Epilepsy surgery planning rarely depends on one test. When MRI does not show a clear lesion, or when scalp EEG does not localise the seizure network well enough, functional and physiological tests can add different kinds of information.
PET: looking at metabolism
Detailed guide: FDG-PET in Epilepsy →
Interictal FDG-PET measures regional glucose metabolism. In focal epilepsy, areas involved in the epileptic network may show reduced metabolism between seizures. PET does not directly record the seizure itself, but it can support a localisation hypothesis when interpreted alongside MRI, EEG and clinical features.
SPECT: looking at blood flow
Ictal SPECT uses a tracer injected around the time of a seizure to capture changes in cerebral blood flow. It can help identify areas activated during the event, although the image may include both the seizure-onset zone and regions recruited as the seizure spreads.
MEG: recording magnetic fields
Magnetoencephalography records the tiny magnetic fields produced by neuronal electrical activity. It has millisecond temporal resolution and can help localise epileptiform activity when scalp EEG is insufficient, particularly in some MRI-negative or neocortical epilepsies.
Why these tests are complementary
PET, SPECT and MEG answer different questions. None is a replacement for MRI, scalp EEG or clinical assessment. Their value is greatest when the findings converge with the rest of the presurgical evaluation and help build a coherent hypothesis about the epileptogenic network.
How these tests fit together
PET, SPECT and MEG answer different questions. FDG-PET maps interictal glucose metabolism; ictal SPECT captures seizure-related perfusion near the time of tracer injection; MEG records the magnetic fields generated by neuronal electrical activity. They are complementary rather than interchangeable.
FDG-PET
Interictal FDG-PET commonly looks for regional hypometabolism. The abnormal region can be broader than the true seizure-onset zone because PET reflects a network rather than a direct recording of seizure onset.
Detailed guide: FDG-PET in Epilepsy →
Ictal and interictal SPECT
Ictal SPECT is highly dependent on timing. A tracer injected early in a focal seizure may demonstrate hyperperfusion near the onset network, whereas delayed injection can increasingly reflect seizure propagation. Subtraction techniques such as SISCOM compare ictal and interictal perfusion and coregister the difference to MRI.
Detailed guide: Ictal and Interictal SPECT →
MEG and magnetic source imaging
MEG provides millisecond-scale recording of epileptiform magnetic activity. Source estimates are mapped onto MRI to create magnetic source imaging. It is especially useful as one component of assessment in MRI-negative or otherwise difficult focal epilepsy.
Detailed guide: MEG and Magnetic Source Imaging →
EEG source imaging is another localisation tool
High-density scalp EEG can be computationally modelled to estimate the cortical generators of spikes or seizure activity. It is often more widely scalable than MEG and can contribute to the same multimodal localisation process.
MRI-negative epilepsy
Advanced functional imaging is particularly valuable when a high-quality epilepsy MRI remains non-lesional. A recent systematic review of MRI-negative epilepsy emphasised that modern assessment should integrate expert MRI review, post-processing and multimodal functional data rather than treating “negative MRI” as the end of the investigation.
Concordance is stronger than any single result
A PET abnormality, SPECT focus or MEG cluster carries more weight when it agrees with seizure semiology, scalp EEG, MRI and neuropsychological findings. Discordance can indicate rapid propagation, multiple networks, an incidental lesion or an incomplete hypothesis.
Planning intracranial EEG
One major role of PET, SPECT, MEG and source imaging is to refine the hypothesis used to place SEEG electrodes. These tests can reduce uncertainty but cannot replace invasive recording when the seizure-onset network remains unclear.
Worldwide access
PET and SPECT require nuclear-medicine infrastructure; MEG requires specialised equipment and shielded facilities. Their availability is therefore concentrated in tertiary centres and varies markedly between countries.
SEEG: Recording Seizures From Inside the Brain →
2024 review — FDG-PET in presurgical focal epilepsy
2024 review — MEG in MRI-negative focal epilepsy
Sources
PET and SPECT in epilepsy: a critical review
Magnetoencephalography for epilepsy presurgical evaluation
MEG in the presurgical evaluation of epilepsy
Evidence checked: September 2026.