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. Phantoms provide well-defined volume conduction configurations with realistic geometries.We aim to characterize the electrical conductivity of materials for modeling head compartments to establish reproducible and stable physical head phantoms. We analyzed sodium chloride (NaCl) solution, agarose hydrogel, gypsum and reed sticks as surrogate materials for the intracranial volume, scalp, skull and anisotropic conductivity structures. We measured the impedance of all materials when immersed in NaCl solution using a four-point setup. The electrical conductivity values of each material were calculated from the temperature compensated impedances considering the sample geometries. Results We obtained conductivities of 0.332 S/m (0.17 % NaCl solution), 0.0425 S/m and 0.0017 S/m (gypsum with and without NaCl), 0.314 S/m, 0.30 S/m, 0.311 S/m (2 %, 3 %, 4 % agarose). The reed sticks were tested in longitudinal and transversal direction and showed anisotropic conductivity with a ratio of 1:2.8. Conclusion We conclude that the tested materials NaCl solution, gypsum and agarose can serve as stable representation of the three main conductivity compartments of the head, intracranial volume, skull and scalp. An anisotropic conductivity structure such as a piece of white matter can be modeled using tailored reed sticks inside a volume conductor.

Investigating Gelatine Based Head Phantoms for Electroencephalography Compared to Electrical and Ex Vivo Porcine Skin Models

“…This phantom was shown to be stable for use over the course of 8 hours. [30] presented a three-layer aragose and gypsum based phantom for testing both EEG and tES.…”

“…The result is that a wide number of different materials have been investigated for making electrophysiology body phantoms: gelatine [2], [5], [13]- [17], wax [31], agar [18], [20], aragose [19], [30], silicon [31], PDMS [21], plastic clay [32], and plastic moulds [24]; with gelatine being by far the most common to date. Based on this, gelatine has also been used as the base material for real-tissue phantoms.…”

“…Thermal and mechanical properties of gelatine phantoms were investigated in [37], with milk added to control these. [30] investigated the a.c. impedance properties of aragose and gypsum based phantoms with different amounts of NaCl present, focusing on measuring the impedance properties of the materials rather than the use as a head phantom.…”