A prototype amorphous silicon array based radiotherapy portal imager

The trend toward highly conformal radiation fields in the treatment of cancer has increased the need for accurate verification of field placement.Conventionally, the placement has ben verified on a weekly basis using a film radiograph, or more recently electronic portal imaging devices (EPIDs). Fluoroscopic EPIDs consisting of a phosphor screen, one or more mirrors, lens, and camera provide reasonable performance but suffer from the poor collection efficiency and bulky nature of their optical components. Large area a-Si:H arrays provide an ideal replacement for the optical subsystem of fluoroscopic EPIDs. The purpose of this work is to (1) characterize the performance and logistics of a prototype a-Si:H array and (2) determine the metal plate/phosphor screen combination which will maximize the system's detective quantum efficiency (DQE) for megavoltage imaging. The prototype imager is based on a 10 X 10 cm² a-Si:H array consisting of 128 X 128 sensor elements coupled to a Gd₂O₂S:Tb screen and 0.5 mm Al plate. The charge signal collected in each diode is digitized to 16 bits at frame rates of up to 25 frames/sec. Dark current and readout noise were determined under controlled conditions. The optical collection efficiency of the photodiodes and the escape fraction of optical quanta from the screen were estimated from the literature. X-ray quantum absorption efficiency, number of optical quanta from the screen were estimated from the literature. X-ray quantum absorption efficiency, number of optical quanta produced, and the Poisson excess associated with energy absorption and conversion in the plate/screen system were calculated using EGS4 Monte Carlo simulations. Using these results, the imaging system performance is estimated using the approach of Cunningham et al for both a 6 MV x-ray spectrum and a ⁶⁰Co spectrum. For both the 6 MV and ⁶⁰Co beams, the predicted system gain was approximately 60 percent of the measured value. For a 6 MV beam, the system DQE (f equals 0) was calculated to be 1.45 percent at clinical doses. Reducing the does had little effect on the DQE (f equals 0), indicating that the dark current and readout noise are not significant factors at f equals 0.