In our series of studies on noninvasive assessment of the regional elasticity of the arterial wall, the displacement gradient (change in thickness) of the arterial wall caused by the heartbeat was measured by the phased tracking method [IEEE Trans. UFFC, 44, 1996]. Since the displacement gradient corresponds to the strain due to the change in blood pressure, the elasticity can be evaluated from the displacement gradient of the arterial wall and the blood pressure, which are noninvasively measured at the upper arm. In the measurement of the elasticity of the arterial wall by our method, the region where the elastic modulus is estimated must be assigned beforehand, and currently, the lumen-intima boundary - of the arterial wall is manually determined by the operator. For the real-time measurement of the elasticity of the arterial wall, a fast, automated method is necessary for detection of the boundary. In this paper, a cost function is proposed for differentiation of the arterial wall from the lumen. The proposed cost function was applied to ultrasound data, which were noninvasively obtained for 5 human carotid arteries. In comparison with the case of detection using only the amplitude of the echo, the root mean square error between the automatically detected lumen-intima boundary and the manually assigned boundary was significantly improved by using the proposed cost function. Furthermore, the lumen-intima boundary was automatically detected in a short period (0.26 s, Pentium III 1.2 GHz).
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