THE NPL DOPPLER FETAL HEART BEAT DETECTOR TEST FACILITY

There are many thousands of Doppler fetal heart beat detectors in medi cal use and many different detector manufacturers but, until recently, there has been no well-defined quantitative method for measuring the sensitivity of these detectors and, therefore, no way of directly comp aring their technical performance under standardised test conditions. At NPL, we have developed a reference test facility for measuring dete ctor sensitivity to meet the needs of manufacturers, and to comply wit h the requirements of an international standard (TEC 1995) that define s methods of measurement of the sensitivity of fetal heart beat detect ors, The test facility has primarily been developed for detectors oper ating at a transmitted frequency of 2 MHz and with Doppler shifts of u p to 1 kHz. The detectors are tested by directing the ultrasound beam at a small moving target being driven at a constant velocity, and then monitoring the output signal from the detector, which will be at the Doppler shift frequency, To determine the sensitivity, attenuators are inserted into the beam until the output signal is reduced to 6 dB abo ve the noise level, The sensitivity is calculated by adding the final signal level above the noise to the total insertion loss of the attenu ators in the ultrasound path and the reflection loss of the target, A crucial aspect of this calculation is the knowledge of the target stre ngths and characteristics. This has already been extensively studied ( Preston and Bond 1997) over the frequency range of interest, The NPL t est facility developed for undertaking the sensitivity measurements is described, including an assessment of the uncertainties in such a mea surement and solutions to problems encountered along the way. (C) 1998 Crown copyright, Reproduced by permission of the Controller of HMSO. Published by Elsevier Science Inc.