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Xiangyang Tang - One of the best experts on this subject based on the ideXlab platform.
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Grating-based x-Ray Differential phase contrast imaging with twin peaks in phase-stepping curves-phase retrieval and dewrapping.
Medical physics, 2016Co-Authors: Yi Yang, Huiqiao Xie, Weixing Cai, Hui Mao, Xiangyang TangAbstract:Purpose: X-Ray Differential phase contrast CT implemented with Talbot interferometry employs phase-stepping to extract information of x-Ray attenuation, phase shift, and small-angle scattering. Since inaccuracy may exist in the absorptiongratingG2 due to an imperfect fabrication, the effective period of G2 can be as large as twice the nominal period, leading to a phenomenon of twin peaks that differ remarkably in their heights. In this work, the authors investigate how to retrieve and dewrap the phase signal from the phase-stepping curve (PSC) with the feature of twin peaks for x-Ray phase contrastimaging. Methods: Based on the paraxial Fresnel–Kirchhoff theory, the analytical formulae to characterize the phenomenon of twin peaks in the PSC are derived. Then an approach to dewrap the retrieved phase signal by jointly using the phases of the first- and second-order Fourier components is proposed. Through an experimental investigation using a prototype x-Ray phase contrastimaging system implemented with Talbot interferometry, the authors evaluate and verify the derived analytic formulae and the proposed approach for phase retrieval and dewrapping. Results: According to theoretical analysis, the twin-peak phenomenon in PSC is a consequence of combined effects, including the inaccuracy in absorptiongratingG2, mismatch between phase grating and x-Ray source spectrum, and finite size of x-Ray tube’s focal spot. The proposed approach is experimentally evaluated by scanning a phantom consisting of organic materials and a lab mouse. The preliminary data show that compared to scanning G2 over only one single nominal period and correcting the measured phase signal with an intuitive phase dewrapping method that is being used in the field, stepping G2 over twice its nominal period and dewrapping the measured phase signal with the proposed approach can significantly improve the quality of x-Ray Differential phase contrastimaging in both radiograph and CT. Conclusions: Using the phase retrieval and dewrapping methods proposed to deal with the phenomenon of twin peaks in PSCs and phase wrapping, the performance of grating-based x-Ray Differential phase contrast radiography and CT can be significantly improved.
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TH-AB-204-08: Phase Retrieval in Grating-Based X-Ray Differential Phase Contrast CT with Twin-Peaks in Phase-Stepping Curves
Medical Physics, 2015Co-Authors: Yi Yang, Huiqiao Xie, Weixing Cai, Xiangyang TangAbstract:Purpose: As a promising new technique with high potential for translation into clinical applications, x-Ray Differential phase contrast (DPC) CT implemented with Talbot interferometry has drawn increasing interest. Usually, the x-Ray DPC-CT employs a phase-stepping procedure to extract the phase signal. Since the fabrication processes may cause defects in analyzer grating G₂, the actual period of G₂ may double the nominal period of G₂, and twin peaks with remarkably different heights can be within a period of the experimental determined phase-stepping curve (PSC). For such a DPC-CT system with twin-peak PSCs, we develop an approach to retrieve the phase contrast for imaging. Methods: Based on the paraxial Fresnel-Kirchhoff theory, an analytical formula is derived to characterize the PSCs of an x-Ray Talbot interferometry with flawed analyzer grating. In addition, an experimental investigation into the phase retrieval in x-Ray DPC-CT with twin-peak PSCs is conducted by utilizing an x-Ray Talbot interferometry with detector cell dimension and nominal period of G₂ being 96×96 micron2 and 4.6 micron, respectively. Results: Theoretical analysis demonstrates that the twin-peak feature of PSCs depends on not only the flaw in G₂, but also the mismatch between the phase grating and the spectrum of x-Ray source. For a DPC-CT system with twin-peak PSCs (Figure 1), experimental results show that in comparison with scanning G₂ over its nominal period g₂ , stepping the analyzer grating over its actual period 2g₂ can provide data to enable a significantly improved reconstruction of the phase-contrast CT images (Figures 2(b) and 2(d)). Conclusion: Our theoretical analysis and experimental investigation show that for an x-Ray DPC-CT imaging system with twin-peak PSCs, the PSCs should be determined by scanning G₂ over the double of its nominal period g₂; and then the PSCs can be utilized to retrieve the phase signal for x-Ray phase contrast imaging. Research Grant: W81XWH-12-1-0138 (DoD).
