Performance analysis of several generations of flat panel x-ray imagers based on polycrystalline silicon TFTs

Active matrix flat-panel imagers (AMFPIs) have become ubiquitous in medical imaging environments. AMFPIs are based on two-dimensional pixelated arrays coupled to various x-ray converter materials that provide either indirect or direct detection of the incident x-ray radiation. However, the capabilities of this technology are severely constrained by the underlying solid-state properties of the amorphous silicon semiconductor material employed in the thin-film transistors present in each array pixel. The considerably higher electron and hole mobilities of polycrystalline silicon, a semiconductor material that (like amorphous silicon) is well suited to fabrication of transistors for large area electronics, provide the potential to overcome these constraints. To explore this potential, a series of prototype arrays based on increasingly complex pixel designs employing polycrystalline silicon transistors is under development by our collaboration. The designs include three generations of active pixel prototypes designed for fluence mode operation that incorporate sophisticated circuits with the goal of improving imaging performance as well as circuit elements created to facilitate initial explorations of single photon counting with polycrystalline silicon circuits. In this presentation, the design and operation of these devices will be described and an early analysis of their signal and noise performance will be presented. The results will be based on various forms of theoretical modeling involving techniques such as cascaded systems analysis and circuit simulations, supplemented with information obtained from empirical studies of prototype poly-Si devices. The research is supported by NIH grant R01 EB000558.