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

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Abstract
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 14000 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.