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The second-order Differential phase contrast and its retrieval for imaging with x-Ray Talbot interferometry
Medical physics, 2012Co-Authors: Yi Yang, Xiangyang TangAbstract:The x-Ray Differential phase contrast imaging implemented with the Talbot interferometry has recently been reported to be capable of providing tomographic images corresponding to attenuation-contrast, phase-contrast, and dark-field contrast, simultaneously, from a single set of projection data. The authors believe that, along with small-angle x-Ray scattering, the second-order phase derivative Φ(") (s)(x) plays a role in the generation of dark-field contrast. In this paper, the authors derive the analytic formulae to characterize the contribution made by the second-order phase derivative to the dark-field contrast (namely, second-order Differential phase contrast) and validate them via computer simulation study. By proposing a practical retrieval method, the authors investigate the potential of second-order Differential phase contrast imaging for extensive applications. The theoretical derivation starts at assuming that the refractive index decrement of an object can be decomposed into δ = δ(s) + δ(f), where δ(f) corresponds to the object's fine structures and manifests itself in the dark-field contrast via small-angle scattering. Based on the paraxial Fresnel-Kirchhoff theory, the analytic formulae to characterize the contribution made by δ(s), which corresponds to the object's smooth structures, to the dark-field contrast are derived. Through computer simulation with specially designed numerical phantoms, an x-Ray Differential phase contrast imaging system implemented with the Talbot interferometry is utilized to evaluate and validate the derived formulae. The same imaging system is also utilized to evaluate and verify the capability of the proposed method to retrieve the second-order Differential phase contrast for imaging, as well as its robustness over the dimension of detector cell and the number of steps in grating shifting. Both analytic formulae and computer simulations show that, in addition to small-angle scattering, the contrast generated by the second-order derivative is magnified substantially by the ratio of detector cell dimension over grating period, which plays a significant role in dark-field imaging implemented with the Talbot interferometry. The analytic formulae derived in this work to characterize the second-order Differential phase contrast in the dark-field imaging implemented with the Talbot interferometry are of significance, which may initiate more activities in the research and development of x-Ray Differential phase contrast imaging for extensive preclinical and eventually clinical applications.
Marco Stampanoni - One of the best experts on this subject based on the ideXlab platform.
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Spline based iterative phase retrieval algorithm for X-Ray Differential phase contrast radiography.
Optics express, 2015Co-Authors: Masih Nilchian, Zhentian Wang, Thomas Thuering, Michael Unser, Marco StampanoniAbstract:Differential phase contrast imaging using grating interferometer is a promising alternative to conventional X-Ray radiographic methods. It provides the absorption, Differential phase and scattering information of the underlying sample simultaneously. Phase retrieval from the Differential phase signal is an essential problem for quantitative analysis in medical imaging. In this paper, we formalize the phase retrieval as a regularized inverse problem, and propose a novel discretization scheme for the derivative operator based on B-spline calculus. The inverse problem is then solved by a constrained regularized weighted-norm algorithm (CRWN) which adopts the properties of B-spline and ensures a fast implementation. The method is evaluated with a tomographic dataset and Differential phase contrast mammography data. We demonstrate that the proposed method is able to produce phase image with enhanced and higher soft tissue contrast compared to conventional absorption-based approach, which can potentially provide useful information to mammographic investigations.
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Constrained regularized reconstruction of X-Ray-DPCI tomograms with weighted-norm
Optics express, 2013Co-Authors: Masih Nilchian, Peter Modregger, Marco Stampanoni, Cedric Vonesch, Stamatios Lefkimmiatis, Michael UnserAbstract:In this paper we introduce a new reconstruction algorithm for X-Ray Differential phase-contrast Imaging (DPCI). Our approach is based on 1) a variational formulation with a weighted data term and 2) a variable-splitting scheme that allows for fast convergence while reducing reconstruction artifacts. In order to improve the quality of the reconstruction we take advantage of higher-order total-variation regularization. In addition, the prior information on the support and positivity of the refractive index is considered, which yields significant improvement. We test our method in two reconstruction experiments involving real data; our results demonstrate its potential for in-vivo and medical imaging.
