Fig 1.
Head localization vs. sensor localization.
Left: classical head localization where a large array of sensors (blue; here: the low-Tc SQUID magnetometer locations of the Elekta Neuromag TRIUX helmet) localizes a set of small, dipolar coils (green) that are attached to the subject’s head. Right: example of the proposed magnetometer localization where an array of dipolar coils (green) is used to localize individual (or small arrays of) on-scalp magnetometers (red).
Fig 2.
Low-Tc vs. on-scalp magnetometer positions.
Left: Magnetometer positions used for the low-Tc system (blue; based on the Elekta Neuromag TRIUX). Right: Magnetometer positions used for the on-scalp system (red). Localization is performed for each magnetometer individually.
Fig 3.
A localization coil used in the Elekta Neuromag TRIUX system (ruler scale is in cm).
Fig 4.
Standard localization coil array (green).
Fig 5.
Local localization coil arrays.
Placement of localization coils (green) in a small (here: 7-coil) array around an on-scalp magnetometer position (red).
Fig 6.
Full-head localization coil arrays.
Localization coil placement in 10- (top-left), 12- (top-right), 21- (bottom-left) and 32- (bottom-right) coil full-head arrays.
Fig 7.
Example of a small array of on-scalp magnetometers (red) that are fit as a group with a 32-coil localization array (green).
Fig 8.
Mean localization error with low-Tc (solid blue) and on-scalp (solid red) systems for different magnetometer noise levels and localization coil magnetic moments (both axes plotted logarithmic). Typical noise levels are marked by vertical blue (low-Tc) and red (on-scalp) dotted lines. The horizontal black dotted line indicates 1 mm localization error.
Fig 9.
Localization error vs. SNR for different measurement time.
Mean localization error with low-Tc (solid) and on-scalp (dashed) systems for different data lengths. Magnetic moment = 1 nAm2. Both axes are plotted logarithmic.
Fig 10.
Localization error vs. a priori errors.
Mean localization error with low-Tc (blue) and on-scalp (red) systems for different a) calibration error ranges, b) position inaccuracies ranges, and c) orientation error ranges. Magnetic moment = 10 nAm2, on-scalp magnetometer noise = 20 fT/Hz1/2, and low-Tc magnetometer noise = 3 fT/Hz1/2. Error bars indicate one standard deviation.
Fig 11.
Localization error vs. number of coils.
Mean localization error with low-Tc (blue) and on-scalp (red) systems for different numbers of localization coils a) in small, local localization arrays b) in full-head localization arrays. Magnetic moment = 10 nAm2, on-scalp magnetometer noise = 20 fT/Hz1/2 and low-Tc magnetometer noise = 3 fT/Hz1/2. Error bars indicate one standard deviation.
Fig 12.
Localization error vs. number of magnetometers.
Mean localization error with low-Tc (blue) and on-scalp (red) systems for different numbers of magnetometers fitted as a group. Magnetic moment = 10 nAm2, on-scalp magnetometer noise = 20 fT/Hz1/2 and low-Tc magnetometer noise = 3 fT/Hz1/2. Error bars indicate one standard deviation.
Fig 13.
An orientation-digitizable dipolar coil prototype.
Photograph of a localization coil mounted on a plate for accurate determination of the coil orientation. At the corners are small indentions for the tip of the Polhemus stylus to assist in the digitization.
Fig 14.
Accuracy of magnetic dipole approximation.
Ratio between magnetic dipole approximation and exact solution of the magnetic field for coils with 2 mm (red, dotted) and 4 mm (blue) radii as a function of distance between sensor and coil.