Theoretical investigation of the count rate capabilities of in-pixel amplifiers for photon counting arrays based on polycrystalline silicon TFTs.

Purpose: Photon counting arrays (PCAs), capable of measuring the spectral information of individual x-ray photons and recording that information digitally, provide a number of advantages compared to conventional, energy-integrating active matrix flat-panel imagers - such as reducing the undesirable effects of electronic readout noise and Swank noise. While contemporary PCAs are based on crystalline silicon, our group has been examining the use of polycrystalline silicon (poly-Si, a semiconductor material better-suited for the manufacture of large-area devices) for such arrays. In this study, a theoretical investigation of the front-end amplifiers of array pixels incorporating photon counting circuits is described - building upon circuit simulation techniques developed in a previous study. Results for amplifier circuit designs corresponding to prototype PCAs currently under development, as well as for hypothetical circuit designs identified in the study, are reported. In the simulations, performance metrics (such as signal gain, linearity of signal response, and energy resolution) as well as various measures of count rate are determined.

Methods: The simulations employed various input energy distributions (i.e., a 72 kVp spectrum as well as monoenergetic x rays) in order to determine circuit performance. To make the results representative of the properties of poly-Si, the simulations incorporated transistor characteristics that were empirically obtained from test devices. Optimal operating conditions for the circuits were determined by applying criteria to the performance metrics and identifying which conditions minimized settling time. Once the optimal operating conditions were identified, trains of input pulses simulating x-ray flux were used to determine two measures of count rate corresponding to dead time losses of 10% and 30% (referred to as CR and CR , respectively).

Results: The best-performing prototype amplifier design (implemented at a pixel pitch of 1 mm) exhibited CR and CR values (expressed in counts per second per pixel) of 8.4 and 21.6 kcps/pixel, respectively. A hypothetical amplifier design was derived by modifying transistor, resistor, and capacitor elements of the prototype amplifier designs. This hypothetical design (implemented at a pitch of 1 mm) exhibited CR and CR values of 154 and 381 kcps/pixel, respectively. When implemented at a pitch of 0.25 mm, the performance of that design increased to 210 and 491 kcps/pixel, respectively (corresponding to counts per second per unit area of 3.4 and 7.9 Mcps/mm ).

Conclusions: The simulation methodology described in this paper represents a useful tool for identifying promising designs for the amplifier component of photon counting arrays, as well as evaluating the analog signal and noise performance of those designs. The results obtained from the current study support the hypothesis that large-area, photon counting arrays based on poly-Si transistors can provide clinically useful count rates. Encouraged by these early results, further development of the methodology to assist in the identification and evaluation of even more promising designs, along with development and empirical characterization of prototype designs, is planned.