REDUCTION OF ARTEFACTS IN A CONE BEAM COMPUTED TOMOGRAPHY

Abstract

The present invention relates to a method and a cone beam computed tomography apparatus for reducing artefacts in an image acquired with the cone beam computed tomography apparatus using a second pass artefact reduction method. Projection data of an object are acquired, wherein the projection data comprises a first subset of data to be used for reconstruction of a first image, and a second subset of data comprising projection data not to be used for the construction of the first image, wherein the second subset of data comprises projection data not comprised in the first subset of data. A first and a second image are reconstructed using the first and the second subset of data, respectively. A second pass artefact reduction method is performed using the second image as input image of the second pass artefact reduction method, thereby reducing artefacts in the first image.

Claims

1. A method for reducing artefacts in a cone beam computed tomography, the method comprising: acquiring projection data of an object to be imaged, wherein the projection data comprises a first subset of data to be used for reconstruction of a first image, and a second subset of data comprising projection data not to be used for the construction of the first image, wherein the second subset of data comprises projection data not comprised in the first subset of data; reconstructing a first image comprising a first resolution using the first subset of data of the projection data; reconstructing a second image comprising a second resolution lower than the first resolution of the first image using the second subset of data of the projection data; and performing a second pass artefact reduction using the second image as input image of the second pass artefact reduction such that artefacts in the first image are reduced.

2. The method according to claim 1, wherein the second subset of data comprises projection data from a region of the object that is not comprised in the first subset of data, or wherein the second subset of data comprises projection data of the object from a projection direction not comprised in the first subset of data.

3. The method according to claim 1, wherein the second pass artefact reduction comprises: determining an artefact-inducing structure in the input image; forward-projecting the artefact-inducing structure into forward projection data; reconstructing an artefact image using the forward projection data; combining the artefact image with a low pass filtered image of the artefact-inducing structure thereby generating a correction image; and combining the correction image with the first image thereby reducing artefacts in the first image.

4. The method according to claim 3, wherein the artefact-inducing structure comprises a high absorption density gradient in a direction parallel to a rotation axis of a computed tomography apparatus.

5. The method according to claim 3, wherein the second pass artefact reduction comprises up-sampling the correction image to a resolution equal to the first resolution of the first image.

6. The method according to claim 1, wherein the second subset of data comprises data acquired in a second scan prior to a brain perfusion scan, or wherein the second subset of data comprises data acquired due to cardiac phase tolerances in a gated cardiac scan.

7. The method according to claim 6, wherein the second scan is a helical scan.

8. The method according to claim 1, wherein a second image contrast of the second image is adjusted to match a first image contrast of the first image.

9. The method according to claim 1, wherein the second image is registered to the first image.

10. The method according to claim 1, wherein the second image is low-pass filtered.

11. The method according to claim 1, wherein the reconstruction of the first image and/or the reconstruction of the second image comprises a frequency split.

12. A computed tomography apparatus for reducing cone beam artefacts in an image using a second pass artefact reduction, the apparatus comprising an acquisition unit configured for acquiring projection data of an object to be imaged, wherein the projection data comprises a first subset of data to be used for reconstruction of a first image, and a second subset of data comprising projection data not to be used for the construction of the first image, wherein the second subset of data comprises projection data not comprised in the first subset of data; and a processor configured for reconstructing a first image comprising a first resolution using the first subset of data of the projection data, wherein the processor is configured for reconstructing a second image comprising a second resolution lower than the first resolution of the first image using the second subset of data of the projection data, and configured for performing the second pass artefact reduction method using the second image as input image of the second pass artefact reduction method, thereby reducing artefacts in the first image.

13. The apparatus according to claim 12, wherein the processor is further configured to determine an artefact-inducing structure in the input image, to forward-project the artefact-inducing structure into forward projection data, reconstruct an artefact image using the forward projection data, combine the artefact image with a low pass filtered image of the artefact-inducing structure thereby generating a correction image, and combine the correction image with the first image thereby reducing artefacts in the first image.

14. (canceled)

15. (canceled)

16. A non-transitory computer-readable medium for storing executable instructions, which cause a method to be performed for reducing artefacts in a cone beam computed tomography, the method comprising: acquiring projection data of an object to be imaged, wherein the projection data comprises a first subset of data to be used for reconstruction of a first image, and a second subset of data comprising projection data not to be used for the construction of the first image, wherein the second subset of data comprises projection data not comprised in the first subset of data; reconstructing a first image comprising a first resolution using the first subset of data of the projection data; reconstructing a second image comprising a second resolution lower than the first resolution of the first image using the second subset of data of the projection data; and performing a second pass artefact reduction using the second image as input image of the second pass artefact reduction such that artefacts in the first image are reduced.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG. 1 shows a block diagram of the method for reducing artefacts in an image acquired with a cone beam computed tomography apparatus using a second pass artefact reduction method according to the invention.

