Intravascular ultrasound
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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.
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[edit] 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 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 remodeling 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.[1] Angiography only reveals the edge of the atheroma that protrudes into the lumen.[2]
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.[citation needed] 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.[citation needed] 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%[citation needed], describing plaques long considered insignificant by many. Only 14% of heart attacks occurred at locations with 75% or more stenosis[citation needed], 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.
[edit] Disadvantages versus angiography
The primary disadvantages of IVUS being used routinely in a cardiac catheterization laboratory are it's 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.
The computerized IVUS echocardiographic recording and display equipment generally costs over $200,000, US, 2004. The disposable catheters used to do each examination typically cost ~$1,500-2,000, US, 2004. In many hospitals, the IVUS recording and display equipment is on loan to the catheterization lab from the manufacturing company, with the understanding that the lab has to purchase the specialized IVUS catheters. Because no standard exists, IVUS catheters cannot be interchanged between different manufacturers.
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.
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.
[edit] 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 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 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).
[edit] 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.
[edit] Intravascular ultrasound in the coronary anatomy
While the routine use of IVUS during percutaneous coronary intervention does not improve short term outcomes[3], there are a number of situations in which IVUS is of particular use in the treatment of coronary artery disease of the heart. In particular in cases when the degree of stenosis of a coronary artery is unclear, IVUS can directly quantify the percentage of stenosis and give insight into the anatomy of the plaque.
One particular use of IVUS in the coronary anatomy is in the quantification of left main disease in cases where routine coronary angiography gives equivocal results. Many studies in the past have shown that significant left main disease can increase mortality[4], and that intervention (either coronary artery bypass graft surgery or percutaneous coronary intervention) to reduce mortality is necessary when the left main stenosis is significant.
When using IVUS to determine whether an individual's left main disease is clinically significant, in terms of the desirability of physical intervention, the two most widely used parameters are the degree of stenosis and the minimal lumen area.[5] A cross sectional area of ≤7 mm² in a symptomatic individual[citation needed] or ≤6 mm² in an asymptomatic individual[6] is considered to be clinically significant and warrants intervention to improve one-year mortality. However, these exact cutoffs are up for debate and different cutoff cross-sectional areas may be used in practice depending on differing interpretations of the trial data.
[edit] Validating the efficacy of new treatments
Because IVUS is widely available in coronary catheterization labs woldwide and can accurately quantify arterial plaque, especially within the coronary arteries, it is increasingly being used to evaluate newer and evolving strategies for the treatment of coronary artery disease, including the statins, torcetrapib and other approaches.[7][8]
[edit] References
- ^ Glagov S, Weisenberg E, Zarins CK, Stankunavicius R, Kolettis GJ. (1987). "Compensatory enlargement of human atherosclerotic coronary arteries.". N Engl J Med 316 (22): 1371-5. PMID 3574413.
- ^ Zarins CK, Weisenberg E, Kolettis G, Stankunavicius R, Glagov S. (1988). "Differential enlargement of artery segments in response to enlarging atherosclerotic plaques.". J Vasc Surg 7 (3): 386-94. PMID 3346952.
- ^ Schiele F, Meneveau N, Vuillemenot A, Zhang DD, Gupta S, Mercier M, Danchin N, Bertrand B, Bassand JP. (1998). "Impact of intravascular ultrasound guidance in stent deployment on 6-month restenosis rate: a multicenter, randomized study comparing two strategies--with and without intravascular ultrasound guidance.". J Am Coll Cardiol 32 (2): 320-8. PMID 9708456.
- ^ Abizaid AS, Mintz GS, Abizaid A, Mehran R, Lansky AJ, Pichard AD, Satler LF, Wu H, Kent KM, Leon MB (1999). "One-year follow-up after intravascular ultrasound assessment of moderate left main coronary artery disease in patients with ambiguous angiograms". J Am Coll Cardiol 34 (3): 707-15. PMID 10483951.
- ^ (2002) "Intravascular Ultrasound", in Robert D. Safian, MD, Mark S. Freed, MD: The Manual Of Interventional Cardiology, Third Edition, Royal Oak, Michigan: Physicians' Press, 712. ISBN 1-890114-39-1.
- ^ Jasti V, Ivan E, Yalamanchili V, Wongpraparut N, Leesar MA. (2004). "Correlations between fractional flow reserve and intravascular ultrasound in patients with an ambiguous left main coronary artery stenosis.". Circulation 110 (18): 2831-6. PMID 15492302.
- ^ Nissen SE (2002). "Who is at risk for atherosclerotic disease? Lessons from intravascular ultrasound.". Am J Med 112 (Suppl 8a): 27S-33S. PMID 12049992.
- ^ Nissen SE, Tsunoda T, Tuzcu EM, Schoenhagen P, Cooper CJ, Yasin M, Eaton GM, Lauer MA, Sheldon WS, Grines CL, Halpern S, Crowe T, Blankenship JC, Kerensky R. (2003). "Effect of Recombinant ApoA-I Milano on Coronary Atherosclerosis in Patients With Acute Coronary Syndromes". JAMA 290 (17): 2292-2300. PMID 14600188.
[edit] External links
- Intravascular Ultrasound from Angioplasty.Org
