Noninvasive assessment of atherosclerosis

N ONINVASIVE evaluation of atherosclerosis has two potential goals: to evaluate blood flow patterns in patients who have symptoms and to detect and evaluate the progress of latent disease. Most noninvasive vascular procedures currently used in clinical medicine are included in a battery of tests to determine 1) who should undergo selective angiography; 2) who should go directly to corrective vascular surgery; and 3) who could safely forego both angiography and surgery. For this purpose, a simple positive or negative test result is usually all that is required. But several noninvasive tests are now beginning to find application in the detection and assessment of atherosclerosis in its presymptomatic phase by measuring rates of growth or change. Feinstein’ has termed this growth rate measurement “auxometry” and one of its most widely known applications to date has been in the prevention of pediatric malnutrition. It may, however, have a potentially powerful role in the treatment of atherosclerosis since atherosclerosis is a disease which has a long latent period during which asymptomatic lesions grow slowly. In addition, a large body of data from animals plus a growing literature of experience in man indicate that the growth of latent lesions can be retarded or reversed by intervention. We predict, therefore, that a major future role for noninvasive procedures which yield vascular images will be in auxometry of atherosclerosis. We also predict that demands for precision and accuracy in imaging procedures will be greater when they are used for auxometry than when they are used clinically: a procedure used to track lesion size over time must be able to detect the spontaneous growth rate of lesions and that has been shown to be up to 4% per year in the femoral arteries.’ Two imaging procedures which offer auxometric potential for preventive treatment of atherosclerosis are ultrasound and digital subtraction angiography. Both seem destined to have a major impact on the future practice of clinical cardiology, but in this article we will concentrate on ultrasound imaging only. The increasing utilization of ultrasound scanning in the clinical setting requires today’s cardiologist to have special appreciation for the physical principles, limitations and potential applications of this developing technology. We will therefore describe in detail the physical principles which are important in determining the quality of ultrasound images.