Image quality in portal imaging suffers significantly from the loss in contrast and spatial resolution that results from the excessive Compton scatter associated with megavoltage x rays. In addition, portal image quality is further reduced due to the poor quantum efficiency (QE) of current electronic portal imaging devices (EPIDs). Commercial video-camera-based EPIDs or VEPIDs that utilize a thin phosphor screen in conjunction with a metal buildup plate to convert the incident x rays to light suffer from reduced light production due to low QE ( for Eastman Kodak Lanex Fast-B). Flat-panel EPIDs that utilize the same luminescent screen along with an a-Si:H photodiode array provide improved image quality compared to VEPIDs, but they are expensive and can be susceptible to radiation damage to the peripheral electronics. In this article, we present a prototype VEPID system for high quality portal imaging at sub-monitor-unit (subMU) exposures based on a thick scintillation crystal (TSC) that acts as a high QE luminescent screen. The prototype TSC system utilizes a thick transparent CsI(Tl) (thallium-activated cesium iodide) scintillator for , resulting in significantly higher light production compared to commercial phosphor screens. The CsI(Tl) screen is coupled to a high spatial and contrast resolution Video-Optics plumbicon-tube camera system (, width at isocenter, ADC). As a proof-of-principle prototype, the TSC system with user-controlled camera target integration was adapted for use in an existing clinical gantry (Siemens ) with the capability for online intratreatment fluoroscopy. Measurements of modulation transfer function (MTF) were conducted to characterize the TSC spatial resolution. The measured MTF along with measurements of the TSC noise power spectrum (NPS) were used to determine the system detective quantum efficiency (DQE). A theoretical expression of DQE(0) was developed to be used as a predictive model to propose improvements in the optics associated with the light detection. The prototype TSC provides with its current imaging geometry, which is an order of magnitude greater than that for commercial VEPID systems and comparable to flat-panel imaging systems. Following optimization in the imaging geometry and the use of a high-end, cooled charge-coupled-device (CCD) camera system, the performance of the TSC is expected to improve even further. Based on our theoretical model, the expected for the TSC system with the proposed improvements, which exceeds the performance of current flat-panel EPIDs. The prototype TSC provides high quality imaging even at subMU exposures (typical imaging dose is ), which offers the potential for daily patient localization imaging without increasing the weekly dose to the patient. Currently, the TSC is capable of limited frame-rate fluoroscopy for intratreatment visualization of patient motion at , since the achievable frame rate is significantly reduced by the limitations of the camera-control processor. With optimized processor control, the TSC is expected to be capable of intratreatment imaging exceeding to monitor patient motion.