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X‐ray imaging using avalanche multiplication in amorphous selenium: Investigation of depth dependent avalanche noise
Author(s) -
Hunt D. C.,
Tanioka Kenkichi,
Rowlands J. A.
Publication year - 2007
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2437097
Subject(s) - avalanche photodiode , detective quantum efficiency , optics , noise (video) , digital radiography , quantum noise , single photon avalanche diode , avalanche breakdown , active matrix , flat panel detector , electric field , physics , materials science , optoelectronics , detector , computer science , image quality , radiography , voltage , quantum , computer vision , breakdown voltage , nanotechnology , thin film transistor , layer (electronics) , quantum mechanics , nuclear physics , image (mathematics)
The past decade has seen the swift development of the flat‐panel detector (FPD), also known as the active matrix flat‐panel imager, for digital radiography. This new technology is applicable to other modalities, such as fluoroscopy, which require the acquisition of multiple images, but could benefit from some improvements. In such applications where more than one image is acquired less radiation is available to form each image and amplifier noise becomes a serious problem. Avalanche multiplication in amorphous selenium ( a ‐ Se ) can provide the necessary amplification prior to read out so as to reduce the effect of electronic noise of the FPD. However, in direct conversion detectors avalanche multiplication can lead to a new source of gain fluctuation noise called depth dependent avalanche noise. A theoretical model was developed to understand depth dependent avalanche noise. Experiments were performed on a direct imaging system implementing avalanche multiplication in a layer of a ‐ Se to validate the theory. For parameters appropriate for a diagnostic imaging FPD for fluoroscopy the detective quantum efficiency (DQE) was found to drop by as much as 50% with increasing electric field, as predicted by the theoretical model. This drop in DQE can be eliminated by separating the collection and avalanche regions. For example by having a region of low electric field where x rays are absorbed and converted into charge that then drifts into a region of high electric field where the x‐ray generated charge undergoes avalanche multiplication. This means quantum noise limited direct conversion FPD for low exposure imaging techniques are a possibility.