Minute changes in thickness of less than 100 μm in the arterial wall cannot be measured by conventional B-mode or M-mode imaging. This paper describes a novel noninvasive method (phased tracking) for measuring such minute changes in arterial wall thickness during one cardiac cycle. A method for evaluation of the regional elastic modulus in the circumference direction, Eθ, from the resulting change in wall thickness is also described. The method was used for two in vivo studies: (1) Eθ was determined in two groups consisting of 31 healthy subjects and 95 subclinical subjects with normal wall thickness of the carotid arteries, and the value in the subclinical group was found to be larger than that in the healthy group. (2) For subjects with hyperlipemia, the spatial distribution of Eθ was evaluated for carotid atherosclerotic plaque. Soft inclusions were found in every plaque, and there were differences in elasticity from the surrounding hard tissues. This novel method is a potentially useful diagnostic technique for analysis of both early-stage atherosclerosis and plaque vulnerability.