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Human hand radiography using X-Ray Differential phase contrast combined with dark-field imaging
Skeletal radiology, 2013Co-Authors: Thomas Thüring, Zhentian Wang, Christian David, Roman Guggenberger, Hatem Alkadhi, Jürg Hodler, Magdalena Vich, Marco StampanoniAbstract:Established X-Ray-based imaging procedures such as conventional radiography and computed tomography (CT) rely on the interaction of photons when passing through tissue, including the Compton scattering and the photoelectric effect, which is influenced by the X-Ray energy and the type of matter. The resulting mean attenuation of X-Rays can be measured and depicted on images with different gRay levels. X-Ray phase contrast imaging (PCI) represents a relatively new imaging technique relying upon the refraction of XRays. As such, PCI relies on a fundamentally different physical contrast mechanism compared with conventional, absorption-based X-Ray imaging. In the energy range of diagnostic imaging (10–120 keV), refraction is the dominant effect over absorption, but more difficult to acquire. Previous studies have demonstrated that PCI can provide considerably higher contrast in soft tissue, giving rise to its application in fields where conventional radiography and CT are usually limited. Among a variety of techniques used to acquire phase contrast images, grating interferometry [1] has recently attracted great attention because of its compatibility with conventional X-Ray tubes [2, 3], which is the key prerequisite for the clinical applicability. In addition, this technique provides a third contrast mode along with absorption and phase contrast, which is the dark-field contrast [4]. Similarly, dark-field imaging again exploits a physically different interaction mechanism and represents the intensity of the scattered X-Rays within the area of a single detector pixel. Image pixels with high gRay values indicate strong scattering. Recent studies have investigated the performance of phase contrast (PC) and dark-field contrast (DC) in the imaging of female breast tissue, indicating promising results for distinguishing microcalcifications and the malignant conversion or extension of the carcinoma into normal breast tissue [5, 6]. Yet, joint pathologies such as rheumatoid arthritis, crystal arthropathies, and connective tissue diseases (e.g., scleroderma), are also associated with soft tissue affection and occasional calcifications. Conventional radiography of the hand is a cornerstone imaging study for the detection and monitoring of joint diseases as subtle changes of joint space and bones (narrowing and erosions or osteophytes) and—if perceivable—of soft tissue (including calcifications and fibrosis) [7, 8], indicating disease activity and/or progress. While tissue evaluation with conventional radiography is based on morphological criteria Electronic supplementary material The online version of this article (doi:10.1007/s00256-013-1606-7) contains supplementary material, which is available to authorized users T. Thuring (*) : Z. Wang :C. David :M. Stampanoni Paul Scherrer Institut, WBBA/213, 5232, Villigen, Switzerland e-mail: thomas.thuering@psi.ch
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Non-linear regularized phase retrieval for unidirectional X-Ray Differential phase contrast radiography.
Optics express, 2011Co-Authors: Thomas Thüring, Zhentian Wang, Peter Modregger, Bernd R. Pinzer, Marco StampanoniAbstract:Phase retrieval from unidirectional radiographic Differential phase contrast images requires integration of noisy data. A method is presented, which aims to suppress stripe artifacts arising from direct image integration. It is purely algorithmic and therefore, compared to alternative approaches, neither additional alignment nor an increased scan time is required. We report on the theory of this method and present results using numerical as well as experimental data. The method shows significant improvements on the phase retrieval accuracy and enhances contrast in the phase image. Due to its general applicability, the proposed method provides a valuable tool for various 2D imaging applications using Differential data.
