PET, SPECT and MEG in Epilepsy
When MRI, EEG and video telemetry do not provide the whole answer
Most people being investigated for epilepsy will never need a PET scan, SPECT scan or MEG.
These are specialist investigations, used particularly when doctors need more information about where seizures are coming from.
They can become especially valuable when:
seizures continue despite appropriate medicines
epilepsy surgery is being considered
MRI appears normal
MRI shows more than one possible abnormality
EEG and MRI do not agree
or specialists need additional evidence before deciding where invasive EEG electrodes should be placed.
NICE recommends referral to tertiary epilepsy services when specialist investigations or surgical assessment are required. These centres provide advanced neuroimaging and neurophysiology in addition to ordinary MRI, EEG and video telemetry.
They are not three versions of the same test
PET, SPECT and MEG measure very different things.
PET
Looks mainly at brain metabolism — how brain tissue is using energy.
SPECT
Looks mainly at blood flow through the brain, particularly around the time of a seizure.
MEG
Records the extremely small magnetic fields generated by brain activity.
These results can then be compared with:
MRI
EEG
video telemetry
seizure symptoms
neuropsychological testing
and other investigations.
No single one normally decides where surgery should take place.
PET — Positron Emission Tomography
PET stands for positron emission tomography.
In epilepsy assessment, the most commonly discussed brain PET technique uses a radiolabelled form of glucose.
Brain cells require glucose for energy.
After a small amount of radioactive tracer is injected into a vein, the scanner detects how the tracer is taken up throughout different areas of the brain.
This creates a map of brain metabolism.
UCLH describes epilepsy PET as using a small amount of radioactively labelled sugar that is taken up by the brain and detected by the scanner.
What is FDG-PET?
The tracer commonly used is called 18F-fluorodeoxyglucose, usually shortened to FDG.
FDG behaves similarly to glucose.
Areas of brain tissue using different amounts of glucose can therefore appear differently on the scan.
For most epilepsy-surgery investigations, PET is performed between seizures rather than during one.
This is called an interictal PET.
What are doctors looking for on PET?
In focal epilepsy, the area associated with seizure generation may use less glucose between seizures than surrounding brain tissue.
This is known as hypometabolism.
UCLH explains that brain areas where epileptic seizures originate can use less glucose between seizures and that PET can help identify these areas, particularly when an MRI is normal.
The specialist is not simply looking for one dark spot.
They consider:
the extent of the metabolic abnormality
which structures are involved
whether it is on one side or both
whether it agrees with EEG
whether it agrees with MRI
and whether it fits what happens during seizures.
Does hypometabolism mean dead brain tissue?
No.
Lower glucose metabolism does not automatically mean brain tissue is dead.
PET is showing a functional metabolic difference, not simply whether tissue exists.
Reduced metabolism around an epileptic network may extend beyond the exact point where seizures begin.
For that reason, PET does not usually provide a precise surgical boundary on its own.
PET can be particularly useful when MRI is normal
A person can have focal epilepsy even when conventional MRI does not reveal a lesion.
This is sometimes described as MRI-negative epilepsy.
PET may provide another clue by showing a metabolic abnormality in a particular region.
The Epilepsy Society's specialist presurgical material describes interictal PET as particularly relevant where epilepsy is non-lesional, MRI and other data disagree, or the extent of an abnormality remains uncertain.
A PET abnormality can then encourage clinicians to:
re-examine that region on MRI
perform more advanced MRI analysis
compare it with EEG localisation
or consider that region when planning intracranial EEG.
Can PET find an MRI abnormality that was previously missed?
Not directly.
PET and MRI show different things.
However, if PET repeatedly points towards one small brain region, a neuroradiologist can look very carefully at that same area on the structural MRI.
Occasionally a very subtle abnormality such as cortical dysplasia becomes more convincing once several investigations all point towards the same location.
This is an example of why epilepsy investigations are interpreted together rather than separately.
Does an abnormal PET prove where seizures start?
No.
A metabolic abnormality may be associated with the epileptic network without representing the exact seizure-onset zone.
PET findings are therefore compared with:
ictal video EEG
interictal EEG
MRI
seizure semiology
neuropsychology
and sometimes intracranial EEG.
A PET result that agrees with all the other evidence is usually more informative than an isolated PET abnormality.
What happens during PET?
Exact procedures vary between centres.
Typically:
A small intravenous cannula is inserted.
The radioactive tracer is administered.
The person rests quietly while the tracer is taken up.
The brain is then scanned.
UCLH advises that its epilepsy PET procedure takes approximately two hours overall, although protocols differ between hospitals.
