Abstract: In a digital intravenous subtraction angiography system an X-ray generator provides low mA continuous X-ray exposures illuminating a standard image intensifier producing an image scanned by a conventional television camera to provide a video signal. An analog-to-digital converter converts the video signal into digital form. The digital frame signals are added together in real time to provide an intermediate digital signal representing the addition of typically 5 to 20 frames. A digital disk receives and stores the intermediate image signals. A subsequent intermediate image signal is added to a second memory while a previously formed intermediate image signal is transferred from a first memory to disk storage. The first and second memories are operated in "ping pong" fashion so that each and every video frame signal during the acquisition period, typically 15 seconds, is summed to form one of the intermediate images.