Discriminating brain tumors from fiber-crossing zones in linear retardance images of brain tissue via depolarization contrast

Accurate delineation of brain tumor borders during neurosurgery is essential for maximizing tumor resection while preserving healthy fiber tracts. Polarized light is sensitive to the optical anisotropy of white matter, caused by densely packed myelinated fibers, and can enhance contrast between anisotropic tumorless white matter of brain and isotropic brain tumor in the images of linear retardance and optical axis azimuth. Using a custom-built wide-field imaging Mueller polarimeter, we previously observed high values of linear retardance and well-aligned azimuth of the optical axis within non-tumoral brain tissue, whereas the tumor regions showed reduced values of linear retardance and randomization of the azimuth. However, crossing of healthy fibers can also disrupt optical anisotropy of brain white matter. To investigate this, we performed polarized Monte Carlo simulations and computed Mueller matrix–derived maps of linear retardance, azimuth of the optical axis, and depolarization. We varied the depth of inclusions, mimicking either crossing fiber or tumor zone, in the optical phantoms of brain tissue. Our results showed that while retardance decreases in both scenarios, depolarization decreases significantly only in tumor-mimicking regions. This depth-dependent depolarization behavior may help differentiate tumor regions from complex fiber architectures during polarimetric image-guided neurosurgery.