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Hele L Reed - One of the best experts on this subject based on the ideXlab platform.
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secondary instability analysis of crossflow on a hypersonic yawed straight Circular Cone
Journal of Fluid Mechanics, 2017Co-Authors: Alexande Moyes, Pedro Paredes, Travis S Kocia, Hele L ReedAbstract:The purpose of this paper is to provide secondary instability analysis of stationary crossflow vortices on a hypersonic yawed straight Circular Cone with a $7^{\circ }$ half-angle at $6^{\circ }$ angle of attack, free-stream Mach number 6 and unit Reynolds number $10.09\times 10^{6}~\text{m}^{-1}$ . At an angle of attack, a three-dimensional boundary layer is developed between the windward and leeward symmetry planes. Under the action of azimuthal pressure gradients, the flow near the surface is deflected more than the flow near the edge of the boundary layer. This results in an inflectional velocity profile that can sustain the growth of crossflow vortices. The stationary crossflow instability is computed by means of the nonlinear parabolized stability equations, including a methodology to predict the stationary-crossflow marching path and variation of the spanwise number of waves in the marching direction solely from the basic state. Secondary instability analysis is performed using spatial BiGlobal equations based on two-dimensional partial differential equations. The secondary instabilities are calculated at different axial locations along two crossflow vortex trajectories selected to complement experiments conducted in the Mach 6 Quiet Tunnel at Texas A&M University and in the Boeing/AFOSR Mach 6 Quiet Tunnel at Purdue University. The secondary instability analysis captures various instability modes. Similar to observations in the low-speed regime for an infinite swept wing, secondary shear-layer instabilities are amplified as a consequence of the three-dimensional shear layer formed by crossflow vortices. Also, low-frequency travelling crossflow and high-frequency second modes coexist with the shear-layer instabilities. These results are shown to be in good agreement with the two sets of hypersonic yawed Cone experiments (one with natural surface roughness and one with artificial discrete roughness) and compare well with experimental measurements of an incompressible swept wing.
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secondary instability analysis of crossflow on a hypersonic yawed straight Circular Cone
Journal of Fluid Mechanics, 2017Co-Authors: Alexander Moyes, Pedro Paredes, Travis S Kocian, Hele L ReedAbstract:The purpose of this paper is to provide secondary instability analysis of stationary crossflow vortices on a hypersonic yawed straight Circular Cone with a half-angle at angle of attack, free-stream Mach number 6 and unit Reynolds number . At an angle of attack, a three-dimensional boundary layer is developed between the windward and leeward symmetry planes. Under the action of azimuthal pressure gradients, the flow near the surface is deflected more than the flow near the edge of the boundary layer. This results in an inflectional velocity profile that can sustain the growth of crossflow vortices. The stationary crossflow instability is computed by means of the nonlinear parabolized stability equations, including a methodology to predict the stationary-crossflow marching path and variation of the spanwise number of waves in the marching direction solely from the basic state. Secondary instability analysis is performed using spatial BiGlobal equations based on two-dimensional partial differential equations. The secondary instabilities are calculated at different axial locations along two crossflow vortex trajectories selected to complement experiments conducted in the Mach 6 Quiet Tunnel at Texas A&M University and in the Boeing/AFOSR Mach 6 Quiet Tunnel at Purdue University. The secondary instability analysis captures various instability modes. Similar to observations in the low-speed regime for an infinite swept wing, secondary shear-layer instabilities are amplified as a consequence of the three-dimensional shear layer formed by crossflow vortices. Also, low-frequency travelling crossflow and high-frequency second modes coexist with the shear-layer instabilities. These results are shown to be in good agreement with the two sets of hypersonic yawed Cone experiments (one with natural surface roughness and one with artificial discrete roughness) and compare well with experimental measurements of an incompressible swept wing.
Xiaochuan Pan - One of the best experts on this subject based on the ideXlab platform.
