What is a Single Photon Emission Computed Tomography (SPECT) ictal plan?

SPECT Ictal Plan: A Critical Imaging Technique for Epilepsy Localization

A SPECT ictal plan is a specialized nuclear medicine imaging protocol that measures regional cerebral blood flow during seizures to identify epileptogenic zones, with the radiopharmaceutical administered during an epileptic discharge and brain uptake completed within 1-2 minutes, providing valuable information for surgical planning in refractory epilepsy. 1

Key Components of SPECT Ictal Planning

Radiopharmaceutical Administration

• The radiopharmaceutical (typically 99mTc-HMPAO or 99mTc-ECD) must be injected as soon as possible after seizure onset, ideally within 20 seconds via a previously placed intravenous line 1
• Brain uptake is irreversibly completed in 1-2 minutes, capturing the cerebral blood flow pattern at the time of injection 1
• Prepared syringes should be stored in the epilepsy monitoring unit to ensure rapid administration 1
• Patients should undergo continuous video-EEG monitoring during the procedure 1

Imaging Protocol

• Both ictal (during seizure) and interictal (between seizures) SPECT images should be obtained for optimal evaluation 1
• Interictal imaging should be performed at least 24 hours after focal aware seizures and 48 hours after focal impaired awareness or focal-to-bilateral tonic-clonic seizures 1
• Typical injected activity in adults is 740 MBq of either 99mTc-HMPAO or 99mTc-ECD 1
• Image acquisition should occur within 40 minutes of tracer administration 2

Image Analysis and Interpretation

Subtraction Techniques

• Subtraction of ictal and interictal SPECT co-registered to MRI (SISCOM) significantly improves sensitivity and specificity of seizure-related perfusion network identification 1
• SISCOM not only shows regions of hyperperfusion but also hypoperfusion, providing comprehensive mapping of ictal networks 1
• Common thresholds used for SISCOM analysis are z=+1.5 and z=+2 1
• SISCOM has shown localization sensitivity higher than 90% in temporal lobe seizures and approximately 70% in extratemporal lobe epilepsy 1

Perfusion Patterns

• During ictal scans, brain regions involved in seizure generation and early propagation demonstrate increased perfusion (hyperperfusion) 1
• Most epileptic networks show hypoperfusion during the interictal state 1
• Ictal SPECT presents a sensitivity of 73% and specificity of 75%, while interictal SPECT alone has lower localization value (50% sensitivity, 75% specificity) 1

Clinical Utility and Outcomes

Surgical Planning Applications

• Ictal SPECT is particularly valuable for lesional epilepsy and in children with focal refractory epilepsies 1
• It serves as a strong predictor of surgical success with an odds ratio of 0.37 for favorable surgical outcome in non-lesional epilepsy 1
• When SISCOM localization is concordant with the surgical resection site, patients have approximately 3 times higher odds of becoming seizure-free 1
• Ictal SPECT/CT hyperperfusion adds predictive value to anatomic imaging and EEG, with 86% frequency of favorable postsurgical outcome after complete removal of the hyperperfusion zone 1

Complementary Role with Other Modalities

• SPECT has higher sensitivity (49-87%) than FDG-PET (56-63%) for seizure focus localization 1
• FDG-PET can provide additional information in 33% of cases where SPECT does not demonstrate the seizure focus 1
• Concordance between ictal SPECT and FDG-PET results is a predictive factor for surgical outcomes in extratemporal epilepsies 1

Limitations and Considerations

Technical Challenges

• Spatial resolution of SPECT is limited compared to other imaging modalities 1
• Capturing the onset of unpredictable seizures presents significant medical and logistical challenges 2
• Accurate co-registration with MRI is essential to avoid mislocalization of seizure foci 3
• Interpretation of late injections or postictal SPECT (after seizure end) can be challenging and less reliable for precise localization 4

Recent Advances

• Cadmium-Zinc-Telluride (CZT) cameras provide more than twofold increase in count sensitivity and better image resolution compared to conventional Anger cameras 1
• Triggered seizures via stereotactic electroencephalography (sEEG) electrodes have been explored as a method to overcome the challenge of capturing unpredictable seizures 2
• Statistical processing methods can further enhance the diagnostic performance of SISCOM 1

By following a structured SPECT ictal plan with proper timing of radiopharmaceutical administration and using advanced image analysis techniques like SISCOM, clinicians can significantly improve seizure focus localization and surgical planning for patients with refractory epilepsy.