Real-Time Neuronavigation

To perform the neuronavigation protocol in real time, we employed an in-house visualization system developed for this specific purpose (Rubianes Silva, 2017). A version runnable on Windows is freely available on our website¹. First, subjects underwent an MRI session on a 3 T MRI scanner (Philips Achieva). The structural MRI image (T1-weighted with 1 × 1 × 1 mm³ resolution, TE = 3.2 s, TR = 7 ms, TI = 900 ms, 8° flip angle) was used as a priori information to guide the movements of the commercial digitizer on the subjects’ scalp during the fNIRS sessions. For this, a rigid transformation matrix that aligns the scanned volume with the corresponding physical head must be computed. Eight pairs of anatomical points (i.e., points from the physical head and its correspondence on the rendered scanned volume) must be provided manually. In this study, we used the inion, right outer corner of eye, nasion, preauricular left, preauricular right, glabella, left alar, and right alar as the anatomical references (Sforza and Ferrario, 2006; Ibrahim et al., 2016). The singular value decomposition (SVD) technique was applied to find the elements of the transformation matrix.

During the fNIRS sessions, the visualization system captures the position of the digitizer on the physical head, transforms it to the anatomical space of the MRI volume using the computed transformation matrix, and displays it as a triad cursor on the visible head’s surface. The setup to calibrate the neuronavigation space employs eight pairs of anatomical points (i.e., points from the physical head and its correspondence on the rendered scanned volume) that must be provided manually. In this study, we used the inion, right outer corner of eye, nasion, preauricular left, preauricular right, glabella, left alar, and right alar as the anatomical references (Sforza and Ferrario, 2006; Ibrahim et al., 2016). The depthmap-based picking algorithm was implemented to afford 3D interactions with rendered images (Wu et al., 2003, 2011). The reproducibility of the procedure was assessed through a one-way analysis of variance (ANOVA) of a series of measurements of randomly chosen seven points on different heads at different instants of time. It only failed when the subject did not have palpable inion.

With the visualization system described above, we used the MRI image to guide the placement of the digitizer over the primary motor cortex at all sessions of each subject (Yousry et al., 1997). In the first session of measurements, we saved the position of the fNIRS optodes that were located above the motor cortex. In the subsequent sessions, small spheres with a radius of less than 6 mm were displayed on these saved positions to guide the placement of optodes. This ensured that all the optodes were placed very close on the head across different sessions.