A 13-year-old with refractory seizures presents for an interictal SPECT scan. The patient has frequent temporal lobe seizures that are refractory to intensive medical treatment. His current medications include levetiracetam, lamotrigine, and clonazepam. Planning for seizure surgery is commencing.
The physical examination reveals a thin, quiet young man with no appreciable disease. He appears afraid and tearful and resists lying down on the table.
Functional neuroimaging techniques utilizing isotopes are routinely used to identify a seizure focus.
When these techniques are coupled with proper electroencephalogram (EEG) analysis and MRI examination, a seizure focus can now be identified with high reliability and sensitivity that may allow for surgical extirpation of the focus.
The term SPECT refers to single-photon emission computed tomography. It is a nuclear medicine technique that maps blood flow. Since cerebral blood flow is usually greatest in areas of seizure activity, this allows us to identify an active seizure focus. In order to reliably do this, two scans often need to be done: an ictal SPECT scan and an interictal SPECT scan. An ictal SPECT scan requires the injection of tracer during a seizure. The seizure origination focus is identified by an increase in gamma emissions in the affected area. An interictal scan involves injection of tracer between seizures and is used as the baseline study for comparison. The tracer radiopharmaceutical usually contains radioactive technetium (Tc 99). The local tracer concentration peaks in the brain tissue about 2 minutes after injection, remains constant for about 2 hours, and degrades with a half-life of 6 hours. Ictal SPECT reveals both the onset zone and the propagation pathways. The ictal and interictal images are then compared to each other and to MRI and analyzed both visually and statistically, usually utilizing a parametric mapping technique in order to precisely delineate the onset zone.
Positron emission tomography (PET) scanning is a different technique. It utilizes fluoro-2-deoxy-D-glucose (FDG) scanning of the brain to assess regional blood glucose uptake and metabolism. A seizure focus usually displays decreased glucose uptake interictally compared to the contralateral side.
PET scanning is also commonly used for the identification of tumors, both within the central nervous system (CNS) and in the body as a whole.
Newer radioligand tracers are being developed to allow for the identification of specific neurotransmitters and receptor activation.
Because of the patient’s high level of anxiety, an anesthesia machine is brought to the scanner area, and an inhalational induction is performed with the mother present. The patient has a good natural airway. All American Society of Anesthesiologists monitors are placed, and an IV is started. A propofol infusion is initiated at 300 μg/kg/min and then reduced to 250 μg/kg/min. A second heparin lock is started, and the radionuclide is injected. The ...