Optical Co-registration of MRI and On-scalp MEG

Abstract

To estimate the neural generators of magnetoencephalographic (MEG) signals, MEG data have to be co-registered with an anatomical image, typically an MR image. Optically-pumped magnetometers (OPMs) enable the construction of on-scalp MEG systems providing higher sensitivity and spatial resolution than conventional SQUID-based MEG systems. We present a co-registration method that can be applied to on-scalp MEG systems, regardless of the number of sensors. We apply a structured-light scanner to create a surface mesh of the subject's head and the sensor array, which we fit to the MR image. We quantified the reproducibility of the mesh and localised current dipoles with a phantom. Additionally, we measured somatosensory evoked fields (SEFs) to median nerve stimulation and compared the dipole positions between on-scalp and SQUID-based systems. The scanner reproduced the head surface with <1 mm error. Phantom dipoles were localised with 2.1 mm mean error. SEF dipoles corresponding to the P35m response for OPMs were well localised to the somatosensory cortex, while SQUID dipoles for two subjects were erroneously localised to the motor cortex. The developed co-registration method is inexpensive, fast and can easily be applied to on-scalp MEG. It is more convenient than traditional co-registration methods while also being more accurate.

Figure 1

Optical co-registration procedure: 1. Initial mesh alignment with manually selected fiducial points, shown as coloured numerals, on the MR (left, green) and structured-light scan (right, red) meshes. Dummy sensors used to fix the head position are seen on either side of the head in the optical scan. 2. Selection of the co-registration area, selected area shown in red. 3. Automatic ICP-based co-registration and visualisation of the surface fit error.

Figure 2

Reproducibility of the surface mesh reconstructed by the optical scanner. Distributions (left) and spatial locations (right) of errors across five scans of the same object. Each coloured density plot represents the error of one repetition.

Figure 3

Left: Phantom measurement setup. Right: Phantom dipole localisation; absolute position, orientation and amplitude errors as well as goodness-of-fit (GOF).

Figure 4

Positions of the OPM (yellow spheres; sensitive axes as red arrows) and SQUID (blue rectangles) sensors in the somatosensory measurement, Subject 1 shown as an example.

Figure 5

Somatosensory evoked fields for the OPM and SQUID measurements for all three subjects. Responses only at a subset of SQUID channels are shown for easier interpretation: 27 magnetometers (mSQUID) and 54 gradiometers (gSQUID) covering the right somatomotor cortex. The stimulus onset is indicated by a grey line. Equivalent current dipoles were fitted at the latency indicated by the red vertical line. Topographic maps at this latency for each sensor type are also shown (SQUID magnetometers and gradiometers above, OPMs below the SEF traces). For the SQUID gradiometer topographic maps, the RMS value of each gradiometer pair is shown. Stimulus artifacts can be seen in SQUID signals at 0 ms, especially for Subject 3.

Figure 6

SNR estimates for OPM and SQUID SEF measurements. For the SQUID measurements, those N magnetometers (mSQUID) and gradiometers (gSQUID) with the highest baseline-normalised signal power were chosen interpretation, where N is the number of OPMs used for that subject. Stimulus artifacts can be seen in SQUID signals at 0 ms, especially for subject 3.

Figure 7

Locations of equivalent current dipoles representing the P35m somatosensory response peak visualised on orthogonal views (lateral, dorsal, rostral) of the right hemisphere. Both OPM (red) and SQUID (green) -based dipoles are shown. For more detailed visualisation, an enlarged section of the dorsal view is also shown.