d Original Contribution RELATIVE BLOOD FLOW CHANGES MEASURED USING CALIBRATED FREQUENCY-WEIGHTED DOPPLER POWER AT DIFFERENT HEMATOCRIT LEVELS SEAN WALLACE,* NICOLA LOGALLO, y KASHIF W. F AIZ, z CHRISTIAN LUND,* RAINER BRUCHER, x and DAVID RUSSELL* *Department of Neurology, Oslo University Hospital, Rikshospitalet, Oslo, Norway; y Department of Neurology, Haukeland University Hospital, Bergen, Norway; z Department of Neurology, Akershus University Hospital, Oslo, Norway; and x Department of Medical Engineering, University of Applied Sciences, Ulm, Germany (Received 10 October 2012; revised 14 April 2013; in final form 21 April 2013) Abstract—In theory, the power of a trans-cranial Doppler signal may be used to measure changes in blood flow and vessel diameter in addition to velocity. In this study, a flow index (FI) of relative changes in blood flow was derived from frequency-weighted Doppler power signals. The FI, plotted against velocity, was calibrated to the zero inter- cept with absent flow to reduce the effects of non-uniform vessel insonation. An area index was also calculated. FIs were compared with actual flow in four silicone tubes of different diameter at increasing flow rates and increasing hematocrit (Hct) in a closed-loop phantom model. FI values were strongly correlated with actual flow, at constant Hct, but varied substantially with changes in Hct. Percentage changes in area indexes, relative to the 4-mm tube, were strongly correlated with tube cross-sectional area. The implications of these results for in vivo use are dis- cussed. (E-mail: seanwallace@live.no) Ó 2014 World Federation for Ultrasound in Medicine & Biology. Key Words: Trans-cranial Doppler, Flow index, Area index, Doppler power,Hematocrit. INTRODUCTION Trans-cranial Doppler (TCD) is a well-established, non- invasive method commonly used to measure blood flow velocity (BFV) in the major intracranial vessels. However, a major limitation of this method is its inability to measure blood flow directly, because blood flow in a vessel is deter- mined by both BFV and vessel cross-sectional area (CSA). In the clinical setting, information regarding changes in blood flow is inferred from changes in BFVon the assump- tion that vessel CSA remains constant. Several studies have validated a relationship between BFV and blood flow in specific clinical situations (Batton et al. 1983; Bishop et al. 1986; Greisen et al. 1984; Haaland et al. 1994; Hansen et al. 1983; Larsen et al. 1994; Lindegaard et al. 1987; Trivedi et al. 1997). In most clinical settings, however, this assumption is either incorrect or unproven (Kontos 1989; Sonesson and Herin 1988; Weyland et al. 1994). This potential source of error may be eliminated if some form of vessel diameter measurement is made together with velocity recordings. Such measurements are, however, difficult in cerebral vessels, as the accuracy of imaging techniques is restricted by the small dimensions of the vessels and any diameter changes that occur. A method that potentially avoids the need for direct measurement of CSA uses changes in the power of the Doppler signal. It has been postulated that each red blood cell (RBC) contributes equally to the power of the reflected Doppler signal, provided the vessel is insonated with uniform intensity. The power of the received signal should therefore be related to the number of RBCs and, hence, the volume of blood within the sample volume (Arts and Roevros 1972). The reflected signal power is, however, also affected by variations in the spatial positioning of RBCs relative to each other. The random positions of RBCs, or aggregates of RBCs, in the Doppler sample volume produces speck- ling in the received Doppler power. With non-turbulent flow, because of the large number of RBCs, there is an average distance between the randomly positioned cells at each hematocrit (Hct). The received Doppler power should depend on the Hct, as well as flow velocity and vessel lumen size. Address correspondence to: Sean Wallace, Department of Neurology, Oslo University Hospital, Rikshospitalet, 0424 Oslo, Norway. E-mail: seanwallace@live.no Conflicts of Interest: The authors have indicated that they have no conflicts of interest regarding the content of this article. 828 Ultrasound in Med. & Biol., Vol. 40, No. 4, pp. 828–836, 2014 Copyright Ó 2014 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/$ - see front matter http://dx.doi.org/10.1016/j.ultrasmedbio.2013.04.016