Effects of head modeling errors on the spatial frequency representation of MEG

TL;DR

This paper investigates how head modeling errors affect MEG signal accuracy, showing that in high SNR scenarios, inaccuracies can impair source localization, especially with closer sensors, but higher SNR can also mitigate these effects.

Contribution

It provides an analytical expression for magnetic field errors due to head geometry inaccuracies and highlights the importance of precise head modeling for high-resolution MEG with close sensors.

Findings

In high SNR, head modeling errors increase localization inaccuracies.

Closer sensors improve SNR but require accurate head models.

Analytical formula for magnetic field errors due to head geometry perturbations.

Abstract

Optically-pumped magnetometers (OPM) -- next-generation magnetoencephalography (MEG) sensors -- may be placed directly on the head, unlike the more commonly used superconducting quantum interference device (SQUID) sensors, which must be placed a few centimeters away. This allows for signals of higher spatial resolution to be captured, resulting in potentially more accurate source localization. In this paper, we show that in the noiseless and high signal-to-noise ratio (SNR) case of approximately $\geq 6$ dB, inaccuracies in boundary element method (BEM) head conductor models (or equivalently, inaccurate volume current models) lead to increased signal and equivalent current dipole (ECD) source localization inaccuracies when sensor arrays are placed closer to the head. This is true especially in the case of deep and superficial sources where volume current contributions are high. In the noisy case however, the higher SNR for closer sensor arrays allows for an improved ECD fit and outweighs the effects of head geometry inaccuracies. This calls for an increase in emphasis in head modeling to reduce inverse modeling errors, especially as the field of MEG strives for closer sensor arrays and cleaner signals. An analytical form to obtain the magnetic field errors for small perturbations in the BEM head geometry is also provided.