Abstracts

Rationale: T2 relaxometry can detect hippocampal sclerosis with high sensitivity, and show minor changes associated with seizures or other pathology [1,2]. In a community cohort of subjects with nonsyndromic focal epilepsies, we previously observed an excess of bilaterally small (<2SD) and large (>2SD) hippocampi relative to controls. Here we examine whether these size anomalies are associated with increased hippocampal T2. Methods: 65 focal epilepsy cases (age 23.1 ± 3.2 years) and 36 healthy controls (age 23.48 ± 3.27 years) were included. Focal epilepsy patients were classified as ‘complicated' (n = 14) when there was an associated lesion or neurodisability, or ‘noncomplicated'(n = 51) in which there was no identified lesion. Clinical information was assessed in each case for indicators suggesting temporal lobe epilepsy. T2-weighted multi-spin echo MRI was acquired using a 1.5T Siemens Sonata MRI scanner. 16 echo times were acquired from TE = 22 ms to 352 ms, incrementally increased by 22 ms. TR = 3000 ms, in- plane resolution = 0.44 x 0.44 mm. Slices were 5mm thick with a 5mm slice gap. 8 slices were acquired per participant, covering the length of the hippocampus. Hippocampal T2 relaxation time was assessed in the hippocampal body. Hippocampal volume was assessed using automated hippocampal segmentation of whole brain T1 MRI acquired in the same imaging session, using Freesurfer 5.1. Hippocampal volumes were corrected for brain volume and categorised into three groups by ranking the volumes and taking the lower 25th percentile (low volume), 25th to 75th percentile (normal volume) and upper 25th percentile (high volume). Results: Relative to controls, T2 was increased in the complicated focal group; 2.9 ms left p < 0.05, 1.6 ms right p > 0.05 (see Table 1 for hippocampal T2 and volumes). There were no general T2 increases in the uncomplicated group, however subgroup analysis suggests a relationship between clinical indication of TLE and increased hippocampal T2; 1.64 ms right p = 0.05, 1.2 ms left p > 0.05. Further, there was no significant relationship between hippocampal volume and hippocampal T2 increases in either of the focal subgroups. Conclusions: Hippocampal T2 increases are limited to complicated focal epilepsy. Subtle hippocampal T2 abnormalities are independent of volumetric abnormalities, suggesting that these two measurements index different epilepsy-related neurological changes in non-mesial temporal lobe epilepsy. The classic MRI-based markers of mesial temporal lobe epilepsy, high T2 in combination with hippocampal atrophy, are specific to hippocampal sclerosis and relatively rare in the population. References: [1] Scott et al, Brain (2003) 126(9): 1968 - 1974 [2] Van Paesschen et al Ann Neurol (1997) 41(1): 41 - 51 Funding: NIH-NINDS R37-NS31146, NHMRC program grant 628952 (Australia)