Quantification of fetal power Doppler: determination of the intravascular standardization point

Abstract

Background: With its sensitivity to low flow, lack of aliasing and relative angle independence, power (amplitude) Doppler ultrasound is well suited to assessment of tissue perfusion. However, the influence of machine settings, depth and other factors related to attenuation means that perfusion measurements taken from deep tissue without standardization to 100% flow are relatively meaningless. Previously published adult research has proposed a technique to estimate a true value for 100% amplitude from a cumulative power distribution across a large local vessel. This standardization value allows measurement of fractional moving vascular volume as an ultrasound index of perfusion.Methods: Fetal renal and aortic blood flow was evaluated using the power Doppler mode of the Acuson Sequoia™. Color flow information was digitally segmented to a linear 8‐bit scale and analyzed using CQ Analysis software as previously reported by our group. Using images from the fetal aorta with and without the kidney, of both segmented and unsegmented images, an attempt was made to plot the cumulative power distribution in eight fetuses (27–37 weeks' gestation). Fetal aortic amplitude information was then examined using a combination of CQ Analysis, Scion Image and Excel software. Peak amplitude was examined between different ultrasound clips in a single fetus and in 50 frames through the cardiac cycle in 10 fetuses. Spatial variation in peak aortic amplitude was examined in 50 frames from four fetuses with varying angle of insonation from 73° to 88°. Temporal variation was examined in 50 frames from a single fetus.Results: In the human fetus, there is no peak vascular value or corresponding ‘knee point’ attributable to the effects of rouleaux. In contrast, the peak aortic value is relatively constant for 100% flow, both between multiple imaging clips and throughout the cardiac cycle in the 10 fetuses. Spatial variation for the 100% flow value increased from an average coefficient of variation of 6.5% at 79° to 28% at 89°. Temporal variation was 8% over 50 frames.Conclusions: The human fetus has a reduced degree of rouleaux formation in part due to an altered form of fibrinogen and reduced concentration of plasma proteins, so the peak amplitude measurement in the center of large vessels represents true 100% flow. This finding is confirmed by the lack of cardiac cycle effects on the amplitude profile. The central value of larger blood vessels, such as the fetal aorta, if measured at an appropriate angle of insonation, may provide a relatively constant standardization value for vessels or regions at a similar depth. From these, true measurements of fractional moving vascular volume may be derived.Acknowledgements: Alec Welsh was supported by a Research Training Fellowship from RCOG/WellBeing, the BUPA Research Foundation Clinical Scientist Award 2000 and the 2001 Bernhard Baron/RCOG Travelling Scholarship.