First unprovoked seizure

ACR AC Seizures and Epilepsy (2020)

The structural survey after a first unprovoked seizure is an MRI, not a CT. CT earns its place only in the emergency department, where the question is whether something needs treating tonight.

A first witnessed convulsion or a first focal seizure, usually in an adult, where the question is whether an underlying structural lesion explains it and whether the risk of recurrence justifies treatment.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

GCS

Anticoagulated or on antiplatelets

Moves the threshold for imaging head injury further than any other single item of history.

Focal neurological deficit

Fever or sepsis

Immunosuppressed

Changes the differential rather than the modality.

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

MRI Brain — routine unenhanced

MRI Brain

What we'd amend, and why

The lesions that cause seizures — cortical dysplasia, hippocampal sclerosis, low-grade glioma, cavernoma, old cortical injury — are precisely the ones CT is worst at. They are small, cortical or mesial temporal, and frequently show no mass effect, no calcification and no enhancement. Only MRI has the tissue contrast and the multiplanar thin-section coverage to find them, and finding one changes both the recurrence risk and the treatment.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwayadult — Adults and children after a first unprovoked seizure
rulerule-mr-device-screening — MR safety screening for implants and foreign bodies; checked by Radiographer at the scanner
rulerule-paeds-sedation — Sedation or anaesthesia for a child; checked by Nurse before the scan

Decision support only. Local protocol takes precedence.

Handled at the scanner(1)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

MR safety screening for implants and foreign bodies
Complete the MR safety questionnaire, verify implant labelling and its stated conditions of use against this scanner and this protocol, and ensure no ferromagnetic object enters Zone IV.
Radiographer at the scannerBefore the scan
Flags back if: An implant or retained foreign body that is MR Unsafe, unlabelled, or cannot be identified; or an MR Conditional device whose stated conditions this scanner or the requested protocol cannot satisfy; or a credible unexcluded intraocular metallic foreign body history.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

Seizure with fever, immunosuppression, anticoagulation, a deficit or a depressed conscious level

Role	Study & protocol	Why this answers the question
First line	CT Head CT Head — Unenhanced usually appropriate	This is the emergency version of the question and the order of the two studies inverts. Any one of these features raises the probability of something that needs treating tonight — haemorrhage into or around a lesion in an anticoagulated patient, an abscess or empyema in an immunosuppressed or febrile one, a mass with shift in a patient who has not returned to baseline — and an unenhanced CT excludes that set within minutes, in a patient who is often postictal and unable to hold still for anything longer. It remains a triage study: a normal CT does not remove the indication for the structural survey, it only says that the survey can wait for a proper protocol.
Second line	MRI Brain MRI Brain — with gadolinium	Contrast is not routine after a first seizure and is the right default in exactly this group, because the differential that opens up here is enhancing: abscess, empyema, tuberculoma, toxoplasmosis and lymphoma in the immunosuppressed, meningeal disease in the febrile, and a tumour that bled in the anticoagulated. Diffusion is what separates an abscess from a necrotic tumour and belongs in the same acquisition. Where none of those is in play once the patient has been reviewed, the plain structural epilepsy survey is the study and the gadolinium can be dropped.

A seizure that followed a head injury belongs on the head injury card, and one with fever and meningism on the meningitis and encephalitis card. This arm covers the patient in whom that has not yet been separated out.

Adults and children after a first unprovoked seizure

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	MRI Brain MRI Brain — routine unenhanced usually appropriate	The lesions that cause seizures — cortical dysplasia, hippocampal sclerosis, low-grade glioma, cavernoma, old cortical injury — are precisely the ones CT is worst at. They are small, cortical or mesial temporal, and frequently show no mass effect, no calcification and no enhancement. Only MRI has the tissue contrast and the multiplanar thin-section coverage to find them, and finding one changes both the recurrence risk and the treatment.
Reasonable alternative	CT Head CT Head — Unenhanced	In the emergency setting an unenhanced CT is an appropriate immediate study, because it excludes quickly the small set of findings that would change management the same night — haemorrhage, a large mass with shift, hydrocephalus. It is a triage study rather than a structural survey, and a normal CT does not remove the indication for MRI.

Pitfalls

Treating a normal emergency CT as having answered the question. The structural survey is still outstanding, and this is where first-seizure pathways most often break down.
Requesting gadolinium routinely. Contrast is for a suspected tumour, infection or inflammatory cause, not for a structural survey after a single seizure.
Ignoring stated focal onset, which raises the yield of MRI and should push a general protocol towards a dedicated epilepsy protocol.
Attributing a seizure to an incidental small vessel lesion. Chronic white matter change is common and rarely epileptogenic.

Priors — what to pull first

Any previous brain imaging, even a CT done for another reason, reframes an incidental finding as old and stable rather than new.
Ask specifically about childhood febrile convulsions, which shift the pre-test probability towards hippocampal sclerosis.

What makes a good request

Two distinct questions hide in the same request. "Is there something acutely dangerous" is answered tonight by CT; "is there a structural cause for epilepsy" is answered by MRI and can usually wait for an outpatient slot.
A seizure with a persistent deficit, fever, head injury, anticoagulation, immunosuppression or failure to return to baseline is the emergency version and belongs with the acute cards rather than here.
Focal onset, however brief, raises the yield of MRI substantially and should be stated on the request.

Scoring this once it is done

The classification and risk tools this question ends in.

Fazekas Scale (White Matter Lesions)

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

CT Head — Unenhanced MRI Brain — with gadolinium MRI Brain — routine unenhanced

Confirm locally

MRI Brain — routine unenhanced: timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.