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region of interest image reconstruction with intensity weighting in Circular Cone beam ct for image guided radiation therapy
Medical Physics, 2009Co-Authors: Seungryong Cho, E Pearson, Charles A. Pelizzari, Xiaochuan PanAbstract:Imaging plays a vital role in radiation therapy and with recent advances in technology considerable emphasis has been placed on Cone-beam CT (CBCT). Attaching a kV x-ray source and a flat panel detector directly to the linear accelerator gantry has enabled progress in target localization techniques, which can include daily CBCT setup scans for some treatments. However, with an increasing number of CT scans there is also an increasing concern for patient exposure. An intensity-weighted region-of-interest (IWROI) technique, which has the potential to greatly reduce CBCT dose, in conjunction with the chord-based backprojection-filtration (BPF) reconstruction algorithm, has been developed and its feasibility in clinical use is demonstrated in this article. A nonuniform filter is placed in the x-ray beam to create regions of two different beam intensities. In this manner, regions outside the target area can be given a reduced dose but still visualized with a lower contrast to noise ratio. Image artifacts due to transverse data truncation, which would have occurred in conventional reconstruction algorithms, are avoided and image noise levels of the low- and high-intensity regions are well controlled by use of the chord-based BPF reconstruction algorithm. The proposed IWROI technique can play an important role in image-guided radiation therapy.
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region of interest image reconstruction in Circular Cone beam microct
Medical Physics, 2007Co-Authors: Seungryong Cho, Charles A. Pelizzari, Junguo Bian, Chintu Chen, Xiaochuan PanAbstract:Cone-beam microcomputed tomography (microCT) is one of the most popular choices for small animal imaging which is becoming an important tool for studying animal models with transplanted diseases. Region-of-interest (ROI) imaging techniques in CT, which can reconstruct an ROI image from the projection data set of the ROI, can be used not only for reducing imaging-radiation exposure to the subject and scatters to the detector but also for potentially increasing spatial resolution of the reconstructed images. Increasing spatial resolution in microCT images can facilitate improved accuracy in many assessment tasks. A method proposed previously for increasing CT image spatial resolution entails the exploitation of the geometric magnification in Cone-beam CT. Due to finite detector size, however, this method can lead to data truncation for a large geometric magnification. The Feldkamp-Davis-Kress (FDK) algorithm yields images with artifacts when truncated data are used, whereas the recently developed backprojection filtration (BPF) algorithm is capable of reconstructing ROI images without truncation artifacts from truncated Cone-beam data. We apply the BPF algorithm to reconstructing ROI images from truncated data of three different objects acquired by our Circular Cone-beam microCT system. Reconstructed images by use of the FDK and BPF algorithms from both truncated and nontruncated Cone-beam data are compared. The results of the experimental studies demonstrate that, from certain truncated data, the BPF algorithm can reconstruct ROI images with quality comparable to that reconstructed from nontruncated data. In contrast, the FDK algorithm yields ROI images with truncation artifacts. Therefore, an implication of the studies is that, when truncated data are acquired with a configuration of a large geometric magnification, the BPF algorithm can be used for effective enhancement of the spatial resolution of a ROI image.
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accurate image reconstruction in Circular Cone beam computed tomography by total variation minimization a preliminary investigation
IEEE Nuclear Science Symposium, 2006Co-Authors: Emil Y Sidky, Xiaochuan PanAbstract:The total variation (TV) minimization image reconstruction algorithm for inverting the divergent-beam x-ray transform is applied to image reconstruction in Circular Cone-beam computed tomography (CT). Reconstructions for the disk phantom are shown and compared with that of projection onto convex sets (POCS). The preliminary results appear to show that TV-minimization yields accurate image reconstructions that are robust against data inconsistencies due to noise or mismatch in the system matrix.
