Intravascular ultrasound

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

The arteriesof the heart(the coronary arteries) have been the most frequent imaging target. IVUS is used in the coronary arteries to determine the amount of diseaseat 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 stenosisof the opening of the coronary arteriesthan possible with angiography.

Advantages over Angiography

Arguably the most valuable use of IVUS has been in research to better understand the behavior of the atherosclerosisprocess in living people. Based on the angiographicview and long popular medical beliefs, it had long been assumed that areas of high grade narrowingof the openingwithin the coronary arteries, visible by angiography, were the likely points at which most heart attackswould occur.

However, IVUS enables more accurately visualizing not only the lumenof the coronary arteriesbut 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-stenosisproblem after angioplastylead to recognition that most of the re-stenosisproblem, as visualized by an angiographyexamination was not true re-stenosis. Instead it was simply a remodeling of atheromatous plaque, still protruding into the lumenof the artery after angioplastycompletion; the stenosisonly appearing to be reduced because radiocontrastagent was now flowing around some of the plaque. The radiocontrastflow around the plaque creates a more open, wider radiocontrastshadow width on the angiographicimage, despite persistent narrowingof the lumenby the plaquewithin. This recognition promoted more frequent use of stentsto 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 lumenand is not recognized on angiographyexaminations. 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 angiographyand IVUS examination, to study which coronary lesions most commonly result in a heart attack. The studies revealed that most heart attacksoccur at areas with extensive atheromawithin the artery wall, however very little narrowingof the artery opening. The range of lumennarrowinglocations at which heart attacksoccurred ranged from areas of mild dilatation all the way to areas of severe, greater than 95%, narrowing. However the average or typical narrowingsat which heart attacksoccurred were found to be small, ~20 to 30%, severities long considered insignificant by many. Only 14% of heart attacksoccurred at locations with 75% or more narrowing, the severe narrowingspreviously thought by many to present the greatest danger to the individual. This research has changed the primary focus for heart attackprevention from severe narrowingto 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-colibacteria, to reduce atheromasize/volume within human coronary arteries of people with unstable angina. This was in contrast to the ususal, without the extra HDLipoproteinsupplements, atheromavolume 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 angioplastyand 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 thrombosisof the artery.

Disadvantages versus angiography only

IVUS technology is not inexpensive. The specialized, computerized IVUS echocardiographicrecording 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 angiographyexamination.

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 angioplastyand stentballoon catheters, a potential major advance, but limited by complexity, cost and increased bulk of the catheters.

Method

To visualize an arteryor vein, angiographictechniques 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 angiographycatheters and into the blood vesselbranch to be imaged.

The ultrasoundcatheter tip is slide in over the guidewire and positioned, using angiographytechniques so that the tip is at the farthest away position to be imaged. The sound waves are emitted from the cathetertip, are usually in the 10-20 MHz range, and the catheteralso receives and conducts the return echo information out to the external computerized ultrasoundequipment which constructs and displays a real time ultrasoundimage of a thin section of the blood vesselcurrently surrounding the cathetertip, usually displayed at 30 frames/second image.

The guidewire is kept stationary and the ultrasoundcatheter 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) atheromatousdisease 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 arterywall in living animals and humans. It has lead to an explosion of better understanding and research on both (a) the behavior of the atherosclerosisprocess and (b) the effects of different treatment strategies for changing the evolution of the atherosclerosisdisease process. This has been important given that atherosclerosisis 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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It uses material from the http://en.wikipedia.org/wiki/Intravascular+ultrasound Wikipedia article Intravascular ultrasound.