The scanning department will provide specific instructions concerning:
eating and drinking
medication
activity
pregnancy
breastfeeding
and diabetes where relevant.
Does PET involve radiation?
Yes.
PET uses a radioactive tracer.
The amount is deliberately small and the radioactive material decays over time.
This differs from:
MRI, which uses magnetic fields and radio waves;
EEG, which records electrical signals;
and MEG, which records magnetic fields generated naturally by brain activity.
The nuclear-medicine team considers whether the expected diagnostic benefit justifies the radiation exposure.
SPECT — Single-Photon Emission Computed Tomography
SPECT stands for single-photon emission computed tomography.
SPECT also uses an injected radioactive tracer.
But instead of primarily showing glucose metabolism, epilepsy SPECT is commonly used to examine brain blood flow.
Blood flow changes as different brain regions become more or less active.
This can provide important information about seizure activity.
Why is SPECT particularly interesting during a seizure?
When a seizure begins, neuronal activity increases dramatically within the seizure network.
That activity requires increased blood supply.
If a suitable radioactive tracer can be injected very soon after a seizure begins, the tracer distribution can capture a snapshot of cerebral blood flow around that time.
This is called ictal SPECT.
Epilepsy Action explains that blood flow generally increases in the areas where the seizure begins and subsequently spreads.
Timing is crucial
The tracer needs to be administered as quickly as possible after seizure onset.
Why?
Because seizures spread.
If the injection occurs late, increased blood flow may already involve regions the seizure has spread into rather than primarily reflecting where it began.
This means ictal SPECT is logistically difficult.
Someone may need to be:
admitted for video telemetry
continuously monitored
connected to EEG
and observed by staff able to administer the tracer rapidly when a typical seizure starts.
UCLH describes ictal SPECT as a tracer injection administered during a seizure while the person is undergoing video EEG telemetry.
Is the brain scanned during the seizure itself?
Usually the important part is the timing of the injection, not getting the person into the scanner while they are convulsing.
The injected tracer is taken up by the brain according to the blood-flow pattern around the time it is administered.
The actual SPECT images can therefore be acquired later, once the seizure has finished and it is safe to perform the scan.
What is interictal SPECT?
Interictal means between seizures.
A second SPECT scan may be performed when the person is not having a seizure.
Doctors can then compare:
blood flow during a seizure
with
blood flow between seizures.
UCLH describes using both ictal and interictal SPECT specifically for this comparison.
Why compare two SPECT scans?
People naturally have different blood-flow patterns.
Looking only at one scan may make subtle abnormalities difficult to recognise.
Comparing the seizure scan with the person's own baseline scan can make seizure-associated changes clearer.
This can help identify a region that deserves closer attention during surgical evaluation.
What is SISCOM?
Some specialist epilepsy centres use computer processing to compare ictal and interictal SPECT scans.
One approach is commonly called SISCOM:
Subtraction Ictal SPECT Co-registered to MRI.
In simplified terms:
blood-flow images obtained during a seizure are compared with images between seizures;
differences are calculated;
those differences are aligned with the person's MRI.
This allows clinicians to see blood-flow changes in relation to the individual's brain anatomy.
It can provide useful localisation evidence, but it still needs to be interpreted with EEG and the other investigations.
Does increased blood flow always show the exact seizure starting point?
No.
One major limitation is seizure spread.
The scan may highlight:
the onset region
areas activated immediately afterwards
or a wider seizure network.
The timing and type of seizure therefore matter greatly.
An abnormality does not automatically become the surgical target.
Why isn't SPECT done for everyone?
It is complicated.
A useful ictal study may require:
a seizure to occur during the admission
the correct seizure type to be recognised quickly
tracer immediately available
rapid injection
simultaneous video EEG
specialised nuclear-medicine facilities
and careful analysis afterwards.
For many people, MRI and video EEG already provide enough information.
SPECT is therefore generally reserved for selected complex presurgical cases. UCLH and Epilepsy Action both describe it as one of the less common or additional investigations used to localise seizures for epilepsy surgery.
PET and SPECT are not interchangeable
Both use radioactive tracers, but their epilepsy applications differ.
PET usually asks:
Is there an area of abnormal metabolism between seizures?
Ictal SPECT usually asks:
Which areas showed increased blood flow around the time this seizure occurred?
One may be more useful than the other depending on:
seizure frequency
seizure duration
MRI findings
EEG findings
and how predictable seizures are.
The specialist multidisciplinary team decides which additional test is most likely to add useful information.
MEG — Magnetoencephalography
MEG stands for magnetoencephalography.