[0047] FIG. 2 shows a block diagram of a second pass artefact reduction method for reducing artefacts in an image acquired with a cone beam computed tomography apparatus.

[0048] FIG. 3 shows a block diagram of the method for reducing artefacts in an image acquired with a cone beam computed tomography apparatus using a second pass artefact reduction method according to an embodiment of the invention.

[0049] FIG. 4 shows a block diagram of the method for reducing artefacts in an image acquired with a cone beam computed tomography apparatus using a second pass artefact reduction method according to an embodiment of the invention.

[0050] FIG. 5 shows a schematic set-up of a cone beam computed tomography apparatus for reducing artefacts in an image using a second pass artefact reduction method according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0051] FIG. 1 shows a block diagram of the method for reducing artefacts in an image acquired with a cone beam computed tomography apparatus 100 using a second pass artefact reduction method according to the invention. In a first step, projection data of an object to be imaged with the cone beam computed tomography apparatus 100 is acquired in one or multiple acquisition scans. The projection data comprises a first subset of data 111 to be used for reconstruction of a first image 113, and a second subset of data 112 comprising projection data not to be used for the construction of the first image 113. The second subset of data 112 comprises projection data not comprised in the first subset of data 111. These projection data can be split in two parts with either data from separate scans or data from the same scan. In a second step, a first image 113 comprising a first resolution using the first subset of data 111 of the projection data is reconstructed, and in a third step, a second image 114 comprising a second resolution using the second subset of data 112 of the projection data is reconstructed. In a fourth step, the second pass artefact reduction method using the second image 114 as input image of the second pass artefact reduction method is performed, thereby reducing artefacts in the first image 113.

[0052] FIG. 2 shows a block diagram of a second pass artefact reduction method for reducing artefacts in an image acquired with a cone beam computed tomography apparatus. The second image 114 reconstructed from the second subset 112 of data can be used as input image for the second pass artefact reduction method. By applying a threshold to the input image, artefact inducing structures can be determined. These artefact inducing structures are forward projected resulting in virtual projection data that would have been detected by the computed tomography apparatus when imaging a phantom only comprising the artefact inducing structures. These virtual projection data are processed in order to reconstruct an image of the artefact inducing structures, for example by a filtered back projection. Therefore, this reconstructed image comprises, in addition to the reconstructed artefact inducing structures, the artefacts induced by the artefact inducing structures. By combining this artefact image with low-pass filtered data of the image of the artefact inducing structures, the artefacts can be isolated. This correction image only comprising the artefacts can be subtracted, preferably after appropriate registration and adjustment of the contrast, from the first image 113. Thus, a high-quality image of the first image 113 depicting the object to be imaged can be provided without being deteriorated by artefacts.

[0053] FIG. 3 shows a block diagram of the method for reducing artefacts in an image acquired with a cone beam computed tomography apparatus 100 using a second pass artefact reduction method according to an embodiment of the invention. The input image to the second pass estimation of the artefacts is replaced by an image that is not the image as it is originally reconstructed (cardiac FOV, gated reconstruction with frequency split). Rather it is a full FOV image obtained from a full FOV reconstruction using all data available. The input image to the second pass estimation uses full-scan data in order to maximize the z-extent of the input image, thus capturing as much of the artefact inducing structures as possible. Optionally, full-scan reconstruction is used only for the image parts where a short scan reconstruction is not possible. Under the assumption that most artefacts are present in the low frequency image, the filtered back projection FBP reconstruction used in the second pass is not necessarily the one used in the first pass (two recons with frequency split) but rather just one, low-frequency reconstruction. The cut-off frequency is not necessarily the same as in the first pass reconstruction. In this case, the low pass filter LP in the second pass method needs to be modified appropriately. The in-plane image resolution can then also be reduced accordingly to save computational effort when performing the second pass forward- and back-projections. In all cases the resulting residual (artefact image) needs to be resampled, assuming that the target resolution within the cardiac FOV, which should be completely reconstructable using the narrow gating window, is higher than what is or should be used for the full FOV reconstruction, being the input to the second pass. In case of strong heart motion, the frequency split method can also be applied beneficially to the full FOV reconstruction. For the narrow gating window (high frequency path of the frequency split method) an angular weighting function is used, for which the weights do not drop to zero but to a small value greater than zero. This results in a merge of high and low frequency images, where in the high frequencies the image using a narrow gating window (high time resolution) is preferred in the cardiac FOV (well covered by the views from the narrow gating window) and the full FOV image is preferred in regions that are only well covered by data from the wide gating window.

