Inhomogeneous acoustic pressure field of CW ultrasound in relation to spectral power density distribution of blood-flow velocity in the carotid artery

As previously reported, a telemetry system for measuring blood velocity in exercising subjects by using continuous-wave (CW) ultrasonic Doppler method has been developed. The spectral power density (SPD) distribution of flow velocity reflects the spatial velocity profile in the blood vessel. In this report, we investigated the acoustic pressure fields of semicircular transducers across the blood vessel in both mediums without attenuation and with attenuation, and their effects on SPD distribution of flow velocity. We calculated the 3-D geometric acoustic pressure fields of 8.0-mm-diameter blood vessels in 20.0 mm and 40.0 mm depth from the skin by a numerical integration of all elements of the divided transducers, There were 25 and 9 dB attenuation of the acoustic pressure field in the peripheries of blood vessels comparing with the centers, respectively. If the ultrasonic Doppler signal is measured by the two semicircular transducers, the SPD of flow velocity, however, would become inhomogeneous. We obtained 55%, and 30% differences in SPDs between the axial and peripheral velocities for blood vessels in 20.0 and 40.0 mm depth by computer simulation. No obvious difference of the SPD distribution of flow velocity between medium with attenuation and without attenuation was obtained by using the parameters in this report. The theoretical results were verified by a model experiment.