Intravascular Ultrasound - Advantages Over Angiography

Advantages Over Angiography

Arguably the most valuable use of IVUS is to visualize plaque, which cannot be seen by angiography. It has been increasingly used in research to better understand the behavior of the atherosclerosis process in living people. Based on the angiographic view and long popular medical beliefs, it had long been assumed that areas of high grade stenosis (narrowing) of the lumen (opening) within the coronary arteries, visible by angiography, were the likely points at which most myocardial infarctions (heart attacks) would occur. Research using IVUS has helped to reveal the fallacy (in most instances) of this belief.

IVUS enables accurately visualizing not only the lumen of the coronary arteries but also the atheroma (membrane/cholesterol loaded white blood cells) "hidden" within the wall. IVUS has thus enabled advances in clinical research providing a more thorough perspective and better understanding.

In the early 1990s, IVUS research on the re-stenosis problem after angioplasty lead to recognition that most of the re-stenosis problem (as visualized by an angiography examination) was not true re-stenosis. Instead it was simply a remodeling of the atheromatous plaque, which was still protruding into the lumen of the artery after completion of angioplasty; the stenosis only appearing to be reduced because blood and contrast could now flow around and through some of the plaque. The angiographic dye column appeared widened adequately; yet considerable plaque was within the newly widened lumen and the lumen remained partially obstructed. This recognition promoted more frequent use of stents to hold the plaque outward against the inner artery walls, out of the lumen.

Additionally, IVUS examinations, as they were done more frequently, served to reveal and confirm the autopsy research findings of the late 1980s, showing that atheromatous plaque tends to cause expansion of the internal elastic lamina, causing the degree of plaque burden to be greatly underestimated by angiography. Angiography only reveals the edge of the atheroma that protrudes into the lumen.

Perhaps the greatest contribution to understanding, so far, was achieved by clinical research trials completed in the United States in the late 1990s, using combined angiography and IVUS examination, to study which coronary lesions most commonly result in a myocardial infarction. The studies revealed that most myocardial infarctions occur at areas with extensive atheroma within the artery wall, however very little stenosis of the artery opening. The range of lumen stenosis locations at which myocardial infarctions occurred ranged from areas of mild dilatation all the way to areas of greater than 95% stenosis. However the average or typical stenosis at which myocardial infarctions occurred were found to be less than 50%, describing plaques long considered insignificant by many. Only 14% of heart attacks occurred at locations with 75% or more stenosis, the severe stenoses previously thought by many to present the greatest danger to the individual. This research has changed the primary focus for heart attack prevention from severe narrowing to vulnerable plaque.

Current clinical uses of IVUS technology include checking how to treat complex lesions before angioplasty and checking how well an intracoronary stent has been deployed within a coronary artery after angioplasty. If a stent is not expanded flush against the wall of the vessel, turbulent flow may occur between the stent and the wall of the vessel; some fear this might create a nidus for acute thrombosis of the artery.