Combined 7-T MRI and histopathologic study of normal and dysplastic samples from patients with TLE

The purpose of the study was to investigate the abnormalities of cortical lamination observed in temporal lobe specimens obtained during surgery for intractable temporal lobe epilepsy (TLE) with hippocampal sclerosis. Specifically, we aimed to 1) correlate high-field ex vivo MRI with histopathologic analysis and 2) evaluate the effect of tissue fixation on image contrast.

A cohort of 13 specimens was considered. T2-weighted imaging and relaxometry were performed during and after fixation using a 7-T experimental scanner. After imaging, the specimens were studied with histopathologic (Black Gold myelin fiber staining) and immunohistochemical (NeuN neuronal staining) methods in order to explore the correspondence between MRI and histopathologic features.

The principal findings of this study are that 1) superior MRI contrast is obtained among the cortical layers using completely fixed specimens as opposed to recently excised tissue, 2) the intensity of the T2-weighted MRI signal is lowest (hypointensity) at the site of highest fiber concentration and cellular density, and highest (hyperintensity) when the density of fibers and cells is lowest, and 3) the MRI signal is altered in presence of abnormal cortical lamination (focal cortical dysplasia type IA).

High resolution ex vivo MRI enables the study of intracortical organization in normal and pathologic areas. Comparisons between MRI, NeuN, and Black Gold indicate that the differences apparent in T2-weighted images are mainly related to fiber concentration, although neuronal density might also play a role.