Theoretical investigation of the noise performance of active pixel imaging arrays based on polycrystalline silicon thin film transistors

Abstract

Purpose Active matrix flat-panel imagers, which typically incorporate a pixelated array with one a-Si:H thin-film transistor (TFT) per pixel, have become ubiquitous by virtue of many advantages, including large monolithic construction, radiation tolerance and high DQE. However, at low exposures such as those encountered in fluoroscopy, digital breast tomosynthesis and breast computed tomography, DQE is degraded due to the modest average signal generated per interacting x-ray relative to electronic additive noise levels of ~1000 e, or greater. A promising strategy for overcoming this limitation is to introduce an amplifier into each pixel, referred to as the active pixel (AP) concept. Such circuits provide in-pixel amplification prior to readout as well as facilitate correlated multiple sampling, enhancing signal-to-noise and restoring DQE at low exposures. In this study, a methodology for theoretically investigating the signal and noise performance of imaging array designs is introduced and applied to the case of AP circuits based on low-temperature polycrystalline silicon (poly-Si), a semiconductor suited to manufacture of large area, radiation tolerant arrays. Methods Computer simulations employing an analog circuit simulator and performed in the temporal domain were used to investigate signal characteristics and major sources of electronic additive noise for various pixel amplifier designs. The noise sources include photodiode shot noise and resistor thermal noise, as well as TFT thermal and flicker noise. TFT signal behavior and flicker noise were parameterized from fits to measurements performed on individual poly-Si test TFTs. The performance of three single-stage and three two-stage pixel amplifier designs were investigated under conditions relevant to fluoroscopy. The study assumes a 20×20 cm2, 150 µm pitch array operated at 30 fps and coupled to a CsI:Tl x-ray converter. Noise simulations were performed as a function of operating conditions, including sampling mode, of the designs. The total electronic additive noise included noise contributions from each circuit component. Results The total noise results were found to exhibit a strong dependence on circuit design and operating conditions, with TFT flicker noise generally found to be the dominant noise contributor. For the single-stage designs, significantly increasing the size of the source-follower TFT substantially reduced flicker noise – with the lowest total noise found to be ~574 e [rms]. For the two-stage designs, in addition to tuning TFT sizes and introducing a low-pass filter, replacing a p-type TFT with a resistor (under the assumption in the study that resistors make no flicker noise contribution) resulted in significant noise reduction – with the lowest total noise found to be ~336 e [rms]. Conclusions A methodology based on circuit simulations which facilitates comprehensive explorations of signal and noise characteristics has been developed and applied to the case of poly-Si AP arrays. The encouraging results suggest that the elect...

Empirical noise performance of prototype active pixel arrays employing polycrystalline silicon thin‐film transistors

“…Computer simulations, based on the formalism detailed in Ref. [29], of the noise of individual pixels having circuits corresponding to the various pixel circuit designs of the prototype arrays were performed for the purposes of comparison with the measured results. The simulations employed the same timings, voltage levels, frame rate, and correlated multiple sampling modes as those used for the array measurements.…”

“…The circuit simulations employed a transistor model based on the same parameter values employed in Ref. [29] where a subset of these values was derived from measurements obtained from individual poly-Si test TFTs. Noise performance was characterized in terms of thermal and flicker (also referred to as 1/f) noise components modeled as current sources between the drain and source of each transistor in the circuit.…”

“…This is an indication that, for TSD-1, TSD-2, TSD-3, and TSD-4, the flicker noise originating from TFT AL dominates that from TFT CSA . 29 Finally, the narrower spread of the measurement results compared to the simulation results may be a consequence of additional noise affecting the measurementswhich is not accounted for in the simulations. One possible candidate for such additional noise is voltage fluctuations on the supply rail (V CSA ) for the common-source amplifier.…”

Performance assessment of active vs passive pixel x‐ray angiography detector systems using a bias‐corrected channelized Hotelling observer and adult patient‐equivalent experimental conditions

“…For the AP array, the electronic gain occurs within an individual detector element. By this method, the desirable signal resulting from x‐ray energy absorption is amplified prior to the ADC, but the electronic readout noise is not amplified …”

“…By this method, the desirable signal resulting from x-ray energy absorption is amplified prior to the ADC, but the electronic readout noise is not amplified. [1][2][3][4][5][6][7][8] Comprehensive assessment of the imaging performance of x-ray angiography systems is required to guide purchase decisions and clinical implementation. However, there are no published works describing x-ray angiography systems which utilize AP detectors.…”