Pattern of Doppler flow indices at the carotid bifurcation - Evaluation by hemodynamic color Doppler imaging

Objective. To visualize the spatial variation of Doppler indices, principally the pulsatility Index, taken proximal to the carotid bifurcation and to evaluate their relationship to the geometry of the carotid bulb. Methods. The pattern of ultrasonographic Doppler indices was studied in healthy volunteers by using hemodynamic color Doppler imaging, which computes and displays a Doppler index at each color pixel from a sequence of color Doppler image frames taken over several cardiac cycles. Results. In carotid bulbs with laminar flow (n = 5), the spatial partitioning between low-resistance internal carotid artery and high-resistance external carotid artery flows could be followed over 5 cm upstream in the common carotid artery. However, normal reverse or vortex flows at the carotid bulb (n = 15) obliterated upstream flow partitioning within 2 cm of the flow divider. The pulsatility index was neither laterally nor axially uniform in the common carotid artery. Conclusions. Localization of core flow where meaningful Doppler indices may be measured is determined by the expansion geometry of the carotid bulb and usually requires positioning of a small sample volume in the center of the lumen at least 3 cm upstream from the flow divider. However, in the absence of reverse or vortex flows, placement of a spectral Doppler sample volume is best guided by hemodynamic color Doppler imaging.