Three-MHz Ultrasound Heats Deeper Into the Tissues Than Originally Theorized

OBJECTIVE: To measure muscle temperature of ultrasound at 1-MHz and 3-MHz frequencies at a depth of 2.5 cm and to compare treatment durations for vigorous heating (increase of 4 degrees C) and for heating to 40 degrees C. DESIGN AND SETTING: A counterbalanced, repeated-measures design with 1 fixed, independent variable, 1.5-W/cm(2) ultrasound treatment (1 MHz, 3 MHz, or control [sham]) using a Theratouch 7.7 ultrasound device. Dependent variables were end-treatment temperature at 2.5 cm, time to vigorous heating, and time to reach 40 degrees C. SUBJECTS: Eighteen healthy volunteers (age = 24.6 +/- 2.3 years, height = 173.0 +/- 9.7 cm, mass = 72.0 +/- 16.3 kg) without a history of lower leg injury. MEASUREMENTS: The medial triceps surae intramuscular temperature at 2.5 cm was measured every 10 seconds using an implantable thermocouple. Each of the 3 ultrasound frequencies was applied in counterbalanced order at 24-hour intervals. RESULTS: Ultrasound of 3 MHz produced both vigorous heating (at 3.4 minutes) and an absolute temperature of 40 degrees C (at 4 minutes). CONCLUSIONS: Our results suggest that 3-MHz ultrasound heats 0.5 cm deeper than suggested by others. With our machine, 3-MHz ultrasound was more effective in heating muscle at this depth than 1-MHz ultrasound.