AN EXPERIMENTAL-STUDY OF THE REFLECTION FROM SPHERICAL AND FLAT ENDEDCYLINDRICAL TARGETS SUITABLE FOR FETAL DOPPLER PERFORMANCE ASSESSMENT

The performance of small-diameter targets suitable for use as oscillat ing targets for testing the sensitivity of Doppler fetal heartbeat det ectors has been systematically studied. Experimental results are prese nted in the 1.6-3.0 MHz frequency range for the plane-wave reflection loss for a total of 16 targets: spherical balls made of stainless stee l; hemispherical-ended rods made of PTFE; and flat-ended rods made of stainless steel, PTFE, polycarbonate, and tungsten carbide. Results sh ow that the fine-structure variation of reflection loss with frequency is greatest in the case of spherical ball targets and least for flat- ended targets. It has been shown that, providing care is taken during manufacture, the reflection loss from a flat-ended target can be predi cted using a simple theory based on a plane disc reflector. Tungsten c arbide targets consisting of a long rod with a diameter of 1.6 mm tape red down to a cylindrical flat end with a diameter of 0.4, 0.5, and 0. 6 mm have been shown to provide reflection losses of between 60 and 40 dB, and to have a smooth variation of reflection loss with frequency. They can also be manufactured in a form that allows no significant in terference from the supporting structure and, therefore, are ideal tar gets to meet the requirements of International Electrotechnical Commis sion 1266:1995. Crown Copyright 1997. Reproduced by permission of the Controller of HMSO.