Baby bugging: Design of a miniature integrated fetal microphone

Dr T D Rowsell

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Why?

  • 16000 fetal deaths per annum in Western Europe
  • Reduce fetal morbidity
  • Improve accuracy of diagnoses of fetal condition
  • Reduce rate of false intervention
  • Methods using ultrasound radiation are expensive and limited

Fetal Phonography

Uses special microphones to record microscopic vibrations of the maternal abdominal wall generated by natural fetal activity

Pros

Cons

Solution?

A Miniature Optical Fetal Microphone

Therefore the ideal fetal microphone does not load the maternal abdomen. Using an optical technique for displacement measurement enables remote sensing of vibration. The system being described uses a CD pickup head.

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This is small, low cost, and houses the necessary opto-electronic elements.

A coherent light beam is projected onto a reflecting surface and focused onto an array of four photo diodes. The distribution of light on these
indicates the position of the surface relative to the point of focus. Two photo diodes become selectively illuminated according to the direction
and magnitude of the required focus correction. The difference in output between photo diode pairs is a measure of surface displacement.

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Another measure of displacement is given by the total photo diode array output.   Attaching a small mirror to the skin surface enables recording of skin vibrations.

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Further Work

1. Design of clinical prototype
2. Improvements to signal conditioning and control electronics
3. Investigate wireless links for cordless monitoring

Suggested Applications


This work was funded by the British Heart Foundation (1993-1994)

Remote measurement of small displacements at compliant surfaces

Dr Tim Rowsell

A problem to be addressed in the design of all instrumentation involves ensuring that the parameter that is to be characterised remains unaffected by the measurement system. A particular instance is the recording of fetal heart 'sounds' at the maternal abdomen. The maternal abdomen is very compliant and the vibrations arising from fetal heart action have a magnitude of the order of just a few microns. For these reasons the use of contacting transducers to sense the vibrations is inappropriate. Instead a method is sought to 'remotely' sense vibrations.

Consider a beam a beam of light which is focussed onto a photodiode array after reflection on a mirrored surface. Displacement of this surface modulates the amount of light reaching the photodiode array. This is the principle currently under investigation as a method to record vibrations of the compliant surface of the maternal abdomen. However, a requirement that the transducer be unobtrusive in use and cheap to produce suggests an alternative be sought to a design incorporating discrete optical componentry.

Low-cost optical componentry of the precision required to resolve micron displacements is found in the domestic Compact Disc (CD) player. A CD pickup subassembly typically contains a high quality lens with actuator enabling small focussing adjustments, a coherent light source and photodiode array. Research is continuing to experimentally explore the limits of such devices and from these results design a prototype optical vibration sensor.

The experimental rig consists of an Sony CD pickup interfaced to an IBM PC. Under software control it is possible to focus the pickup and record the optical output. Focussing the pickup onto a micrometer provides a basic calibration facility.

Other applications of this type of sensor include measurement of the deformation of compliant materials used in aerodynamic investigations.

Further information :

The funding for this project was provided by the British Heart Foundation.

'Co-workers' on this project are Dr. David Holburn (CUED) and Dr. K.J. Dalton of the University Department of Obstetrics & Gynaecology, Rosie Maternity Hospital, Addenbrookes Hospital


tdr11@medschl.cam.ac.uk