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region of interest reconstruction from truncated data in Circular Cone beam ct
IEEE Transactions on Medical Imaging, 2006Co-Authors: Yu Zou, Charles A. Pelizzari, Emil Y Sidky, Peter R T Munro, Xiaochuan PanAbstract:The Circular scanning trajectory is one of the most widely adopted data-acquisition configurations in computed tomography (CT). The Feldkamp, Davis, Kress (FDK) algorithm and its various modifications have been developed for reconstructing approximately three-dimensional images from Circular Cone-beam data. When data contain transverse truncations, however, these algorithms may reconstruct images with significant truncation artifacts. It is of practical significance to develop algorithms that can reconstruct region-of-interest (ROI) images from truncated Circular Cone-beam data that are free of truncation artifacts and that have an accuracy comparable to that obtained from nontruncated Cone-beam data. In this work, we have investigated and developed a backprojection-filtration (BPF)-based algorithm for ROI-image reconstruction from Circular Cone-beam data containing transverse truncations. Furthermore, we have developed a weighted BPF algorithm to exploit "redundant" information in data for improving image quality. In an effort to validate and evaluate the proposed BPF algorithms for Circular Cone-beam CT, we have performed numerical studies by using both computer-simulation data and experimental data acquired with a radiotherapy Cone-beam CT system. Quantitative results in these studies demonstrate that the proposed BPF algorithms for Circular Cone-beam CT can reconstruct ROI images free of truncation artifacts
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a unified analysis of fbp based algorithms in helical Cone beam and Circular Cone and fan beam scans
Physics in Medicine and Biology, 2004Co-Authors: Xiaochuan Pan, Dan Xia, Yu ZouAbstract:A Circular scanning trajectory is and will likely remain a popular choice of trajectory in computed tomography (CT) imaging because it is easy to implement and control. Filtered-backprojection (FBP)-based algorithms have been developed previously for approximate and exact reconstruction of the entire image or a region of interest within the image in Circular Cone-beam and fan-beam cases. Recently, we have developed a 3D FBP-based algorithm for image reconstruction on PI-line segments in a helical Cone-beam scan. In this work, we demonstrated that the 3D FBP-based algorithm indeed provided a rather general formulation for image reconstruction from divergent projections (such as Cone-beam and fan-beam projections). On the basis of this formulation we derived new approximate or exact algorithms for image reconstruction in Circular Cone-beam or fan-beam scans, which can be interpreted as special cases of the helical scan. Existing algorithms corresponding to the derived algorithms were identified. We also performed a preliminary numerical study to verify our theoretical results in each of the cases. The results in the work can readily be generalized to other non-Circular trajectories.
Pedro Paredes - One of the best experts on this subject based on the ideXlab platform.
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secondary instability analysis of crossflow on a hypersonic yawed straight Circular Cone
Journal of Fluid Mechanics, 2017Co-Authors: Alexande Moyes, Pedro Paredes, Travis S Kocia, Hele L ReedAbstract:The purpose of this paper is to provide secondary instability analysis of stationary crossflow vortices on a hypersonic yawed straight Circular Cone with a $7^{\circ }$ half-angle at $6^{\circ }$ angle of attack, free-stream Mach number 6 and unit Reynolds number $10.09\times 10^{6}~\text{m}^{-1}$ . At an angle of attack, a three-dimensional boundary layer is developed between the windward and leeward symmetry planes. Under the action of azimuthal pressure gradients, the flow near the surface is deflected more than the flow near the edge of the boundary layer. This results in an inflectional velocity profile that can sustain the growth of crossflow vortices. The stationary crossflow instability is computed by means of the nonlinear parabolized stability equations, including a methodology to predict the stationary-crossflow marching path and variation of the spanwise number of waves in the marching direction solely from the basic state. Secondary instability analysis is performed using spatial BiGlobal equations based on two-dimensional partial differential equations. The secondary instabilities are calculated at different axial locations along two crossflow vortex trajectories selected to complement experiments conducted in the Mach 6 Quiet Tunnel at Texas A&M University and in the Boeing/AFOSR Mach 6 Quiet Tunnel at Purdue University. The secondary instability analysis captures various instability modes. Similar to observations in the low-speed regime for an infinite swept wing, secondary shear-layer instabilities are amplified as a consequence of the three-dimensional shear layer formed by crossflow vortices. Also, low-frequency travelling crossflow and high-frequency second modes coexist with the shear-layer instabilities. These results are shown to be in good agreement with the two sets of hypersonic yawed Cone experiments (one with natural surface roughness and one with artificial discrete roughness) and compare well with experimental measurements of an incompressible swept wing.
