Head phantoms for bioelectromagnetic applications: a material study

Abstract

Background: Assessments of source reconstruction procedures in electroencephalography and computations of transcranial electrical stimulation profiles require verification and validation with the help of ground truth configurations as implemented by physical head phantoms.. For these phantoms, synthetic materials are needed, which are mechanically and electrochemically stable and possess conductivity values similar to the modeled human head tissues. Typical three-compartment head models comprise a scalp layer with a conductivity range from 0.137 S/m to 2.1 S/m, a skull layer with conductivity values between 0.066 S/m and 0.00275 S/m, and an intracranial volume with an often-used average conductivity value of 0.33 S/m. To establish a realistically shaped physical head phantom with a well-defined volume conduction configuration we here characterize the electrical conductivity of synthetic materials for modeling head compartments. We analyze agarose hydrogel, gypsum, and sodium chloride (NaCl) solution as surrogate materials for scalp, skull, and intracranial volume. We measure the impedance of all materials when immersed in NaCl solution using a four-electrode setup. The measured impedance values, temperature compensated to 25°C, were used to calculate the electrical conductivity values of each material. Further, the conductivities in the longitudinal and transversal directions of reed sticks immersed in NaCl solution were measured to test their suitability for mimicking the anisotropic conductivity of white matter tracts.Results: We obtained conductivities of 0.314 S/m, 0.30 S/m, 0.311 S/m (2 %, 3 %, 4 % agarose), 0.0425 S/m and 0.0017 S/m (gypsum with and without NaCl in the compound), and 0.332 S/m (0.17 % NaCl solution). These values are within the range of the conductivity values used for EEG and TES modeling. The reed sticks showed anisotropic conductivity with a ratio of 1:2.8. Conclusion: We conclude that the tested materials agarose, gypsum, and NaCl solution can serve as stable representations of the three main conductivity compartments of the head scalp, skull, and intracranial volume. An anisotropic conductivity structure such as a fiber track in white matter can be modeled using tailored reed sticks inside a volume conductor.