Abstracts

Rationale: Histological studies have shown that neuron loss in TLE-MTS predominantly affects CA1 and dentate/hilus but spares CA2. High-resolution images at 4T depict details of the internal structure of the hippocampus not visible at 1.5T which can be used for in vivo parcellation of different hippocampal subfields. The aim of this study was to test if the subfield specific volume loss and distinct atrophy pattern can be detected in TLE-MTS and TLE-no. Methods: T1-weighted gradient echo images and a high resolution T2-weighted fast spin echo images (0.4 x 0.5 x 2 cmm), aimed at the hippocampus were obtained in 34 controls, 13 TLE-MTS and 15 TLE-no. Entorhinal cortex (ERC), subiculum (SUB), CA1, CA2 and CA3&dentate were marked on both sides on five consecutive slices of the T2 image using a manual parcellation scheme (1). Total hippocampal volumes were determined from the T1 using FreeSurfer. Multiple regression analyses and Tukey post hoc tests were used to test for significant group effects (TLE-MTS, TLE-no, controls), z-scores were used to compare the severity of volume loss between different subfields. Results: Significant group effects were found for ipsilateral CA1 (p<0.001) CA2 (p<0.001) and CA3&DG (p<0.001) and total hippocampal volume (p<0.001). Post hoc analyses showed that TLE-MTS had significantly smaller CA1, CA2, CA3&DG and total hippocampal volume than controls or TLE-no. Contralateral subfields were not different between the 3 groups. Mean z-scores of the ipsilateral CA1 and CA3&DG were significantly lower than ipsilateral CA2, ERC, SUB or total hippocampus z-scores. Contralateral z-scores were not different. There were no significant differences between the ipsi- or contralateral subfield and hippocampal z-scores in TLE-no. If a z-score ≤-2.0 was used as cutoff to identify severe volume loss, the following atrophy patterns emerged in TLE-MTS: CA1 atrophy only: 2; CA3&DG atrophy only: 1; CA1 and CA3&DG atrophy: 3; CA1 and CA3&DG and other subfields: 6. No significant volume loss: 1 with bilateral DG hyperintensity and CA3&DG z-score of -1.4 on both sides. 10 TLE-MTS met the criterion for significant total hippocampal atrophy. None had a significant contralateral volume loss. Three TLE-no had significant subfield atrophy: isolated contralateral CA3&DG: 1; bilateral CA3&DG atrophy: 1; ipsilateral ERC and SUB atrophy: 1. None had significant total hippocampal volume loss. Conclusions: Using a high resolution MRI acquired on a 4T magnet and a manual subfield parcellation scheme, we were able to find the characteristic ipsilateral CA1 and CA3&DG volume losses and different atrophy patterns described by histopathological studies in TLE-MTS. We also found subfield atrophy in 20% of the TLE-no despite normal total hippocampal volume. These findings indicate that high resolution MRI and subfield volumetry provide superior information compared to standard hippocampal volumetry. Imaging/histopathological correlation studies will be needed to validate these preliminary findings. Reference: 1. Neurobiol Aging. 2007 28:719-26