3 MHz CONTINUOUS ULTRASOUND CAN ELEVATE TISSUE TEMPERATURE AT 3-CM DEPTHS

PURPOSE: The purpose of this study was to determine the rate of tissue temperature rise at known depths with the application of 3 MHz continuous ultrasound. SUBJECTS: Four healthy subjects (2 women, 2 men) participated in the study. METHODS: Using techniques previous described by Draper et al (1995), two 5-cm thermistor probes were inserted in the lateral calf, resulting in two target depths, 1.6 cm and 3.0 cm below the apex of the posterior triceps surae muscle belly. After the depth of the thermistors was determined by diagnostic US, two 10-min applications of 3 MHz ultrasound were applied to each subject. The ultrasound intensity was 1.5 W/[cm.sup.2] for two subjects, while the intensity for the other two subjects was 1.0 W/[cm.sup.2]. The two ultrasound applications were preceded by 10-min baselines, with an -30-min recovery period between applications. ANALYSIS: Single-subject experimental design was used to assess the change in tissue temperature rise across time, which included calculations of autocorrelation coefficients, trend lines, time series analyses, and rates of tissue temperature rise. RESULTS: Seven of the eight probe placements across the 4 subjects were within 0.4 cm of the target depth. Tissue temperature rises at superficial and deep levels increased linearly across the 10-min application for each ultrasound intensity and were significantly elevated from baseline. For the higher intensity ultrasound conditions, three of the four applications resulted in temperature increases of at least 3.8 [degrees] C at the 3.0 cm depth. CONCLUSIONS: Tissue temperature changes were consistent across the two ultrasound applications. These results indicate that ultrasound applications using 3 MHz frequencies can significantly elevate tissue temperature at tissue depths of at least 3 cm.