210. The detector 212 is arranged opposite to the X-ray tube 211, and a plurality of detection elements are two-dimensionally arranged in the plane opposite to 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. After that, various correction processes are performed on the digital signal to obtain projection data.

[0028] The drive unit 214 rotates the X-ray tube 211 and the detector 212 around the subject 210 according to the control signal sent from the scanner control unit 218. By irradiating and detecting X-rays as the X-ray tube 211 and the detector 212 rotate, projection data is obtained from a plurality of projection angles. The data collection unit for each projection angle is called a view. In the arrangement of the respective detection elements of the two-dimensionally arranged detector 212, the rotation direction of the detector 212 is called the channel, and the direction orthogonal to the channel is called the column. As Figure 2 illustrated, the projection data is identified by the view, the channel, and the column. And, the opening angle in the channel direction of the X-rays irradiated from the X-ray tube 211 is called the fan angle.

[0029] The bed control unit 219 controls the movement of the bed 240, and during the irradiation and detection of X-rays, 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. The scan performed with the bed 240 kept stationary is called an axial scan, and the scan performed while moving the bed 240 is called a helical scan.

[0030] The central control unit 215 is a device that controls the operation of the above-described scanner 200 according to an instruction from the operation unit 250. Specifically, it is a CPU (Central Processing Unit), an MPU (Micro Processor Unit), etc.

[0031] 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.

[0032] The image generation unit 251 is a device that reconstructs a tomographic image using the projection data acquired by the scanner 200. Specifically, it is a CPU or a GPU (Graphics Processing Unit), etc. The image processing unit 252 is a device that performs various image processing operations to make the tomographic image suitable for diagnosis. Specifically, it is a CPU or a GPU, etc.

[0033] The storage unit 254 is a device for storing projection data, tomographic images, or images after image processing, specifically an HDD (Hard Disk Drive) or an SSD (Solid State Drive), etc. The display unit 256 is a device for displaying tomographic images or 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, scanning speed, etc.) of projection data or reconstruction conditions (reconstruction filter, FOV size, etc.) of tomographic images, specifically a keyboard, a mouse, a touch panel, etc. The mouse may be another pointing device such as a trackpad or a trackball.

[0034] Use Figure 3 , and an example of the processing flow executed in Example 1 will be described for each step.

[0035] (S301)

[0036] The image generation unit 251 acquires projection data. The projection data can be calculated based on the detector output output by the detector 212, or can be read from the storage unit 254.

[0037] (S302)

[0038] The image generation unit 251 uses the projection data acquired in S301 to reconstruct a tomographic image. In addition, the tomographic image reconstructed in S302 is corrected to suppress cone-beam artifacts and noise.

[0039] Use Figure 4 , and an example of the processing flow of image reconstruction with correction in S302 will be described for each step.

[0040] (S401)

[0041] The image generation unit 251 obtains 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 with a projection angle range of 180 degrees + φ. The full-scan data is projection data with a projection angle range wider than the half-scan data, for example, projection data with a projection angle range wider than 180 degrees + φ. An example of the half-scan data and the full-scan data is shown in Figure 5 . In the half-scan data and the full-scan data exemplified in Figure 5 , the centers of the views are the same.

[0042] (S402)

[0043] 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.

[0044] (S403)

[0045] The image generation unit 251 generates a first corrected image by reducing the high-frequency components of the half-scan image reconstructed in S402. In reducing the high-frequency components, an arbitrary low-pass filter, such as a Gaussian filter, is used. Additionally, the high-frequency components and the low-frequency components are divided, for example, by the Nyquist frequency of the X-ray CT apparatus.

[0046] The cone-beam artifacts of the half-scan image are relatively few, but the noise is relatively much. Therefore, the first corrected image generated by reducing the high-frequency components becomes an image with relatively few cone-beam artifacts and suppressed noise.

[0047] (S404)

[0048] 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.

