Towards a portable system for the measurement of thermal and mechanical indices

Users of diagnostic ultrasound equipment are provided with on-screen indices (TI and MI) to indicate the potential thermal and non-thermal hazards. It is prudent that these indices be checked for accuracy. Indeed, this is a recommendation of the Safety Guidelines of the British Medical Ultrasound Society. Knowledge of the variation with depth of the derated temporal-average intensity is required for two of the thermal index formulae for non-scanned beams — TIS (large aperture) and TIB. A portable system has been designed and built to permit a plot of derated temporal average intensity versus depth to be quickly made for any ultrasound diagnostic scanner. Signals from a membrane hydrophone are amplified and input to an RF power meter. The use of an analogue method for measuring temporal intensity obviates the need for trigger signals from the scanner. The output from the power meter and the output from a hydrophone depth measuring potentiometer are input to a laptop computer, which calculates and displays the plots of derated intensities (ITA·3 and ITA·6) versus depth. By entering the separately measured acoustic output power of the scanner via the laptop keyboard, the laptop can produce and display MI and TI values.