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A High-Resolution CMOS Single-Photon-Counting Spectrographic X-Ray Imager

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Date

2023-09-01

Authors

Wright, Michael

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Publisher

University of Waterloo

Abstract

Digital X-ray imagers for clinical mammography and computed tomography (CT) have been recently taking advantage of the noise-rejection properties of single-photon-counting (SPC) architectures. However, conventional SPC imagers are limited in spatial and energy resolution due to the substantial circuit area required to accommodate signal processing and digitization components in each pixel. To enable higher spatial resolution using smaller pixels, while simultaneously increasing energy resolution for multi-spectral imaging, new circuit architectures are required for SPC X-ray imagers. This thesis presents the design and experimental characterization of the first comple- mentary metal-oxide-semiconductor (CMOS) SPC X-ray imager readout integrated circuit (ROIC) to use frequency-division-multiplexed (FDM) readout for improved spatial and energy resolution. FDM readout allows the pixels in our ROIC to share analog-to-digital converter (ADC) resources, so that fewer components are integrated in each pixel. This enables us to shrink the pixel area, improving spatial resolution. Shared ADCs also have higher resolution than those in conventional implementations, enabling higher overall en- ergy resolution. Our proposed ROIC, designed in a 180-nm CMOS technology, includes an 8 × 16 pixel array and 16 ADCs. To achieve high spatial resolution and sufficient count rate for mam- mography and CT, we design our pixels with an area of only 35 × 35 μm^2, approximately 50% smaller than commercial SPC imagers. Our ROIC uses FDM readout to multiplex banks of eight pixels into a shared 8-bit 160-MHz-sample-rate pipelined ADC, enabling full digitization of pixel analog outputs. We achieve six bins of energy resolution, which exceeds the resolution of recently-reported conventional SPC imagers that use larger-area pixels. Our ROIC also has a measured equivalent-noise charge (ENC) of 101 e−rms, which is comparable to conventional imagers, and a maximum count-rate of 10^8 photons/mm^2/s, suitable for mammography and CT.

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Keywords

single photon counting, analog circuit design, x-ray imaging

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