Carotid Ultrasound in One, Two and Three Dimensions

Ultrasound measurements of the carotid wall are widely used surrogate markers for atherosclerosis with the intima-media thickness (IMT) being the most commonly measured variable. However, ultrasound has recently defined carotid artery wall structures in higher spatial dimensions, such as total area of carotid plaques (TPA) and total volume of carotid plaques (TPV). It is possible that IMT, TPA and TPV measure different aspects of the atherosclerotic process and thus have different clinical implications. IMT, TPA and TPV have never been correlated in the same individuals. We had a unique dataset in which IMT, TPA and TPV were measured concurrently in 272 subjects. We evaluated the left and right carotid arterial systems in each subject as independent variables and performed three pairwise correlation analyses between IMT and transformed TPA and TPV from the 544 carotid arterial systems. We found pairwise correlations between IMT and square root TPA, IMT and cubic root TPV and square root TPA and cubic root TPV; these correlation coefficients were 0.667, 0.586 and 0.963, respectively (all P<0.0001). However, correlation graphs showed marked scatter of individual data points. Thus, IMT was only moderately well correlated with TPA and TPV, but TPA and TPV were very strongly correlated with each other. This suggests that IMT is less related to TPA and TPV than they are to each other and could reflect somewhat different aspects of atherosclerosis. Furthermore, the high correlation between TPA and TPV suggests that both might not need to be measured together in future analyses.