Diffusional anisotropy in cranial nerves with maturation: Quantitative evaluation with diffusion MR imaging in rats

PURPOSE: To investigate the correlation between diffusional anisotropy and developmental changes in anatomy, which include myelination, in central and peripheral nerves in an animal model by using quantitative diffusion magne tic resonance (MR) imaging and electron microscopy. MATERIALS AND METHODS: In vivo transverse and longitudinal apparent diffusi on coefficients (ADCs) of the optic and trigeminal nerves in 2-10-week-old rats were measured with MR imaging. Then the animals were sacrificed at eac h time point, and transverse and longitudinal sections of optic and trigemi nal nerves were studied with electron microscopy. RESULTS: In the optic nerve, the ADC parallel to the neurofibers increased with development and increased contemporaneously with myelination, while th e ADC perpendicular to the nerve did not change. This resulted in a signifi cant increase in diffusional anisotropy. There were no significant changes in ADCs in either direction in the trigeminal nerve. Longitudinal sections of optic nerve showed a marked change in the orientation of each fiber. As development proceeded, the axons, which initially followed tortuous courses , assumed straighter and more parallel orientations. Trigeminal nerves disp layed straight parallel courses at 2 weeks that did not change over the stu dy period. CONCLUSION: Changes in fiber anatomy in maturation from tortuous to straigh ter and more parallel orientation can account for changes in longitudinal A DC and in diffusional anisotropy.