Grating Technology and System Development for High-Resolution X-Ray Phase-Contrast Imaging
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Date
2023-02-16
Authors
Pil-Ali, Abdollah
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
X-ray imaging is a workhorse of medical imaging, and in particular, high-resolution X-ray imaging is used in the detection of carcinomas and micro-calcifications in the breast, small fractures, and early stages of cancerous tumors. In addition, high-resolution X-ray imaging is essential for material characterization, material differentiation, and minute crack detection in industrial imaging. Although high-resolution X-ray imaging has traditionally been performed using X-ray attenuation-contrast imaging, X-ray phase-contrast imaging is nascent. X-ray phase-contrast (XPCi) imaging can visualize and image materials that are transparent to X-ray attenuation methods with higher sensitivity and contrast, and even highlight sub-pixel resolution features. XPCi methods in medical imaging have been demonstrated to provide significantly better soft tissue contrast and could potentially offer a viable high-speed X-ray imaging alternative to slower high-resolution MRI scans in particular use cases such as stroke imaging.
Grating-based XPCi techniques provide three images for each single X-ray exposure: transmission (absorption), refraction (phase-shift), and dark-field (ultra-small angle scattering). These images help quantify an object of interest. X-ray absorption gratings play a central role in grating-based XPCi systems as they directly determine the quality of imaging and enable the acquisition of the transmission, refraction, and dark-field images in a single X-ray exposure. An impediment to commercial development and adoption of XPCi systems is the lack of scalable manufacturing technology for large area, high-resolution, high aspect ratio X-ray absorption gratings. Grating technology development, primarily due to technological limitations, has lagged system development and today prevents the scaling up of the XPCi system into a footprint and price point acceptable for the medical market.
Towards that end, we investigate the fabrication of high-resolution high aspect ratio X-ray absorption gratings. We present the first bridge-assisted high-resolution high aspect ratio silicon-micropillar design for X-ray absorption grating fabrication in silicon wafers. Furthermore, we report on a novel fabrication process that relies on a self-aligned single-sided multilayer grating fabrication technique based on a conventional UV-LIGA lithography process to fabricate scalable high aspect ratio micron-scale feature X-ray absorption gratings. The process we invented and developed does not require any specialized processing steps or equipment such as access to synchrotron facilities for X-ray LIGA, deep reactive ion etching (DRIE), or even atomic layer deposition (ALD) commonly seen in silicon-based fabrication processes. By stacking multiple layers of the same grating design (i.e., multi-layer structures) through a backside self-aligned UV flood exposure technique, larger-area X-ray gratings can be realized in an established and mature large-area thin-film electronics processing facility. We demonstrate the first prototype grating with an absorption thickness of 40 μm with SU-8 micropillars of 4 μm in diameter and 16.2 μm period (with three layers), with an average visibility of 28% at 60 kVp, all fabricated in a university lab with no specialized equipment. The development of such a grating creates new opportunities for translation of grating-based XPCi into clinical imaging applications where contrast, resolution, and dose efficiency are important, such as screening and diagnostic mammography.
We also present the design of an unoptimized single-mask edge-illumination (EI) XPCi system employing an in-house high-resolution prototype X-ray detector alongside the high-resolution prototype X-ray absorption gratings reported in this work. The compact single-mask high-resolution EI-XPCi system developed in this thesis is demonstrated to image various test samples including nano-scale powders. The imaging is performed using only a single X-ray exposure and subsequently results in dose reduction compared to two-exposure approaches. Moreover, our 2D X-ray grating system design is beneficial because it eliminates the object orientation dependency associated with line grating systems. We also briefly explore three previously reported algorithms to retrieve the three established contrast modalities, namely transmission, phase-shift, and dark-field images, and present first, unoptimized results with our in-house high-resolution grating.
The large area fabrication process for X-ray gratings and results demonstrated can potentially expedite the development of commercial, single-grating, high-resolution, X-ray phase contrast systems, especially for mammography medical imaging.
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Keywords
X-ray, X-ray phase-contrast imaging, X-ray imaging, high-resolution x-ray detector, high-resolution x-ray grating, high aspect ratio, high aspect ratio micropillars, self-aligned multi-layer, SU8, grating fabrication, X-ray detector, scattering, dark-field imaging, phase-contrast, refraction, tranmission, silicon