Limited interchangeability of methods of applying 1MHz ultrasound

Objective: To investigate the interchangeability of two different methods used to apply 1MHz ultrasound for deep tissue heating, namely, the direct contact and the subaqueous methods. Design: An intervention study with a crossover design. Setting: The setting was a laboratory. Participants: Four specimens, cross-sections of nonliving pig tissue, were used. Interventions: Specimens were each exposed to 1MHz ultrasound under five different conditions: in contact with gel, and suaqueous at 0, 1, 2, and 4cm from the applicator to the skin. Main Outcome Measures: Tissue temperatures were measured at six thermocouple sites at different depths from the skin surface. Results: Subaqueous ultrasound applied at a distance of 0cm from the skin produced significantly less tissue heating than direct contact (with gel) ultrasound at the same intensity. Increasing the distance from the applicator to the skin from 0 to 1, 2 and 4cm resulted in significantly lower mean maximum temperature increases of 69%, 56%, and 44%, respectively, of the 0cm values. Conclusions: The methods used clinically to apply ultrasound have only limited interchangeability. Large corrections to the power output are required to compensate when using subaqueous rather than direct (in air) ultrasound, and for applicator to skin distances of beyond 0cm in subaqueous ultrasound.