MR signal intensity of the optic radiation.

PURPOSE To determine whether a hyperintense layer adjacent to the lateral ventricle on T2-weighted MR images represents the optic radiation. METHODS We reviewed 11 brain specimens from patients with nonneurologic diseases and MR images from 43 healthy volunteers. The MR images in a patient with cerebral infarction involving the lateral geniculate body were also reviewed to evaluate wallerian degeneration of the optic radiation. RESULTS The external sagittal stratum, composed of the optic radiation, showed a pale layer in specimens stained by Bodian's method. On high-power microscopic views of the specimens, the axons of the external sagittal stratum were large and separated by wide translucent spaces. In the volunteers, the external sagittal stratum appeared hyperintense on T2-weighted MR images and hypointense on T1-weighted images. The MR images in a patient with cerebral infarction showed hyperintensity within the layer corresponding to the external sagittal stratum. CONCLUSIONS The hyperintense layer on T2-weighted images represents the external sagittal stratum, or optic radiation. The signal intensity of the external sagittal stratum reflects histologic characteristics of low axonal density.