Measurements of regional propagation velocities of forward and reflected pulse waves by high frame rate ultrasonic imaging

Pulse wave velocity (PWV) is widely used to evaluate artery-wall elasticity. In the traditional PWV method, the average PWV is calculated between 2 points, the carotid and femoral arteries, at an interval of several tens of centimeters. However, it is preferable to measure PWV in a more local region because PWVs of carotid and femoral arteries are different and atherosclerosis often forms regional lesions. To estimate regional PWV, in the present study, minute vibration velocities of the carotid arterial wall were measured at intervals of 0.2 mm at M = 72 points assigned along the arterial longitudinal direction using the phased-tracking method at a high temporal resolution of 3472 Hz. In the estimation of PWV, acceleration waveforms were obtained by applying time differentiation to measured velocities to enhance high frequency components because the use of high frequency components will improve the temporal resolution in estimation of time delays among the vibration waveforms occurred by pulse wave propagation. Time delays were estimated using analytic signals of acceleration waveforms obtained by Hilbert transform. PWV was estimated from the slope and intercept of the relationship between the phase angle of the complex correlation function of analytic signals and lag. Carotid arteries of three healthy subjects were measured in vivo. PWVs in short longitudinal segments of 14.4 mm were estimated to be 5.6, 6.4, and 6.7 m/s for the pulse wave component propagating from the heart to the periphery. Estimated PVWs were in good agreement with those reported in literature. Furthermore, in one of the subjects, there was a component reflected by the peripheral arteries. Propagation speed of the reflection component could be also separately estimated to be -8.4 m/s. The higher PWV of the reflection component was considered to be the difference in blood pressure at the arrival times of the forward and reflection components. The proposed method for measurement of regional PWV would be useful for more sensitive evaluation of the change in elasticity due to progress of atherosclerosis.