The variation of heating depth with therapeutic ultrasound frequency in physiotherapy.

In patient treatment, different ultrasound (US) frequencies are attributed to differences in penetration and, as an effect of that, to different heating depths in tissues. A set of 13 experiments was carried out with US frequencies of 0.86, 2 and 3 MHz. A dynamic treatment protocol and a stationary treatment protocol were used. The temperature increase patterns were thermally imaged with a 1-min interval during an insonation of 5 min. At every data point, the temperature in the reference image was subtracted from the thermal image after 1, 2, 3, 4 and 5 min. In every difference thermal image, the distance between the US applicator and the deepest point of heat increase was measured. Results show that US frequencies do not affect the depth limit for the different temperature ranges, in either the static treatment protocol or the dynamic treatment protocol. Copyright 2003 World Federation for Ultrasound in Medicine & Biology