A NEW PERIVASCULAR MULTIELEMENT PULSED DOPPLER ULTRASOUND SYSTEM FOR IN-VIVO STUDIES OF VELOCITY-FIELDS AND TURBULENT STRESSES IN LARGE VESSELS

A pulsed Doppler ultrasound (PDU) multi-element system was developed f or perivascular registration of velocity fields and turbulence in larg e vessels. In vivo evaluation and comparison with hot-film anemometry (HFA) was performed. C-shaped shells were designed with holes to fit f ive small 10 MHz ultrasonic probes directed at five measuring points a long a diameter perpendicular to the vessel axis. By rotating the shel l in 45 degrees steps, blood velocities were measured in 17 points cov ering the entire cross-sectional vessel area. Measurements were perfor med in the ascending aorta and at three axial locations in the descend ing thoracic aorta in pigs. Simultaneous PDU and HFA measurements were performed distal in induced vascular stenoses of different degrees. T hree-dimensional visualisation of velocity profiles was made, and Reyn olds normal stresses (RNS) were calculated for different levels of tur bulence intensities based on the simultaneous PDU and HFA measurements . The velocity profiles in the ascending aorta were skewed at top syst ole with the highest velocities towards the posterior wall. In the des cending thoracic aorta at the ligamentum of Botalli, the velocity prof iles were skewed wall. Further downstream in the descending aorta the velocity profiles appeared blunter. The frequency response of the modi fied PDU system was determined by a 'random noise test' revealing an u pper -3dB cut-off frequency of approximately 200 HZ. Regression analys is showed a linear relationship between RNS measured with PDU and RNS measured with HFA (r = 0.93). Two vessel diameters distal to a 75% ste nosis RNS up to 28 N m(-2) were measured. The present perivascular PDU system is able to register velocity profiles covering the entire vess el area in a plane perpendicular to the flow axis, as well as turbulen t velocity fluctuations within the restrictions imposed by the Doppler ambiguity process. Compared with HFA, PDU is easier to calibrate, eas ier to handle, semi-invasive, direction-sensitive, but still suffers f rom range-velocity limitations a limited frequency response.