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high resolution large field of view x Ray Differential phase contrast imaging on a compact setup
Applied Physics Letters, 2011Co-Authors: T Thuering, Peter Modregger, Christian David, T. Grund, Johannes Kenntner, Marco StampanoniAbstract:X-Ray grating interferometry is a well established technique to perform Differential phase contrast imaging on conventional x-Ray tubes. So far, the application of this technique in commercial micro computed tomography scanners has remained a major challenge due to the compact setup geometry. In this letter, we report on the design of a compact imaging setup using a microfocus source. Due to the extreme wave front curvature, the gratings are fabricated on a flexible substrate, enabling precise cylindrical shaping. A laboratory setup and a modified SCANCO μCT100 scanner have been built, allowing high resolution and large field of view imaging.
Franz Pfeiffer - One of the best experts on this subject based on the ideXlab platform.
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Penalized maximum likelihood reconstruction for x‐Ray Differential phase‐contrast tomography
Medical physics, 2015Co-Authors: Bernhard Brendel, Franz Pfeiffer, Maximilian Von Teuffenbach, Peter B. Noël, Thomas KoehlerAbstract:Purpose: The purpose of this work is to propose a cost function with regularization to iteratively reconstruct attenuation, phase, and scatterimages simultaneously from Differential phase contrast (DPC) acquisitions, without the need of phase retrieval, and examine its properties. Furthermore this reconstruction method is applied to an acquisition pattern that is suitable for a DPC tomographic system with continuously rotating gantry (sliding window acquisition), overcoming the severe smearing in noniterative reconstruction. Methods: We derive a penalized maximum likelihood reconstruction algorithm to directly reconstruct attenuation, phase, and scatterimage from the measured detector values of a DPC acquisition. The proposed penalty comprises, for each of the three images, an independent smoothing prior. Image quality of the proposed reconstruction is compared to images generated with FBP and iterative reconstruction after phase retrieval. Furthermore, the influence between the priors is analyzed. Finally, the proposed reconstruction algorithm is applied to experimental sliding window data acquired at a synchrotron and results are compared to reconstructions based on phase retrieval. Results: The results show that the proposed algorithm significantly increases image quality in comparison to reconstructions based on phase retrieval. No significant mutual influence between the proposed independent priors could be observed. Further it could be illustrated that the iterative reconstruction of a sliding window acquisition results in images with substantially reduced smearing artifacts. Conclusions: Although the proposed cost function is inherently nonconvex, it can be used to reconstructimages with less aliasing artifacts and less streak artifacts than reconstruction methods based on phase retrieval. Furthermore, the proposed method can be used to reconstructimages of sliding window acquisitions with negligible smearing artifacts.
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penalized maximum likelihood reconstruction for x Ray Differential phase contrast tomography
Medical Physics, 2015Co-Authors: Bernhard Brendel, Franz Pfeiffer, Maximilian Von Teuffenbach, Peter B. Noël, Thomas KoehlerAbstract:Purpose: The purpose of this work is to propose a cost function with regularization to iteratively reconstruct attenuation, phase, and scatterimages simultaneously from Differential phase contrast (DPC) acquisitions, without the need of phase retrieval, and examine its properties. Furthermore this reconstruction method is applied to an acquisition pattern that is suitable for a DPC tomographic system with continuously rotating gantry (sliding window acquisition), overcoming the severe smearing in noniterative reconstruction. Methods: We derive a penalized maximum likelihood reconstruction algorithm to directly reconstruct attenuation, phase, and scatterimage from the measured detector values of a DPC acquisition. The proposed penalty comprises, for each of the three images, an independent smoothing prior. Image quality of the proposed reconstruction is compared to images generated with FBP and iterative reconstruction after phase retrieval. Furthermore, the influence between the priors is analyzed. Finally, the proposed reconstruction algorithm is applied to experimental sliding window data acquired at a synchrotron and results are compared to reconstructions based on phase retrieval. Results: The results show that the proposed algorithm significantly increases image quality in comparison to reconstructions based on phase retrieval. No significant mutual influence between the proposed independent priors could be observed. Further it could be illustrated that the iterative reconstruction of a sliding window acquisition results in images with substantially reduced smearing artifacts. Conclusions: Although the proposed cost function is inherently nonconvex, it can be used to reconstructimages with less aliasing artifacts and less streak artifacts than reconstruction methods based on phase retrieval. Furthermore, the proposed method can be used to reconstructimages of sliding window acquisitions with negligible smearing artifacts.
