Deep brain stimulation (DBS), a surgical procedure that can be used to treat Parkinson’s, obsessive-compulsive disorder, and other neurological disorders, involves implanting electrodes in specific brain regions to regulate abnormal neural activity. The precise placement of these electrodes is crucial for a successful clinical outcome. Magnetic resonance imaging (MRI), the tool commonly used for DBS mapping, lacks the resolution and contrast needed to accurately pinpoint the small, deep brain nuclei targeted for electrode placement. Consequently, researchers are exploring optical imaging techniques with better contrast, higher resolution, and lower costs than MRI to serve as supplementary tools in intraoperative DBS. A study by Laval University and Harvard Medical School explores one such tool, polarization-sensitive optical coherence tomography (PS-OCT), and demonstrates its potential as a complementary imaging technique for guiding DBS surgery. PS-OCT provides higher resolution and contrast images than MRI, as demonstrated in three common DBS targets: the subthalamic nucleus (right), the caudate nucleus (center), and the globus pallidus (left). Courtesy of S. Masoumi et al., doi 10.1117/1.NPh.12.3.035001. Unlike MRI, which provides mm-scale resolution, PS-OCT can visualize brain structures at the μm level. This enables it to provide detailed information essential to accurately target electrodes used in DBS surgery. The researchers tested PS-OCT on three primary DBS targets in a postmortem animal. To simulate a DBS procedure, they inserted a PS-OCT probe into the brain along predefined trajectories. As the probe was pulled through the tissue, it collected data and captured high-resolution images of the brain’s internal structure. The researchers matched these images with MRI scans and anatomical references to assess their accuracy. The PS-OCT system used a rotating catheter with a tiny lens and prism to direct light into the tissue and measure how the light’s polarization changed as it passed through different structures. This change, or birefringence, reflects the alignment and density of fibers in the brain’s white matter. The use of polarized light to detect subtle structural differences in tissue and capture birefringence could enable more accurate identification of white matter fiber tracts — bundles of nerve fibers in the brain that are crucial landmarks for DBS targeting. The researchers used a simplified segmentation approach to compare the performance of PS-OCT with MRI. They averaged data along the probe path and applied clustering to separate tissue types. This allowed them to create “tissue barcodes” showing transitions between white and gray matter. The results showed that PS-OCT was able to distinguish between white and gray matter more clearly than MRI. PS-OCT also captured fine fiber structures that MRI missed, such as the internal capsule, a dense bundle of fibers important for DBS planning. In one case, PS-OCT identified highly organized fiber tracts near the external pallidum that were invisible in MRI scans. Overall, PS-OCT’s polarization-sensitive reconstruction algorithms provided more detailed, accurate information than MRI, while remaining consistent with MRI findings. PS-OCT could provide surgeons with supplementary intraoperative feedback during DBS procedures, improving accuracy and reducing the risk of electrode misplacement. Moreover, the system’s compact form factor and imaging paradigm could be integrated seamlessly into the surgical workflow. “Catheter-based PS OCT shows strong promise as a tool complementary to MRI in DBS neurosurgery,” researcher Shadi Masoumi said. “By providing high-resolution structural information and visualizing critical fiber pathways, it could help surgeons target brain regions more precisely.” Although PS-OCT offers superior resolution, future advancements could further broaden its applicability. It currently measures fiber orientation in 2D only, and the ability to capture fiber orientations in 3D would increase its value as a visualization tool. The PS-OCT probe used in the study was slightly larger than standard DBS electrodes, but smaller probes are now available and could be adapted for clinical use. Next steps include live testing, integration into surgical workflows, and direct comparisons with diffusion MRI, another technique used to map brain fibers. If successful, PS-OCT could become a valuable addition to the neurosurgical toolkit, improving outcomes for patients undergoing DBS. The research was published in Neurophotonics (www.doi.org/10.1117/1.NPh.12.3.035001).