Electrical activity in brain cells produces extremely weak magnetic fields.
MEG uses highly sensitive sensors to detect those fields outside the head.
Great Ormond Street Hospital describes MEG as measuring the weak magnetic fields created by electrical signalling in the brain and using them to help create a detailed map of seizure-related activity.
Is MEG basically another EEG?
They are related, but not identical.
EEG measures:
electrical voltage differences at the scalp.
MEG measures:
magnetic fields generated by neuronal electrical activity.
Because electrical and magnetic signals interact differently with:
brain tissue
skull
and scalp,
MEG may sometimes provide localisation information that is difficult to obtain from ordinary scalp EEG.
Epilepsy Action notes that MEG findings are often used alongside EEG and that MEG can sometimes identify abnormal activity that was not detected by EEG.
Does MEG require anything to be injected?
No radioactive tracer is required simply to record MEG brain activity.
The sensors measure magnetic fields that the brain is already producing.
That makes MEG fundamentally different from PET and SPECT.
What is the person actually doing during MEG?
With conventional MEG, the person usually sits or lies with their head positioned within a large sensor helmet.
The sensors do not need to be surgically attached to the brain.
Traditional MEG systems require the person to remain relatively still because movement can make accurate localisation more difficult.
Recording may take a substantial period because the system needs enough useful brain activity to analyse.
What is MEG looking for in epilepsy?
Often, specialists are looking for interictal epileptiform activity — abnormal brain activity occurring between recognised seizures.
If repeated epileptiform magnetic discharges arise from a similar region, computer modelling can estimate where in the cortex those signals may have originated.
The resulting localisation can then be compared with MRI and EEG.
Does MEG have to record an actual seizure?
Not necessarily.
One of MEG's strengths is that useful information may come from epileptiform discharges occurring between seizures.
That can be easier than waiting for a person's habitual seizure to occur while they are inside the MEG system.
However, usefulness varies greatly between individuals because some people do not produce enough detectable interictal activity during the recording.
Can MEG pinpoint epilepsy exactly?
Not by itself.
MEG is a source-localisation technique.
It estimates where detected magnetic activity most likely originated.
That estimate depends on:
signal quality
the pattern of epileptiform activity
head position
modelling assumptions
and the anatomy shown on MRI.
The result is therefore another source of localisation evidence rather than a guaranteed identification of the epileptogenic zone.
Why can MEG sometimes help when MRI is normal?
An MRI can look structurally normal while abnormal electrical networks are still present.
MEG is measuring function, not brain structure.
The Epilepsy Society's specialist presurgical material notes that MEG may be particularly useful in selected MRI-negative cases and in helping identify focal abnormalities for further investigation.
A MEG abnormality can potentially help clinicians decide:
where to examine MRI again
where seizure-generating cortex might lie
or where intracranial electrodes should be considered.
Why isn't MEG available everywhere?
The signals generated by the brain are extraordinarily weak.
Traditional MEG systems require:
highly sensitive sensors
specialised equipment
a magnetically shielded room
trained scientists
advanced computer processing
and considerable expense.
Epilepsy Action notes that MEG scanners are available only in very specialist centres.
This means a person may be referred to another specialist centre if MEG is considered important to their surgical assessment.
What is wearable OPM-MEG?
A newer development is optically pumped magnetometer MEG, or OPM-MEG.
Instead of relying on the traditional fixed helmet arrangement, small magnetic sensors can be positioned much closer to the person's scalp within a wearable helmet.
Researchers in the UK have been developing this particularly for children and people who find conventional MEG difficult.
Young Epilepsy describes OPM-MEG as a wearable system that measures brain magnetic fields while allowing substantially more freedom of movement than traditional MEG.
This is a developing technology and availability is still limited.
PET, SPECT and MEG do not replace video telemetry
A sophisticated scan can seem more impressive than watching someone have seizures on video EEG.
But video telemetry remains extremely important because it can demonstrate:
what the seizure actually looks like
whether EEG changes occur
when the seizure begins
how it spreads
what functions are affected
and what happens during recovery.
PET, SPECT and MEG generally add to this evidence rather than replacing it.
Related Information Hub page:
Video Telemetry: Recording Seizures in Hospital
They also do not replace MRI
MRI shows detailed anatomy.
PET can show metabolic differences.
SPECT can show blood-flow differences.
MEG can localise abnormal magnetic activity.
A metabolic or magnetic abnormality becomes much more meaningful when clinicians can place it accurately onto the person's structural brain images.
Related Information Hub page:
MRI and Epilepsy: What Doctors Are Looking For
Why do doctors want several tests to agree?
