In the first 20 years of the modern era of cardiology, an enormous fund of knowledge and understanding was generated by the development and application of invasive cardiac catheterization and angiographic techniques to the problems of clinical cardiology. Coincident with the ability to evaluate the function of the heart and to visualize the form of the diseased cardiovascular system, new advances in anesthesia and surgery made it possible within a relatively few years to palliate and even, in limited situations, to cure patients whose lesions only shortly before were inoperable.
Because of the complexity, discomfort, and small but substantial morbidity and even mortality, invasive examination of patients with cardiovascular disease has its limitations. This is especially true for the screening of populations for suspected disease and for sequential follow-up evaluation of patients with known cardiac disease. For these reasons, in the past decade there has been a vast increase in