d Original Contribution DESIGN AND CHARACTERIZATION OF A CLOSE-PROXIMITY THERMOACOUSTIC SENSOR JIDA XING,* MICHAEL CHOI,* WOON ANG, y XIAOJIAN YU,* and JIE CHEN* z * Electrical and Computer Engineering Department, University of Alberta, Edmonton, Alberta, Canada; y IntelligentNano Inc., Edmonton, Alberta, Canada; and z Biomedical Engineering Department, University of Alberta, Edmonton, Alberta, Canada (Received 17 September 2012; revised 30 January 2013; in final form 7 March 2013) Abstract—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 ultra- sound generated at a 1.5-MHz frequency, 20% duty cycle, 1-kHz pulse repetition frequency and intensities between 30 and 120 mW/cm 2 . The sensor captures the beam, converts the ultrasound power into heat and indirectly measures the spatial-average time-average ultrasound intensity (I sata ) 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 distribu- tion across the back face. An embedded system design was implemented using an Atmel microcontroller pro- grammed 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 ultra- sound intensity with an average error of 5.46% compared with readings taken using a radiation force balance. (E-mail: jc65@ualberta.ca) Ó 2013 World Federation for Ultrasound in Medicine & Biology. Key Words: Low-intensity pulsed ultrasound, Thermoacoustic sensor, Ultrasound intensity measurement. INTRODUCTION Ultrasound has a wide range of biomedical applications, from imaging to promoting cell growth (Shaw and Hodnett 2008). For biological experiments, it is important to regulate the acoustic output to ensure the quality and consistency of each trial. If not monitored properly, the under-application of ultrasound in high-intensity focused ultrasound (HIFU)-based kidney stone disintegration can lead to incomplete treatment, whereas the over- application of ultrasound in low-intensity pulsed ultra- sound (LIPUS) applications can lead to cell death (Shaw and Hodnett 2008). Acoustic output parameters are typically evaluated using a hydrophone or a radiation force balance. Hydrophones are considered the universal instrument for characterization of acoustic field parame- ters, such as pressure waveforms and beam profiles. For determining ultrasound output power, the accepted tech- nique is the use of a radiation force balance (Shaw and Hodnett 2008). However, both of these techniques have their limitations. Operation of a hydrophone can be tech- nically difficult, time consuming and expensive (Wilkens 2004, 2010a). On the other hand, a radiation force balance is constrained by the setup apparatus required; the ultrasound beam must be transmitted into a chamber containing degassed water onto an absorbing or reflecting target, which must intercept the entire beam (Shaw and Hodnett 2008). Because of these draw- backs, the development of another sensor design is desirable. Thermoacoustic sensors that measure the transforma- tion of the incident ultrasonic energy into heat have the potential to be an alternative approach to determination of ultrasound intensity. These sensors are based on the transformation of incident ultrasonic energy into heat inside a small cylindrical absorber and the detection of the temper- ature rise on the rear side of the absorber (Wilkens 2010a). Previous thermoacoustic sensor operation has required the Address correspondence to: Jie Chen, Electrical and Computer Engineering Department, University of Alberta, W6–019, ECERF, Edmonton, AB T6 G 2 V4, Canada. E-mail: jc65@ualberta.ca 1613 Ultrasound in Med. & Biol., Vol. 39, No. 9, pp. 1613–1622, 2013 Copyright Ó 2013 World Federation for Ultrasound in Medicine & Biology Printed in the USA. All rights reserved 0301-5629/$ - see front matter http://dx.doi.org/10.1016/j.ultrasmedbio.2013.03.010