Atheromatous plaque cap thickness can be determined by quantitative color analysis during angioscopy: Implications for identifying the vulnerable plaque

Background: Coronary angioscopy in acute myocardial infarction has frequently revealed disrupted yellow lesions. Furthermore, postmortem studies have demonstrated that these lesions have thin collagenous caps with underlying lipid-rich cores. Hypothesis: We hypothesized that the yellow color is due to visualization of reflected light from the lipid-rich yellow core through a thin fibrous cap. Thus, quantification of yellow color saturation may estimate plaque cap thickness and identify vulnerable plaques. Methods: To test this hypothesis, the feasibility of detecting cap thickness was tested using both a model of lipid-rich plaque and human atherosclerotic plaque. The model was constructed by injecting a yellow beta-carotene-lipid emulsion subendothelially into normal bovine aorta. Human plaque was obtained from cadaver aorta. Digitized images were obtained by angioscopy, and percent yellow saturation was analyzed using a custom computer program. Plaque cap thickness was measured by planimetry of digitized images on stained tissue sections. Percent yellow saturation was then correlated with plaque cap thickness. Results: In the bovine model, plaque cap thickness and percent yellow saturation correlated inversely (r(2) = 0.91; p = 0.0001). In human plaques, yellow saturation was significantly greater in atheromatous than in white plaques (p<0.0004). Also, there was a high correlation between plaque cap thickness and yellow saturation at various angles of view between 40degrees and 90degrees, the greatest between 50degrees and 80degrees (r(2) = 0.75 to 0.88). Conclusion: Plaque cap thickness is a determinant of plaque color, and this can be assessed by quantitative colorimetry. Thus, plaque color by angioscopy may be useful for detecting vulnerable plaques.