  More children and adults under the age of 40 die of brain tumor than from any
other cancer. Brain surgery constitutes the first and decisive step for the
treatment of such tumors. It is extremely crucial to achieve complete tumor
resection during surgery, however, this is a highly challenging task, as it is
very difficult to visually differentiate tumorous cells from the surrounding
healthy white matter. The nerve fiber bundles constitutive of the white matter
are organized in such a way that they exhibit a certain degree of structural
anisotropy and birefringence. The birefringence exhibited by such aligned
fibrous tissue is known to be extremely sensitive to small pathological
alterations. Indeed, highly aligned anisotropic fibers exhibit higher
birefringence than structures with weaker alignment and anisotropy, such as
cancerous tissue. In this study, we performed experiments on thick coronal
slices of a healthy human brain to explore the possibility of (i) measuring,
with a polarimetric microscope (employed in the backscattering geometry to
facilitate non-invasive diagnostics), the birefringence exhibited by the white
matter and (ii) relating the measured birefringence to the fiber orientation
and the degree of alignment. This is done by analyzing the spatial distribution
of the degree of polarization of the backscattered light and its variation with
the polarization state of the probing beam. We demonstrate that polarimetry can
be used to reliably distinguish between white and gray matter in the brain,
which might help to intraoperatively delineate unstructured tumorous tissue and
well organized healthy brain tissue. In addition, we show that our technique is
able to sensitively reconstruct the local mean nerve fiber orientation in the
brain, which can help to guide tumor resections by identifying vital nerve
fiber trajectories thereby improving the outcome of the brain surgery.

Medical Physics