Frequency dependence of thresholds for ultrasonic production of thermal lesions in tissue

Abstract

The ultrasonic intensity threshold for producing lesions in mammalian brain tissue is not a strong function of frequency (over the range of 1–10 MHz) for exposures longer than 1 sec. A model is presented to explain this apparent lack of frequency dependence. It is assumed that the maximum temperature developed in the lesion volume for a given pulse duration is determined (1) by the absorption coefficient of that tissue, and (2) by the distribution of the acoustic intensity over the treated volume (sharpness of the beam). The former is observed experimentally to be nearly linearly dependent upon frequency in the range 1–10 MHz, and the latter, for a good lens, is related inversely to frequency. Temperature calculations which account for heat loss by diffusion as well as the frequency dependence for the beam geometry and the absorption coefficient are presented. These lead to nearly frequency independent curves for threshold dosages beyond 1-sec exposure, giving credence to the suggestion that thermal processes may be predominant for such exposures.