Aim of structural imaging
- To identify on structural Imaging for abnormality
- The success of surgery is directly related to the ability to localize epileptogenic cortex, and to precisely resect or isolate this.
- Téllez-Zenteno 2010: The odds of being seizure-free after surgery in temporal lobe and extra-temporal epilepsy are 2- to 3-fold higher in lesional (lesion on MRI or histopathology) compared to non-lesional epilepsy
- Results by age group
Categories (n of studies) | Non-lesional | ㅤ | ㅤ | Lesional | ㅤ | ㅤ |
ㅤ | Total N patients | Seizure-free % | 95%CI | Total N patients | Seizure-free % | 95%CI |
Children | ㅤ | ㅤ | ㅤ | ㅤ | ㅤ | ㅤ |
Temporal and extratemporal (n = 9) | 93 | 45 | 35–55 | 317 | 74 | 69–79 |
Temporal epilepsy (n = 5) | 48 | 45 | 31–59 | 146 | 81 | 75–87 |
Extratemporal epilepsy (n = 4) | 31 | 46 | 30–62 | 97 | 73 | 64–82 |
Type of structural imaging
- Other sequences:
- Susceptibility weighted images
- Diffusion tensor imaging
- Arterial spin labelling (perfusion)
- Paediatric MRI epilepsy protocol
- See Pearce 2020
0 to 2 years old | > 2 years old | Diagnostic role |
Inversion recovery coronal T1w | Volumetric FLAIR e.g. T2w SPACE FLAIR | - Coronal reconstruction for hippocampal sclerosis - Focal cortical dysplasia, tumours and sites of damage |
Volumetric T1w e.g. T1 MPRAGE | Volumetric T1w e.g. T1 MPRAGE | - Excellent grey-white matter contrast. - Cortical malformation and migration abnormalities - Hippocampal segmentation and volumetry |
Axial DESTIR | Axial T2w | - Focal cortical dysplasia, tumours and sites of damage |
Coronal DESTIR | Coronal T2w | - Focal cortical dysplasia, tumours and sites of damage |
Axial DWI | Axial DWI | - Tumour characterisation - Evidence of recent seizure activity |
SWI | SWI | - Detection of calcification, haemorrhage and vascular malformations |
± Post-contrast T1w | ± Post-contrast T1w | - Tumour characterisation |
Example
- Cortical dysplasia
- Cavernomas
Identification (lateralization and localization) of epileptogenic focus or Ictal onset zone (IOZ)
- Ictal & Interictal SPECT
- Inject immediately during ictus and taken to scan within 4hrs.
- Injected isotope
- Non-invasive localization and lateralization of epileptogenic focus in intractable epilepsy prior to surgery.
- Seizures are associated with increased glucose metabolism, which is closely coupled to cerebral blood flow.
- Hence ictal SPECT scans show increased perfusion in the region of seizure onset.
- PET (FDG)
- A gamma imaging technique that uses radiotracers that emit positrons
- PET relies on the simultaneous detection of the pair of gamma photons that are released from an annihilation of a positron and an electron.
- By mapping gamma rays that arrive at the same time, the PET system can produce an image with high spatial resolution.
- 18F-FDG
- Measures glucose metabolism related to the synaptic and neuronal activity of the brain tissue
- Interictal 18F-FDG PET typically shows reduced radiotracer uptake (hypometabolism) in the epileptogenic region
- Challenges:
- Timing in relationship to seizures
- GA in children
- Hyperglycemia
- Can help identify where the lesion is if there is no abnormalities in the MRI
- Seizure semiology
- Neurophysiology
- Neuropsychology & deficits
Pre-operative planning
fMRl (eloquent areas/ language lateralization)
- Aim
- Assess relationship between lesion and eloquent areas of brain
- Motor mapping
- Visual mapping
- Language mapping/lateralization
- Left temporal lobe epilepsy (TLE) and temporal lobectomy may affect
- Language and cognitive function,
- In particular verbal and visual memory
- Association between neocortical language and hippocampal memory regions in patients with left-sided epilepsy.
