Abstract
A high persistence reduces the differences caused by variations in blood flow due to systole and diastole in the displayed color Doppler signal. Through this the level of persistence might mask the effect of systole and diastole on the measurement of the fractional moving blood volume (FMBV) causing loss of spatial resolution when FMBV measurement is done in the 3D volume in a multi-slice technique. The objective of this study was to investigate the impact of the persistence of power Doppler ultrasound (PDU) during different periods of the cardiac cycle on the estimation of FMBV in the fetal brain. Five consecutive PDU examinations of the cerebral posterior fossa in four different fetuses were performed. For each examination a gradual increase in persistence was selected: 0 (none), 1, 2, 3 and 4 (maximal). The transducer was strictly kept in the same position while the Doppler sequence was videotaped. Each video sequence comprised at least five cardiac cycles. The video sequences were converted to a series of images and in each image FMBV was calculated in a constant and well defined region of interest. Cyclic changes of FMBV were analyzed for all consecutive images from the same video sequence. When persistence 0 was used FMBV values of a defined ROI showed significant changes during the different periods of the cardiac cycle. These changes diminished progressively with increasing persistence. Frame-to-frame variations of FMBV measurements were significantly larger with respect to the mean with a 0 persistence in comparison to measurements with persistence 1 through 4 (P < 0.001). Real changes in blood flow during cardiac systole and diastole influence the calculation of FMBV. These differences are hidden when a high persistence is used. Whereas a high PDU persistence could be applied for 2D FMBV calculations it might diminish the spatial resolution in a multi-slice 3D PDU technique.
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