The Search for Vulnerable Plaque: Mission Accomplished?

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 - Plaque

The hunt for vulnerable plaque has been the holy grail of cardiology for years. The theory driving this mission seems nice and tight. If a cardiologist can detect vulnerable plaque, he or she can undertake some action to treat it and prevent a future myocardial infarction. However, the mission remains a work-in-progress, and the end goals may be off the mark.

The first issue is a good news/bad news fact that relates to detection of vulnerable plaque. The good news is there are multiple imaging modalities with some utility at detecting vulnerable plaque. The bad news? None of them are ideal.

The other challenge relates to the goal. Can one really prevent a future myocardial infarction by detecting vulnerable plaque? According to Armin Arbab-Zadeh, MD, PhD, of the department of medicine, cardiology division at Johns Hopkins University in Baltimore, the answer is no. In a review article (J Am Coll Cardiol 2015; 65:846-55), Zadeh challenges colleagues to drop their focus on vulnerable plaque and instead focus on atherosclerotic disease burden.

Cardiovascular Business unveils the advances and issues regarding imaging vulnerable plaque.

Stable plaque, vulnerable plaque: A brief review

Stable plaque is just that—stable. It typically progresses slowly with each decade of life until symptoms appear, and then a physician attempts to address it as best as he or she can.

In contrast, unstable or vulnerable plaque does not have to become significant to cause problems. It can impinge on the lumen of the vessel and rupture, releasing lipids and igniting the inflammatory process within the vessel as the body tries to heal itself. If the vessel is large enough, a heart attack results.

Thus, the cardiology community is on a quest for a way to identify these vulnerable plaques. Several options are on the table. 

Many arrows in the quiver

“We have so many imaging modalities, and every one of them has plus and negative points,” says Annapoorna S. Kini, MD, from the division of cardiology at Mount Sinai Hospital in New York City.

Intravascular ultrasound (IVUS) has the longest history. For nearly 30 years, IVUS has served as a standard method for cardiovascular imaging. It’s routinely used and readily available in most cath labs. It can image deep into vessels. On the downside, IVUS does not allow assessment of cap thickness, one of the most important features of vulnerable plaque.

Optical coherence tomography (OCT) is a newer imaging modality that provides better imaging resolution than IVUS; OCT proponents suggest the modality can accurately image plaque components and calcium with greater resolution than IVUS. However, OCT cannot image as deeply into the vessel as IVUS, nor does it have decades of randomized study data that IVUS has.

Near-infrared spectral imaging (NIRS) produces a chemogram that quantifies the underlying chemical composition of the vessel wall and displays a probability of lipid core plaque presence using a color-coded scale. But NIRS does not provide information on the depth of lipid, nor does it provide geographic orientation inside the coronary artery.

Experts agree no single modality has emerged as optimal for imaging vulnerable plaque. In fact, it may be a hybrid approach that ultimately meets the goal of imaging vulnerable plaque.

“The combination of IVUS, NIRS, OCT, fluorescence imaging and photoacoustic tomography technology in hybrid catheters will provide new synergistic information,” wrote Pranav M. Patel, MD, from the division of cardiology at the University of California, Irvine in Catheterization and Cardiovascular Interventions online October 20, 2014.

Is hybrid imaging the answer?    

Patel and his colleagues have developed a prototype hybrid OCT-IVUS imaging technique. “Our thought was that combining the two modalities might give us the best of both worlds,” he explains. “What happens if you approach any interventional cardiologist and tell him or her you have three systems? The first is an ultrasound device that looks inside the vessel, but the images have yellow haze and the resolution is not so great. The other system gives you great images, but you can’t image as deeply or identify lipid. The third gives you the features of both. Which will he choose?” asks Patel. Most, if not all, will opt for the hybrid approach.

Early research backs up the merit of this approach.

Patel and his colleagues have reviewed IVUS, OCT and hybrid OCT-IVUS images of autopsy vessels with cardiologists and asked them to