- Correlation between language lateralization and verbal memory performance
- Eloquent cortical language areas
- Broca's area:
- Production of language
- (Left) inferior frontal gyrus/ anterior insula
- Insult leads to expressive dysphasia
- Wernicke's area:
- Comprehension of human language
- (Left) posterior superior temporal gyrus
- Insult leads to receptive dysphasia
- Hemispheric dominance:
- Normal:
- Right handedness: left dominance
- Left handedness: 70% left-hemispheric dominance
- In pathology, e.g. left TLE:
- Everts 2010 "Atypical" language lateralization can occur in left TLE
- This benefits verbal memory performance, via secondary to "remodelling" & transfer of verbal memory function
- Plasticity & remodelling → "atypical" language representation
- Co-dominance;
- Dominance differs for Broca's & Wernicke's area
- False negative haemodynamic activation in vascular lesions and tumour with BBB breakdown
- Used in epilepsy tumor and AVM surgery
DTI
- Aim: Assessment of tract proximity to lesion and tract integrity
- Assess white matter tracts pre-operatively in case of tumours causing epilepsy (e.g. neuroglial/ DNET):
- Deviation, infiltration, destruction by tumours
- May provide insight into network dysfunction in TLE epilepsy
- Leyden KM et al. What does diffusion tensor imaging (DTI) tell us about cognitive networks in temporal lobe epilepsy? Quant Imaging Med surg 2015;
- Pre-operative planning epilepsy surgery, in particular in TLE
- IFOF (inferior fronto-occipital fasciculus)
- Subserves semantic language processing
- Arcuate fasciculus:
- Connects the temporal and inferior parietal cortex frontal lobe connecting Broca's and area.
- Critical for language: spontaneous speech, word retrieval, repetition, comprehension
- Meyer's loop:
- Optic radiation fibres from inferior retina, passing through temporal lobe looping around the temporal horn of the lateral ventricle to occipital lobe.
- Variable in anterior extent.
- Carries information from the superior visual field; damage causes homonymous superior quadrantinopia (DVLA!)
Summary
Investigation | Comments |
Sleep-deprived EEG | Many routine EEGs are normal, especially if the patient has been taking antiepileptic medication before the study. A prolonged (1 h) sleep-deprived EEG may show epileptiform discharges during nonrapid-eye-movement sleep. |
Video EEG | Admission for monitoring while medication is tapered with the aim of recording a minimum number of seizures. May also require sleep deprivation, hyperventilation, photic stimulation, caffeine, vigorous exercise. Sphenoidal electrodes should be placed when mesial temporal lobe origin is suspected. Can be complicated by concomitant nonepileptic seizures. |
MRI | Standard sequences include T1, T2, and FLAIR with coronal sequences through mesial temporal lobe. Contrast is given if inflammatory, infectious or neoplastic etiology suspected. |
PET | Seizure focus is often hypometabolic on FDG-PET. In patients with frequent seizures, if a seizure happens immediately before or during the scan the epileptogenic focus may appear hypermetabolic and be missed—hence concurrent EEG should be used in these patients. |
Neuropsychological testing | Identifies epileptogenic region (and associated dysfunctioning areas), lateralization/dominance of language and memory, prediction of postoperative deficits, and preoperative psychological issues needing treatment (depression, anxiety). |
Psychiatric evaluation | Aim to identify and treat psychological problems which require surgery to be delayed/withheld or arise postoperatively, e.g. depression/anxiety, nonepileptic seizures, adjustment to seizure freedom. |
Wada study | Indicated preoperatively in patients with TLE or involvement of areas critical for language. It aids localizing language and memory, assess risk of postoperative amnesia, risk of memory decline and likelihood of seizure-free outcome. Angiography must exclude cerebral vessel cross filling before injection of sodium amobarbital otherwise results will be affected. Ipsilateral carotid injection allows testing of contralateral hemisphere memory and language for 15 min. |
fMRI | Aids in localizing language and motor function, and interictal spikes. Not sufficient for predicting postoperative verbal memory impairment. |
Ictal SPECT with SISCOM | Technically difficult hence used when preceding workup has not identified an epileptogenic focus—usually in the case of extratemporal nonlesional epilepsy. Interictal SPECT is performed and then the patient undergoes video EEG. As soon as seizure onset is detected radioactive tracer injected which concentrates at the focus (delays in tracer injection result in poorer distinction between onset zone and propagation zone). Subtraction Ictal SPECT Coregistered to MRI (SISCOM) allows better localization of the seizure focus and operative targeting. |
Magnetoencephalography | Records interictal magnetic brain waves and maps them in 3D onto MRI images of the patient’s brain. Aids localization of seizure focus in extratemporal nonlesional epilepsy and preoperative functional mapping. |
Visual field testing | Testing should be done where seizure foci are in temporal, parietal or occipital lobes; preoperative testing may reveal deficits unnoticed by patients, and new postoperative deficits. |
Data - what would you do?
- EEG left frontocentral spikes in sleep
- Ictal discharge - frontal, non lateralising
- SPECT increased perfusion in left frontal and left anterior temporal areas
- fMRI - Left language dominant but language areas distinct from lesion
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