The NPL Doppler fetal heart beat detector test facility

There are many thousands of Doppler fetal heart beat detectors in medical 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 comparing their technical performance under standardised test conditions. At NPL, we have developed a reference test facility for measuring detector sensitivity to meet the needs of manufacturers, and to comply with the requirements of an international standard (IEC 1995) that defines methods of measurement of the sensitivity of fetal heart beat detectors. The test facility has primarily been developed for detectors operating at a transmitted frequency of 2 MHz and with Doppler shifts of up 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 above the noise level. The sensitivity is calculated by adding the final signal level above the noise to the total insertion loss of the attenuators in the ultrasound path and the reflection loss of the target. A crucial aspect of this calculation is the knowledge of the target strengths and characteristics. This has already been extensively studied (Preston and Bond 1997) over the frequency range of interest. The NPL test facility developed for undertaking the sensitivity measurements is described, including an assessment of the uncertainties in such a measurement and solutions to problems encountered along the way.