Calibrating therapeutic ultrasound transducers: corrections for the effects of acoustic cavitation and acoustic streaming

Commercial ultrasound power meters based on the radiation force technique are ubiquitously used in both the clinic and in laboratory settings for calibrating therapeutic ultrasound transducers. Despite their popularity, these devices are inherently inaccurate in that they do not compensate for the effects of acoustic cavitation and acoustic streaming. These factors can alter the displacement generated on the meter’s target, and hence the power being sensed. In the present study we built a low cost power meter comprised of a non-reflecting target suspended from an analytical balance in a water tank. Investigations in to the effects of cavitation and streaming were performed, where the former was shown to significantly lower the measured power and the latter was shown to increase it. Both effects were found to be proportional to the applied power as predicted by theory. A modified device was then constructed, where an acoustic permeable membrane was positioned directly over the target and shown to effectively eliminate the streaming effect. For the effects of cavitation, a pair of ultrasound transmitting and receiving transducers was positioned across the beam path, and custom software automatically calculated the attenuation coefficient of the water in the beam column. This was then used to correct for the attenuating effect of cavitation on the power being measured. In addition to correcting for sources of error associated with commercial devices, the setup can easily be constructed for a much lower cost using existing, off-the-shelve components found typically in the laboratory environment. The system may also be employed for research on the effects of water borne phenomena associated with the application of ultrasound in a fluid medium.