Multimodality image coregistration for MRI-negative epilepsy surgery

Resective surgery in MRI-negative epilepsy results in poorer outcomes and a higher rate of recurrence compared to cases with a structural lesion visible on magnetic resonance imaging (MRI). As many as 26% overall and 46% of extratemporal (1) patients undergoing epilepsy surgery have negative MRI, and in these cases functional studies, including positron emission tomography (PET), ictal and interictal single photon emission computed tomography (SPECT), diffusion-weighted MRI (DWI), functional MRI (fMRI), and chronic intracranial EEG (icEEG) monitoring are often essential for localization of the epileptogenic zone. In order to coalesce the results from these disparate modalities around a coherent epileptogenic hypothesis to guide resective surgery, the functional data must be spatially aligned into a single coordinate system, typically corresponding to the patient’s high-resolution T1-weighted MRI, to ultimately guide the resection plan. In addition, improvements to the sensitivity and specificity of functional modalities are often achievable by comparison of patient scans to spatially varying statistical metrics of normality derived from measurements of normal, or nonepileptic, subjects. For a group analysis, it is necessary to transform all data to one common template space. Recent results in cerebral blood flow mapping with ictal–interictal subtraction SPECT show significant improvement in localization when patient scans are evaluated in the context of paired resting scans of normal individuals (2,3). Statistical parametric mapping of [18F] fluorodeoxyglucose (FDG) PET scans in comparison to normals has been shown to improve the localizing capability of PET in epilepsy (4,5). The development of novel receptor PET tracers has also furthered the study of epilepsy via PET, and SPM will likely prove useful as normal scans become available.