Reversible edema following electric drilling of macaque craniotomy

In-vivo electrophysiology requires direct access to brain tissue, necessitating the development and refinement of surgical procedures and techniques that promote the health and well-being of the animal subjects. Here, we report a series of findings noted on structural magnetic resonance imaging (MRI) scans in monkeys with MRI-compatible implants following small craniotomies that provide access for intracranial electrophysiology. We found distinct brain regions exhibiting hyperintensities in T2-weighted scans that were prominent underneath the sites at which craniotomies had been performed. We interpreted these hyperintensities as edema of the neural tissue and found that they were predominantly present following electric drilling, but not when manual, hand-operated drills were used. Further, the anomalies subsided within 2-3 weeks following surgery. Our report highlights the utility of MRI-compatible implants that promote clinical examination of the animals brain, sometimes revealing findings that may go unnoticed when incompatible implants are used. We show replicable differences in outcome when using electric vs. mechanical devices, both ubiquitous in the field. If electric drills are used, our report cautions electrophysiological recordings from tissue directly underneath the craniotomy for the first 2-3 weeks following the procedure due to putative edema.