[0049] (S405)

[0050] The image generation unit 251 generates a second corrected image by reducing the low-frequency components of the full-scan image reconstructed in S404. In reducing the low-frequency components, an arbitrary high-pass filter is used. For example, the second corrected image can also be generated by subtracting the image obtained by applying a Gaussian filter to the full-scan image from the full-scan image. Additionally, when a Gaussian filter is used in reducing the high-frequency components in S403, the same Gaussian filter is also used in reducing the low-frequency components in S405.

[0051] The noise of the full-scan image is relatively few, but the cone-beam artifacts are relatively much. Therefore, the second corrected image generated by reducing the low-frequency components becomes an image with relatively little noise and suppressed cone-beam artifacts.

[0052] (S406)

[0053] 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. Additionally, a synthesized image can also be generated by adding the first corrected image and the second corrected image with weighting. The weight coefficients for weighted addition can be set in advance for each part.

[0054] By Figure 4 the processing flow exemplified in, 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 suppressed cone-beam artifacts and noise can be generated. Return to Figure 3 the description of.

[0055] (S303)

[0056] The image generation unit 251 causes the tomographic image generated in S302 to be displayed on the display unit 256. The tomographic image displayed on the display unit 256 is used for image diagnosis.

[0057] Through Figure 3 the processing flow exemplified in, a tomographic image in which cone-beam artifacts and noise are suppressed can be displayed on the display unit 256, and thus image diagnosis can be smoothly performed.

[0058] [Example 2]

[0059] In Example 1, band-corrected image reconstruction based on the synthesis of a half-scan image with reduced high-frequency components and a full-scan image with reduced low-frequency components was described. The band-corrected image reconstruction is not limited to Figure 4 the processing flow exemplified in. In Example 2, band-corrected image reconstruction based on the reconstruction of scan data obtained by synthesizing half-scan data with reduced high-frequency components and full-scan data with reduced low-frequency components is described. In addition, except for the processing flow of the band-corrected image reconstruction, it is the same as in Example 1, and thus the description is omitted.

[0060] Using Figure 6 , an example of the processing flow of the band-corrected image reconstruction of Example 2 is described step by step.

[0061] (S601)

[0062] Similar to S401, the image generation unit 251 obtains half-scan data and full-scan data from the projection data obtained in S301.

[0063] (S602)

[0064] The image generation unit 251 generates first correction data by reducing the high-frequency components of the half-scan data obtained in S601. The reduction of the high-frequency components is performed for each view, and an arbitrary low-pass filter is used.

[0065] (S603)

[0066] The image generation unit 251 generates second correction data by reducing the low-frequency components of the full-scan data obtained in S601. The reduction of the low-frequency components is performed for each view, and an arbitrary high-pass filter is used. Among them, the high-pass filter and the low-pass filter are set such that their sum becomes 1 when the high-pass filter used in S603 and the low-pass filter used in S602 are added together.

[0067] (S604)

[0068] The image generation unit 251 synthesizes the first correction data generated in S602 and the second correction data generated in S603. For example, the synthesized data is generated by adding the first correction data and the second correction data. Alternatively, the synthesized data may be generated by weighted addition of the first correction data and the second correction data. The weight coefficients for weighted addition may be set in advance for each part.

[0069] (S605)

[0070] The image generation unit 251 reconstructs a tomographic image by back-projecting the synthesized data generated in S604.

[0071] By Figure 6 the processing flow exemplified in, a tomographic image with suppressed cone-beam artifacts and noise can be reconstructed. Additionally, in Figure 6 the processing flow, the number of back-projections is 1, which is less than 2 in Figure 4 , so the operation time can be shortened. However, in Figure 4 the processing flow, the high-frequency components or low-frequency components of the tomographic image are reduced, and compared with reducing the high-frequency components or low-frequency components of the projection data as in Figure 6 , the operation can be performed with less memory.

[0072] [Embodiment 3]

[0073] In Embodiment 1, the corrected image reconstruction based on the synthesis of a half-scan image with reduced high-frequency components and a full-scan image with reduced low-frequency components was described. In Embodiment 3, a case where a full-scan pseudo-image equivalent to a full-scan image is generated using two half-scan images and the full-scan pseudo-image with reduced low-frequency components is used for corrected image reconstruction is described. Additionally, except for the processing flow of corrected image reconstruction, it is the same as in Embodiment 1, so the description is omitted.