[0054] FIG. 4 shows a block diagram of the method for reducing artefacts in an image acquired with a cone beam computed tomography apparatus 100 using a second pass artefact reduction method according to an embodiment of the invention. The second pass artefact reduction method of FIG. 3 is performed with an input image comprising a lower resolution compared to the first image 113. Thus, the correction image is up-sampled before combined with the first image 113 to the final image comprising reduced artefacts.

[0055] FIG. 5 shows a schematic set-up of a cone beam computed tomography apparatus 100 for reducing artefacts in an image using a second pass artefact reduction method according to the invention. The computed tomography apparatus 100 comprises an acquisition unit 110 for acquiring a first subset of data 111 and a second subset of data 112. The computed tomography apparatus 100 further comprises a processing unit 120 configured for controlling the acquisition unit and for reconstructing a first image 113 and a second image 114. The processing unit 120 is further configured for performing the second pass artefact reduction method with the second image as input image.

[0056] Provided a perfusion scan protocol that involves a time series of axial scans (perfusion scan) plus, ideally, one scan covering a larger region including the region for the perfusion scan (native scan), the method comprises: A registration method to register the volume image of the native scan to each of the volume images of the time series produced in the perfusion scan. If the native scan was performed at different tube settings (kVp), the image contrast needs to be adjusted to match the contrast of the perfusion scans. This can be easily done if the native scan is a dual energy scan. Usage of the registration parameters to determine the relative system-to-patient configuration. Given a relative system-to-patient configuration, any method to estimate cone-beam artefacts (residual image) for an axial cone-beam CT acquisition is performed, e.g. the second pass method described above. This is preferably done using the native scan volume. For this purpose, the native scan volume is warped onto the perfusion scan volume.

[0057] Further modifications/improvements: [0058] 1. In order to reduce computational burden, the artefacts are only estimated once for a specific system to patient configuration and the residual image is applied to all volumes of the perfusion scan time series. [0059] 2. If registration of the native scan is performed with respect to each image in the time series, the system-to-patient configuration can be tracked. This can be used to [0060] a. e.g. use a mean of registration parameters to estimate a mean system to patient configuration for the estimation of the residual image. [0061] b. to perform a number of residual image estimations as needed for significantly different system to patient configurations. [0062] i. this can include some measure for significance [0063] ii. plus possibly a clustering of deviations of system to patient configurations This may result in a number of residual images to be estimated smaller than the number of images in the time series from the perfusion scan. [0064] c. correct registration of the residual image to each of the perfusion scan images in case of slight variations. [0065] 3. This method may or may not be performed using either the native scan image, a single or multiple images from the time series, the native scan promising best results. A different method for producing the correction image can be realized in the frequency domain without an explicit forward- and back projection procedure. [0066] The second image is transformed into the frequency domain (FFT) [0067] From the projection geometry and using the Fourier slice theorem a region in the Fourier domain is identified that contains the missing data, which has effectively not been measured during acquisition of the first image. [0068] From this missing data, the correction image can be obtained by inverse FFT. [0069] The region can be spatially varying, depending on the position in the image relative to the system geometry. Thus multiple inverse FFTs may need to be performed for different positions within or regions of the image.

[0070] In case one or both scans are spectral scans, the correction can be performed independently using the corresponding material basis image, using the same registration parameters, e.g. taken from registering the combined (conventional) or some basis material image. In case of spectral image acquisitions, mismatches between the images in terms of contrast (different keV setting, contrast medium present or not present), specific reconstructions, possibly differing from the diagnostic images, can be used for registration. One specific example: First and second scan may possibly be done at different keVs, a perfusion scan typically done at 80 keV (first scan), a typical native or CTA scan done at 120 keV (second scan). The keV mismatch results in different contrast levels in the images and may deteriorate registration results. In case, however, the second scan is a spectral scan, a virtually conventional image at the keV of the first scan can be reconstructed and used for registration instead of the diagnostic image. Another possibility could be to use a virtual non-contrast image in case the second scan is a CTA scan. Thus, a conventional image based on a kVp switching dual energy acquisition can be generated. This is based on an intermediate material decomposition followed by a re-composition at the desired conventional tube spectrum. The presence of a conventional image will improve customer acceptance of the dual energy acquisition protocol.

[0071] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing a claimed invention, from a study of the drawings, the disclosure, and the dependent claims.

[0072] In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are re-cited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SIGNS

[0073] 100 cone beam computed tomography apparatus [0074] 110 acquisition unit [0075] 111 first subset of data [0076] 112 second subset of data [0077] 113 first image [0078] 114 second image [0079] 120 processing unit