This is called concordance.
Imagine that:
video EEG suggests seizures begin in the left temporal lobe;
MRI shows left hippocampal sclerosis;
PET shows reduced metabolism in the left temporal region;
and neuropsychology shows a pattern compatible with dysfunction in the same network.
Several independent investigations are now pointing in a similar direction.
That is much stronger evidence than one isolated abnormal test.
By contrast, if every test points somewhere different, the case needs further investigation.
What does discordant mean?
Discordant means the evidence does not agree.
For example:
MRI abnormality on the right
EEG seizure onset apparently on the left
PET abnormality involving both sides.
That does not automatically mean any one test is wrong.
Possible explanations include:
rapid seizure spread
an MRI abnormality unrelated to the epilepsy
a larger seizure network
more than one seizure onset region
limitations of scalp EEG
or limitations of the imaging methods.
Discordant findings are one reason intracranial EEG may eventually be considered.
Can these tests tell doctors where to put SEEG electrodes?
They can help.
Stereo-EEG, or SEEG, uses electrodes implanted into selected areas of the brain.
Doctors cannot realistically sample every brain region.
They need a carefully developed hypothesis about where seizures might originate.
PET, SPECT and MEG can all contribute to that hypothesis.
Specialist literature describes these techniques as useful for guiding invasive-electrode placement when non-invasive information is incomplete.
What if PET, SPECT or MEG points somewhere different from EEG?
The result is not simply ignored.
The multidisciplinary team asks why.
They may:
re-review the video telemetry
re-examine MRI
analyse seizure symptoms again
compare several seizures
perform advanced image processing
obtain another test
or consider intracranial EEG.
Sometimes the disagreement itself reveals that the seizure network is more complex than initially thought.
Can these scans show seizures spreading?
SPECT can particularly demonstrate blood-flow changes associated with the onset and spread of an actual seizure.
PET usually provides a broader picture of abnormal metabolism between seizures rather than a moment-by-moment recording of seizure spread.
MEG often focuses on interictal epileptiform sources rather than following an entire clinical seizure.
Therefore the three tests provide complementary rather than identical information.
What is the epileptogenic zone?
The epileptogenic zone is a conceptual term used in epilepsy surgery.
In simplified terms, it refers to the brain tissue that would need to be removed or disconnected for seizures to stop.
No single investigation can directly display a box labelled:
“This is the epileptogenic zone.”
Instead, doctors build a hypothesis from several forms of evidence.
Those may include:
seizure semiology
scalp EEG
video telemetry
MRI
PET
SPECT
MEG
neuropsychology
and intracranial EEG.
A scan abnormality is not automatically the surgical target
This is especially important.
Suppose PET shows a large region of reduced metabolism.
That does not mean the entire abnormal-looking region should be removed.
Likewise:
high blood flow on SPECT
a MEG source
or an MRI lesion
cannot automatically be converted into surgical boundaries.
The team must also protect brain systems responsible for:
language
memory
movement
vision
sensation
and other important functions.
Why are these tests usually associated with drug-resistant epilepsy?
Most people with epilepsy achieve sufficient control with antiseizure treatment and do not need advanced localisation investigations.
NICE says people with drug-resistant epilepsy should be referred to a tertiary centre for consideration of whether resective surgery could help — including people whose MRI does not reveal an abnormality.
Epilepsy Action describes drug-resistant epilepsy in this context as seizures not being stopped after appropriate trials of two or more antiseizure medicines.
Advanced investigations become relevant when specialists need to know whether the epilepsy could be treated in another way.
Having one of these tests does not mean surgery is definitely going ahead
No.
The purpose of presurgical assessment is partly to discover whether surgery is suitable at all.
The outcome might be:
surgery appears possible;
more investigations are needed;
invasive monitoring is required;
the suspected region is unsafe to remove;
seizures arise from more than one region;
or another treatment would be more appropriate.
Getting PET, SPECT or MEG therefore means doctors are gathering information — not that a decision has already been made.
What if none of these tests finds a clear focus?
That can happen.
Some epilepsy networks remain difficult to localise even with sophisticated testing.
The team may then consider:
advanced MRI
repeat video telemetry
neuropsychology
functional imaging
SEEG
another surgical strategy
neuromodulation
ketogenic therapy
or continued medication treatment.
A negative PET, SPECT or MEG does not mean the seizures are not real.
It means that that particular method did not provide sufficiently useful localisation information.
PET, SPECT and MEG in children
These techniques can also form part of paediatric epilepsy-surgery assessment.
