Cerebral Metabolic Changes Associated With Thalamic Stimulation

• Brain Abnormalities [ Time Frame: within 30 days of enrollment in the study ] [ Designated as safety issue: No ]
  One Magnetic Resonance Imaging (MRI) scan will be used to take detailed images of the brain and detect whether there are any abnormalities.
• Ability to learn and understand [ Time Frame: within 30 days of enrollment in the study ] [ Designated as safety issue: No ]
  One session of neuropsychological testing will be done in order to assess the subject's ability to learn and understand.

The purpose of this research study is to evaluate the brain circuits function and circuits involved in the mechanism of thalamic DBS in patients with medically refractory epilepsy.

Deep brain stimulation (DBS) of the anterior nucleus of the thalamus in patients with medically refractory epilepsy has been shown to be efficacious. Studies have estimated a 46-76% reduction in seizures relative to the baseline. However, the exact anatomic circuitry involved in the mechanism of DBS in epilepsy remains unknown. Positron emission tomography (PET) studies can provide clues to the metabolic activity associated with DBS activation. In our study, patients will be scanned with their DBS electrodes turned "on" and "off". PET data captured during these two distinct states may show characteristic differences in cerebral glucose metabolism and modulatory effects of surrounding cerebral structures, thus providing clues to how DBS works to prevent seizures.

• Procedure: Positron Emission Tomography (PET) Scan

  The PET procedure consists of an intravenous injection of 18F-FDG followed by a 45-minute period of quiet resting. PET images will be acquired on an integrated PET/CT scanner.

  This process will be repeated with the stimulator "on" during the first PET image acquisition and "off" during the second PET image acquisition. "On" and "off" PET image acquisitions will be performed on two separate days.

• Procedure: Magnetic Resonance Imaging (MRI)
  MRI will be performed on a 1.5T system, using a combination of different sequences including sagittal T1, coronal and axial T2, FLAIR and proton density and axial 3D T1 weighted images.
• Other: Neuropsychological Testing
  On the same day or separate visit, depending on the timing of the PET imaging, they will be required to attend one half-day session for neuropsychological testing. Patients will arrive at the Toronto Western Hospital, the stimulator will be turned "off" for 30 minutes prior to neuropsychological testing with testing repeated with the stimulator on.

Inclusion Criteria:

• 18-85 years old
• currently have DBS implanted for medically refractory epilepsy
• Women of childbearing age will be required to use established contraceptive methods while enrolled in the study.

Exclusion Criteria:

Patients who:

• are < 18 years of age or > 85 years of age
• are medically unstable
• are women who are potentially pregnant, pregnant or nursing
• have had PET imaging within 1 year.