 Successful epilepsy surgery depends on accurate identification of the area of brain from which seizures originate the epileptogenic zone. There are many non-invasive tests including EEG and MRI to indicate this area. Some patients require implantation of intracranial electrodes in order to determine the epileptogenic zone. A test known as SISCOM subtraction ictal-interictal SPECT co-registered with MRI has been validated as a non-invasive means of indicating the epileptogenic zone. However, this test is not widely available in the US, and it is not known how often the additional information provided by SISCOM makes a difference to clinical decision-making. 