Intravascular ultrasound

From Academic Kids

Intravascular ultrasound (IVUS) is an medical imaging methodology using (a) specially designed long thin complex manufactured catheters attached to (b) computerized ultrasound equipment. It enables applying ultrasound technology to see from inside blood vessels out through the surrounding blood column, visualize the inner wall of blood vessels and especially the anatomy of the walls of blood vessels in living individuals, while awake and without pain.

The arteries of the heart (the coronary arteries) have been the most frequent imaging target. IVUS is used in the coronary arteries to determine the amount of disease at any particular point in one of the coronary arteries regardless the presence or absence of any stenosis. Additionally, IVUS also facilites much more accurate measurement of the degree of stenosis of the opening of the coronary arteries than possible with angiography.

Contents

Advantages over Angiography

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 visualizing not only the lumen of the coronary arteries but also the atheroma "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 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.

Additionally, IVUS examinations, as they were done more frequently, especially at research centers, served to reveal and confirm the autopsy research findings published by Steven Glasgov, M.D. in 1987, that most of the atherosclerotic process, contrary to many popular illustrations, exists primarily outside the lumen and is not recognized on angiography examinations. Angiography, only reveals the tips of protruding atheroma, "icebergs", and usually not the ones representing the greatest danger to the individual's health.

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 heart attack. The studies revealed that most heart attacks occur at areas with extensive atheroma within the artery wall, however very little narrowing of the artery opening. The range of lumen narrowing locations at which heart attacks occurred ranged from areas of mild dilatation all the way to areas of severe, greater than 95%, narrowing. However the average or typical narrowings at which heart attacks occurred were found to be small, ~20 to 30%, severities long considered insignificant by many. Only 14% of heart attacks occurred at locations with 75% or more narrowing, the severe narrowings 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.

Research coordinated by the Cleveland Clinic, and published in the Journal of the American Medical Association in January 2004, relied on IVUS technology to evaluate the beneficial effects of five small weekly intravenous doses of a genetic human variant of HDLipoprotein, called apo-a1-Milano, synthesized in E-coli bacteria, to reduce atheroma size/volume within human coronary arteries of people with unstable angina. This was in contrast to the ususal, without the extra HDLipoprotein supplements, atheroma volume progression, also demonstrated by the trail in those people who were randomized to the placebo arm (i.e. saline infusion only without the synthetic apo-a1-Milano HDLipoprotein) of the treatment comparison trial.

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.

Disadvantages versus angiography only

IVUS technology is not inexpensive. The specialized, computerized IVUS echocardiographic recording and display equipment generally costs over $200,000, US, 2004. The specialized, one-time-use catheters, used to do each examination, typically cost ~$1,500-2,000, US, 2004. Additionally, IVUS adds significant additional examination time and some increased risk to the patient beyond performing only an angiography examination.

Thus, as of 2004, IVUS remains a better research tool than clinical tool.

IVUS continues to improve and some manufacturers have proposed building IVUS technology into angioplasty and stent balloon catheters, a potential major advance, but limited by complexity, cost and increased bulk of the catheters.

Method

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 computerized 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 guidewire 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 motorized pullback tends to be smoother than hand movement by the physician.)

The (a) blood vessel wall inner lining, (b) atheromatous disease within the wall and (c) connective tissues covering the outer surface of the blood vessel are echogenic, i.e. they return echos 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).

Uses

IVUS, as outlined above, has been the best technology, so far, to demonstrate the anatomy of the artery wall in living animals and humans. It has lead to an explosion of better understanding and research on both (a) the behavior of the atherosclerosis process and (b) 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.

Because IVUS is also complex, time consuming and expensive, beyond research uses, it is not commonly available or used as a clinical tool in arterial disease treatment, including in most tertiary medical centers in the US.

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