The calculation of induced currents and absorbed power in a realistic, heterogeneous model of the lower leg for applied electric fields from 60 Hz to 30 MHz.

The ankle consists mainly of bone and tendon with little muscle. Currents will tend to preferentially flow through the high-conductivity muscle and this can result in very high local values of the specific energy-absorption rate (SAR). This paper presents a finite-difference method to calculate SAR in a realistic, heterogeneous model of the leg below the knee. The structure of the leg is defined by cross-sectional slices from an anatomical atlas which are converted into a 3D model of over 14,000 cells. Four types of tissue are included in the model--muscle, cortical bone, trabecular bone and connective tissue. Displacement as well as ionic currents are considered in a complex potential representation. The current to be injected into the limb model is obtained from the computed coupling of an applied vertical electric field with a 1.8 m tall, homogeneous phantom. Values of the maximum current density and SAR in the ankle from 60 Hz to 30 MHz are presented. Field limits based on a maximum SAR of 20 W kg-1 averaged over 1 g of tissue are given. Sensitivity analyses with regard to the range of tissue electric properties and the ankle cross-sectional area are performed.