Improved Localization Accuracy in Magnetic Source Imaging Using a 3-D Laser Scanner

Abstract

Brain source localization accuracy in magnetoencephalography (MEG) requires accuracy in both digitizing anatomical landmarks and coregistering to anatomical magnetic resonance images (MRI). We compared the source localization accuracy and MEG-MRI coregistration accuracy of two head digitization systems-a laser scanner and the current standard electromagnetic digitization system (Polhemus)-using a calibrated phantom and human data. When compared using the calibrated phantom, surface and source localization accuracy for data acquired with the laser scanner improved over the Polhemus by 141% and 132%, respectively. Laser scan digitization reduced MEG source localization error by 1.38 mm on average. In human participants, a laser scan of the face generated a 1000-fold more points per unit time than the Polhemus head digitization. An automated surface-matching algorithm improved the accuracy of MEG-MRI coregistration over the equivalent manual procedure. Simulations showed that the laser scan coverage could be reduced to an area around the eyes only while maintaining coregistration accuracy, suggesting that acquisition time can be substantially reduced. Our results show that the laser scanner can both reduce setup time and improve localization accuracy, in comparison to the Polhemus digitization system.

Reliability for non-invasive somatosensory cortex localization: Implications for pre-surgical mapping

“…This co-registration is usually achieved via the spatial localization of head position indicator coils attached to anatomical landmarks on the patient's head. Recent data has suggested that new head digitization technologies can improve MEG source localization accuracy from 5 mm to 2 mm, and improve intersession reliability approximately by a factor of 2, as compared to the commonly used methodology described in the current study (Polhemus) [14], [15]. Thus, novel digitization technologies can have a major impact on the intersession reliability reported herein.…”

A comparison of automated and manual co-registration for magnetoencephalography

“…The co-registration error obtained for both processes in the present study is also consistent with that of other studies. For instance, a study using bite bars to reduce motion found a mean co-registration error of 1.16 mm (5), while a study using a 3D scanner found mean error of 2.2 mm (7), and one using a 3D camera (Kinect) observed a mean error of 1.62 mm (6). Despite the ready availability of co-registration error metrics, reporting of these metrics in MEG studies has not yet become standard practice (29,30).…”

“…The headshape points are collected primarily from the brow, bridge of the nose, and skull, avoiding the lower jaw and cartilaginous or fatty tissue that would be expected to shift when the participant moves or might be compressed by the head coil during the participant's MRI scan. Because this preparation process can be somewhat labor-intensive and subject to variability in technician skill, alternatives such as use of a bite bar (5), 3-D camera (6), or 3-D laser scanner (7) have also been explored but are not widely used. transform the participant's MRI into the MEG head coordinate space.…”

Head movement compensation in real-time magnetoencephalographic recordings

“…The selected current sources allowed for maximum spatial disparity between sources, to investigate activation across the sensor array. These sources can be localized with millimeter accuracy based on the MEG data using a current dipole model [1]. The HPI coils on the phantom are wire loops that can be activated to generate a magnetic field source that is easily localized by the MEG sensor array.…”

“…In MEG, sources are localized in the coordinate frame of the MEG device. A conversion to the coordinate frame of the participant requires the accurate tracking of head position within the scanner [1]. To calculate head position, HPI coils, placed at common anatomical landmarks on the head, are localized during the scan, generating a continuous estimate of head position.…”