Empirical performance of polycrystalline silicon pixel circuit components for monolithic, large-area photon counting arrays

Abstract

Photon counting detectors (PCDs) offer several advantages for medical x-ray imaging compared to conventional imagers. However, current PCDs, whose circuits are fabricated using crystalline silicon semiconductor material, are not well-suited for large-area imaging applications such as breast CT and kilovoltage cone-beam CT (kV CBCT) in radiotherapy. To address this challenge, prototype PCDs based on polycrystalline silicon (a semiconductor better suited for manufacture of large-area devices) were created and, in this paper, an empirical determination of the maximum count rate of individual pixel circuit components (amplifier, comparator, clock generator and counter) corresponding to those prototypes is reported. For each circuit component, test input pulses (which were generated so as to approximate those in a complete pixel circuit) were used to obtain output response waveforms from which count rate was determined. The maximum count rate (in units of counts per second, cps) for the amplifier was determined to be 20.8 kcps while that of the comparator, clock generator and counter components were determined to be 1.2 Mcps, 98 kcps and 4.9 Mcps, respectively. The comparator and counter components provide count rates beyond that required for breast CT (∼108 kcps at a pixel pitch of 330 μm) while those components as well as the clock generator exceed the rate required for kV CBCT in radiotherapy (∼72 kcps at a pitch of 400 μm). It is anticipated that new polycrystalline silicon circuit designs for the amplifier and clock generator could provide count rates sufficient for both applications.