An in vivo Comparison of Principal and Polar Strains in Carotid Atherosclerotic Plaques

Rupture of carotid atherosclerotic plaques is one of the major causes of cerebrovascular events. Polar strains in the cross-sectional carotid elastography have been demonstrated to identify vulnerable plaques. However, the vessel center used to calculate the polar strains is difficult to estimate precisely in carotid arteries with plaques. To solve this problem, principal strains which can be measured without identification of the vessel center are proved consistent with polar strains in healthy volunteers. In this study, we compared the performance of principal and polar strains in assessing the elasticity of carotid atherosclerotic plaques. Ultrasound radiofrequency data were acquired from 13 carotid plaques of 9 patients, in the transversal imaging views. The inter-frame axial and lateral strains were obtained using an affine model-based spatial angular compounding method, which were further transformed into the polar (radial and circumferential) and principal (minor and major) strains. The maximum, mean, median, standard deviation (SD) and 99^th percentile were calculated from the absolute value of each strain for comparison. Statistically significant correlations were found between the maximum, mean, median, SD and 99^th percentile calculated from minor principal strain and radial polar strain, as well as between the indices from major principal strain and circumferential polar strain. These findings demonstrate significant correlations between polar and principal strains, and principal strains may be useful indices for assessing the vulnerability of carotid atherosclerotic plaques.