Ratio-images calculated from interictal positron emission tomography and single-photon emission computed tomography for quantification of the uncoupling of brain metabolism and perfusion in epilepsy

Purpose: Image processing techniques were applied to interictal positron em ission tomography (PET) and single-photon emission computed tomography (SPE CT) brain images to aid in the localization of epileptogenic foci by calcul ating a functional image that represents the degree of coupling between per fusion and metabolism. Uncoupling of these two functions has been demonstra ted to be a characteristic of epileptogenic tissue in temporal lobe epileps y and has the potential to serve as a diagnostic measure for localization i n other areas as well. Methods: Interictal PET (F-18-FDG) and interictal SPECT (Tc-99m-HMPAO) scan s were acquired from II epilepsy patients. The metabolism and perfusion ima ges were three-dimensionally spatially registered, and a functional ratio-i mage was computed. These functional maps are overlaid onto a three-dimensio nal rendering of the same patient's magnetic resonance imaging anatomy. Results: In all patients, an average uniform perfusion-to-metabolism ratio showed approximately constant values throughout most of the whole brain. Ho wever, the epileptogenic area (confirmed on surgery) demonstrated an area o f elevated perfusion/metabolism in the grey matter. Conclusions: Although hypometabolism in the PET image was observed in most of these patients, the calculation of a functional ratio-image demonstrated localized foci that in some cases could not be observed on the PET image a lone. The ratio-image also yields a quantitative measure of the uncoupling phenomenon.