We have been developing a digital fluoroscopic imaging system to replace the portal films that are currently used to verify patient positioning during radiotherapy treatments. Our system differs from previously reported devices in the construction of the detector and in the operation of the TV camera. The signal, noise, and signal-to-noise properties of this system have been determined by measuring the modulation transfer function [MTF(f)], the noise power spectra [NPS(f)], and by calculating the detective quantum efficiency [DQE(f)] of the system. The results show: (i) that the spatial resolution of the system is determined largely by the lens of the TV camera and by frame grabber; and (ii) that the noise in the system is dominated by the secondary light quanta, due to the poor light collection efficiency of the optical chain. Despite these physical limitations, a contrast-detail study shows that the fluoroscopic system is better at detecting large, low contrast objects than portal films. Therefore the system is already a reasonable alternative to portal films and modifications to the metal plate/phosphor detector, lens, TV camera, and frame grabber should improve the performance of the system further.