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Flow is defined as the amount of fluid, ordinarily blood, passing per unit time (usually per minute). Perfusion is defined as the volume of blood flowing through a mass of tissue per unit time. The standard measure of perfusion is millilitres of fluid per second per 100 g of tissue (or 100 ml in volume) i.e. the fractional blood volume in an area divided by the mean transit time through the area, where the area is scaled to a 100 ml of tissue. The fractional vascular volume is an index of the physiological vascularity of a region and represents the proportion of tissue occupied by blood. Several ultrasound based techniques have been used in an attempt to quantify perfusion. Vessel dimensions of capillaries and arterioles are below the spatial resolution of even the latest ultrasound machines, and capillary flow is slower than that required to generate a Doppler signal which leads to underestimation of true perfusion. For all ultrasound-based techniques movement artefacts are probably the greatest obstacle to obtain quantitative information on perfusion. This review will discuss the technology and use of the ultrasound mode of three dimensional power Doppler, its benefits and limitations in estimating vascularity focusing on the field of obstetrics.
A three year study was undertaken during 2002 to 2004 from May to September to estimate abundance and density of harbour porpoises on the north coast of Anglesey, Wales, UK. There were no ecological data regarding the harbour porpoises in Anglesey waters so the ability to influence conservation measures was highly constrained.
Boat based transects using distance sampling techniques were applied so a robust estimate of density and abundance could be attained. The study area consisted of a block approximately 489 km2 extending from the east of Point Lynas to the west of South Stack on north coast of Anglesey. The study area was divided into 5 blocks consisting of 31 perpendicular transect lines to the shore. Each of the transect lines were surveyed 1–5 times by the end of the three year study.
Based on the assumption that g(0) = 1 the density of harbour porpoises for the 489 km2 study site was estimated to be 0.630 individuals/km2 (CV = 0.20) and the abundance is estimated to be 309 individuals (CV = 0.20). Heterogeneity in density and abundance was observed across the 5 blocks which showed Point Lynas and South Stack to have the highest densities. This distribution was closely associated to fine-scale oceanographic features which cause prey to be concentrated and may facilitate foraging for harbour porpoises. The study showed that Anglesey provides coastal habitats for the harbour porpoise and was the first study of this kind in North Wales, UK.
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