Longitudinal Hippocampal Diffusion-Weighted Imaging and T2 Relaxometry Demonstrate Regional Abnormalities Which Are Stable and Predict Subfield Pathology in Temporal Lobe Epilepsy.

OBJECTIVE
High-resolution (1 mm isotropic) diffusion tensor imaging (DTI) of the hippocampus in temporal lobe epilepsy (TLE) patients has shown patterns of hippocampal subfield diffusion abnormalities which were consistent with hippocampal sclerosis (HS) subtype on surgical histology. The objectives of this longitudinal imaging study were to determine the stability of focal hippocampus diffusion changes over time in TLE patients, compare diffusion and quantitative T2 abnormalities of the sclerotic hippocampus and correlate pre-surgical mean diffusivity (MD) and T2 maps with post-surgical histology.


METHODS
Nineteen TLE patients and 19 controls underwent two high-resolution (1 mm isotropic) DTI and 1.1 × 1.1 × 1 mm3 T2 relaxometry scans (in a subset of 16 TLE patients and 9 controls) of the hippocampus at 3T, with a 2.6±0.8 year inter-scan interval. Within-participant hippocampal volume, MD and T2 were compared between the scans. Contralateral hippocampal changes 2.3±1.0 years after surgery and ipsilateral pre-operative MD maps versus post-operative subfield histopathology were evaluated in 8 patients who underwent surgical resection of the hippocampus.


RESULTS
Reduced volume and elevated MD and T2 of sclerotic hippocampi remained unchanged between longitudinal scans. Focal regions of elevated MD and T2 in bilateral hippocampi of HS TLE were detected consistently at both scans. Regions of high MD and T2 correlated and remained consistent over time. Volume, MD and T2 remained unchanged in post-operative contralateral hippocampus. Regional elevations of MD identified subfield neuron loss on post-surgical histology with 88% sensitivity and 88% specificity. Focal T2 elevations identified subfield neuron loss with 75% sensitivity and 88% specificity.


SIGNIFICANCE
Diffusion and T2 abnormalities in ipsilateral and contralateral hippocampi remained unchanged between the scans suggesting permanent micro-structural alterations. MD and T2 demonstrated good sensitivity and specificity to detect hippocampal subfield neuron loss on post-surgical histology, supporting the potential that high-resolution hippocampal DTI and T2 could be used to diagnose HS subtype before surgery.