On the Volume Conduction Model Validation with Stereo EEG Data
Creators: name: PIASTRA, MARIA CARLA
Publisher: name: Universit� degli studi di Genova
Year: 2019

description: Volume conduction can be defined as the transmission of electric potential and magnetic field
generated by a primary current source of brain activation in the surrounding medium, i.e.,
the human head.
Volume conduction simulations are based on sophisticated models whose construction
represents a current challenge within the neuroscientific community.
Volume conduction models are used in various applications such as electroencephalography
(EEG) or magnetoencephalography (MEG) source reconstruction, or in the optimization
of the electrode placement in a transcranial electrical stimulation session. Clinical applications
based on volume conduction models are, for example, the localization of the
epileptogenic zone, i.e., the brain area responsible for the generation of seizures, in the presurgical
assessment of focal drug-resistant epilepsy patients, and the antidepressant effects
given by transcranial electrical stimulation. Increasing the accuracy of volume conduction
simulations is therefore crucial.
To the best of our knowledge, the accuracy of volume conduction models have never
been validated directly with actual measurements in human patients.
The main goal of this thesis is to describe a first attempt to validate volume conduction
modeling using electric stimulation stereo-encephalografic (sEEG) data.
This work therefore is focused on the research, investigation and test of tools and methods
which can be used to describe the accuracy of volume conduction models used in both clinical
and basic research.
Given a dataset of one pharmaco-resistant epilepsy patient, composed by the anatomical
T1 weighted magnetic resonance image (MRI), the electrophysiological signal recorded
during electric brain stimulation sessions with sEEG technique and sEEG contact positions
extracted by post-implantation CT image, the analysis conducted in this work can be split
into three main steps.
First, we built volume conduction head models and we simulated the electric potentials
during the electric brain stimulations. In this step, we solved the so-called (s)EEG forward
problem by means of the finite element method in its classical formulation, and we considered
three different conductivity profile to assign to the computational domain, individually
extracted by the T1-w MRI. Moreover we computed the solution in meshes with two different
resolution, i.e., 1 mm and 2 mm, with three different ways to model the source term, i.e., the
partial integration approach, the subtraction approach and Venant?s approach.
Second, we extracted the responses to the electric brain stimulations from the actual
sEEG measurements. Particular emphasis in this step was given to the optimal referencing
systems of sEEG electrodes.
Third, we compared the simulated and measured potentials for each of the three volume
conduction head models, both in a single shaft and global comparison.
The comparison results in overall high relative differences, with only slight modulations
given by the distance from the stimulation site, the underlying volume conduction head
model used and the compartment where the dipolar source is located.
Simulation results show that the computation of sEEG forward problem solution is
feasible with the same scheme adopted for scalp EEG in the duneuro software (http://
duneuro.org/), and it is stable for different mesh resolutions and source models also for
intracranial electrodes, i.e., for electrodes close to the source positions.
From this first validation attempt, we can conclude that the distance contact-source
modulates the relative error between measured and simulated potential; for the contacts in
the white matter compartment we observed the most accurate results, and the results relative
to the three and four compartment results were more accurate than the ones relative to the
five compartment results. While we achieved topographical errors within 10% for most of
the shafts, the amplitude of simulated and measured potentials notably differs.