Abstract: A method for compression of digital data representing a series of time-dependent images (frames) of an object of interest, as might be produced by an X-ray angiography imaging system of a cardiovascular process. The amount of data is reduced with only an insignificant reduction of the relevant information content therein. According to one aspect of the invention, the method compresses the data by isolating only data pertaining to an object of interest. The excess data, that which is not isolated, is discarded or otherwise compressed. According to a second aspect of the invention, if the data represents a series of periodic time-dependent images the period of the series and an optimal first frame are determined. Thereafter, only one such period is retained. The period is determined by determining the period of a closeness function of consecutive frames. The optimal first image is determined from an extremum in the closeness function.