04 Aug 2025
Polarized light method provides more detailed and accurate structural information than MRI.
Deep brain stimulation (DBS) involves implanting electrodes into specific brain areas to regulate abnormal activity, as a surgical treatment for neurological disorders such as Parkinson’s disease.
Placing these electrodes with high positional accuracy is a vital aspect of the procedure, but imaging tools such as MRI often fail to reveal small deep-brain structures, making precise targeting difficult.
A project group from Quebec’s Laval University and Harvard Medical School has now developed a better method, using catheter-based polarization-sensitive optical coherence tomography (PS-OCT).
The results, published in Neurophotonics, show that PS-OCT could be an effective intraoperative imaging tool for guiding DBS neurosurgery, complementing MRI methods.
By detecting the polarization properties of light PS-OCT can yield additional contrast data over standard OCT, and in the case of the brain it can capture the intrinsic birefringence characteristics of the brain’s white-matter tissues.
“In birefringent materials such as brain white matter, light encounters two slightly different refractive indices depending on whether the light is polarized parallel or perpendicular to the tissue fiber direction,” noted the project in its paper. “The amount of birefringence detected in tissue is directly related to the alignment and density of the fibrous structures within it.”
Since PS-OCT can visualize brain structures at the micron level rather than the millimeter resolution provided by MRI, it can detect fine details in white matter fiber tracts, the bundles of nerve fibers that are crucial landmarks for DBS targeting.
Visualizing tissue structures during neurosurgery
In trials using a postmortem animal model, researchers inserted a PS-OCT probe into the brain along planned trajectories, and then captured high-resolution images of the brain’s internal structure as the probe was withdrawn back through the tissue. These images were then matched with MRI scans to assess accuracy.
Results showed that PS-OCT could distinguish between white and gray matter more clearly than MRI and reveal 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 that were invisible in MRI scans.
While PS-OCT offers clear advantages, noted the project, it currently measures fiber orientation only in two dimensions; future improvements enabling full 3D mapping would further enhance its usefulness. The ultimate goal would be a catheter-based system suitable for use by surgeons during implantation of DBS electrodes, able to offer real-time guidance.
“Surgeons would be able to receive feedback on the structural details of brain tissues during DBS procedures, facilitating precise targeting and localization of brain structures,” wrote the project in its paper.
“The ability to visualize fine tissue structures during neurosurgery could significantly improve surgical accuracy and reduce the risk of errors due to misalignment or incomplete targeting.”