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Background: Yellow plaques of higher color grades are regarded as more vulnerable. We tried to elucidate the characteristics of yellow color that are regarded as the sign of vulnerable plaques by an objective and quantitative plaque color analysis.

Methods: The color of yellow plaques was quantitatively analyzed using LCH color space and was presented by the maximum values of lightness (Lmax), chroma (Cmax), and hue (Hmax). Effect of light intensity on these parameters was experimentally examined (five plaque models, six light intensities). Relation between conventional yellow-color grade and LCH parameters was examined (31 plaques). Color analysis with LCH color space was applied (1) to compare the culprit lesions of unstable angina (eight patients) and of stable effort angina (nine patients) and (2) to evaluate the regression of yellow plaque by 80-week statin treatment (four patients, 19 plaques).

Results: Cmax and Hmax were not influenced by light intensity so far as it was within appropriate range. Cmax (13 ± 3, 25 ± 5, and 28 ± 4) became higher and Hmax (178 ± 26, 134 ± 10, and 91 ± 16) became lower as the grade became higher (P < 0.0001). Culprit lesions of unstable angina had higher Cmax (22.8 ± 7.9 vs 14.8 ± 7.5, P = 0.04) and lower Hmax (73.3 ± 19.7 vs 168.0 ± 71.5, P = 0.002) than stable effort angina. Statin treatment reduced Cmax (21.3 ± 9.6 vs 14.6 ± 3.1, P = 0.004) and increased Hmax (82.6 ± 25.8 vs 142.3 ± 54.1, P = 0.0005).

Conclusions: Plaque color was consistently and quantitatively measurable using LCH color space uninfluenced by light intensity of appropriate range. Vulnerable yellow color had high chroma and low hue, indicating vivid and reddish-yellow color.