Intravascular ultrasound (IVUS) is a medical imaging methodology using a specially designed catheter with a miniaturized ultrasound probe attached to the distal end of it.
The proximal end of the catheter is attached to computerized ultrasound equipment. It allows the application of ultrasound technology to see from inside blood vessels out through the surrounding blood column, visualizing the endothelium (inner wall) of blood vessels in living individuals. The arteries of the heart (the coronary arteries) are the most frequent imaging target for IVUS.
IVUS is used in the coronary arteries to determine the amount of atheromatous plaque built up at any particular point in the epicardial coronary artery. This build up of plaque leads to stenosis (narrowing) of the artery (known as coronary artery lesions). IVUS is of particular use to determine the degree of stenosis in situations in which angiographic imaging is considered unreliable, such as for ostial lesions and when angiography cannot visualize the segment adequately, particularly if there is a lot of overlapping arterial segments.
Arguably the most valuable use of IVUS has been in research to better understand the behaviour 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.
However, IVUS enables more accurately visualizing not only the lumen of the coronary arteries but also the atheroma (cholesterol plaque) "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 remodelling of the atheromatous plaque, still protruding into the lumen of the artery after angioplasty completion; the stenosis only appearing to be reduced because the plaque burden was shifted somewhat into the wall of the artery. This recognition promoted more frequent use of stents to hold the plaque 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.
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.
The primary disadvantages of IVUS being used routinely in a cardiac catheterization laboratory are its expense, the increase in the time of the procedure, and the fact that it is considered an interventional procedure, and should only be performed by angiographers that are trained in interventional cardiology techniques. In addition, there may be additional risk imposed by the use of the IVUS catheter. Additionally, IVUS adds significant additional examination time and some increased risk to the patient beyond performing a standard diagnostic angiographic examination. This increase is significantly less when IVUS is part of a percutaneous coronary intervention, since much of the setup is the same for the intervention as for the IVUS imaging.
To visualize an artery or vein, angiographic techniques are used and the physician positions the tip of a guidewire, usually 0.014" diameter with a very soft and pliable tip and about 200 cm long. The physician steers the guidewire from outside the body, though angiography catheters and into the blood vessel branch to be imaged. The ultrasound catheter tip is slide in over the guidewire and positioned, using angiography techniques so that the tip is at the farthest away position to be imaged. The sound waves are emitted from the catheter tip, are usually in the 10-20 MHz range, and the catheter also receives and conducts the return echo Information out to the external computerised ultrasound equipment which constructs and displays a real time ultrasound image of a thin section of the blood vessel currently surrounding the catheter tip, usually displayed at 30 frames/second image.
The guide wire is kept stationary and the ultrasound catheter tip is slid backwards, usually under motorized control at a pullback speed of 0.5 mm/s. (The motorised pullback tends to be smoother than hand movement by the physician.) The blood vessel wall inner lining, atheromatous disease within the wall and connective tissues covering the outer surface of the blood vessel are echogenic, i.e. they return echoes making them visible on the ultrasound display. By contrast, the blood itself and the healthy muscular tissue portion of the blood vessel wall is relatively echolucent, just black circular spaces, in the images. Heavy calcium deposits in the blood vessel wall both heavily reflect sound, i.e. are very echogenic, but are also distinguishable by shadowing. Heavy calcification blocks sound transmission beyond and so, in the echo images, are seen as both very bright areas but with black shadows behind (from the vantage point of the catheter tip emitting the ultrasound waves).
IVUS as outlined above, has been the best technology, so far, to demonstrate the anatomy of the artery wall in living animals and humans, and is now available in multi-colour format to enable better clinical interventions. It has lead to an explosion of better understanding and research on both the behaviour of the atherosclerosis process and the effects of different treatment strategies for changing the evolution of the atherosclerosis disease process. This has been important given that atherosclerosis is the single most frequently devastating disease process for the greatest percentage of individuals living in first world countries.
Compiled by JD Sandham IEng MIET MIHEEM