1. A data compression-expansion method for use in digital subtraction angiographic systems for improving the signal to noise ratio of the expanded data while maintaining the fidelity of the data; said method comprising the steps of: acquiring data, digitizing the acquired data to provide uncompressed digital data, using a transfer function separable digital filter which filters out noise and has a minimal effect on the digital data, said separable filter comprising a filter that has frequency response of approximately 1 over the applicable frequency range with a slight fall off at the high end of the range, said separable filter having a first portion and a second portion, filtering the uncompressed digital data to smooth the uncompressed digital data with said first portion of the separable filter, compressing the smooth digital data by coding to reduce the quantity of the smooth digital data while maintaining the amplitude of the retained data, operating on the compressed smooth digital data, expanding the compressed smooth digital data after said operating step to restore the quantity of the smooth digital data present prior to the compressing step, and inherently restoring most of the digital data removed by the first portion of the separable filter by filtering the expanded digital data with said second portion of the separable filter to enhance the expanded digital data.