Peyton Harmon¹, Bruno Averbeck¹, Reza Azadi¹; ¹Laboratory of Neuropsychology, National Institute of Mental Health, Bethesda, MD

Traditional MRI-guided neurosurgical targeting in nonhuman primates (NHPs) often relies on MRI-visible earbars in the stereotaxic devices. However, variability in repositioning can compromise the accuracy of these methods. To address this, we developed a reliable approach using fiducial markers implanted into the skull, enabling direct coregistration of MRI scans with stereotaxic coordinates based on skull position. Human fiducial markers are often unsuitable for NHPs due to their large size. Therefore, we developed a novel method using MRI-compatible cranial screws made for NHP surgeries. These screws are detectable on T1-weighted MRI scans due to the contrast with surrounding soft tissue. To evaluate this method, we first implanted 28 ceramic screws into a 3D-printed macaque skull surrounded by a contrast agent gel. The coordinates of each fiducial were manually measured on T1-weighted MRI and in a stereotaxic device, both with and without altering the skull’s position. These coordinates were cooregistered using a 3D affine transformation. The results showed reliable and accurate alignment even after repositioning the 3D printed skull within the stereotaxic device. The average error was xyz using all 28 fiducial markers. Moreover, a resampled analysis showed that using as few as six screws provided nearly equivalent reliability, with an average error of xyz. This novel approach eliminates dependence on earbar coordinates and their associated variability, providing more accurate and reproducible reference points for brain regions. Furthermore, similar methods can be used for coregistering MRI and CT scans.

Acknowledgements: This work was supported by the Intramural Research Program of the National Institute of Mental Health (ZIA MH002928) to B.A.