MRI protocol
A specialized scanning protocol is used for patients with epilepsy, aimed at detailed visualization of the medial temporal lobes and the cortex.
Key MRI requirements:
- 3D T1-weighted images with an isotropic voxel (1 mm) for volumetric assessment of structures.
- T2, STIR, and FLAIR with high resolution and a slice thickness of no more than 3 mm.
- Coronal acquisition (perpendicular to the hippocampal axis) is critical for detecting temporal lobe pathologies.
- Use of sequences sensitive to blood degradation products (hemosiderin) and calcifications.
- Strict patient positioning symmetry: the internal auditory meatuses should be at the same level.
Administration of gadolinium is not strictly mandatory as a routine step, but helps differentiate vascular and infectious processes.
Structural and congenital alterations
Up to half of pharmacoresistant epilepsy cases in children are associated with malformations of cortical development. These include disorders of cellular proliferation, migration, or cortical organization.
Key pathologies:
- Focal cortical dysplasia (FCD): cortical thickening, blurring of the grey-white matter junction, and hyperintense signal on T2/FLAIR. The area of metabolic disturbance (on PET) is often broader than the visible defect.
- Gray matter heterotopia: clusters of normal neurons in atypical locations (e.g., in the white matter). Their signal is identical to normal cortex on all MRI sequences.
- Tuberous sclerosis: a phacomatosis characterized by cortical tubers, subependymal nodules, and lines of radial glial migration.
- Sturge-Weber syndrome: pial angiomas, cortical calcifications, and hemiatrophy on the affected side.
Acquired pathologies
Hippocampal sclerosis is the most common cause of temporal lobe epilepsy requiring surgery. On MRI, it manifests as hippocampal volume loss, disruption of its internal architecture, and increased T2/FLAIR signal due to neuronal death and replacement by glial scar tissue.
Tumors account for about 4% of epilepsy cases (most commonly in the temporal location). Slow-growing lesions (low-grade astrocytomas, gangliogliomas, DNTs) are most typical. Functional MRI (fMRI) helps assess the distance from the tumor to critical areas (speech, motor) before surgery.
Seizures may also result from:
- TBI: development of post-traumatic encephalomalacia, hemosiderin deposition, and glial scar formation.
- Cerebrovascular disease: strokes, arteriovenous malformations, cavernomas (especially in the elderly).
- Infections: for example, neurocysticercosis, which progresses from cysts containing a scolex to calcification accompanied by inflammatory edema.
Briefly: answers to topical questions
Which specific MRI pulse sequences are used to detect hemosiderin and calcifications?
Sequences sensitive to susceptibility and calcification are used to detect hemosiderin and calcifications. T2* GRE and SWI are used to visualize hemosiderin deposits; hemosiderin causes a magnetic susceptibility artifact ("blooming").
What stages of neurocysticercosis development can be visualized on MRI?
Four stages of parasite development are distinguished on MRI of the parenchymal form of neurocysticercosis.
- Vesicular stage (vesicular stage) — thin-walled cysts are visualized containing a viable larva with a scolex, without perifocal edema.
- Colloidal vesicular stage (colloidal vesicular stage) — reflects larval degeneration, characterized by ring enhancement, perifocal edema, and mass effect.
- Granular nodular stage (granular nodular stage) — represents a partially collapsed cyst with a thick, strongly enhancing wall and prominent surrounding edema.
- Calcified nodular stage (calcified nodular stage) — an inactive stage indicated by the presence of calcification.
Class and exam questions
What is "dual pathology" in epilepsy?
This is a combination of hippocampal sclerosis with another structural anomaly, most commonly focal cortical dysplasia in an extrahippocampal location.
What is the signal characteristic of heterotopia on MRI?
Heterotopic areas consist of normal neurons, therefore on all pulse sequences (T1, T2, FLAIR) they exhibit a signal completely identical to the normal cerebral cortex.
Why is functional MRI (fMRI) performed in patients with epilepsy?
The method is necessary during neurosurgical planning to identify the location of functionally eloquent areas (e.g., motor cortex) relative to the tumor or dysplasia zone, thereby minimizing the risk of neurological deficit.
What else to study on this topic
- Metabolic and immune causes of epilepsy (Rasmussen encephalitis, anti-NMDA receptor encephalitis)
- FDG-PET and perfusion MRI in the search for occult epileptogenic zones
- Differential MRI diagnosis of epileptogenic tumors (DNT vs. Ganglioglioma)
- Stages of neurocysticercosis on MRI
- Classification of malformations of cortical development (schizencephaly, lissencephaly, polymicrogyria)
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