Theoretical and practical aspects of digital angiography for quantitative coronary flow studies

Summary

Digital angiography offers the potential benefits of densitometric parameter extraction for functional studies of the coronary circulation. Special emphasis is placed on coronary flow and flow reserve measurements for assessment of the severity of critical coronary stenoses. Basically, two approaches are discussed: (1) the use of a large region-of-interest (ROI) for summated density measurements of the total amount of contrast medium that traverses the vascular bed of a myocardial perfusion segment; and (2) the analysis of temporal variations in contrast mass at each picture element allowing the display of functional information in a parametric image.

ROI-densitometry is used to derive numerical data on contrast flow, vascular volume and myocardial transit-time, while functional imaging provides information on myocardial appearence time of the contrast bolus as well as regional thickness of the perfused vasculature. There is theoretical and practical evidence, that the reliability of angiographically determined flow is essentially dependent on specific angiographic requirements, among which selective coronary injections with high flow rates and concomitant backflow of contrast medium into the aorta are of particular importance. Nevertheless, experimental studies with nonstenosed vessels demonstrate great variability in flow data indicating that the extent of coronary flow reserve is not solely determined by the degree of maximal vasodilatation but also by the spontaneous and highly variable resistance in the control state. In view of these findings it is perhaps now appropriate to reconsider the need for methodologic improvements in quantitative flow studies and to focus on the physiologic and angiographic factors affecting the flow.