SPECT scan (single photon emission computed tomography)
Unlike the PET scan which measures the brain cells’ use of glucose; the SPECT scan measures the amount of blood that goes to the brain (blood flow). The basis for this study is that during a partial seizure most of the blood will go to the part of the brain that is seizing since those brain cells need more energy (blood carries oxygen and glucose). The part of the brain that is more active than the rest of the brain and takes more blood flow is called “hyperperfusion area” and shows up highlighted when a tracer is injected during the seizure (ictal SPECT).
Technically: a tracer which is a dye composed of Technetium (radiation agent) and a compound that takes it to the brain (HMPAO or Neurolite) is injected at the time of the seizure. This tracer enters and then becomes trapped in the brain cells. A scan is taken later which shows where the dye got trapped. Since most of the blood flow goes to where the seizure is originating, this provides an image of where the seizure started.
The same study is done while the patient is not having a seizure (interictal SPECT). This is then compared to the one that was performed during the seizure. One is subtracted from the other to compare the change in blood flow during the seizure.