Current video-based electronic portal imaging devices (VEPIDs) that
utilize thin phosphor screens have poor image quality due to low light
collection. Expensive flat-panel imagers with the same screen yield higher
quality imaging at 2–4 monitor-units (MU) exposures, but are susceptible to
radiation damage to the peripheral electronics. A prototype thick scintillation
crystal (TSC) VEPID for subMU exposures is presented here. Utilizing a 12 mm
thick CsI(Tl) crystal scintillator for greater light production, the detective
quantum efficiency (DQE) is significantly increased for the prototype TSC (17
× 17 cm
$^{2}$
field-of-view at
isocenter) while spatial resolution is not affected by the increased optical
path. A high-resolution plumbicon camera with 12-bit ADC and
computer-controlled target integration provides variable exposure imaging.
Modulation transfer function (MTF) and DQE measurements along with phantom
imaging are presented for the TSC, commercial VEPIDs, flat-panels and film. The
DQE for TSC is significantly higher than current VEPIDs, and comparable to
flat-panel at low frequency. TSC provides portal imaging superior to other
VEPIDs at subMU exposures. The TSC provides high quality portal imaging and
with further optimization is expected to be better or comparable to
flat-panels. TSC subMU imaging has potential for intra-treatment patient
position monitoring, and increased portal verification frequency for patient
localization.