England's Children's Epilepsy Surgery Service describes:
PET for identifying brain regions functioning or using energy differently;
SPECT for comparing blood flow during and between seizures;
and MEG for measuring magnetic brain activity and helping identify where seizures may originate.
Which tests are needed depends on the child's epilepsy and on how clearly ordinary MRI and video EEG identify the seizure network.
Radiation and children
PET and SPECT involve radioactive tracers.
Any investigation involving ionising radiation in a child is justified carefully.
The aim is always to obtain information likely to influence management while keeping exposure as low as reasonably practicable.
MEG does not use an injected radioactive tracer.
MRI likewise does not use ionising radiation.
Why one person might have PET but another has SPECT
There is no universal checklist saying everyone being considered for surgery must have every available scan.
One person may have:
MRI + video EEG + neuropsychology
and already have enough concordant evidence.
Another may need:
PET
because MRI is normal.
Another may need:
ictal SPECT
because seizure-onset localisation remains uncertain.
Another may undergo:
MEG
because interictal electrical source localisation could help guide further investigation.
Specialist presurgical pathways are deliberately individual.
Questions to ask if PET is recommended
Useful questions include:
What are you hoping the PET scan will show?
Is my MRI normal or unclear?
Are you looking for reduced metabolism?
Does PET require me to fast?
Should I take my epilepsy medication normally?
What happens if I have a seizure around the scan?
How will PET be compared with my MRI and EEG?
Will the result affect whether I need invasive monitoring?
Questions to ask if SPECT is recommended
Useful questions include:
Are you planning ictal and interictal scans?
How quickly does the tracer need to be injected after seizure onset?
Will I already be undergoing video telemetry?
What happens if I do not have a seizure during admission?
Which seizure type are you trying to capture?
How will you distinguish seizure onset from spread?
Will the scan be compared directly with my MRI?
Questions to ask if MEG is recommended
Useful questions include:
What question are you hoping MEG will answer?
Did my EEG fail to localise the activity clearly?
Is my MRI normal?
Are you looking for interictal epileptiform activity?
How long will the recording take?
Do I need to remain completely still?
How will the MEG findings be combined with MRI?
Could the result affect placement of SEEG electrodes?
The most important message
PET, SPECT and MEG are specialist tools for answering difficult localisation questions.
They do not compete with MRI, EEG or video telemetry.
They provide different information.
PET examines brain metabolism and can reveal areas using less glucose between seizures.
SPECT examines cerebral blood flow and can be particularly useful when tracer is injected very soon after a seizure begins.
MEG records the tiny magnetic fields produced by brain activity and can help localise epileptiform activity with high spatial precision in selected cases.
None of these tests independently proves exactly which tissue should be removed.
Their greatest value comes when their findings are compared with:
seizure behaviour
video EEG
MRI
neuropsychology
and other investigations.
In complex epilepsy, the question is rarely:
“Which test is right?”
The more useful question is:
“Do the different pieces of evidence point towards the same seizure network?”
That agreement — or disagreement — is what helps the specialist epilepsy team decide what investigation or treatment should come next.
Sources and further reading
NICE — Epilepsies in children, young people and adults (NG217).
Current NICE guidance sets out referral to tertiary epilepsy services for people requiring specialist investigation and assessment for surgery, including people with drug-resistant epilepsy whose MRI is normal. Last updated January 2025.
UCLH National Hospital for Neurology and Neurosurgery — Epilepsy surgery assessment.
Current NHS information describes PET as measuring uptake of radiolabelled glucose and SPECT as comparing cerebral blood flow during and between seizures as part of specialist presurgical investigation.
Epilepsy Action — Epilepsy surgery for adults.
Current UK information on specialist presurgical investigations, including SPECT and MEG, and the role of these tests alongside MRI, EEG, video telemetry and neuropsychology. Updated November 2025.
Epilepsy Action — Children's Epilepsy Surgery Service.
Information on PET, SPECT and MEG within paediatric epilepsy-surgery evaluation in England.
Epilepsy Society — Presurgical evaluation.
Specialist educational material describing the complementary roles of PET, ictal/interictal SPECT and MEG when MRI or EEG information is incomplete or discordant.
Great Ormond Street Hospital — MEG scans.
NHS information explaining how MEG records weak magnetic fields generated by neuronal electrical activity and can contribute to mapping seizure-related activity before epilepsy surgery.
Information reviewed: September 2026.
This page provides general educational information. PET, SPECT and MEG are specialist investigations, and whether any of them is useful depends on the individual's epilepsy, previous test results and the specific clinical question being investigated.
