Yan Xi

Shanghai Jiao Tong University, Shanghai, Shanghai Shi, China

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Publications (17)35.92 Total impact

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    ABSTRACT: Imaging techniques for visualizing cerebral vasculature and distinguishing functional areas are essential and critical to the study of various brain diseases. In this paper, with the X-ray phase-contrast imaging technique, we proposed an experiment scheme for the ex vivo mouse brain study, achieving both high spatial resolution and improved soft-tissue contrast. This scheme includes two steps: sample preparation and volume reconstruction. In the first step, we use heparinized saline to displace the blood inside cerebral vessels and then replace it with air making air-filled mouse brain. After sample preparation, X-ray phase-contrast tomography is performed to collect the data for volume reconstruction. Here, we adopt a phase-retrieval combined filtered backprojection method to reconstruct its three-dimensional structure and redesigned the reconstruction kernel. To evaluate its performance, we carried out experiments at Shanghai Synchrotron Radiation Facility. The results show that the air-tissue structured cerebral vasculatures are highly visible with propagation-based phase-contrast imaging and can be clearly resolved in reconstructed cross-images. Besides, functional areas, such as the corpus callosum, corpus striatum, and nuclei, are also clearly resolved. The proposed method is comparable with hematoxylin and eosin staining method but represents the studied mouse brain in three dimensions, offering a potential powerful tool for the research of brain disorders.
    Full-text · Article · Nov 2015 · Computational and Mathematical Methods in Medicine
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    ABSTRACT: We propose a one-shot thickness measurement method for sponge-like structures using a propagation-based X-ray phase-contrast imaging (P-PCI) method. In P-PCI, the air-material interface refracts the incident X-ray. Refracted many times along their paths by such a structure, incident X-rays propagate randomly within a small divergent angle range, resulting in a speckle pattern in the captured image. We found structure thickness and contrast of a phase-contrast projection are directly related in images. This relationship can be described by a natural logarithm equation. Thus, from the one phase-contrast view, depth information can be retrieved from its contrast. Our preliminary biological experiments indicate promise in its application to measurements requiring in vivo and ongoing assessment of lung tumor progression.
    Preview · Article · Apr 2015
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    ABSTRACT: Current bio-medical imaging researches aim to detect brain micrometastasis in early stage for its increasing incidence and high mortality rates. Synchrotron phase-contrast imaging techniques, such as in-line phase-contrast (IPC) and grating-based phase-contrast (GPC) imaging, could provide a high spatial and density imaging study of biological specimens' 3D structures. In this study, we demonstrated the detection efficiencies of these two imaging tools on breast cancer micrometastasis in an ex vivo mouse brain. We found that both IPC and GPC can differentiate abnormal brain structures induced by micrometastasis from the surrounding normal tissues. We also found that GPC was more sensitive in detecting the small metastasis as compared to IPC.
    Full-text · Article · Mar 2015 · Scientific Reports
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    ABSTRACT: Current bio-medical imaging researches aim to detect brain micrometastasis in early stage for its increasing incidence and high mortality rates. Synchrotron phase-contrast imaging techniques, such as in-line phase-contrast (IPC) and grating-based phase-contrast (GPC) imaging, could provide a high spatial and density imaging study of biological specimens' 3D structures. In this study, we demonstrated the detection efficiencies of these two imaging tools on breast cancer micrometastasis in an ex vivo mouse brain. We found that both IPC and GPC can differentiate abnormal brain structures induced by micrometastasis from the surrounding normal tissues. We also found that GPC was more sensitive in detecting the small metastasis as compared to IPC.
    Full-text · Article · Mar 2015 · Scientific Reports
  • Yi Chen · Yan Xi · Jun Zhao
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    ABSTRACT: The temporal resolution of current computed tomography (CT) systems is limited by the rotation speed of their gantries. A helical interlaced source detector array (HISDA) CT, which is a stationary CT system with distributed X-ray sources and detectors, is presented in this paper to overcome the aforementioned limitation and achieve high temporal resolution. Projection data can be obtained from different angles in a short time and do not require source, detector, or object motion. Axial coverage speed is increased further by employing a parallel scan scheme. Interpolation is employed to approximate the missing data in the gaps, and then a Katsevich-type reconstruction algorithm is applied to enable an approximate reconstruction. The proposed algorithm suppressed the cone beam and gap-induced artifacts in HISDA CT. The results also suggest that gap-induced artifacts can be reduced by employing a large helical pitch for a fixed gap height. HISDA CT is a promising 3D dynamic imaging architecture given its good temporal resolution and stationary advantage.
