Design and Characterization of a Close-Proximity Thermoacoustic Sensor

Although the radiation force balance is the gold standard for measuring ultrasound intensity, it cannot be used for real-time monitoring in certain settings, for example, bioreactors or in the clinic to measure ultrasound intensities during treatment. Foreseeing these needs, we propose a close-proximity thermoacoustic sensor. In this article, we describe the design, characterization, testing and implementation of such a sensor. We designed a 20-mm-diameter plexiglass sensor with a 2-mm-long absorber and tested it against low-intensity pulsed ultrasound generated at a 1.5-MHz frequency, 20% duty cycle, 1-kHz pulse repetition frequency and intensities between 30 and 120 mW/cm2. The sensor captures the beam, converts the ultrasound power into heat and indirectly measures the spatial-average time-average ultrasound intensity (Isata) by dividing the calculated power by the beam cross section (or the nominal area of the transducers). A thin copper sheet was attached to the back face of the sensor with thermal paste to increase heat diffusivity 1000-fold, resulting in uniform temperature distribution across the back face. An embedded system design was implemented using an Atmel microcontroller programmed with a least-squares algorithm to fit measured temperature-versus-time data to a model describing the temperature rise averaged across the back side of the sensor in relation to the applied ultrasound intensity. After it was calibrated to the transducer being measured, the thermoacoustic sensor was able to measure ultrasound intensity with an average error of 5.46% compared with readings taken using a radiation force balance.