Ultrasonic measurement of arterial wall for quantitative diagnosis of atherosclerosis--elasticity imaging of arterial wall and atherosclerotic plaque

To provide useful information for the diagnosis of atherosclerosis in addition to the imaging of morphology using B-mode ultrasonography, we have developed a method, namely, the phased-tracking method, to measure the small change in the thickness of the arterial wall due to the heartbeat. This change in thickness corresponds to strain due to the change in internal pressure, and the elasticity of the arterial wall is obtained by the simultaneous measurement of the change in thickness and pulse pressure. Furthermore, an elasticity image can be classified into tissue components using the reference data obtained by in vitro experiments because the elastic properties are different among the tissue components in the arterial wall. We have measured the elasticity distributions of lipids, blood clots, fibrous tissue, and calcified tissue in vitro. From these results, it was found that arterial tissues can be classified into soft tissues (lipids, blood clots) and hard tissues (fibrous tissue, calcified tissue) on the basis of their elasticity. However, it is difficult to differentiate lipids from blood clots and fibrous tissue from calcified tissue because the elasticity distributions of these components overlap each other. To overcome this problem, we proposed a tissue classification method in which, the elasticity distribution of each small region of interest (not a single pixel) in an elasticity image was used in the classification of lipids, blood clots, fibrous tissue, and calcified tissue, respectively. Tissue classification results obtained by this method showed good agreement with the pathological image of the corresponding section.