Localization of a Myelin Layer within the Hippocampal Formation: MRI and Histologic Correlation

Abstract

A white matter layer within the hippocampal formation can be reliably identified in fixed brain specimens with high resolution MR imaging. Correlation with histologic specimens indicates that the myelin layer is located within the stratum moleculare and lacunosum. This m yelin layer might be called either the internal medullarly lamina or the internal myelin layer.Identification of this internal layer of white matter is the initial step in radio logically evaluating the internal architecture of the hippocampal formation since it has different signal characteristics from the surrounding gray matter in the hippocampus and dentate gyrus.The ability of MRI to detect subtle atrophic abnormalities in the internal myelin layer has been limited by its ability to resolve small structures. Recent evolution in MRI technology has resulted in improved gradient coils (25mT/m) and imaging sequences (T2 weighted fast spin echo). Further improvements have also been made through the use of either surface or phase array coils. These developments have made it possible to improve resolution from a 5 mm thick 256 x 256 matrix to a 3 mm thick 512 x 512 matrix.