    No preview · Article · Nov 2014 · Journal of X-Ray Science and Technology
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    ABSTRACT: Spectral computed tomography (CT) has attracted considerable attention because of its energy-resolving capability in identifying and discriminating materials. The use of a narrow energy bin can improve energy resolution. However, a narrow energy bin has high noise ratio, which degrades the imaging quality of spectral CT. To address this problem, this study exploits the structure correlations of images in the energy domain and proposed two types of united iterative reconstruction (UIR) algorithms. One type uses the well-reconstructed broad-spectrum image, with all available photons, as a constraint, whereas the other type uses a pseudo narrow-energy image, which is estimated with the use of our proposed structure-coupling (SC) method, as a constraint. The SC method utilizes local structures to connect images that are reconstructed with broad-spectrum and narrow-energy CT datasets. Given a broad-spectrum image, the SC method can accurately estimate its corresponding narrow-energy image. Results show that UIR algorithms significantly outperform conventional iterative reconstruction algorithms for narrow-energy image reconstruction in spectral CT. Among the UIR algorithms, SC-UIR yields the best results.
    No preview · Article · Jul 2014 · IEEE Transactions on Medical Imaging
  • Yi Chen · Yan Xi · Jun Zhao
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    ABSTRACT: With distributed sources along a helix trajectory, the helical interlaced source detector array (HISDA) requires no rotation while scanning, thus improving temporal resolution and ensuring mechanical stability. However, in this system, gaps exist in the detector array where sources are placed, resulting in the non-measurement of projection data in gaps. To solve this problem, we propose the semi-stationary CT, which can move the table or gantry based on the HISDA CT. This additional movement can reshape the effective helix trajectory, reducing the required detector area, so that the gaps are out of this area. We discuss the minimum moving velocity to meet the theoretical data sufficiency condition. Furthermore, the velocity condition is also investigated to apply the exact reconstruction algorithm. Simulation studies verified that the proposed scanning method can solve the data absence problem and improve the imaging quality. This semi-stationary CT can be designed with less cost and scattering in comparison with the stationary HISDA CT.
    No preview · Conference Paper · Apr 2014
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    ABSTRACT: In this paper, we extend the spectrography method to visualize 3D structures of complex samples from only one spectral view. Utilizing a weighted difference map and the Fourier central slice theorem, a number of Fourier planes are reconstructed, which go through the origin of the 3D Fourier space and interact with a region formed by the Ewald spheres. Thus, the complex x-ray wave fronts can be recovered at small tilting angles from the incident x-ray beam. Patterns from various computed projections can generate perception of 3D structure features inside the sample. To demonstrate the feasibility of the proposed spectrographic imaging method, numerical simulations are performed and analyzed. The results suggest that spectrography is an effective method for 3D structure studies by a single spectral exposure.
    No preview · Article · Sep 2013 · Proceedings of SPIE - The International Society for Optical Engineering
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    Yan Xi · Jun Zhao
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    ABSTRACT: Grating-based phase-contrast imaging has been a hot topic for several years due to its excellent imaging capability for low-density materials and easy implementation with a laboratory x-ray source. Compared with traditional x-ray computed tomography (CT) systems, the standard data collection procedure, "phase-stepping" (PS), in the grating-based phase-contrast CT (GPC-CT) is time consuming. The imaging time of a GPC-CT scan is usually up to hours. It is unacceptable in clinical CT examinations, and will cause serious motion artifacts in the reconstructed images. Additionally, the radiation dose delivered to the object with the PS-based GPC-CT is several times larger than that by a conventional CT scan. To address these problems, in this paper, we followed the interlaced PS method and proposed a novel image reconstruction method, namely the inner-focusing (IF) reconstruction method. With the interlaced PS method, the sample rotation and the grating stepping in GPC-CT occur at the same time. Thus, the interlaced GPC-CT scan can have a comparable temporal resolution with existing CT systems. Without any additional requirements, the proposed IF reconstruction method can prevent the artifacts existing in the conventional interlaced PS method. Both numerical simulations and real experiments were carried out to verify the proposed IF reconstruction method. And the results demonstrated it was effective in archiving a fast and low-dose GPC-CT.
    Preview · Article · Mar 2013 · Optics Express
  • Yan Xi · Ge Wang · Hengyong Yu · Yuxin Wang · Jun Zhao
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    ABSTRACT: A scheme is proposed here for spectrography that is for 3D analysis of an object from a single spectral view. In this scheme, we first reconstruct some planes that go through the origin of the Fourier space and interact significantly with Ewald spheres. Since only amplitude data are measured, the corresponding phase information is estimated using a hybrid-input-output algorithm. Finally, each of the specified planes is reconstructed from the highly under-sampled Fourier data using a dictionary learning technique. According to the Fourier central slice theorem, the reconstructed image is equivalent to a parallel-beam projection of the object. Hence, the reconstructed projections can be used to analyze internal structures of the object via stereo imaging. The numerical experiments suggest that our method is promising for studies on 3D structures from one spectral view.
    No preview · Conference Paper · Oct 2012
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    ABSTRACT: An X-ray grating interferometer was installed at the BL13W beamline of Shanghai Synchrotron Radiation Facility (SSRF) for biomedical imaging applications. Compared with imaging results from conventional absorption-based micro-computed tomography, this set-up has shown much better soft tissue imaging capability. In particular, using the set-up, the carotid artery and the carotid vein in a formalin-fixed mouse can be visualized in situ without contrast agents, paving the way for future applications in cancer angiography studies. The overall results have demonstrated the broad prospects of the existing set-up for biomedical imaging applications at SSRF.
