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RADIOLOGY: HEAD: Case# 33666: HYPOMETABOLIC SEIZURE FOCUS LEFT ANTERIOR TEMPORAL LOBE. Patient is a 17 y.o. young man with a history of complex partial seizures. Selected axial and coronal images from an interictal18-FDG PET study reveal hypometabolism of the left anterior temporal lobe. MRI T2 weighted image through the temporal lobes reveals an area of increased signal intensity in the left hippocampus. 18-FDG is a glucose analoge that competes with glucose for uptake. Once it is actively transported, it is then phosphorylated but not further metabolized. Consequently its accumulation within a cell is a function of the cells metabolic rate. Preoperative planning for epilepsy surgery often includes an interictal PET study to localize the epilepsy site. Focal hypometabolism relative to the contralateral structures suggests a seizure focus. PET will demonstrate interictal temporal lobe hypometabolism in 70% of patients with temporal lobe epilepsy (1). Proper interpretation requires knowledge of any seizure activity that may have occurred during the uptake of the 18-FDG, in which case a hypermetabolic focus may be seen. Another approach to localization of seizure foci has been ictal Tc-99m HMPAO (or ECD) brain SPECT. This also has a sensitivity of 70% of depicting epileptic foci but requires rapid administration of a radiopharmaceutical immediately upon seizure activity (1). An interictal SPECT study is usually obtained as a baseline study for comparison with the ictal scan. Also, the interictal SPECT study has a 55% incidence of showing hypoperfusion at the epilepsy site (2). Patient Prep: The patients EEG was monitored shortly before, during uptake, and during imaging. Since ingested glucose could compete with the 18-2 flouro-2 deoxyglucose(18-FDG), the patient should be NPO 4 hours prior to injection. The patient was injected with 10 mCi of 18-flouro-2deoxy-glucose IV and was taken to a quiet, dark room. The patient was instructed not to sleep and to keep his eyes open. After allowing 30 minutes for radiopharmaceutical localization, the patient was taken to the PET imaging suite and emission data was acquired and attenuation was calculated assuming a uniform attenuation coefficient of .088 cm-1.

Peter Anderson
head, radiology