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secondary instability analysis of crossflow on a hypersonic yawed straight Circular Cone
Journal of Fluid Mechanics, 2017Co-Authors: Alexander Moyes, Pedro Paredes, Travis S Kocian, Hele L ReedAbstract:The purpose of this paper is to provide secondary instability analysis of stationary crossflow vortices on a hypersonic yawed straight Circular Cone with a half-angle at angle of attack, free-stream Mach number 6 and unit Reynolds number . At an angle of attack, a three-dimensional boundary layer is developed between the windward and leeward symmetry planes. Under the action of azimuthal pressure gradients, the flow near the surface is deflected more than the flow near the edge of the boundary layer. This results in an inflectional velocity profile that can sustain the growth of crossflow vortices. The stationary crossflow instability is computed by means of the nonlinear parabolized stability equations, including a methodology to predict the stationary-crossflow marching path and variation of the spanwise number of waves in the marching direction solely from the basic state. Secondary instability analysis is performed using spatial BiGlobal equations based on two-dimensional partial differential equations. The secondary instabilities are calculated at different axial locations along two crossflow vortex trajectories selected to complement experiments conducted in the Mach 6 Quiet Tunnel at Texas A&M University and in the Boeing/AFOSR Mach 6 Quiet Tunnel at Purdue University. The secondary instability analysis captures various instability modes. Similar to observations in the low-speed regime for an infinite swept wing, secondary shear-layer instabilities are amplified as a consequence of the three-dimensional shear layer formed by crossflow vortices. Also, low-frequency travelling crossflow and high-frequency second modes coexist with the shear-layer instabilities. These results are shown to be in good agreement with the two sets of hypersonic yawed Cone experiments (one with natural surface roughness and one with artificial discrete roughness) and compare well with experimental measurements of an incompressible swept wing.
Emil Y Sidky - One of the best experts on this subject based on the ideXlab platform.
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image reconstruction in Circular Cone beam computed tomography by constrained total variation minimization
Physics in Medicine and Biology, 2008Co-Authors: Emil Y SidkyAbstract:An iterative algorithm, based on recent work in compressive sensing, is developed for volume image reconstruction from a Circular Cone-beam scan. The algorithm minimizes the total variation (TV) of the image subject to the constraint that the estimated projection data is within a specified tolerance of the available data and that the values of the volume image are non-negative. The constraints are enforced by the use of projection onto convex sets (POCS) and the TV objective is minimized by steepest descent with an adaptive step-size. The algorithm is referred to as adaptive-steepest-descent-POCS (ASD-POCS). It appears to be robust against Cone-beam artifacts, and may be particularly useful when the angular range is limited or when the angular sampling rate is low. The ASD-POCS algorithm is tested with the Defrise disk and jaw computerized phantoms. Some comparisons are performed with the POCS and expectation-maximization (EM) algorithms. Although the algorithm is presented in the context of Circular Cone-beam image reconstruction, it can also be applied to scanning geometries involving other x-ray source trajectories.
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accurate image reconstruction in Circular Cone beam computed tomography by total variation minimization a preliminary investigation
IEEE Nuclear Science Symposium, 2006Co-Authors: Emil Y Sidky, Xiaochuan PanAbstract:The total variation (TV) minimization image reconstruction algorithm for inverting the divergent-beam x-ray transform is applied to image reconstruction in Circular Cone-beam computed tomography (CT). Reconstructions for the disk phantom are shown and compared with that of projection onto convex sets (POCS). The preliminary results appear to show that TV-minimization yields accurate image reconstructions that are robust against data inconsistencies due to noise or mismatch in the system matrix.
