Reduction of influence of decrease in SNR of ultrasonic pulse during cardiac cycle in measurement of small change in thickness of arterial wall

We have previously proposed a method for measuring the small change in thickness of the arterial wall during the cardiac cycle, which cannot be measured from conventional B-mode or M-mode images because of its small amplitude of less than 100 micrometer. However, in spite of its great usefulness for in vivo assessment of the local elasticity of the arterial wall, there are cases where successful measurements based on only two consecutive echo signals fail because of low signal-to-noise ratio (SNR) during the cardiac cycle. To realize the proposed method in practical use, this problem must be overcome. In this paper, we propose a method that is more robust in the presence of noise. In this method, more than two echo signals are employed in estimation of their phase shift for reducing the influence of low SNR. Simulation experiments provide the optimal value of the estimation period (the number of echo signals) for each SNR. By evaluating the SNR of in vitro experimental data, the optimal estimation period is determined. In in vivo experiment, by using the optimal estimation period of 20 ms, the change in thickness can be measured with good reproducibility even in the case of low SNR of 28 dB.