Towards a portable system for the measurement of thermaland mechanical indices

Abstract

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