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3D algebraic iterative reconstruction for cone-beam x-Ray Differential phase-contrast computed tomography.
PloS one, 2015Co-Authors: Astrid Velroyen, Martin Bech, Ming Jiang, Franz PfeifferAbstract:Due to the potential of compact imaging systems with magnified spatial resolution and contrast, cone-beam x-Ray Differential phase-contrast computed tomography (DPC-CT) has attracted significant interest. The current proposed FDK reconstruction algorithm with the Hilbert imaginary filter will induce severe cone-beam artifacts when the cone-beam angle becomes large. In this paper, we propose an algebraic iterative reconstruction (AIR) method for cone-beam DPC-CT and report its experiment results. This approach considers the reconstruction process as the optimization of a discrete representation of the object function to satisfy a system of equations that describes the cone-beam DPC-CT imaging modality. Unlike the conventional iterative algorithms for absorption-based CT, it involves the derivative operation to the forward projections of the reconstructed intermediate image to take into account the Differential nature of the DPC projections. This method is based on the algebraic reconstruction technique, reconstructs the image Ray by Ray, and is expected to provide better derivative estimates in iterations. This work comprises a numerical study of the algorithm and its experimental verification using a dataset measured with a three-grating interferometer and a mini-focus x-Ray tube source. It is shown that the proposed method can reduce the cone-beam artifacts and performs better than FDK under large cone-beam angles. This algorithm is of interest for future cone-beam DPC-CT applications.
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Fast one-dimensional wave-front propagation for x-Ray Differential phase-contrast imaging.
Biomedical optics express, 2014Co-Authors: Johannes Wolf, Andreas Malecki, Jonathan I. Sperl, Michael Chabior, Markus Schüttler, Dirk Beque, Cristina Cozzini, Franz PfeifferAbstract:Numerical wave-optical simulations of X-Ray Differential phase-contrast imaging using grating interferometry require the oversampling of gratings and object structures in the range of few micrometers. Consequently, fields of view of few millimeters already use large amounts of a computer’s main memory to store the propagating wave front, limiting the scope of the investigations to only small-scale problems. In this study, we apply an approximation to the Fresnel-Kirchhoff diffraction theory to overcome these restrictions by dividing the two-dimensional wave front up into 1D lines, which are processed separately. The approach enables simulations with samples of clinically relevant dimensions by significantly reducing the memory footprint and the execution time and, thus, allows the qualitative comparison of different setup configurations. We analyze advantages as well as limitations and present the simulation of a virtual mammography phantom of several centimeters of size.
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Phase Unwrapping in Spectral X-Ray Differential Phase-Contrast Imaging With an Energy-Resolving Photon-Counting Pixel Detector
IEEE transactions on medical imaging, 2014Co-Authors: F. M. Epple, Pierre Thibault, Peter B. Noël, Thomas Koehler, Sebastian Ehn, Guillaume Potdevin, Julia Herzen, David Pennicard, Heinz Graafsma, Franz PfeifferAbstract:Grating-based Differential phase-contrast imaging has proven to be feasible with conventional X-Ray sources. The polychromatic spectrum generally limits the performance of the interferometer but benefit can be gained with an energy-sensitive detector. In the presented work, we employ the energy-discrimination capability to correct for phase-wrapping artefacts. We propose to use the phase shifts, which are measured in distinct energy bins, to estimate the optimal phase shift in the sense of maximum likelihood. We demonstrate that our method is able to correct for phase-wrapping artefacts, to improve the contrast-to-noise ratio and to reduce beam hardening due to the modelled energy dependency. The method is evaluated on experimental data which are measured with a laboratory Talbot–Lau interferometer equipped with a conventional polychromatic X-Ray source and an energy-sensitive photon-counting pixel detector. Our work shows, that spectral imaging is an important step to move Differential phase-contrast imaging closer to pre-clinical and clinical applications, where phase wrapping is particularly problematic.
Yi Yang - One of the best experts on this subject based on the ideXlab platform.
