Regional variation in power Doppler perfusion measurements within normal placenta

Abstract

Background: Power Doppler ultrasound is well suited to the evaluation of placental perfusion, with its sensitivity to low flow, lack of aliasing and relative angle independence. Previously published work from our group has proposed the use of power Doppler evaluation of abnormal placentation to monitor progress after significant placental abruption. Commercially available software (CQ Analysis, Kinetic Imaging Ltd., UK) is able to segment color flow information from compound color/greyscale images and express flow information in relation to vessels (mean color velocity or energy) or tissue (percentage integrated velocity or energy). Care must be taken to either standardize power Doppler values to local regions of 100% blood flow or alternatively to compare regions at the same depth and machine settings sat the same examination.Methods: Ten patients with uncomplicated pregnancies were recruited for additional power Doppler ultrasound examination. All were in the third trimester with anterior placentas. The placental location was initially mapped, and then separate regions were chosen, approximating to different placental quadrants. With constant machine settings for each case, and depth being constant, multiple clips were stored from each region. After image segmentation, all frames were examined individually to exclude any with movement or flash artefact. The percentage of ICE was measured in each frame: firstly, carefully tracing the fetal placental vessels away from the surface and secondly, using an oval shaped region of interest as defined by CQ Analysis. From these two regions, two different examinations were performed, averaging over the whole clip to remove cardiac cycle effects, and looking manually at systolic frames only.Results: A total of 8156 frames were analyzed, of these 872 selected as systolic. The variation in regional perfusion assessed with power Doppler from normal placenta ranged from 19 to 132%. There were no statistically significant differences in reproducibility for the techniques for region selection and for region selection.Conclusions: In this study, an attempt was made to show the high degree of variation in normal perfusion within the human placenta as a function of space. Unlike other organs where power Doppler perfusion evaluation may be linked to specific planes or landmarks, this is not technically possible with the placenta. An exception to this may be within areas of pathological placentation that are followed prospectively. However, caution should be exercised in the use of power Doppler for evaluation of normal human placenta perfusion.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.