Intravascular Ultrasound (IVUS)
New imaging method to evaluate coronary arteries more accurately
Dr S M Mustafa Zaman
The international gold standard technique for the detection and evaluation of coronary artery disease is contrast angiography. In recent years a number of limitations of this technique have become apparent; these include the two-dimensional nature of the images, the absence of information about the blood vessel wall, insensitivity to substantial plaque burden in outwardly remodeled vessels and inability to detect vessel wall disruption during angioplasty. To overcome these limitations, intravascular ultrasound (IVUS) was developed.It is a medical imaging methodology using (a) specially designed long thin complex manufactured catheters attached to (b) computerised ultrasound equipment. It enables applying ultrasound technology to see from inside blood vessels out through the surrounding blood column, visualise the inner wall of blood vessels and especially the anatomy of the walls of blood vessels in living individuals, while awake and without pain. In clinical practice IVUS is most often used as an adjunct to balloon angioplasty to detect dissection, stent underdeployment, stent thrombosis and to predict restenosis risk in developed countries. It is also used as an accessory to diagnostic angiography to evaluate lesions of uncertain severity (especially in the left main coronary artery) and to detect disease which is not visible on an angiogram (as in the case of transplant coronary artery disease). The technique is similar to balloon angioplasty -- a 6 to 8 French access sheath and guiding catheter are used through which a conventional angioplasty guidewire is passed to the distal part of the artery under scrutiny. Arguably the most valuable use of IVUS has been 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 narrowing of the opening within the coronary arteries, visible by angiography, were the likely points at which most heart attacks would occur. However, IVUS enables more accurately visualising not only the lumen of the coronary arteries but also the atheroma "hidden" within the wall. IVUS has thus enabled advances 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 visualised by an angiography examination was not true re-stenosis. Instead it was simply a remodeling of atheromatous plaque, still protruding into the lumen of the artery after angioplasty completion; the stenosis only appearing to be reduced because radiocontrast agent was now flowing around some of the plaque. The radiocontrast flow around the plaque creates a more open, wider radiocontrast shadow width on the angiographic image, despite persistent narrowing of the lumen by the plaque within. This recognition promoted more frequent use of stents to hold the plaque out of the lumen. In developed countries, interventional cardiologists hardly think an angioplasty without IVUS. Some conscious patients (who can afford) are going abroad for angioplasty since there is no facility of IVUS in the country. So the widely used imaging technique should also be introduced in our country as soon as possible to assure quality service in coronary artery diseases. The writer is an Assistant Professor of Cardiology of Bangabandhu Sheikh Mujib Medical University.
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