Educational scope notice: This is a study note for medical students, not medical advice, diagnosis, or treatment guidance. Clinical management should follow local protocols and current guidelines.
SPECT exploits the perfusion flip between seizure states. During a seizure, the firing onset zone demands more oxygen and glucose, driving cerebral blood flow up by at least half from a baseline near 50 mL per 100 g per minute. Between seizures the same region runs hypoperfused. Because SPECT tracers distribute with blood flow at the instant of injection, an ictal scan marks the focus as a hot spot of hyperperfusion while an interictal scan shows a cool zone. Comparing the two states localizes the onset zone.
The technique images the seizure itself, which is its edge over FDG-PET: PET accumulates signal over 30 to 45 minutes into a blended metabolic picture, while SPECT freezes one perfusion moment. That advantage costs logistics. The tracer must be injected within seconds of seizure onset, before activity propagates, so the patient waits in an epilepsy monitoring unit under continuous EEG with a prepared dose at the bedside, and a second interictal scan follows on a seizure-free day. Delays beyond roughly half a minute to a minute risk mapping propagated activity instead of the origin.
SPECT resolution is coarser than PET, around 1 cm against 3 to 4 mm, yet when interictal PET is non-diagnostic or diffusely non-localizing, a focal ictal hyperperfusion can still supply the evidence surgery needs. The two techniques complement rather than compete. The source material covers SPECT in less depth than PET, so this note stays introductory: the perfusion concept, the timing requirement, and the complementary role are the durable points.
Evidence anchors
- NICE. Epilepsies in children, young people and adults (NG217): https://www.nice.org.uk/guidance/ng217