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Grating-based x-Ray Differential phase contrast imaging with twin peaks in phase-stepping curves-phase retrieval and dewrapping.
Medical physics, 2016Co-Authors: Yi Yang, Huiqiao Xie, Weixing Cai, Hui Mao, Xiangyang TangAbstract:Purpose: X-Ray Differential phase contrast CT implemented with Talbot interferometry employs phase-stepping to extract information of x-Ray attenuation, phase shift, and small-angle scattering. Since inaccuracy may exist in the absorptiongratingG2 due to an imperfect fabrication, the effective period of G2 can be as large as twice the nominal period, leading to a phenomenon of twin peaks that differ remarkably in their heights. In this work, the authors investigate how to retrieve and dewrap the phase signal from the phase-stepping curve (PSC) with the feature of twin peaks for x-Ray phase contrastimaging. Methods: Based on the paraxial Fresnel–Kirchhoff theory, the analytical formulae to characterize the phenomenon of twin peaks in the PSC are derived. Then an approach to dewrap the retrieved phase signal by jointly using the phases of the first- and second-order Fourier components is proposed. Through an experimental investigation using a prototype x-Ray phase contrastimaging system implemented with Talbot interferometry, the authors evaluate and verify the derived analytic formulae and the proposed approach for phase retrieval and dewrapping. Results: According to theoretical analysis, the twin-peak phenomenon in PSC is a consequence of combined effects, including the inaccuracy in absorptiongratingG2, mismatch between phase grating and x-Ray source spectrum, and finite size of x-Ray tube’s focal spot. The proposed approach is experimentally evaluated by scanning a phantom consisting of organic materials and a lab mouse. The preliminary data show that compared to scanning G2 over only one single nominal period and correcting the measured phase signal with an intuitive phase dewrapping method that is being used in the field, stepping G2 over twice its nominal period and dewrapping the measured phase signal with the proposed approach can significantly improve the quality of x-Ray Differential phase contrastimaging in both radiograph and CT. Conclusions: Using the phase retrieval and dewrapping methods proposed to deal with the phenomenon of twin peaks in PSCs and phase wrapping, the performance of grating-based x-Ray Differential phase contrast radiography and CT can be significantly improved.
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TH-AB-204-08: Phase Retrieval in Grating-Based X-Ray Differential Phase Contrast CT with Twin-Peaks in Phase-Stepping Curves
Medical Physics, 2015Co-Authors: Yi Yang, Huiqiao Xie, Weixing Cai, Xiangyang TangAbstract:Purpose: As a promising new technique with high potential for translation into clinical applications, x-Ray Differential phase contrast (DPC) CT implemented with Talbot interferometry has drawn increasing interest. Usually, the x-Ray DPC-CT employs a phase-stepping procedure to extract the phase signal. Since the fabrication processes may cause defects in analyzer grating G₂, the actual period of G₂ may double the nominal period of G₂, and twin peaks with remarkably different heights can be within a period of the experimental determined phase-stepping curve (PSC). For such a DPC-CT system with twin-peak PSCs, we develop an approach to retrieve the phase contrast for imaging. Methods: Based on the paraxial Fresnel-Kirchhoff theory, an analytical formula is derived to characterize the PSCs of an x-Ray Talbot interferometry with flawed analyzer grating. In addition, an experimental investigation into the phase retrieval in x-Ray DPC-CT with twin-peak PSCs is conducted by utilizing an x-Ray Talbot interferometry with detector cell dimension and nominal period of G₂ being 96×96 micron2 and 4.6 micron, respectively. Results: Theoretical analysis demonstrates that the twin-peak feature of PSCs depends on not only the flaw in G₂, but also the mismatch between the phase grating and the spectrum of x-Ray source. For a DPC-CT system with twin-peak PSCs (Figure 1), experimental results show that in comparison with scanning G₂ over its nominal period g₂ , stepping the analyzer grating over its actual period 2g₂ can provide data to enable a significantly improved reconstruction of the phase-contrast CT images (Figures 2(b) and 2(d)). Conclusion: Our theoretical analysis and experimental investigation show that for an x-Ray DPC-CT imaging system with twin-peak PSCs, the PSCs should be determined by scanning G₂ over the double of its nominal period g₂; and then the PSCs can be utilized to retrieve the phase signal for x-Ray phase contrast imaging. Research Grant: W81XWH-12-1-0138 (DoD).
