[Development of a fetal breathing movement tracking system using pulsed Doppler ultrasound]

Fetal breathing movements (FBM) form a useful means of monitoring fetal conditions and functional development. To develop a continuous and quantitative FBM monitoring system for clinical use, a non-imaging method using pulsed Doppler was devised. A pulsed Doppler cardiotocograph module was extended to obtain low frequency Doppler signals, by the addition of a 90 degree phase shifter, analog multipliers, quadrature detector, sample and hold circuits and low pass filters, to produce five simultaneous outputs representing movement at depths separated by 1.5cm intervals. These outputs represent the displacement inside tissue at the various depths. Signal processing was executed on a 32-bit computer with an A/D converter. On-line processing by means of a zero-crossing method and off-line arc tangent method with high accuracy displacement estimation were applied for calculation. With this system, FBM was detected as cyclic and regular displacements of a few hundred microns ranging from 0.5 to 1.5Hz. With digital signal processing of 2/3Hz high pass filtering, precise evaluation FBM was possible while rejecting gross movements due to maternal breathing, etc. Although further studies are required for clinical application, it appears that quantitative assessments of FBM should be possible with this technique.