What is the best imaging modality for evaluating seizures, specifically in patients with epilepsy?

Best Imaging Modality for Seizure Evaluation

MRI brain without IV contrast is the imaging modality of choice for evaluating seizures in non-emergent settings, with superior sensitivity (70-80%) compared to CT (30%) for detecting epileptogenic lesions. 1, 2, 3

Clinical Context Determines Imaging Selection

Emergent Presentation (New-Onset Seizure in ED)

• CT head without contrast is appropriate for initial rapid assessment when immediate intervention may be needed, the patient requires close monitoring during scanning, or acute trauma is suspected 2, 4
• CT rapidly identifies life-threatening pathology including intracranial hemorrhage, stroke, vascular malformation, hydrocephalus, or tumors requiring neurosurgical intervention 1, 4
• However, CT has critical limitations: it detects abnormalities in only 18-30% of cases versus 55% with MRI, has poor visualization of orbitofrontal and medial temporal regions, and misses 29% of abnormalities later found on MRI 2

Non-Emergent Evaluation (Outpatient or Stable Patients)

• MRI brain without IV contrast is the definitive study due to excellent gray-white matter differentiation and multiplanar imaging capability 1, 3
• The American College of Radiology recommends dedicated epilepsy protocols using 3T scanners whenever possible for improved lesion detection over 1.5T systems 1, 2
• MRI provides superior visualization of hippocampal abnormalities (the most common cause of temporal lobe seizures) with 70-80% detection rates 3

Dedicated MRI Seizure Protocol Components

A proper epilepsy MRI protocol must include 2:

• Coronal T1-weighted imaging (3mm) perpendicular to the long axis of the hippocampus
• High-resolution 3D T1-weighted gradient echo (GRE) with 1mm isotropic voxels
• Coronal T2-weighted sequences
• Coronal and axial FLAIR sequences for optimal lesion detection

When to Add IV Contrast

IV contrast is NOT routinely necessary for seizure evaluation 2, 3. Consider adding contrast only when 2:

• Initial non-contrast images are insufficient
• Neoplasm is suspected
• Inflammatory condition is suspected
• Infection is suspected

Advanced Functional Imaging for Surgical Planning

FDG-PET/CT Brain

• FDG-PET is complementary to MRI for presurgical evaluation, particularly when MRI is normal but seizures persist 1, 2
• Sensitivity: 87-90% for temporal lobe epilepsy, 38-55% for extra-temporal lobe epilepsy 1
• FDG-PET identifies lesions missed on CT or MRI in refractory cases and improves detection when coregistered with MRI 1
• Limitation: hypometabolism often extends beyond the epileptogenic zone, limiting precise surgical margin identification 1

SPECT/CT Brain (Ictal and Interictal)

• SPECT provides assessment of regional cerebral blood flow, showing hypoperfusion interictally and hyperperfusion ictally 1
• Statistical ictal SPECT coregistered to MRI identifies hyperperfusion focus in 84% of patients versus 66% with subtraction techniques alone 1
• SPECT has higher sensitivity (49-87%) than FDG-PET (56-63%) but the two modalities are complementary, with FDG-PET providing additional information in 33% of cases where SPECT is negative 1

Critical Pitfalls to Avoid

• Never assume a normal CT excludes structural abnormality: MRI reveals significant epileptogenic lesions in 22% of patients with normal neurologic examinations and normal CT 2, 4
• 20-30% of temporal lobe epilepsy and 20-40% of extra-temporal lobe epilepsy cases have no clear lesion on MRI despite having epileptogenic foci 1, 3
• Patients with focal circumscribed lesions identified on presurgical MRI have 60-80% seizure-free rates post-surgery versus lower rates without identified lesions 1, 3

Priority Patients for MRI

Prioritize MRI for patients with 2:

• Focal neurological deficits on examination
• Persistent headache
• Recent history of head trauma
• Abnormal EEG findings
• Focal seizure features