Impact of skull sutures, spongiform bone distribution, and aging skull conductivities on the EEG forward and inverse problems

Hannah Mccann,Leandro Beltrachini

doi:10.1088/1741-2552/ac43f7

Abstract

Objective. Source imaging is a principal objective for electroencephalography (EEG), the solutions of which require forward problem (FP) computations characterising the electric potential distribution on the scalp due to known sources. Additionally, the EEG-FP is dependent upon realistic, anatomically correct volume conductors and accurate tissue conductivities, where the skull is particularly important. Skull conductivity, however, deviates according to bone composition and the presence of adult sutures. The presented study therefore analyses the effect the presence of adult sutures and differing bone composition have on the EEG-FP and inverse problem (IP) solutions. Approach. Utilising a well-established head atlas, detailed head models were generated including compact and spongiform bone and adult sutures. The true skull conductivity was considered as inhomogeneous according to spongiform bone proportion and sutures. The EEG-FP and EEG-IP were solved and compared to results employing homogeneous skull models, with varying conductivities and omitting sutures, as well as using a hypothesised aging skull conductivity model. Main results. Significant localised FP errors, with relative error up to 85%, were revealed, particularly evident along suture lines and directly related to the proportion of spongiform bone. This remained evident at various ages. Similar EEG-IP inaccuracies were found, with the largest (maximum 4.14 cm) across suture lines. Significance. It is concluded that modelling the skull as an inhomogeneous layer that varies according to spongiform bone proportion and includes differing suture conductivity is imperative for accurate EEG-FP and source localisation calculations. Their omission can result in significant errors, relevant for EEG research and clinical diagnosis.

Highlights

Electroencephalography (EEG) is a non-invasive imaging modality used for characterising the electrical activity of the brain (Henry, 2006)
This paper aims at evaluating the contribution that adult sutures have on the EEG forward and inverse solutions and their effect compared to neglecting deviations in bone composition
When accounting for a “ground truth” of both variation in bone composition and the presence of sutures the electrical impedance tomography (EIT) estimated homogeneous model conductivity was estimated as 0.0072 S/m

Summary

Introduction

Electroencephalography (EEG) is a non-invasive imaging modality used for characterising the electrical activity of the brain (Henry, 2006). The majority of current studies incorporate realistic geometries from complementary medical images, the conductivities of all tissues, including the skull, are typically assumed from existing literature This poses a limitation to model personalisation as it is accepted that most tissues, and in particular the skull, are subject to large variability between participants and measurement methods and protocols (McCann et al, 2019). Misspecification of this conductivity, when considered as one compartment, has resulted in significant EEG forward and inverse solution errors (Vallaghé et al, 2008; Acar er al., 2013; Chen et al, 2010; Vorwerk et al, 2019).

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