The Effect of Back Reflections on the Acoustic Power Delivered by Physiotherapy Ultrasound Machines

Physiotherapy ultrasound is used widely for the treatment of soft tissue injuries. The ultrasonic treatment heads are all highly resonant devices and may therefore be sensitive to the levels of acoustic back reflection that they experience. However, the extent to which reflections affect acoustic power during clinical use has not been reported in the literature and is not addressed in current technical standards. This study investigated the effect of physiological levels of acoustic reflection on 29 physiotherapy treatment heads from a total of 21 machines and 6 manufacturers. A range of membranes were constructed and used to mimic the levels of acoustic reflections that occur during treatment. The results obtained showed that almost half of the heads tested (45%) had deviations in acoustic power of more than 15% compared with free-field measurements. Four heads (17%) had deviations in power of more than 25%. We recommend that the susceptibility of physiotherapy ultrasound machines to acoustic reflections be addressed in the relevant technical standards. Also, it is appropriate for tests to be carried out during design and manufacture, and by the purchaser during their acceptance testing. (E-mail: karne.mcbride@luht.scot.nhs.uk)