Assessment of Optic Pathway Structure and Function in Patients With Compression of the Optic Chiasm: A Correlation With Optical Coherence Tomography.

PURPOSE: The purpose of this study was to investigate correlations between retinal fiber thickness measured by optical coherence tomography (OCT) and anterograde functional and structural differences in the optic pathway of patients with compression of the optic chiasm. Our hypothesis was that loss of visual acuity caused by chronic compressive pathologies may lead to an irreversible decline in vision because of permanent neurodegeneration of the optic radiations and visual cortex. METHODS: Quantitative OCT, functional magnetic resonance imaging (MRI) and diffusion tensor MRI measurements were made in 17 patients being surgically treated for chiasmal compression. RESULTS: In our study we found that surgically irreversible visual field defects and reduced retinal nerve fiber layer thickness were significantly associated with lower fractional diffusion anisotropy and higher diffusivities in optic radiations and less functional MRI activation in the visual cortex. CONCLUSIONS: Damage to the retinal nerve fiber layer is associated with downstream structural and functional degradation of the optic pathway. This may be related to trans-synaptic degeneration and the fact that these factors are important potential imaging biomarkers for predicting visual recovery after surgical decompression.

Probabilistic MRI tractography of the optic radiation using constrained spherical deconvolution: a feasibility study.

BACKGROUND AND PURPOSE: Imaging the optic radiation (OR) is of considerable interest in studying diseases affecting the visual pathway and for pre-surgical planning of temporal lobe resections. The purpose of this study was to investigate the clinical feasibility of using probabilistic diffusion tractography based on constrained spherical deconvolution (CSD) to image the optic radiation. It was hypothesized that CSD would provide improved tracking of the OR compared with the widely used ball-and-stick model. METHODS: Diffusion weighted MRI (30 directions) was performed on twenty patients with no known visual deficits. Tractography was performed using probabilistic algorithms based on fiber orientation distribution models of local white matter trajectories. The performance of these algorithms was evaluated by comparing computational times and receiver operating characteristic results, and by correlation of anatomical landmark distances to dissection estimates. RESULTS: The results showed that it was consistently feasible to reconstruct individual optic radiations from clinically practical (4.5 minute acquisition) diffusion weighted imaging data sets using CSD. Tractography based on the CSD model resulted in significantly shorter computational times, improved receiver operating characteristic results, and shorter Meyer's loop to temporal pole distances (in closer agreement with dissection studies) when compared to the ball-and-stick based algorithm. CONCLUSIONS: Accurate tractography of the optic radiation can be accomplished using diffusion MRI data collected within a clinically practical timeframe. CSD based tractography was faster, more accurate and had better correlation with known anatomical landmarks than ball-and-stick tractography.