Finite Element study of skin and fat delineation in an obese subject for transcranial Direct Current Stimulation

Because of pilot data suggesting the efficacy of transcranial Direct Current Stimulation (tDCS) in treating a range of neuropsychiatric disorders as well as in controlling cravings, there is interest to apply to obese subjects. The abnormal thickness of fat that exist in obese subjects may influence current delivery from scalp electrodes to the brain. MRI-derived Finite Element (FE) models of a morbidly obese subject were created with and without fat delineated. The inclusion of fat to the FE model reduced the effective volume of the relatively conductive skin. This led to greater current penetration to the cortical surface. Electric field was substantially greater (60%) in magnitude and a difference in the spatial profile was noted in the model with fat. Additional models testing the effect of varying fat conductivity revealed an inflection in current penetration as fat conductivity is varied. It was postulated that this may be due to a shunting effect both when the shell of fat surrounding the skull is too resistive for penetration and when the fat is so conductive as to lead current around rather than through the head. Precise FE tDCS model of obese patients requires the precise inclusion of fat.