The measurement of the acoustic output of diagnostic ultrasound equipment is important with regard to the safety of patients. Commonly, the sound-field characterization is performed with the aid of hydrophone measurements using raster scanning, and the overall emitted acoustic power is measured using radiation-force balances. The determination of the local, time-averaged ultrasound intensity, a measure mainly important for the estimation of potentially harmful thermal effects, requires a very great technical effort, particularly in the case of combined imaging modes of modern sonography machines using complex pulse-emitting sequences. To include all ultrasound waves contributing to the overall intensity at the measurement location, well-elaborated synchronization methods have to be applied. In this context, deep insight in the details of the operation modes of the individual sonography machine is also necessary, which is commonly available only for the manufacturer.

Thermoacoustic sensors form a significantly less complex alternative for the determination of intensities. Due to the thermal operation principle, these sensors inherently perform temporal averaging of all incoming sound waves without the need for synchronization to individual pulses. The thermoacoustic measurement technique developed in the working group was used for the first time for exposimetry on sonography machines. To enable quantitative intensity measurements, an acoustic calibration of the sensor was performed, using a specifically designed substitution measurement setup. The calibration was carried out in the frequency range from 1 MHz to 9 MHz, thus covering the center frequencies commonly used in sonography. The transfer function obtained (see Figure 1) shows the temperature increase produced within the sensor per incident sound intensity. Preliminary intensity measurements on a sonography machine, at first using simple pulse-emitting sequences, showed the favourable applicability of the method. Comparison with the results of hydrophone measurements shows good agreement if the spatial averaging effect of the thermoacoustic sensors is taken into account. At the moment, the spatial resolution is restricted by the diameter of the receiver of 3 mm and is planned to be improved in the future.