Measuring MEG closer to the brain: Performance of on-scalp sensor arrays

Abstract

Optically-pumped magnetometers (OPMs) have recently reached sensitivity levels required for magnetoencephalography (MEG). OPMs do not need cryogenics and can thus be placed within millimetres from the scalp into an array that adapts to the individual head size and shape, thereby reducing the distance from cortical sources to the sensors. Here, we quantified the improvement in recording MEG with hypothetical on-scalp OPM arrays compared to a 306-channel state-of-the-art SQUID array (102 magnetometers and 204 planar gradiometers). We simulated OPM arrays that measured either normal (nOPM; 102 sensors), tangential (tOPM; 204 sensors), or all components (aOPM; 306 sensors) of the magnetic field. We built forward models based on magnetic resonance images of 10 adult heads; we employed a three-compartment boundary element model and distributed current dipoles evenly across the cortical mantle. Compared to the SQUID magnetometers, nOPM and tOPM yielded 7.5 and 5.3 times higher signal power, while the correlations between the field patterns of source dipoles were reduced by factors of 2.8 and 3.6, respectively. Values of the field-pattern correlations were similar across nOPM, tOPM and SQUID gradiometers. Volume currents reduced the signals of primary currents on average by 10%, 72% and 15% in nOPM, tOPM and SQUID magnetometers, respectively. The information capacities of the OPM arrays were clearly higher than that of the SQUID array. The dipole-localization accuracies of the arrays were similar while the minimum-norm-based point-spread functions were on average 2.4 and 2.5 times more spread for the SQUID array compared to nOPM and tOPM arrays, respectively.

Keywords: Atomic magnetometer; Lead field; Magnetoencephalography; Optically-pumped magnetometer; Sensor array; Superconducting quantum interference device.

Figure 1

The constructed SQUID (left) and OPM (right) sensor positions. Top: Frontal and lateral views of the sensor locations for one subject. Other rows: Lateral views for the rest of the subjects.

Figure 2

The relative powers of the arrays. The average values of the relative power across ten subjects are presented in the average brain of the subjects. Histograms represent the distribution of the values of relative power for sources in both hemispheres. Histograms are normalized by the total number of sources.

Figure 3

Contributions of primary and volume currents. The ratios of the norms of (A) total- and primary-current topographies (TP) and (B) primary- and volume-current topographies (PV). C: The correlation coefficient between the topographies of the primary and volume currents (CC_PV). For further details, see the caption of Fig. 2.

Figure 4

Similarity of source topographies. (A) Peak position errors (PPEs) and (B) cortical areas (CAs) of topography correlations. CA is measured as a percentage of the total area of cortex. For details, see the caption of Fig. 2.

Figure 5

The effective field-of-view (eFOV) of the sensors and correlation of lead fields. A: Histograms of eFOV with values from different subjects pooled. eFOV is measured as a percentage of the total area of cortex. Each histogram has been normalized by the total number of sensors across the subjects. B: Histograms presenting the correlation coefficient between sensor lead fields as a function of sensor distance in degrees. The lead-field correlations across the subjects and sensors have been pooled. Each histogram has been normalized so that the sum over the bins gives 100%.

Figure 6

The total information capacities of the sensor arrays. Total information capacity is measured in units of bits per sample (bits/Sa). Top: Total information capacity averaged across the subjects and normalized by the number of sensors. Bottom: Total information capacity for each subject.

Figure 7

Characteristics of the minimum-norm-based point-spread functions. (A) Peak position error (PPE) and (B) cortical area (CA) relative to the total area of cortex. For details about the illustration, see the caption of Fig. 2.