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region of interest reconstruction from truncated data in Circular Cone beam ct
IEEE Transactions on Medical Imaging, 2006Co-Authors: Yu Zou, Charles A. Pelizzari, Emil Y Sidky, Peter R T Munro, Xiaochuan PanAbstract:The Circular scanning trajectory is one of the most widely adopted data-acquisition configurations in computed tomography (CT). The Feldkamp, Davis, Kress (FDK) algorithm and its various modifications have been developed for reconstructing approximately three-dimensional images from Circular Cone-beam data. When data contain transverse truncations, however, these algorithms may reconstruct images with significant truncation artifacts. It is of practical significance to develop algorithms that can reconstruct region-of-interest (ROI) images from truncated Circular Cone-beam data that are free of truncation artifacts and that have an accuracy comparable to that obtained from nontruncated Cone-beam data. In this work, we have investigated and developed a backprojection-filtration (BPF)-based algorithm for ROI-image reconstruction from Circular Cone-beam data containing transverse truncations. Furthermore, we have developed a weighted BPF algorithm to exploit "redundant" information in data for improving image quality. In an effort to validate and evaluate the proposed BPF algorithms for Circular Cone-beam CT, we have performed numerical studies by using both computer-simulation data and experimental data acquired with a radiotherapy Cone-beam CT system. Quantitative results in these studies demonstrate that the proposed BPF algorithms for Circular Cone-beam CT can reconstruct ROI images free of truncation artifacts
Yu Zou - One of the best experts on this subject based on the ideXlab platform.
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region of interest reconstruction from truncated data in Circular Cone beam ct
IEEE Transactions on Medical Imaging, 2006Co-Authors: Yu Zou, Charles A. Pelizzari, Emil Y Sidky, Peter R T Munro, Xiaochuan PanAbstract:The Circular scanning trajectory is one of the most widely adopted data-acquisition configurations in computed tomography (CT). The Feldkamp, Davis, Kress (FDK) algorithm and its various modifications have been developed for reconstructing approximately three-dimensional images from Circular Cone-beam data. When data contain transverse truncations, however, these algorithms may reconstruct images with significant truncation artifacts. It is of practical significance to develop algorithms that can reconstruct region-of-interest (ROI) images from truncated Circular Cone-beam data that are free of truncation artifacts and that have an accuracy comparable to that obtained from nontruncated Cone-beam data. In this work, we have investigated and developed a backprojection-filtration (BPF)-based algorithm for ROI-image reconstruction from Circular Cone-beam data containing transverse truncations. Furthermore, we have developed a weighted BPF algorithm to exploit "redundant" information in data for improving image quality. In an effort to validate and evaluate the proposed BPF algorithms for Circular Cone-beam CT, we have performed numerical studies by using both computer-simulation data and experimental data acquired with a radiotherapy Cone-beam CT system. Quantitative results in these studies demonstrate that the proposed BPF algorithms for Circular Cone-beam CT can reconstruct ROI images free of truncation artifacts
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a unified analysis of fbp based algorithms in helical Cone beam and Circular Cone and fan beam scans
Physics in Medicine and Biology, 2004Co-Authors: Xiaochuan Pan, Dan Xia, Yu ZouAbstract:A Circular scanning trajectory is and will likely remain a popular choice of trajectory in computed tomography (CT) imaging because it is easy to implement and control. Filtered-backprojection (FBP)-based algorithms have been developed previously for approximate and exact reconstruction of the entire image or a region of interest within the image in Circular Cone-beam and fan-beam cases. Recently, we have developed a 3D FBP-based algorithm for image reconstruction on PI-line segments in a helical Cone-beam scan. In this work, we demonstrated that the 3D FBP-based algorithm indeed provided a rather general formulation for image reconstruction from divergent projections (such as Cone-beam and fan-beam projections). On the basis of this formulation we derived new approximate or exact algorithms for image reconstruction in Circular Cone-beam or fan-beam scans, which can be interpreted as special cases of the helical scan. Existing algorithms corresponding to the derived algorithms were identified. We also performed a preliminary numerical study to verify our theoretical results in each of the cases. The results in the work can readily be generalized to other non-Circular trajectories.