Rhodes, Natalie
(2024)
Development and Application of a Paediatric OPM-MEG System.
PhD thesis, University of Nottingham.
Abstract
The first few years of life see rapid changes in brain function, as we grow and learn, yet the developing brain remains poorly understood. This is in part due to instrumentational constraints. Magnetoencephalography (MEG) is a non-invasive neuroimaging technique that assesses human brain function through the detection of magnetic fields generated by neural currents. As a direct measure of electrophysiology, MEG can detect changes in brain activity with a high temporal resolution. Further, by measuring across the whole head, MEG is able to probe the dynamics of brain networks as they evolve in support of cognition. MEG is therefore, in principle, the ideal tool to understand how the brain changes in the first years of life. However, conventional MEG systems are based on sensors that must be cryogenically cooled to reach the sensitivities required to detect the neuromagnetic field. As such, sensors are held in one-size-fits-all systems that require participants to remain still during data acquisition, which is not ideal for scanning young participants. MEG systems are also built for adults and therefore lack sensitivity for scanning infants and children. Recently, optically pumped magnetometers (OPMs) have emerged as a potential replacement for cryogenic sensors in MEG. OPMs are lightweight, flexible and operate without the requirement for cryogenic cooling. As such, OPMs can be placed close to the scalp surface, enabling the development of wearable MEG systems that can conform to different head sizes and allow naturalistic movement.
This thesis aims to develop and implement the first whole-head paediatric OPM-MEG system for studying neurodevelopment. We first test a whole-head adult system for sensitivity to neural activity in the theta band in comparison to conventional MEG instrumentation, assessing whether OPM-MEG is sensitive to low frequency signals. This is particularly important in neurodevelopment, where previous studies have found elevated low frequency activity in younger participants. Next, we propose and demonstrate a method of warping template anatomical MRI scans to 3D structure scans of the participant’s head shape to generate “pseudo-MRIs”, enabling the analysis of OPM-MEG data without the acquisition of an individual’s brain structure. This is important for children who may find the MRI scanning environment challenging.
Following this, we describe two of the first neurodevelopmental studies using OPMMEG. We first investigate the developmental trajectory of cortical beta band oscillations using a simple sensory task across a group of healthy children and adults. We use data-driven analysis approaches to investigate how beta band activity arises and analyse the evolution of the spectral properties of MEG signals with age. Finally, we use data collected from two systems at different sites to form (to our knowledge) the largest (to date) OPM-MEG study of 102 participants aged from 2 years to adulthood. We analyse data during a visual task to investigate how visual gamma oscillations change with age. Overall, the thesis demonstrates how OPM-MEG can be used as a new, viable platform for the study of neurodevelopment.
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