[0074] Using Figure 7 , an example of the processing flow of corrected image reconstruction in Embodiment 3 is described step by step.

[0075] (S701)

[0076] The image generation unit 251 obtains first half-scan data and second half-scan data from the projection data acquired in S301. Using Figure 8 , an example of the first half-scan data and the second half-scan data is described. 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.

[0077] (S702)

[0078] The image generation unit 251 reconstructs the first half-scan image as the first tomographic image by back-projecting the first half-scan data acquired in S701.

[0079] (S703)

[0080] 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. In reducing the high-frequency components, an arbitrary low-pass filter, such as a Gaussian filter, is used.

[0081] The cone-beam artifacts of the first half-scan image are relatively few, but the noise is relatively much. Therefore, the first corrected image generated by reducing the high-frequency components becomes an image with relatively few cone-beam artifacts and suppressed noise.

[0082] (S704)

[0083] The image generation unit 251 reconstructs the second half-scan image as the third tomographic image by back-projecting the second half-scan data acquired in S701.

[0084] (S705)

[0085] The image generation unit 251 generates an image equivalent to the full-scan image, i.e., a full-scan pseudo-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.

[0086] (S706)

[0087] 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. In reducing the low-frequency components, an arbitrary high-pass filter is used. For example, the second corrected image can also be generated by subtracting the image obtained by applying a Gaussian filter to the full-scan pseudo-image from the full-scan pseudo-image. Additionally, when a Gaussian filter is used in reducing the high-frequency components in S703, the same Gaussian filter is also used in reducing the low-frequency components in S706.

[0088] The noise of the full-scan pseudo-image is relatively few, but the cone-beam artifacts are relatively much. Therefore, the second corrected image generated by reducing the low-frequency components becomes an image with relatively few noise and suppressed cone-beam artifacts.

[0089] (S707)

[0090] The image generation unit 251 synthesizes the first corrected image generated in S703 and the second corrected image generated in S706. For example, a synthesized image is generated by adding the first corrected image and the second corrected image. Alternatively, a synthesized image may be generated by weighted addition of the first corrected image and the second corrected image. The weight coefficients for weighted addition may be preset for each part.

[0091] By Figure 7 the processing flow exemplified in, 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 suppressed cone-beam artifacts and noise can be generated. Additionally, in Figure 7 the processing flow, the number of backprojections is 2 times, but both are backprojections of half-scan data. Therefore, compared with Figure 4 the case where backprojections are performed on both half-scan data and full-scan data, the operation time can be shortened.

[0092] The embodiments of the present invention have been described above. Additionally, the present invention is not limited to the above embodiments, and constituent elements can be modified and embodied without departing from the gist of the invention. Also, a plurality of constituent elements disclosed in the above embodiments can be appropriately combined. Moreover, several constituent elements can be deleted from all the constituent elements shown in the above embodiments.

Claims

1. An X-ray CT device, characterized in that: have: 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 a projection angle range 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 The image synthesis unit synthesizes the first corrected image and the second corrected image.

2. An X-ray CT device, characterized in that: have: 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 a projection angle range wider than the first projection angle range; a data correction unit that reduces high-frequency components of the first projection data to generate first correction data, and reduces low-frequency components of the second projection data to generate second correction data; and The reconstruction unit reconstructs a tomographic image based on synthesized data obtained by synthesizing the first corrected data and the second corrected data.

3. The X-ray CT apparatus according to claim 1, characterized in that: The data acquisition unit further acquires third projection data obtained by subtracting the first projection data from the second projection data. The image correction unit generates the second corrected image by combining a third tomographic image reconstructed from the third projection data and the first tomographic image.

4. A method for generating a tomographic image, characterized in that: have: 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 which is a projection angle range 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 is to synthesize the first corrected image and the second corrected image.

5. The tomographic image generating method according to claim 4, characterized in that: In the data acquisition step, third projection data is further acquired by subtracting the first projection data from the second projection data. In the image correction step, the second corrected image is generated by synthesizing a third tomographic image reconstructed from the third projection data and the first tomographic image.

Images