A new perivascular multi-element pulsed Doppler ultrasound system forin vivo studies of velocity fields and turbulent stresses in large vessels

A pulsed Doppler ultrasound (PDU) multi-element system was developed for perivascular registration of velocity fields and turbulence in large vessels. In vivo evaluation and comparison with hot-film anemometry (HFA) was performed. C-shaped shells were designed with holes to fit five small 10 MHz ultrasonic probes directed at five measuring points along a diameter perpendicular to the vessel axis. By rotating the shell in 45° steps, blood velocities were measured in 17 points covering the entire cross-sectional vessel area. Measurements were performed in the ascending aorta and at three axial locations in the descending thoracic aorta in pigs. Simultaneous PDU and HFA measurements were performed distal to induced vascular stenoses of different degrees. Three-dimensional visualisation of velocity profiles was made, and Reynolds normal stresses (RNS) were calculated for different levels of turbulence intensities based on the simultaneous PDU and HFA measurements. The velocity profiles in the ascending aorta were skewed at top systole with the highest velocities towards the posterior wall. In the descending thoracic aorta at the ligamentum of Botalli, the velocity profiles were skewed throughout the entire systole with the highest velocities at the right anterior vessel wall. Further downstream in the descending aorta the velocity profiles appeared blunter. The frequency response of the modified PDU system was determined by a ‘random noise test’ revealing an upper −3dB cut-off frequency of approximately 200 Hz. Regression analysis showed a linear relationship between RNS measured with PDU and RNS measured with HFA (r=0.93). Two vessel diameters distal to a 75% stenosis RNS up to 28 N m−2 were measured. The present perivascular PDU system is able to register velocity profiles covering the entire vessel area in a plane perpendicular to the flow axis, as well as turbulent velocity fluctuations within the restrictions imposed by the Doppler ambiguity process. Compared with HFA, PDU is easier to calibrate, easier to handle, semi-invasive, direction-sensitive, but still suffers from range-velocity limitations and a limited frequency response.