AbstractID: 9755 Title: A Prototype High Quantum Efficiency Thick Scintillator Video EPID for Sub-MU
Imaging
Megavoltage x-ray imaging inherently suffers from poor spatial and contrast resolutions compared to diagnostic x-ray imaging due to
the dominance of Compton scattering in the former. Hence it is necessary to minimize image degradation within the electronic portal
imaging device (EPID) used for imaging. Conventional video EPIDs (VEPIDs), which typically use a powder gadolinium oxysulfide
screen as a scintillator, have poor image quality compared to film. Flat panel imagers generally yield high quality imaging but are
susceptible to radiation damage to the electronics and are expensive. A prototype VEPID, with a 26x24 cm2 field of view at isocenter,
utilizing a thick crystal and a vidicon camera on a clinical gantry has been developed. A 12mm thick clear CsI(Tl) crystal is used as a
scintillator to overcome the low light output common to conventional VEPIDs. The spatial resolution is not adversely affected by the
use of a thick scintillator as seen from measurements of MTF. A high resolution vidicon camera with a 12-bit ADC is used to further
improve image contrast and spatial resolution (250 µm pixel size at isocenter). Such a system offers not only an alternative to flat
panel imagers but also sub-MU exposure imaging, which is not possible with current EPIDs. Sub-MU imaging offers the potential of
intra-treatment imaging to monitor patient movements. Also it permits daily portal imaging for patient localization without exposing
the patient to higher levels of dose than currently received with weekly portal imaging using film.