RELATION OF THE FLOW-FIELD DISTAL TO A MODERATE STENOSIS TO THE DOPPLER POWER

An experimental investigation was undertaken to establish how differen t flow regimes affect the Doppler signal. A rigid tube model consistin g of a 70% asymmetric area stenosis was used with steady and pulsatile flow conditions. The characteristics of the flow field at various sit es was determined using a photochromic flow visualization method. Cont inuous-wave Doppler measurements were made using a 41% suspension of h uman red blood cells (RBCs) in saline as well as a dilute suspension o f 4% fixed RBCs. For steady flow, the photochromic results indicated t hat for Reynolds numbers (Re) of 545 and 1410, turbulence was generate d and the length of the turbulent region was found to increase with in creasing Re. Under pulsatile flow conditions, turbulence was triggered around peak systole and began to dissipate in late deceleration, and by the end of diastole the flow field almost relaminarized. During the turbulent phase of the flow cycle, the poststenotic flow field was se en to consist of four distinct flow regimes similar to those observed for steady flow. For higher Womersley parameters and Reynolds numbers the turbulent zone was found to be larger and to occupy a greater frac tion of the flow cycle. These flow visualization results were compared with the Doppler power measurements made at the same locations and un der similar flow conditions. At physiological hematocrits (41%) the on set of turbulence for both steady and pulsatile flow increased the bac kscattered Doppler power. The location of the peak Doppler power coinc ided with the region of maximum turbulence observed using the photochr omic technique. Copyright (C) 1997 World Federation for Ultrasound in Medicine & Biology.