Digital angiography

The great increase in data achieved for a given dose of radiation as compared with screen film recording, leaves no doubt that progressively more radiographic imaging will be recorded and manipulated using digital electronics and displayed on high resolution television systems. Digital techniques are particularly useful for rapid manipulation of multiple images. If the failure of interest does not change it can be clarified by adding or averaging the images which will even out the distribution of noise. If X-ray absorption in the feature of interest differs in two images while the background is unchanged, subtraction of the images leaves an unobstructed view of the region of interest; the relative contrast gain more than overcomes the inevitable increase in noise in the subtracted image (X/2) thus increasing 'conspicuity'. Differences in X-ray absorption are induced in three ways: (1) by making images with X-rays at different energy levels (dual or multiple energy subtraction) or (2) by making images before and after intravascular injection of contrast medium (temporal subtraction) or (3) combining both procedures (hybrid subtraction). Although some systems in use have a lower specification, that quoted here is the current recommendation