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The second-order Differential phase contrast and its retrieval for imaging with x-Ray Talbot interferometry
Medical physics, 2012Co-Authors: Yi Yang, Xiangyang TangAbstract:The x-Ray Differential phase contrast imaging implemented with the Talbot interferometry has recently been reported to be capable of providing tomographic images corresponding to attenuation-contrast, phase-contrast, and dark-field contrast, simultaneously, from a single set of projection data. The authors believe that, along with small-angle x-Ray scattering, the second-order phase derivative Φ(") (s)(x) plays a role in the generation of dark-field contrast. In this paper, the authors derive the analytic formulae to characterize the contribution made by the second-order phase derivative to the dark-field contrast (namely, second-order Differential phase contrast) and validate them via computer simulation study. By proposing a practical retrieval method, the authors investigate the potential of second-order Differential phase contrast imaging for extensive applications. The theoretical derivation starts at assuming that the refractive index decrement of an object can be decomposed into δ = δ(s) + δ(f), where δ(f) corresponds to the object's fine structures and manifests itself in the dark-field contrast via small-angle scattering. Based on the paraxial Fresnel-Kirchhoff theory, the analytic formulae to characterize the contribution made by δ(s), which corresponds to the object's smooth structures, to the dark-field contrast are derived. Through computer simulation with specially designed numerical phantoms, an x-Ray Differential phase contrast imaging system implemented with the Talbot interferometry is utilized to evaluate and validate the derived formulae. The same imaging system is also utilized to evaluate and verify the capability of the proposed method to retrieve the second-order Differential phase contrast for imaging, as well as its robustness over the dimension of detector cell and the number of steps in grating shifting. Both analytic formulae and computer simulations show that, in addition to small-angle scattering, the contrast generated by the second-order derivative is magnified substantially by the ratio of detector cell dimension over grating period, which plays a significant role in dark-field imaging implemented with the Talbot interferometry. The analytic formulae derived in this work to characterize the second-order Differential phase contrast in the dark-field imaging implemented with the Talbot interferometry are of significance, which may initiate more activities in the research and development of x-Ray Differential phase contrast imaging for extensive preclinical and eventually clinical applications.
Guang-hong Chen - One of the best experts on this subject based on the ideXlab platform.
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Improving radiation dose efficiency of X-Ray Differential phase contrast imaging using an energy-resolving grating interferometer and a novel rank constraint.
Optics express, 2016Co-Authors: Ran Zhang, Guang-hong ChenAbstract:In this paper, a novel method was developed to improve the radiation dose efficiency, viz., contrast to noise ratio normalized by dose (CNRD), of the grating-based X-Ray Differential phase contrast (DPC) imaging system that is integrated with an energy-resolving photon counting detector. The method exploits the low-dimensionality of the spatial-spectral DPC image matrix acquired from different energy windows. A low rank approximation of the spatial-spectral image matrix was developed to reduce image noise while retaining the DPC signal accuracy for every energy window. Numerical simulations and experimental phantom studies have been performed to validate the proposed method by showing noise reduction and CNRD improvement for each energy window.
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X-Ray Differential phase contrast imaging using a grating interferometer and a single photon counting detector
Medical Imaging 2016: Physics of Medical Imaging, 2016Co-Authors: Ran Zhang, Guang-hong ChenAbstract:For grating interferometer-based x-Ray Differential phase contrast (DPC) imaging systems, their noise performance is strongly dependent on both the visibility of the interference fringe pattern and the total number of photons used to acquire and extract the DPC signal. For a given interferometer, it is usually designed to work at a specific x-Ray energy, therefore any deviation from the designed energy may result in certain visibility loss. In this work, a single photon counting detector (PCD) was incorporated into a DPC imaging system, which enabled photons with energies close to the designed operation energy of the interferometer to be selectively used for DPC signal extraction. This approach led to significant boost in the fringe visibility, but it also discarded x-Ray photons with other energies incident on the detector and might result in degradations of the overall radiation dose efficiency of the DPC imaging systems. This work presents a novel singular value decomposition (SVD)-based method to leverage the entire spectrum of x-Ray photons detected by the PCD, enabling both fringe visibility improvement and reduction in image noise. As evidenced by the results of experimental phantom studies, the contrast-to-noise ratio of the final DPC images could be effectively improved by the proposed method.
