Historically, several different electronic technologies have been utilized to replace the role of radiographic films in capturing the images, including the scanning liquid ionization chamber array and camera-based systems viewing a phosphor scintillating screen. By modern standards, this first generation of electronic portal imaging devices (EPIDs) produced somewhat poor quality radiological images and possessed some promising but very limited dosimetric characteristics. The modern a-Si flat-panel EPID is based on thin-film semiconductor technology. Desirable characteristics of any dosimeter include linearity, reproducibility, high spatial resolution, no dead time, and real-time readout. The incident beam fluence shape is a dosimetric characteristic of the beam and may be of interest in EPID dosimetric applications. Image ghosting refers to the change in individual pixel gains due to the trapped charge modifying the electric field strength in the photodiode, thus allowing prior irradiations to effect charge-collection efficiency and therefore effect EPID response.