Ultrasound power measurement with a water-based calorimeter

Abstract

Therapeutic ultrasound equipment typically employs frequency from 1 to 3 MHz and intensities ranging from 0.1 to 3.0 W cm−2 in continuous and pulsed modes. Ultrasound is among the physical resources most used by professional physiotherapists in the treatment in a wide diverse disorder of the musculoskeletal system. The intensity of the ultrasonic radiation is an essential factor for the success of any therapy, as well as its time of application. The ultrasound power should be calibrated to avoid lack of efficiency of the treatment. A fast and reliable ultrasonic power measurement is necessary to a rapidly assess the output of a physiotherapy equipment. This paper presents a comparison between an ultrasound power measurement based on a radiation force balance and a calorimeter. The ultrasound power source is a calibrated 3.5 MHz continuous-wave instrument, with a robust NDT transducer. The calorimeter was specially developed to assess ultrasonic power up to 1 W, using deionized and degassed pure water as sonication medium. The power measurement outcome from the calorimeter were equivalent to the power measurement undertaken with a calibrated radiation force balance. The expanded uncertainty (p = 0.95) achieved with the calorimeter was 60 mW, whilst with the calibrated microbalance the uncertainty was 32 mW. The results obtained so far indicates that the use of a calorimeter for ultrasonic power measurement is a fast and metrologically reliable way to assure the output power of ultrasonic physiotherapy equipment.