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In a new study, researchers compared the orientations of nerve fibers in a human brainstem using two advanced imaging techniques: diffusion magnetic resonance imaging (dMRI)-based tractography and polarization sensitive optical coherence tomography (PS-OCT). The findings could aid in combining these techniques, which each offer unique advantages, to advance our understanding of the brain's microstructure and help inform new techniques for early diagnosis of various brain disorders. Isabella Aguilera-Cuenca from the University of Arizona presents this research at Frontiers in Optics + Laser Science (FiO LS), held 23–26 September 2024 at the Colorado Convention Center in Denver.
"Neurodegenerative diseases are becoming increasingly widespread as lifespans increase and populations age—better understanding the link between brain microstructure and these diseases could lead to developing improved methods for prevention, detection, and management," said Aguilera-Cuenca. Nerve fiber orientation is an important aspect of brain microstructure due to its influence on the connectivity and communication pathways in the brain. One way to study this microstructure is by using dMRI, a non-invasive imaging method that uses water molecule diffusion to reveal structural connectivity.
A specialized application of dMRI known as diffusion tensor imaging (DTI) can be used to reconstruct nerve fiber pathways through a process known as tractography. Although DTI is sensitive to differences across brain tissue environments, it can't detect specific cellular changes and can only resolve nerve tracts, not individual axon orientations. PS-OCT is also useful for studying brain microstructure .
It uses the properties of back-scattered light and variations in polarization to create depth-resolved cross-sectional images of tissue microstructures. This information can be used to identify fiber tracts with micrometer-scale resolution and distinguish between white and gray matter. However, in scattering media such as brain tissue, PS-OCT can only image up to a few millimeters deep.
To conduct a quantitative comparison of nerve fiber orientation distributions with dMRI-based tractography and PS-OCT, the researchers used both techniques to image a human brainstem sample fixed in paraformaldehyde and then stored in PBS with sodium azide. The results showed that polarization properties of phase retardation and optic axis can be used to map nerve fiber presence and orientation in brain tissue , similar to results obtained via dMRI. This indicates the strong potential for PS-OCT to validate dMRI data, providing valuable insights about the microstructural organization of nerve fibers, which is crucial for understanding normal physiology and the changes that may occur with neurodegenerative conditions.
"To further advance this work, we will study the microstructural alterations in a range of brain regions from patients with different neurodegenerative conditions, with the goal of identifying alterations that occur with the onset of disease," said Aguilera-Cuenca. "We hope this work eventually translates into new approaches for early detection of these pathologies.".
