Abstract
Optically pumped magnetometers (OPMs) are quickly widening the scopes of noninvasive neurophysiological imaging. The possibility of placing these magnetic field sensors on the scalp allows not only to acquire signals from people in movement, but also to reduce the distance between the sensors and the brain, with a consequent gain in the signal‐to‐noise ratio. These advantages make the technique particularly attractive to characterise sources of brain activity in demanding populations, such as children and patients with epilepsy. However, the technology is currently in an early stage, presenting new design challenges around the optimal sensor arrangement and their complementarity with other techniques as electroencephalography (EEG). In this article, we present an optimal array design strategy focussed on minimising the brain source localisation error. The methodology is based on the Cramér‐Rao bound, which provides lower error bounds on the estimation of source parameters regardless of the algorithm used. We utilise this framework to compare whole head OPM arrays with commercially available electro/magnetoencephalography (E/MEG) systems for localising brain signal generators. In addition, we study the complementarity between EEG and OPM‐based MEG, and design optimal whole head systems based on OPMs only and a combination of OPMs and EEG electrodes for characterising deep and superficial sources alike. Finally, we show the usefulness of the approach to find the nearly optimal sensor positions minimising the estimation error bound in a given cortical region when a limited number of OPMs are available. This is of special interest for maximising the performance of small scale systems to ad hoc neurophysiological experiments, a common situation arising in most OPM labs.
Highlights
Pumped magnetometers (OPMs) are revolutionising the way we measure magnetic brain signals
The main advantage of this approach lies in its capability to provide optimal performance metrics on the solution of the electromagnetic source localisation problem based on the sensor type, number, and arrangement, but independently of the algorithm employed
The Cramér-Rao bound (CRB) framework was used to compare the capabilities of Optically pumped magnetometers (OPMs) arrays with others generally utilised in the field, such as high density EEG (hdEEG) and SQUID-based systems, as well as to generate optimal sensor arrangements suited to different needs
Summary
Pumped magnetometers (OPMs) are revolutionising the way we measure magnetic brain signals. With regard to OPM-MEG, this means to compare their performance with conventional SQUID-MEG for characterising sources of brain activity This task is of particular importance at this early developmental stage for determining optimal array designs that allow minimal source reconstruction errors. To assess the performance of wholehead standard OPM-based MEG arrays based on existing on-scalp sensor positioning setups Such large arrays are currently scarce (Hill et al, 2020), the CRB allows to quantify the improvements on source localisation compared with SQUID-based montages resembling commercially available systems, as well as with high density EEG (hdEEG). Red dots indicate gradiometer coils, whereas yellow dots represent magnetometers or EEG electrodes (whenever corresponds) axial and one tangential direction (randomly chosen in this study; see Section 4 and Figure S1) and located at a distance of 6 mm from the scalp, emulating the second generation sensors provided by QuSpin Inc. 40 Hz (Vrba & Robinson, 2002)
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