INTRAVASCULAR ULTRASOUND : THREE-DIMENSIONAL APPLICATIONS AND FORWARD VIEWING

Atherosclerosis is a diffuse disease process that results in the accumulation of fatty, fibrous, calcific, and inflammatory tissue within the arterial wall. As atherosclerosis expands, it causes narrowing of the arterial lumen with resultant decreased blood flow. This blood flow further decreases to a point that there is markedly diminished flow reserve beyond the atherosclerotic narrowing. The end result is a myocardial infarction, stroke, or organ failure. The pathogenesis of atherosclerosis is multifactorial, being in part caused by genetic chromosomal alterations, in part by metabolic causes (the classic examples are increased cholesterol or renal failure), and in part by other concurrent pathologic processes occurring in the body such as hypertension and endothelial cell dysfunction. Regardless of the numerous factors that cause atherosclerosis, the end result is devastating. Determination of the extent and severity of atherosclerosis is extremely important. In individual patients, the severity of atherosclerosis is necessary to decide the way treatment approaches are planned. If, for example, severe atherosclerosis is formed in a certain way, medical therapy could be given to remove lipid components or take away the potential of the atheroma to rupture and occlude that artery (i.e., aspirin). If the atheroma consists of fibrous and calcific components, other techniques such as interventions (angioplasty, atherectomy, or bypass) may be necessary. In groups of individuals, it is important to identify the extent and severity of atherosclerosis to evaluate new therapies aimed at decreasing atheroma. For many years, the hallmark of coronary and peripheral atherosclerosis was believed to be an apparent localized obstruction of the artery. For more than three decades, the significance of these obstructions had been detected by angiography as percent stenosis. Unfortunately, one of the concepts developed using animal studies is that percent stenosis can identify the severity of arterial obstructions. 9 Pathologists for many years have suggested that atherosclerosis is much more extensive a disease process than the angiogram predicts. 1,18 The authors' group in an arterial imaging study using high-frequency ultrasound evaluated the extent and severity of coronary atherosclerosis and compared the findings to what angiography would predict. 12 In those patients in whom angiography predicted that there would be little or no atherosclerotic disease, in the vast majority of arterial segments, there was diffuse atherosclerosis that could not be identified by angiography. These data were some of the first patient data to demonstrate that diffuse atherosclerosis is present and much more widespread even when angiography demonstrates only discrete lesions. Why does this occur? Arterial remodeling is the thinning and remodeling of the arterial media in an attempt to allow atherosclerosis to continue to enlarge while preserving the luminal area of the underlying artery. Pathologists since the late 1950s suggested that this phenomenon may actually occur in humans. 2,5 This phenomenon was not demonstrated in vivo in animals and patients, however, until the late 1980s and 1990s. The authors 13 and others including Hermiller and colleagues 6 have demonstrated the phenomenon of in vivo coronary arterial remodeling using high-frequency epicardial and intravascular ultrasound (IVUS). This remodeling process allows extensive atherosclerosis to form and be present before techniques such as angiography can demonstrate that atheroma has occurred. Remodeling has been demonstrated to occur in many arteries, including the coronaries, carotids, and iliacs. Therefore, because of its inability to identify the extent of atherosclerosis in the presence of arterial remodeling, it is not surprising that angiography has a poor correlation to the functional and anatomic severity of arterial disease. Additionally, atherosclerosis is a markedly variable disease. Not only are the components variable, but also the shape and deposition of atherosclerotic lesions are markedly variable within different segments of an artery. Pathologists and physiologists have demonstrated markedly irregular and severe atherosclerotic plaques in one portion of an artery surrounded by little, diffuse, or irregular disease throughout the rest of that artery. 11,19 New imaging techniques have been developed that can evaluate the extent of atherosclerosis. Modifications of these techniques allow three-dimensional reconstruction (3DR) of the arterial segment such that a true picture of the artery, including the lumen, wall, features of the wall, and atheroma, can be identified. The implications of providing 3DR of arterial segments are profound in that a true picture of the arterial bed can be presented such that determination of the components and severity of the atherosclerotic process can be made in individual patients at diagnosis. Exciting new developments with IVUS imaging catheters allow the device to view ahead of the catheter without narrowing or obstructing the arterial segment. This article provides the reader with some idea of the principles and techniques of 3DR using intravascular imaging data and describes new IVUS imaging devices that have the ability to interrogate the arterial wall ahead of the imaging catheter.