Additive noise properties of active matrix flat‐panel imagers

Abstract

A detailed theoretical and empirical investigation of additive noise for indirect detection, active matrix flat-panel imagers (AMFPIs) has been performed. Such imagers comprise a pixelated array, incorporating photodiodes and thin-film transistors (TFTs), and an associated electronic acquisition system. A theoretical model of additive noise, defined as the noise of an imaging system in the absence of radiation, has been developed. This model is based upon an equivalent-noise-circuit representation of an AMFPI. The model contains a number of uncorrelated noise components which have been designated as pixel noise, data line thermal noise, externally coupled noise, preamplifier noise and digitization noise. Pixel noise is further divided into the following components: TFT thermal noise, shot and 1/f noise associated with the TFT and photodiode leakage currents, and TFT transient noise. Measurements of various additive noise components were carried out on a prototype imaging system based on a 508 microm pitch, 26 x 26 cm2 array. Other measurements were performed in the absence of the array, involving discrete components connected to the preamplifier input. Overall, model predictions of total additive noise as well as of pixel, preamplifier, and data line thermal noise components were in agreement with results of their measured counterparts. For the imaging system examined, the model predicts that pixel noise is dominated by shot and 1/f noise components of the photodiode and TFT at frame times above approximately 1 s. As frame time decreases, pixel noise is increasingly dominated by TFT thermal noise. Under these conditions, the reasonable degree of agreement observed between measurements and model predictions provides strong evidence that the role of TFT thermal noise has been properly incorporated into the model. Finally, the role of the resistance and capacitance of array data lines in the model was investigated using discrete component circuits at the preamplifier input. Measurements of preamplifier noise and data line thermal noise components as a function of input capacitance and resistance were found to be in reasonable agreement with model predictions.

Image quality vs radiation dose for a flat-panel amorphous silicon detector: a phantom study

“…For lower doses, such as in fluoroscopy, the DQE falls rapidly due to the influence of the additive noise levels [18]. The role of the flat-panel detector 1 2 3 1 2 3 1 2 3 1 2 3 Flat-panel detector 400 speed 34 29 29 34 37 36 31 29 30 27 26 24 600 speed 36 39 32 52 49 47 34 39 33 33 28 30 800 speed 47 35 40 58 53 56 41 43 43 35 32 31 Storage phosphor plates 40 41 41 47 48 52 41 41 40 33 33 28 Screen film 37 39 32 46 42 40 40 39 35 22 24 22 in fluoroscopic imaging has not been evaluated in this study.…”

Theoretical investigation of the design and performance of a dual energy (kV and MV) radiotherapy imager

“…In this equation, the value of the data line capacitance (C data ) was conservatively estimated using the capacitance per unit length of an array with a large pixel pitch, 47 scaled to the data line length of typical MV AMFPIs (i.e., 40 cm).…”

Active pixel imagers incorporating pixel‐level amplifiers based on polycrystalline‐silicon thin‐film transistors

“…This results in a significant reduction in detective quantum efficiency ͑DQE͒ under conditions of low exposure or high spatial frequencies 1 and is particularly pronounced for fluoroscopy 1,2 as well as for applications requiring small pixels, such as mammography and breast tomosynthesis. 3,4 In the case of fluoroscopy, it has been shown that the DQE of AMFPIs is inferior to that of image intensifiers ͑XRIIs͒ at the lower end of the exposure range ͑0.1-1 R͒ 5 -where the limited system gain of AMFPIs results in the additive noise 6 becoming significant relative to x-ray quantum noise.…”