    No preview · Article · Sep 2012 · Journal of Synchrotron Radiation
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    ABSTRACT: An X-ray grating interferometer has been installed at Shanghai Synchrotron Radiation Facility (SSRF). Three sets of phase gratings were designed to cover the wide X-ray energy range needed for biological and soft material imaging capabilities. The performance of the grating interferometer has been evaluated by a tomography study of a PMMA particle packing and a new born mouse chest. In the mouse chest study, the carotid artery and carotid vein inside the mouse can be identified in situ without contrast agents.
    No preview · Article · Jul 2012
  • Yang Fu · Yan Xi · Yixin Cao · Yujie Wang
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    ABSTRACT: We present an x-ray microtomography study of the compaction process of cylindrical rods under tapping. The process is monitored by measuring the evolution of the orientational order parameter, local, and overall packing densities as a function of the tapping number for different tapping intensities. The slow relaxation dynamics of the orientational order parameter can be well fitted with a stretched-exponential law with stretching exponents ranging from 0.9 to 1.6. The corresponding relaxation time versus tapping intensity follows an Arrhenius behavior which is reminiscent of the slow dynamics in thermal glassy systems. We also investigated the boundary effect on the ordering process and found that boundary rods order faster than interior ones. In searching for the underlying mechanism of the slow dynamics, we estimated the initial random velocities of the rods under tapping and found that the ordering process is compatible with a diffusion mechanism. The average coordination number as a function of the tapping number at different tapping intensities has also been measured, which spans a range from 6 to 8.
    No preview · Article · May 2012 · Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics
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    ABSTRACT: The suture middle cerebral artery occlusion (MCAO) model is used worldwide in both academia and industry. However, the variable occurrence of dysplasia in posterior communicating arteries (PcomAs) induces high mortality and instability in permanent MCAO models, limiting the model's application to transient focal ischemia. In particular, high mortality in intraluminal suture MCAO models is associated with the dysplasia of PcomAs in C57BL/6 mice. Optimization of silicone coating length is critical for reducing mortality and generating stable infarct in this model. The aim of our study is to reduce mortality and improve the reproducibility of the intraluminal suture MCAO model in C57BL/6 mice, which have high variation in PcomA dysplasia. Adult male C57BL/6 mice (n=38) underwent MCAO using sutures with various diameters and silicone coating lengths. The occlusion of cerebral vessels was examined by synchrotron radiation live angiography. The morphology of PcomAs was examined under a microscope after MICROFIL(®) infusion. Neurological outcome, infarct volume, and mortality were examined within 28 days. Optimizing the silicone coating on an 8-0 suture tip, we were able to reduce the model mortality to zero after permanent occlusion in C57BL/6 and produce stable brain infarct volume independent of the patency of PcomAs.
    Full-text · Article · Nov 2011 · Journal of neurotrauma
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    ABSTRACT: In the present study, we investigated the potential of gas-filled microbubbles as contrast agents for in-line x-ray phase-contrast imaging (PCI) in biomedical applications. When imaging parameters are optimized, the microbubbles function as microlenses that focus the incoming x-rays to form bright spots, which can significantly enhance the image contrast. Since microbubbles have been shown to be safe contrast agents in clinical ultrasonography, this contrast-enhancement procedure for PCI may have promising utility in biomedical applications, especially when the dose of radiation is a serious concern. In this study, we performed both numerical simulations and ex vivo experiments to investigate the formation of the contrast and the effectiveness of microbubbles as contrast agents in PCI.
    No preview · Article · Jul 2011 · Applied Physics Letters
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    ABSTRACT: The purpose of the study was to evaluate the feasibility of microbubbles as phase contrast imaging (PCI) agents for angiography applications. The hypothesis was that the introduction of microbubbles into tissue produces a significant change in the refractive index and highlights the lumen of the vessel in PCI. The absorption and phase contrast images of commercially available microbubbles were obtained and compared in vitro. A further increase in contrast was observed in PCI. Microbubbles highlighted the lumen of the renal microvessels, acting as a positive contrast medium in ex vivo imaging. In addition, home-made microbubbles with larger diameters were introduced for image contrast enhancement in living tumor-bearing mice, demonstrating the feasibility of microbubble-based x-ray phase-contrast imaging for tumor vasculature in vivo.
    Full-text · Article · Jun 2011 · Physics in Medicine and Biology
  • Jun Zhao · Yan Xi
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    ABSTRACT: This paper presents a helical interlaced-source-detector-array (HISDA) CT approach for superior temporal resolution. A Feldkamp-type algorithm is first proposed for HISDA CT reconstruction. Then, schemes for recovering missing projection data are studied. The performance of HISDA CT is evaluated in the numerical simulation, which is better than the previously proposed circular interlaced-source-detector-array (CISDA) CT in terms of accuracy and uniformity.
    No preview · Conference Paper · Jan 2010