A High-Resolution Frequency-Division-Multiplexed Single-Photon-Counting CMOS X-Ray Imager

Abstract

Spectral X-ray imaging based on single photon counting (SPC) has clear benefits for reducing radiation dose and improving tissue contrast in medical imaging. However, fitting the spectral readout electronics into a small pixel area in order to provide high spatial resolution is still challenging in readout integrated circuit (ROIC) design. This thesis covers the modeling, design, and measurement of a CMOS SPC X-ray imager intended for mammography and computed tomography (CT) that implements, for the first time, frequency-division multiplexed (FDM) readout to achieve high spatial resolution. Reducing pixel dimensions to 50 x 50 µm² is critical to leveraging the "small-pixel effect", which is a phenomenon that minimizes the impact of slow charge trapping in some X-ray sensors to drastically improve energy resolution. However, this stringent area constraint renders conventional per-pixel digitization architectures impractical. The proposed architecture, therefore, uses FDM, which allows the analog-to-digital converter (ADC) to be moved outside the pixel so that one ADC can serve an entire column of pixels. This architectural change helps mitigate the area and count-rate limitations inherent in X-ray imagers with in-pixel ADCs. In our proposed ROIC, each pixel contains a charge-sensitive amplifier (CSA) followed by a CR-RC shaper, which then feeds a mixer driven by a local oscillator. The shaped pulses are modulated onto orthogonal carrier frequencies, summed using a transimpedance amplifier (TIA), and then digitized by a shared 8-bit 200 MHz pipelined ADC. Measurements from our ROIC, fabricated in a 1.8-V 180-nm CMOS process, confirm that our proposed multi-carrier FDM readout scheme functions as intended. The ADC achieves a 7.28 bit effective number of bits (ENOB), providing a theoretical maximum of 176 distinct energy bins. In addition the average equivalent noise charge (ENC) is measured to be 96 e- per pixel, and 306 e- per column. Our experimental results demonstrate that FDM is a viable approach for spectral X-ray imaging and point toward a scalable path for future high-resolution SPC imagers.