Statistical analysis of Doppler ultrasound data: A study of umbilical blood flow velocity waveforms for prediction of fetal distress

Abstract

Doppler ultrasound is used to study fetoplacental and uteroplacental circulations to assess fetal and maternal well-being, and to enhance the understanding of intrauterine physiology. The information obtained by Doppler ultrasound consists largely of the maximal blood flow velocity waveforms. The umbilical waveforms, in particular, are studied to recognise fetal compromise due to placental insufficiency - an obstetric term for functional abnormality resulting in failure of fetal growth. The growth restricted fetuses are at risk of perinatal mortality and morbidity and the early identification of intrauterine compromise may be vital for their development, especially since recent medical research indicates a connection between low birthweight and subsequent illness in adulthood. Current methods of umbilical monitoring and assessment are used only in the management of high risk pregnancies in late pregnancy from 28 pregnancy weeks onward. Further applications of Doppler velocimetry, either in earlier gestation or in low risk obstetric population, have significant potential for favourable clinical effect providing that new methods of umbilical waveform analysis are developed to accommodate these situations. A new method of waveform extraction from noisy Doppler measurements and models for prediction of fetal growth restriction are presented in this thesis. The waveform identification method, based on time series techniques and parametric shape constraints, utilises estimated periodicities of the Doppler signal and the smoothness of the image boundaries. The shape of the maximal waveform envelope is modeled as a gamma function which incorporates the hidden underlying baseline of the blood flow not considered in the gamma waveform model introduced by a previous study. This gamma estimated waveform envelope is used to construct new waveform indices which are then considered for prediction of fetal distress. The underlying baseline of the blood flow appears to be one of the significant predictors of suboptimal fetal growth. Prediction models for detection of abnormal velocity waveforms associated with the incidence of intrauterine growth restriction are derived at 18 and 28 pregnancy weeks. The prediction model based on the waveform information obtained at 18 pregnancy weeks has significant predictive ability and potential for clinical use even at this early gestational age