IMAGING MODALITIES USED IN SEIZURES
- CT Scan
- MRI (and advanced MRI techniques s.a. DTI, magnetization transfer imaging, voxel-based analysis, T2 mapping) and functional MRI (fMRI)
- PET Scan
- SPECT Studies
- magneto encephalography / magnetic source imaging
CT Scan
– to exclude acute neuro problems that require urgent intervention
– hemorrhages, gross structural malformations, large tumors, calcified lesions
7 Structural Causes to look for in MRI:
- Mesial temporal sclerosis
- Cortical dysplasia
- Brain tumors
- Vascular malformations
- Cerebral infarction / hemorrhage
- Traumatic brain injury
- Infections (encephalitis, cerebral access, granulomas, cysts)
MESIAL TEMPORAL SCLEROSIS
– AKA hippocampal sclerosis
– most commonly diagnosed structural abnormality in epilepsy
– presents in childhood
– surgery is curative
– MRI characteristics: hippocampal atrophy , increased t2 and flair signal intensity
– look for MRI changes in coronal T2W images and coronal FLAIR
Figure. Subtle gliosis of left hippocampus (blue arrow) and atrophy (yellow arrow).
CORTICAL DYSPLASIA
– second most common structural etiology for epilepsy
– lesions congenital, usually presents in childhood
MRI findings suggestive of cortical dysplasia
- cortical thickening
- blurring of gray-white margin
- increased signal on FLAIR
- subtle tapering bands of gray matter extending from the cortex towards the ventricles
Brain tumors and cerebrovascular disease
– more common in elderly
NEUROCYSTICERCOSIS
– caused by Taenia solium
– common etiology in endemic populations (Mexico, Latin America, Russia, India, Pakistan, Southeast Asia, China, West Africa)
– MRI with contrast, but CT is more sensitive for detecting small areas of calcification
Note: most individuals with new onset epilepsy will not have a structural lesion on MRI, yield is 14%.
EPILEPSY PROTOCOL:
– Standard T1-weighted images
– T2 weighted fast spin echo sequences
– Gradient echo sequences
– FLAIR sequences
– 3d volume acquisition sequences with high def of grey-white junction including magnetization prepared rapid acquisition gradient-echo (or MP RAGE), 3-D fast spoiled gradient recalled echo acquisition at steady state (or 3-D fast-spoiled GRASS or 3-D SPGR)
Note: MRI evidence of hippocampal atrophy is a strong predictor of excellent postoperative seizure control after anterior temporal lobectomy.
Advanced MRI techniques
– high field strength MRI: 3 Tesla
– use of multichannel phase array surface coils
These techniques allow for a higher signal to noise ratio, improved imaging uniformity, and better spatial resolution.
DIFFUSION TENSOR IMAGING (DTI)
– reveals white matter tracts
– delineate epileptogenic substrate and surrounding tissue
SUSCEPTIBILITY-WEIGHTED IMAGING (SWI)
– exploits magnetic properties of blood or hemosiderin
– more sensitive in detecting cavernous malformations
– identifies epileptogenic, post-infectious, calcified lesions eg Cryptococcus, tuberculosis, cysticercosis
MRI findings that are not known to be epileptogenic:
- punctate foci of T2 signal change in the white matter
- many cystic lesions such as arachnoid cysts, choroidal fissure cysts
- lacunar strokes
- ventricular asymmetry
- diffuse atrophy
- isolated venous anomalies
MRI changes after seizures
- local swelling
- increased T2 signal intensity
- restricted diffusion
- focal and/or leptomeningeal contrast enhancement
FUNCTIONAL MRI
- detect focal changes in blood flow and oxygenation levels that occurs when an area of the brain is activated
- change in neuronal activity accompanied by change in ratio of oxy to deoxyhemoglobin in blood
measured as the blood-oxygen-level-dependent (BOLD) effect - used to noninvasively map motor, sensory and language functions; surgical planning
- may eventually replace carotid amobarbital (Wada) test for language lateralization
- *Powerpoint show: fMRI simplified <linkout>
PET SCAN
- 2[18f] fluoro-2-deoxy-d-glucose positron emission tomography or FDG-PET
- images topographic distribution of glucose uptake in brain
- provides a picture of brain metabolism
- performed in interictal state
- goal is to detect focal areas of decreased metabolism (functional disturbances of cerebral activity associated with epileptogenic tissue)
- sensitivity increased when seizures are more frequent or performed soon after seizure has occurred
PET scan. The arrow points to where the seizures are coming from.
SPECT STUDY
- single photon emission computed tomography study
- radiolabeled tracer (99mTc-hexamethylpropyleneamineoxime or 99mTc-HMPAO) injected which binds on first-pass through brain
- provides snapshot of cerebral circulation
- ictal SPECT – shows hyperperfusion at seizure focus with surrounding hypoperfusion
- post-ictal and interictal SPECT – shows regional hypoperfusion
- SISCOM (subraction ictal SPECT scan coregistered with MRI) improves localization
- limitations: injection timing is critical
Ictal SPECT perfusion exam demonstrates a hyperperfused (metabolic) area in the right temporo-parietal region which corresponds to a hypoperfused region on the inter-ictal exam.
MEG/MSI
- magnetoencephalography (MEG) and magnetic source imaging (MSI)
- MEG – recording of magnetic fields generated by intraneuronal electrical currnets
- MSI – combination of MEG source localization with coregistered anatomical imaging in which magnetic dipole representing an epileptiform discharge is placed on patient’s MRI scan
- approved for presurgical localization for epilepsy and for localization of neuronal function
References
AuntMinnie.com,. ‘SPECT Imaging In Seizure Disorders Discussion’. N.p., 2015. Web. 27 Sept. 2015.
Radiologyassistant.nl,. ‘The Radiology Assistant : Role Of MRI In Epilepsy’. N.p., 2012. Web. 26 Sept. 2015.
Seattlechildrens.org,. ‘Epilepsy Symptoms And Diagnosis | Seattle Children’S Hospital’. N.p., 2015. Web. 26 Sept. 2015.
Slideshare.net,. ‘Fmri Terms: HRF And BOLD’. N.p., 2015. Web. 26 Sept. 2015.
Uptodate.com,. ‘Neuroimaging In The Evaluation Of Seizures And Epilepsy’. N.p., 2015. Web. 26 Sept. 2015.
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