ubtraction ictal SPECT co-registered to MRI or SISCOM

Examples of subtraction ictal SPECT co-registered to MRI (SISCOM)

Measuring Blood Flow in the Brain

Single-photon emission computed tomography (SPECT) is a nuclear radiology study that measures the blood flow in the brain.

  • A safe, short-lived radioactive substance is injected into the arm and a CT (computed tomography) scan is performed shortly afterwards.
  • The more blood flowing through an area in the brain, the brighter that area lights up on the scan.
  • During a seizure, more blood flows to the area where the seizure comes from, making this test valuable in finding the seizure onset zone.

Protocol for Finding Where Seizures Start

A special protocol has been developed to try and obtain more specific information about where seizures start in the brain. The test compares blood flow during and in between seizures, and then superimposes those images on an MRI (magnetic resonance imaging). This protocol is called subtraction ictal SPECT co-registered to MRI, or SISCOM.

  • This study is performed during an admission to the Epilepsy Monitoring Unit, as the injection needs to be performed during a seizure by specially certified staff.
  • The SPECT is then performed to look at the blood flow during that seizure.
  • A second injection is performed at least 24 hours later, in between seizures, to look at blood flow between seizures.
  • These images are then combined and placed on an MRI, so that epilepsy team can see what regions in the brain the seizure came from. This allows for more detailed information regarding seizure onset zone when the scalp EEG (electroencephalogram) is not definitive enough.

SISCOM studies are performed only in certain epilepsy centers and only during an admission in the Epilepsy Monitoring Unit. The information obtained needs to be considered along with the EEG data and MRI to determine the seizure onset zone.

Authored by: Amy Crepeau MD | Diagnostics Editor on 3/2017
Reviewed by: Joseph I. Sirven MD | Editor-in-Chief on 3/2017