Stochastic Global Optimization: Two-Phase Methods.

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. A selected mask image signal is subtracted from any or all intermediate image signals to provide an enhanced subtracted image signal. Various mask image signals may be subtracted so that the operator may decide by visual inspection of the subtracted image signals which mask image signal minimizes misregistration artifacts. Each intermediate image signal or each subtraction intermediate image signal is weighted approximately proportional to the contrast agent intensity to form a weighted sum signal. The operator may exclude from this combining process any intermediate signal which the operator has determined is not suitably registered. An intermediate image signal may be processed to provide a phantom image signal through translation and/or rotation of the intermediate image signal.