Analysis of the auditory perception of ultrasound doppler signals to improve pregnancy risk assessment

Abstract

In high-risk pregnancies, the transport of oxygen and nutrients from maternal to fetal blood via the placenta is often impaired. To assess the risk, pulsed Doppler ultrasound (US) is used to evaluate the flow velocity waveform in the umbilical artery with the pulsatility index (PI), which is derived from the velocity envelope of the Doppler power spectrum. However, simply listening to the Doppler signal can indicate to an experienced sonographer that the type of the blood flow is worse than the PI suggests. This is however dependent on the operator's experience and it may be difficult to estimate what influences the subjective judgement. Motivated by the description of the Doppler sounds by an experienced operator (AT) as having a "timbre", this study describes an analysis of Doppler sounds in search for an index or method with capacity to better evaluate the blood flow in the umbilical artery in high-risk pregnancies. A test was designed, where synthetically produced Doppler sounds with various spectral contents were played together with a variable sinusoidal sound signal. The task for the five test persons was to match the frequency of the sinusoidal signal to the Doppler sounds. The tests indicated that the human ear is most sensitive to the lower frequencies of Doppler sounds. An analysis of prerecorded sounds showed a difference in the lower frequencies of a sound considered to emanate from the umbilical blood flow of healthy fetuses with normally functioning placenta as compared to a pathological one. This might explain the difference between the sounds experienced by an operator. As a suggestion to extract more information than the maximum envelope, the minimum frequency envelope was extracted from pre-recorded clinical sounds. Based on the pilot tests presented here, this shows to be a promising strategy. © 2011 Springer-Verlag.