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Measurement of quantum noise in fluoroscopic systems for portal imaging
Author(s) -
Mah Dennis W.,
Rowlands J. A.,
Rawlinson J. Alan
Publication year - 1996
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 - 10.1118/1.597794
Subject(s) - detective quantum efficiency , shot noise , quantum noise , optics , physics , noise (video) , image noise , noise spectral density , amplifier , optoelectronics , quantum , noise figure , image quality , computer science , detector , cmos , quantum mechanics , artificial intelligence , image (mathematics)
In fluoroscopic portal imaging systems, a metal plate is bonded to a phosphor screen and together these act as the primary x‐ray sensor. The light from the screen is collected and imaged by a lens on the target of a video camera. The demagnification ( M ) between the large area of the phosphor being imaged and the small active area of the video camera results in poor optical coupling between the screen and the video camera. Consequently x‐ray quantum noise is small compared to other noise sources. By reducing the demagnification, the light from the screen is collected more efficiently, so we were able to increase the x‐ray quantum noise relative to other noise sources and thus unambiguously identify it. The noise power spectrum was measured as a function of M to determine the relationship between the x‐ray quantum noise, shot noise, and amplifier noise. It was found by extrapolation to clinical demagnifications that the amplifier noise dominates x‐ray quantum noise at all spatial frequencies, but the shot noise was less than the x‐ray quantum noise at low spatial frequencies. For low spatial frequencies, this implies that a secondary quantum sink can be avoided. If amplifier noise could be sufficiently reduced, x‐ray quantum limited images could be obtained in clinical systems at low spatial frequencies.