Chapter 18 – High-Resolution Fiber and Fiber Tract Imaging Using Polarized Light Microscopy in the Human, Monkey, Rat, and Mouse Brain

Polarized light imaging (PLI) enables ultra-high resolution visualization of nerve fibers in postmortem brains by the birefringent property of myelin. Using the Jones calculus, 3D orientation of fibers can be determined in serial sections throughout the brain. We present examples of fiber architecture with an unprecedented spatial resolution both in the white matter and within the cerebral cortex of human, vervet monkey, and rodent brains. Crossing of fibers can be directly visualized without any assumptions or modeling, and fibers can be followed from the white matter into the most superficial layers of the cortex. Even extremely small fiber tracts are visible. Furthermore, the fiber architecture within the cerebral cortex provides a new approach for its parcellation into areas with distinct distribution, orientation, and density patterns of nerve fibers. PLI also provides the anatomical ground truth for the evaluation of results derived from diffusion weighted imaging.