Jun 7, 2019
Room 218, Whitaker Hall
Joint Reconstruction for Single-Shot Edge Illumination Phase-Contrast Tomography (EIXPCT)
Edge illumination X-ray phase-contrast tomography (EIXPCT) is an emerging X-ray phasecontrast tomography technique for estimating the complex-valued X-ray refractive index distribution of an object with laboratory-based X-ray sources. Conventional image reconstruction approaches for EIXPCT require multiple images to be acquired at each tomographic view angle. This contributes to prolonged data-acquisition times and elevated radiation doses, which can hinder in-vivo applications. In this dissertation, a new “single-shot” method without restrictive assumptions related to the object, imaging geometry or hardware is proposed for joint reconstruction (JR) of the real and imaginary-valued components of the refractive index distribution from a tomographic data set that contains only a single image acquired at each view angle. The proposed method is predicated upon a non-linear formulation of the inverse problem that is solved by use of a gradient-based optimization method. The potential usefulness of this method is validated and investigated by use of computer-simulated and experimental EIXPCT data sets. The convexity, cross-talk properties and noise properties of the JR method are also investigated.
One important advantage of EIXPCT is that its flexibility enables novel flexible dataacquisition designs. In this dissertation, two aspects of data-acquisition designs are explored in two separate studies. The first study focuses on where the masks in EIXPCT should be placed during the data-acquisition process. In this study, several promising mask displacement strategies are proposed, such as the constant aperture position (CAP) strategy and the alternating aperture position (AAP) strategies covering different angular ranges. In computer-simulation studies, candidate designs are analyzed and compared in terms of image reconstruction stability and quality. Experimental data are employed to test the designs in real-world applications. All candidate designs are also compared for their implementation complexity. The tradeoff between data acquisition time and image quality is discussed. The second study focuses on a resolution-enhancement method called dithering. Dithering requires that multiple projection images per tomographic view angle are acquired as the object is moved over sub-pixel distances. The EIXPCT resolution is mainly determined by the grating period of a sample mask, but can be significantly improved by the dithering technique. However, one main drawback of dithering is the increased data-acquisition time. Motivated by the flexibility in data acquisition designs enabled by the JR method, a novel partial dithering strategy for data acquisition is proposed. In this strategy, dithering is implemented at only a subset of the tomographic view angles. This results in spatial resolution that is comparable to that of the conventional full dithering strategy where dithering is performed at every view angle, but the acquisition time is substantially decreased. The effect of dithering parameters on image resolution is explored.
Finally, a bench-top EIXPCT system has been set up in the lab. The components are designed to fit the need of in-vivo imaging of small animal models. However, thick objects such as animals pose unique challenges for the EIXPCT system, including the potential phase-wrapping problem, limited signal sensitivity, and elevated noise. The components of the system were designed to tackle these challenges, and some initial images obtained from the system show promising potential.
Organizer / Host: Dr. Yuan-Chuan Tai and Dr. Mark Anastasio