3D-Printed Head-Cast for Optimized OPM Placement

There are two practical problems with using a single OPM to detect neuromagnetic fields: first, the convoluted shape of the cortex means that, even if the approximate location of the region of interest in the subject's head (somatosensory cortex in our case) is known, the precise orientation of the local normal to the cortical surface is unknown. Since the locations of field maxima outside the head are strongly dependent on the direction of current flow, this means that it is difficult to predict where on the scalp sensors should be optimally placed. Second, in order to mathematically model the measured fields, and hence derive 3D images of changing cortical current flow, the precise location and orientation of the sensors relative to the brain anatomy must be known. We solved both problems via the use of 3D printing (see Fig. 2). We used an anatomical magnetic resonance image (MRI) of the subject's head (as described in Meyer et al. (2017) (link)) (Fig. 2A) in order to extract a 3D mesh, representing the outer surface of the head and face (Fig. 2B). Following this, a nylon cast of the outer head surface was fabricated using 3D printing (http://www.chalkstudios.co.uk/), resulting in a head-cast that is moulded to the shape of the individual subject's head (Fig. 2C/D). As part of the head-cast design, we digitally placed a nominally hexagonal array of slots for 13 radially-oriented OPMs over the subject's right somatosensory cortex. The array was located and oriented so as to sample the field maxima and minima expected at 6.5 mm from the scalp surface due to a dipole in the somatosensory cortex (whose location and orientation was estimated from previous SQUID-based dipole fits to the same subject). Importantly, since the head-cast was generated directly from the subject's MRI, the precise location and orientation of the slots for the sensors, with respect to the brain anatomy, was known. The multiple slots meant that the single OPM could be placed in any one of the 13 locations in order to sample the spatially variation of scalp-level magnetic fields.Fig. 2

Head-cast design and fabrication: A) Single sagittal slice from the anatomical MRI. B) Outer head surface extracted from MRI. C) CAD model of the head-cast with slots designed to house the OPM sensors over sensorimotor cortex. Slot positions are based on a-priori prediction of the spatial topography of scalp field pattern, derived from previous SQUID measurements in the same subject. D) 3D-printed head-cast on subject. E) Subject in situ with the OPM attached. Note that the head-cast is not only fixed rigidly to the subject's scalp, but is also fixed relative to the MSR, thus eliminating any sensor motion relative to the subject, and subject motion relative to the MSR.

Fig. 2