Ictal and Interictal SPECT in Epilepsy

SPECT—single-photon emission computed tomography—can map cerebral blood flow during or between seizures. In presurgical epilepsy evaluation, the most important application is ictal perfusion SPECT, where a radiotracer is injected as early as possible during a seizure.

What SPECT measures

Perfusion SPECT uses a radiotracer that is taken up in proportion to regional cerebral blood flow around the time of injection.

The scan itself can be performed later because the tracer distribution effectively preserves a snapshot of perfusion at the injection period.

Ictal SPECT

During a focal seizure, cerebral blood flow usually increases in regions involved early in the seizure network.

Ictal SPECT therefore looks for areas of hyperperfusion.

Why injection timing is critical

The seizure may spread rapidly.

If tracer injection is delayed, the scan may reflect propagation rather than the true seizure-onset region.

Interictal SPECT

Interictal scans are acquired away from seizures and may show relative hypoperfusion in the epileptogenic region.

On their own, interictal scans are generally less localising than well-timed ictal studies.

SISCOM

SISCOM stands for subtraction ictal SPECT coregistered to MRI.

The interictal image is subtracted from the ictal image and the difference map is placed onto the patient’s structural MRI.

Why subtraction helps

Raw perfusion varies naturally between people and brain regions.

Subtracting the interictal baseline can make seizure-related perfusion increases more conspicuous.

Statistical processing

More advanced statistical methods can compare ictal and interictal perfusion against normal reference distributions.

Different centres use methods such as SISCOM and statistical ictal SPECT analysis.

When SPECT is most useful

SPECT is often reserved for difficult drug-resistant focal epilepsy when MRI, EEG and other tests do not provide a sufficiently clear localisation hypothesis.

It can be especially helpful when seizures are frequent enough to make planned tracer injection practical.

Operational challenge

Ictal SPECT requires the radiotracer and trained staff to be immediately available during inpatient video-EEG monitoring.

This logistical requirement is one of the main reasons the technique is not available everywhere.

What recent evidence shows

A 2024 study found that ictal perfusion information contributed substantially more high-confidence lateralisation than interictal imaging alone.

Another 2024 study compared SISCOM with statistical post-processing approaches, reflecting continued refinement of the technique.

False localisation

Hyperperfusion may reflect early propagation rather than seizure onset.

Injection delay, very brief seizures and rapid bilateral spread can complicate interpretation.

SPECT and SEEG

SPECT can help define hypotheses for stereoelectroencephalography implantation.

It does not eliminate the need for invasive EEG when the non-invasive evidence remains uncertain.

Radiation

SPECT involves ionising radiation, so use must be justified, particularly in children.

Worldwide access

Ictal SPECT requires nuclear medicine infrastructure plus continuous epilepsy-monitoring capability and rapid injection procedures.

For many countries, this makes access even more restricted than standard PET.

The central point

Ictal SPECT is a time-sensitive perfusion snapshot. Its localising value depends heavily on how quickly the tracer is injected after seizure onset and how the scan is integrated with EEG and MRI.

Sources

Evidence checked: September 2026.