Rate of temperature increase in human muscle during 1 MHz and 3 MHz continuous ultrasound

To achieve the thermal effects of ultrasound, the tissue temperature must be raised from 1 to > or = 4 degrees C, depending on the desired outcome of the treatment. In the past 25 years, there have been no in vivo studies that have measured rate of change in temperature during 1-MHz ultrasound treatments, and none have ever been performed with the 3-MHz frequency. Thus, we are left to pure speculation regarding how long to administer an ultrasound treatment. We performed this study to plot the rate of temperature increase during ultrasound treatments delivered at various intensities and frequencies. We inserted two 23-gauge thermistors into each subjects' medial triceps surae at the following depths: 1 MHz at depths of 2.5 and 5.0 cm (12 subjects) and 3 MHz at depths of .8 and 1.6 cm (12 subjects). Each subject received a total of four 10-minute treatments, one each at .5, 1.0, 1.5, and 2.0 W/cm2, and temperature was measured every 30 seconds. No significant difference was found in the rate of heating at the two depths (p = .987) within the same frequency and dose levels. The 3-MHz frequency heated significantly faster than the 1-MHz frequency at all doses tested (p < .001). On average, the rate of temperature increased per minute at the two depths of the 1-MHz frequency was: .04 degrees C at .5 W/cm2; .16 degrees C at 1.0 W/cm2; .33 degrees C at 1.5 W/cm2; and .38 degrees C at 2.0 W/cm2. The rate of temperature increase per minute at the two depths of the 3-MHz frequency was: .3 degrees C at .5 W/cm2; .58 degrees C at 1.0 W/cm2; .89 degrees C at 1.5 W/cm2; and 1.4 degrees C at 2.0 W/cm2. The results of this research should enable clinicians to choose the correct frequency, intensity, and treatment time when using thermal ultrasound.