Abstracts

Rationale: A combined PET/MR scanner with simultaneous acquisition permits simultaneous imaging of physiologic & pathophysiologic processes and provides both anatomical & functional information on the same subject at the same time. It allows direct correlations of PET data with MR-detected patterns of neural synchrony in both cortical and subcortical regions. This multi-modal analysis will facilitate the identification of an optimal biomarker. To demonstrate this feasibility, we initiated a comparative study in healthy controls (HC) and epilepsy patients (Epi) using F-18-fluorodeoxyglucose (FDG). This pilot study evaluated metabolic activity differences between epilepsy patients and healthy controls and directly correlated FDG uptake with MR regional abnormality. Methods: Six HC and 11 Epi patients (avg. age 26 and 36, respectively) were enrolled so far and imaging was performed on a whole-body simultaneous PET/MR scanner (Biograph mMR, Siemens). After injection of approx. 370 MBq FDG, dynamic brain PET scans were acquired for ~90 minutes. Simultaneously, MR imaging, including T1, T2, resting state, DKI, MRS, field map and other sequences, were performed. Dixon sequence was acquired to obtain a μ-map for attenuation correction (AC) of PET data. Standard uptake values (SUV) were mapped via MATLAB on a summed PET image (127 slices) of each subject. A total of 117 masks (regions of interest, ROIs), including left and right, for cortical (96) and subcortical regions (21) based on Harvard-Oxford Atlas were generated. Image preprocessing was carried out using Mango and FSL. The SUV maps were registered to the subject's T1 images that, in turn, are registered to the MNI152 2mm template. After applying the masks, averaged SUV values for all masks were derived for all subjects. Statistical analyses on SUV values derived from two normalization methods, by subject's mean cortical SUV (SUV_{COR_norm}) or by white matter (SUV_{WM_norm}), were compared. Results: Based on Mann-Whitney U test (SPSS) and pair-t-tests, it appeared that when SUV_{COR_norm} values were used, more hypometabolic regions and more asymmetry regions in Epi patients, as compared to HC, were detected than when using SUV_{WM_norm} values. Left and right postcentral gyri were found to be consistently hypermetabolic regions, while right temporal pole and planum polare were consistently hypometabolic regions by the two normalization methods (Fig. 1). Using the asymmetry index (AI>10% or SUV ratios>1.2), more metabolic asymmetry regions were detected in patients than in controls, with 96.2% agreement. The presence of hippocampal abnormalities or cortical tubers detected via T2 FLAIR in patients correlated well with the hypometabolism detected via FDG-PET (Fig. 2). Conclusions: Our results showed specific patterns of metabolic abnormality and asymmetry over 117 brain regions in epilepsy patients, as compared to controls, suggest that simultaneous PET/MR imaging provides a useful tool to help understand etiopathogenesis and localize seizure foci.