Polarized Light Imaging
Cortical areas and subcortical nuclei are interconnected via short- and long-distance fiber tracts, which form neural networks. To understand the functionality of these networks, it is indispensable to know their anatomical connections. Three-dimensional polarized light imaging (3D-PLI) represents a novel neuroimaging technique to map nerve fibers, i.e. myelinated axons, and their pathways in human postmortem brains with a resolution at the sub-millimeter scale, i.e. at the mesoscale. 3D-PLI utilizes an optical property of the myelin sheaths of nerve fibers known as birefringence. It is caused by the regular arrangement of lipids and proteins in myelin. Birefringence results in a distinct optical anisotropy reflecting the spatial fiber architecture of nerve fibers.
Birefringence can be quantified by passing linearly polarized light through thin brain tissue (≤100 µm) and by measuring local changes in the polarization state of light. The linearly polarized light interacts locally with the radially oriented myelin lipids, becomes elliptically polarized, and serves as a direct measure of the 3D spatial orientation of a myelinated axon. This type of measurement is widely known as polarimetry (cf. figure).
Signal analysis and 3D reconstruction of a contiguous series of microtome brain sections yields a fundamental volume data set describing the local prevailling fiber orientations in terms of orientation vectors (fiber orientation map, 3D-FOM). Fiber tractography applied to this data set finally reveals the spatial courses of fiber tracts as well as their connections to distinct cortical and subcortical areas (cf. Movie).