Abstracts

Rationale: The orbitofrontal (OF) cortex is one of the least explored and least understood regions of the cerebral cortex. There are not many studies on patients with OF epilepsy which is electrophysiologically and surgically confirmed. Studies on OF epilepsy with a negative pre-surgical MRI are even rarer. The aim of our study is to examine the use of voxel-based MRI post-processing technique in studying surgically proven nonlesional OF epilepsy. Methods: From the surgical series of Cleveland Clinic Epilepsy Center, 10 MRI negative (MRI-) patients with OF epilepsy were identified who met the following criteria: 1) surgical resection of the OF region with or without resection of adjacent brain structure; 2) Engel class 1 outcome with minimum follow-up duration of 12 months; 3) negative pre-surgical MRI by radiology report. The post-processing was implemented in a Morphometric Analysis Program (MAP) with methodology published by Huppertz et al. MAP study was performed on T1 high-resolution scans (MPRAGE) in Matlab SPM5. Three feature maps were evaluated: cortical junction, extension and thickness files. Z-score was determined by comparing individual feature files with a normal, scanner-specific database. Regions with z-score greater than 4 standard deviations were considered to be significant. These significant areas were then presented to the neuroradiologist (SEJ), who was the final judge of whether they present true lesions. These images were presented as part of a large ongoing retrospective study, and the reviewer was not given prior information about the type of epilepsy or whether it was a patient or control. Concordance between MAP and surgical resection area was then evaluated. Results: Five patients had MAP+ abnormality in the OF region. Overall, there is 50% positive rate of MAP. Four patients had single-focus lesion. Resection included area of lesion in all 4 patients. Given that all patients were seizure-free, these lesions were considered true positives. The Figure shows one patient whose MAP finding was concordant with PET, SPECT and ICEEG. We also found a patient who had multiple (3) lesions. Two of the three lesions were resected; patient has been seizure-free after surgery in 2007. The 5 MAP+ patients had pathology including FCD 1, FCD 2a, remote infarct, Rosenthal fiber formation, and nonspecific gliosis. The 5 MAP- patients all had MCD/CD. Conclusions: We present here the largest series to date of MRI- surgically proven OF epilepsy. Conventional MRI visual analysis of OF cortex can be challenging because of the dense convexity in that area of the brain. We found that voxel-based morphometric MRI post-processing can positively indentify subtle abnormalities in 50% of the nonlesional OF patients we studied. Lesions detected by MAP, when present, was very specific in this cohort. We hypothesize that MAP is not sensitive strictly to FCD pathology per se, but to pathological substrates (or combinations of them) causing T1 signal alteration leading to a blurred gray-white junction. Further studies on MRI post-processing in MRI- patients are ongoing in our laboratory.