'Napkin-ring Sign' on CT Identifies High-Risk Coronary Lesions

January 25, 2013

BOSTON — Researchers are proposing a new way of classifying angiography findings on CT that they say may do a better job of distinguishing high-risk, unstable plaques from those that are less likely to pose acute problems. Dr Pál Maurovich-Horvat (Semmelweis University, Budapest, Hungary), while working in Dr Udo Hoffmann's group at Massachusetts General Hospital, has and colleagues have identified a ringlike attenuation pattern of coronary atherosclerotic plaque, dubbed the "napkin-ring sign," that they say can identify advanced coronary lesions with a specificity of 99%.

"Early CT scans did not have very good spatial and temporal resolution," lead investigator Maurovich-Horvat told heartwire . "With those scanners, they were able to identify calcified plaques, noncalcified plaques, and mixed plaque. It's a very simple way of classifying the plaque, but the challenge with the existing technology has been to further differentiate these plaques into low-risk and high-risk, such as plaques that are rich in lipids."

The study, conducted along with US researchers from Massachusetts General Hospital and the CVPath Institute, is published in the December 2012 issue of the Journal of the American College of Cardiology: Imaging.

The introduction of 256-slice and 320-slice CT scanners has improved temporal and spatial resolution and this, along with better reconstruction algorithms and postprocessing tools, has helped operators  to better identify structures within the coronary plaque. Maurovich-Horvat and colleagues previously published data identifying novel plaque attenuation patterns. The researchers identified a ringlike attenuation of the noncalcified portion of the coronary atherosclerotic lesions, which they called the napkin-ring sign. The napkin-ring sign is indicative of an advanced lesion rich in lipids and has been associated with high-risk plaques in other studies.

In the new study, the researchers performed CT angiograms in seven explanted hearts (six donors had died of natural causes, one of suicide), simulating the physiological conditions of living hearts, and intraluminal contrast to mime the kind of enhancement seen in in vivo hearts. In all, 21 coronary arteries were imaged using CCTA and categorized according to conventional plaque categories (noncalcified, mixed, or calcified plaque) as well as according to plaque patterns (homogenous or heterogeneous with or without napkin-ring sign). After the imaging studies, the arteries were excised, prepared, and sectioned for histological analysis to confirm the findings derived from the plaque pattern seen on CCTA and to see whether that plaque pattern could differentiate between early and advanced coronary lesions.

In total, 611 histological sections were obtained and coregistered with the CCTA images. No plaques were identified in 21.9% of the sections, noncalcified plaques were identified in 41.6% of the sections, and mixed plaques were identified in 31.3% of histological sections. These mixed and noncalcified plaques were then further classified according to plaque patterns.

With the histological results used as benchmarks, researchers were able to identify plaques categorized on CCTA as noncalcified or mixed as advanced lesions with a moderate specificity of 57.9% and 72.1%, respectively, similar to the specificity yielded by plaques categorized as homogenous (with no different attenuation patterns within the plaque) and heterogeneous plaques (with at least two regions of visually distinguished attenuation on CCTA).

By contrast, the specificity of the napkin-ring sign to identify coronary lesions confirmed histologically was 98.9%.

"This mean that if you see the napkin-ring sign on coronary CTA, it's almost 100% that this is a high-risk lesion or even a thin-cap fibrous atheroma," said Maurovich-Horvat. "This is important if you like to use CT as a gatekeeper." He added that if the napkin-ring sign is identified with CT, clinicians might opt for optical coherence tomography (OCT) to confirm that the lesion is indeed high risk. Some have even proposed a role for prophylactic stenting in this setting. A bioabsorbable stent, for example, could be implanted to eliminate the risk associated with the vulnerable plaque, Maurovich-Horvat suggested.

However, such strategies are still some ways down the road. While there is a hope that the novel nomenclature--homogenous, heterogeneous, and napkin-ring sign plaques--can help identify at-risk patients, there is still a need to conduct large-scale prospective clinical trials to assess the prognostic ability of the napkin-ring sign lesion on CCTA to predict future clinical events.