In Doppler systems which automatically calculate the maximum frequency
envelope and pulsatility index (PI) of umbilical artery Doppler waveforms there is
the possibility of error in these parameters when the technical quality of the
acquired waveform is low. Low quality waveforms may arise when there is an
inappropriate set of physical parameters or when there are other sources of noise
such as overlying vessels signals. In this thesis the effect of physical parameters on
the envelope and on PI are investigated, and also methods for the detection of low
quality waveforms are described and tested.
A flow phantom which is able to produce realistic looking umbilical artery
Doppler waveforms is described. This is based upon microcompruter control of a
stepping motor / gear pump combination. The statistics of the Doppler spectra
produced using artificial blood and human blood in the phantom are found to be
identical.
The effect of a number of physical parameters on the simulated umbilical artery
waveforms produced using the phantom is investigated. The accuracy of estimation
of the envelope and the PI is similar over a wide range of physical conditions.
A suitable image processing algorithm for speckle reduction of Doppler
waveforms is developed and tested using simulated waveforms from the phantom.
Using the flow device it was found that both filtering of the envelope and also
speckle suppression of the spectrum improved the accuracy of estimation of the
envelope and of the PI.
A number of quality indices based upon the degree of noise of the envelope are
described. Using the flow device there is found to be a high correlation between
the quality index values, and the errors in PI and errors in envelope estimation
respectively. In a clinical trial the quality index values from umbilical arteries were
compared with the waveform quality as assessed by a skilled observer. The clinical
results show that quality indices are able to separate high and low quality
waveforms when the indices are calculated from the unprocessed envelope, but not
when calculated from the filtered envelop
