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Thick, segmented CdWO 4 ‐photodiode detector for cone beam megavoltage CT: A Monte Carlo study of system design parameters
Author(s) -
Monajemi T. T.,
Fallone B. G.,
Rathee S.
Publication year - 2006
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 - 10.1118/1.2370503
Subject(s) - optics , detective quantum efficiency , photodiode , detector , physics , monte carlo method , optical transfer function , photon , beam (structure) , beam divergence , materials science , beam diameter , image quality , laser , statistics , mathematics , artificial intelligence , laser beams , computer science , image (mathematics)
Megavoltage (MV) imaging detectors have been the focus of research by many groups in recent years. We have been working with segmentedCdWO 4crystals in contact with photodiodes in our lab. The present study uses both x‐ray and optical photon transport Monte Carlo simulations to analyze the effects of scintillation crystal height, septa material, beam divergence, and beam spectrum on the modulation transfer function, MTF ( f ) and zero frequency detective quantum efficiency, DQE(0), of a theoretical area detector. The theoretical detector is comprised of tall, segmentedCdWO 4crystals and two dimensional photodiode arrays with a pitch of 1 mm and a fill factor of 72%. Increasing the crystal height above 10 mm does not result in an improvement in the DQE(0) if the reflection coefficient of the septa is less than 0.8. For a reflection coefficient of 0.975 for the septa, there is a continual gain in the DQE(0) up to 30 mm tall crystals. Similar calculations show that employing a 3.5 MV beam without a flattening filter increases the DQE(0) for 20 mm tall crystals by 9% compared to a typical 6 MV beam with a flattening filter. The severe degradations due to beam divergence on MTF ( f ) are quantified and suggest the use of focused detectors in MV imaging. It is found that when the effect of optical photons is considered, the presence of divergence can appear as a shift in the location of the input signal as well as loss of spatial resolution.