“…The proportional power/radiation force relationship assumes plane wave propagation. In practice, corrections are applied to the measured power to account for deviations from plane wave behaviour, and for the attenuation of ultrasound in the water path [36,37]. Acoustic streaming [38] is a well-known phenomenon that occurs during ultrasound propagation, due to attenuation in the medium, which can affect measurements of radiation force, but its influence at low MHz frequencies used in physiotherapy ultrasound is small, and so it will not be considered here.…”