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Grating based x-Ray Differential phase contrast imaging without mechanical phase stepping
Optics express, 2014Co-Authors: John Garrett, Guang-hong ChenAbstract:Grating-based x-Ray Differential phase contrast imaging (DPCI) often uses a phase stepping procedure to acquire data that enables the extraction of phase information. This method prolongs the time needed for data acquisition by several times compared with conventional x-Ray absorption image acquisitions. A novel analyzer grating design was developed in this work to eliminate the additional data acquisition time needed to perform phase stepping in DPCI. The new analyzer grating was fabricated such that the linear grating structures are shifted from one detector row to the next; the amount of the lateral shift was equal to a fraction of the x-Ray diffraction fringe pattern. The x-Ray data from several neighboring detector rows were then combined to extract Differential phase information. Initial experimental results have demonstrated that the new analyzer grating enables accurate DPCI signal acquisition from a single x-Ray exposure like conventional x-Ray absorption imaging.
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Fast data acquisition method in X-Ray Differential phase contrast imaging using a new grating design
Medical Imaging 2014: Physics of Medical Imaging, 2014Co-Authors: John Garrett, Guang-hong ChenAbstract:Grating-based x-Ray Differential phase contrast imaging (DPCI) often uses a phase stepping procedure that involves sequential grating motion and multiple x-Ray exposures to obtain x-Ray phase information. Such a data acquisition process breaks the continuous data acquisition into several step-and-shoot data acquisition sequences. Between two neighboring x-Ray pulses, the acquisition will have to be stopped for the grating to translate into the next phase stepping position. This setup also requires that the grating not be fixed. If the gratings are to be mounted onto a fast-rotating gantry (such as those used in x-Ray CT), this translation of the grating would add another potential source of mechanical instability. To accelerate the data acquisition speed and improve the mechanical stability of of DPCI data acquisitions, a new grating design was developed. In this method, one of the gratings used in DPCI was divided into four-row groups, within each group, grating structures have a designed offset with respect to their neighboring rows. This design allows the acquired data from any adjacent four detector rows to be combined in order to retrieve the needed x-Ray Differential phase information from a single x-Ray exposure. Both numerical simulations and initial phantom experiments have demonstrated that the new interferometer design can enable DPCI image acquisitions without this well-known overhead in data acquisition time.
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Interior tomography in x-Ray Differential phase contrast CT imaging
Physics in medicine and biology, 2012Co-Authors: Pascal Thériault Lauzier, Nicholas Bevins, Joseph Zambelli, Guang-hong ChenAbstract:Differential phase contrast computed tomography (DPC-CT) is an x-Ray imaging method that uses the wave properties of imaging photons as the contrast mechanism. It has been demonstrated that DPC images can be obtained using a conventional x-Ray tube and a Talbot-Lau-type interferometer. Due to the limited size of the gratings, current data acquisition systems only offer a limited field of view, and thus are prone to data truncation. As a result, the reconstructed DPC-CT image may suffer from image artifacts and increased inaccuracy in the reconstructed image values. In this paper, we demonstrate that a small region of interest (ROI) within a large object can be accurately and stably reconstructed using fully truncated projection datasets provided that a priori information on electron density is known for a small region inside the ROI. The method reconstructs an image iteratively to satisfy a group of physical conditions by using a projection onto convex set (POCS) approach. In this work, this POCS algorithm is validated using both numerical simulations and physical phantom experimental data. In both cases, the root mean square error is reduced by an order of magnitude with respect to the truncated analytic reconstructions. Truncation artifacts observed in the latter reconstructions are eliminated using the POCS algorithm.
