Hippocampal structures: anteroposterior N-acetylaspartate differences in patients with epilepsy and control subjects as shown with proton MR spectroscopic imaging.

PURPOSE: To determine the distribution of proton metabolites along the long axis of the hippocampus.
MATERIALS AND METHODS: Proton magnetic resonance (MR) spectroscopic imaging measurements were performed in the hippocampi of 14 control subjects and nine patients with unilateral mesial temporal lobe epilepsy.
RESULTS: Control subjects showed significantly lower ratios of N-acetylaspartate (NAA) to choline-containing compounds (Ch) and creatine plus phosphocreatine (CR) (NAA/[Cr + Ch]) in the anterior as compared with the posterior part of the hippocampus. Furthermore, a similar anteroposterior (AP) difference in NAA/(Cr + Ch) values was found in both ipsilateral and contralateral hippocampi of patients. In the patients compared with the control subjects, ipsilateral NAA/(Cr + Ch) levels were reduced in every part of hippocampal tissue with an average reduction of 17%, and contralateral NAA/(Cr + Ch) was reduced by about 10%. In the patients compared with the control subjects, the proportional reduction in ipsilateral NAA/(Cr + Ch) was greatest in voxels from anterior hippocampal regions.
CONCLUSION: AP differences could be a result of fewer neurons in the anterior compared with the posterior hippocampus or of the increasing thickness of the hippocampus from posterior to anterior, which leads to different contributions from adjacent tissue. Measurements of T2 showed that T2 differences are probably not responsible for these changes.