What is the recommended approach to brain mapping in patients with a history of neurological conditions such as epilepsy or brain tumors?

Brain Mapping in Neurological Conditions: Epilepsy and Brain Tumors

Recommended Approach

For patients with epilepsy or brain tumors requiring brain mapping, MRI with dedicated epilepsy protocols on 3T magnets is the primary structural imaging modality, supplemented by functional MRI (fMRI) for eloquent cortex localization and FDG-PET for metabolic mapping of epileptogenic zones, particularly in MRI-negative cases. 1

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Structural Brain Mapping

Primary Modality: MRI

• MRI is the modality of choice for high-resolution structural imaging in epilepsy and brain tumor patients due to superior gray-white matter differentiation and multiplanar capability 1
• Dedicated seizure protocols should include 1:
  • Coronal T1-weighted imaging (3mm) perpendicular to hippocampal long axis
  • High-resolution 3D T1-weighted gradient echo with 1mm isotropic voxels
  • Coronal T2 and coronal/axial FLAIR sequences
  • 3T magnets are preferred over 1.5T for improved lesion detection 1
• MRI sensitivity reaches 84% with dedicated epilepsy protocols, significantly outperforming CT's 30% detection rate for focal epilepsies 1, 2
• IV contrast is not routinely necessary but useful when neoplasm or inflammatory conditions are suspected 1

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Functional Brain Mapping

Eloquent Cortex Localization

fMRI has replaced the intracarotid amobarbital procedure (IAP) as the standard for language lateralization, demonstrating 89% concordance with IAP in right temporal lobe epilepsy and 85% in left temporal lobe epilepsy 1

• For language mapping: fMRI shows 83% concordance with IAP in extratemporal epilepsy and can predict postsurgical language deficits 1
• For memory lateralization: fMRI shows moderate correlation (r=0.31, P=.007) with IAP memory laterality, though conflicting data exists 1
• Strong left frontal activation on fMRI predicts postresection language decline, making it valuable for surgical planning 1

Magnetoencephalography (MEG)

• MEG achieves 85% seizure-free rates when findings are concordant and specific with the resection region, compared to only 37% with nonspecific/discordant findings 1
• MEG demonstrates 70% sensitivity in detecting epileptic activity in large case series 1
• MEG is particularly useful as a complementary modality for identifying eloquent cortex and determining safe resection margins, though it remains performed in only a minority of presurgical evaluations 1
• MEG signals penetrate skull without distortion, unlike EEG which suffers signal deterioration 1, 3

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Metabolic Brain Mapping

FDG-PET: Primary Metabolic Modality

FDG-PET is essential for presurgical evaluation, particularly in MRI-negative epilepsy, identifying focal abnormalities when structural imaging is normal 1, 4

Indications and Performance

• FDG-PET identifies the functional deficit zone showing abnormal interictal brain function 1
• Sensitivity of 63-67% for detecting epileptogenic foci, with higher yield when combined with other modalities 4
• FDG-PET co-registered with MRI gray matter segmentation shows higher correspondence to intracranial EEG than without segmentation 1
• In pediatric epilepsy, FDG-PET defines lesions with higher sensitivity and larger extent than MRI, especially for focal cortical dysplasia type 2 1

Prognostic Value

• Bilateral temporal lobe hypometabolism predicts poor surgical outcomes in temporal lobe epilepsy 1
• Concordance between FDG-PET and other localizing modalities (MRI, EEG, SPECT) strongly predicts seizure-free outcomes 1, 4
• Contralateral thalamic hypometabolism indicates poorer surgical prognosis compared to ipsilateral involvement 4

Technical Considerations

• Visual analysis should evaluate all brain regions using asymmetry as the main criterion 1
• PET/MRI fusion is essential and consistently improves detection rates across patient series 1
• Movement artifacts and EEG electrode attenuation effects must be checked before interpretation 1
• In children under 6 years, age-matched controls are necessary for semi-quantitative analysis due to developmental glucose metabolism patterns 1

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Perfusion Brain Mapping

SPECT Imaging

Subtraction ictal SPECT co-registered to MRI (SISCOM) is the preferred SPECT technique, improving sensitivity and specificity of seizure localization 1, 4

Performance Characteristics

• SISCOM achieves >90% sensitivity in temporal lobe seizures but only 70% in extratemporal epilepsy 1
• SISCOM localizes hyperperfusion foci in 71% of cases versus 47.4% with side-by-side SPECT evaluation 1
• When SISCOM is concordant with surgical resection site, seizure-free odds ratio is 3-times higher than in non-concordant cases 1

Technical Requirements

• Tracer (Tc-99m-HMPAO or Tc-99m-ECD) should be injected within 20 seconds of seizure onset via pre-placed IV line 1
• Continuous video-EEG monitoring is mandatory during ictal SPECT 1
• Best results obtained during focal impaired awareness seizures 1
• Statistical SPECT processing methods further enhance SISCOM diagnostic performance 1

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Integrated Multimodal Approach

Algorithmic Decision Framework

For epilepsy presurgical evaluation:

1. Start with dedicated 3T MRI epilepsy protocol 1, 2
2. If MRI positive: Add fMRI for eloquent cortex mapping + FDG-PET for metabolic confirmation 1
3. If MRI negative: FDG-PET becomes essential, consider adding SISCOM and MEG 1
4. For temporal lobe epilepsy: Prioritize hippocampal imaging sequences + FDG-PET for bilateral assessment 1, 4
5. For extratemporal epilepsy: MEG and SISCOM have higher relative value 1

For brain tumor mapping:

• MRI with and without contrast is primary for structural delineation 1
• Add fMRI for all tumors near eloquent cortex (language, motor, sensory areas) 1
• Consider MEG for tumors in eloquent regions to define safe resection margins 1

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Critical Pitfalls to Avoid

Common Errors

• Relying on CT alone: CT detects only 30% of focal epilepsies versus MRI's 84% sensitivity 1, 2
• Using standard MRI protocols: Dedicated epilepsy protocols on 3T magnets are essential for optimal detection 1
• Ignoring bilateral findings on FDG-PET: Bilateral temporal hypometabolism predicts poor surgical outcomes 1
• Performing isolated interictal SPECT: Only 60% abnormal; SISCOM is far superior 1
• Skipping functional mapping in eloquent regions: fMRI prevents unexpected postoperative deficits 1

Technical Considerations

• Check for EEG electrode artifacts on PET/MRI that can affect quantification 1
• Verify attenuation correction alignment before interpreting PET images 1
• Ensure injection timing documentation for SISCOM interpretation 1
• Use age-matched controls for pediatric FDG-PET semi-quantification 1

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Special Population Considerations

Pediatric Patients

• Malformations of cortical development are the most common cause of medically refractory focal epilepsy in children 1, 2
• FDG-PET is particularly valuable for defining lesion extent beyond MRI findings 1
• Bilateral FDG-PET abnormalities are independent predictors of seizure recurrence after hemispherotomy 1

MRI-Negative Epilepsy

• FDG-PET becomes the primary localizing modality when MRI is negative 1, 4
• Prognosis for seizure-free outcome is worse than MRI-positive cases, making multimodal mapping critical 1
• Combined FDG-PET, SISCOM, and MEG provide complementary localization in this challenging population 1