Pixel distribution analysis of B-mode ultrasound scan images predicts histologic features of atherosclerotic carotid plaques.

BACKGROUND The correlation of B-mode ultrasonographic morphology with histologic characteristics of atherosclerotic carotid plaques remains ill-defined. The classification of plaques with recently reported measures of plaque echogenicity and heterogeneity has been unsatisfactory. We used computer-assisted duplex ultrasound (DU) scan image analysis to determine echogenicity of specific tissues in control subjects. This information was used to quantify each tissue in imaged carotid plaques with pixel distribution analysis (PDA). These objective observations then were quantitatively compared with plaque histology in symptomatic and asymptomatic patients. METHODS We performed standardized DU scanning of healthy tissues in 10 volunteer subjects and of 20 carotid artery plaques (7 symptomatic and 13 asymptomatic) in 19 patients with carotid stenosis. The plaques underwent histologic analysis after carotid endarterectomy. The grayscale intensity ranges of blood, lipid, fibromuscular tissue, and calcium were calculated in the control subjects. With computer-assisted image analysis, B-mode images of plaques were linearly scaled to normalize data. Pixel distribution within the images then was analyzed. The grayscale ranges of known tissues obtained from control subjects helped define the amount of intraplaque hemorrhage, lipid, fibromuscular tissue, and calcium within carotid plaque images. This analysis was correlated with tissue composition measurements on histologic sections of excised plaques. RESULTS The median grayscale intensity (range) in control subjects was 2 (0 to 4) for blood, 12 (8 to 26) for lipid, 53 (41 to 76) for muscle, 172 (112 to 196) for fibrous tissue, and 221 (211 to 255) for calcium. PDA-derived predictions for blood, lipid, fibromuscular tissue, and calcium within carotid plaques correlated significantly with the histologic estimates of each tissue respectively (blood: P =.012; lipid: P =.0006; fibromuscular: P =.035; and calcium: P =.0001). A significantly higher amount of blood and lipid was seen within symptomatic plaques compared with asymptomatic ones (P =.0048 and P =.026, respectively). Conversely, a larger amount of calcification was noted within asymptomatic plaques (P =.0002). CONCLUSION Computer-assisted PDA of DU scan images accurately quantified intraplaque hemorrhage, fibromuscular tissue, calcium, and lipid. Symptomatic plaques had lower calcium content but larger amounts of intraplaque hemorrhage and lipid. Quantitative PDA may be used to determine carotid plaque tissue composition to assist in the identification of symptomatic and potentially unstable asymptomatic plaques.