Neuroimaging in Seizure

IMAGING MODALITIES USED IN SEIZURES

  1. CT Scan
  2. MRI (and advanced MRI techniques s.a. DTI, magnetization transfer imaging, voxel-based analysis, T2 mapping) and functional MRI (fMRI)
  3. PET Scan
  4. SPECT Studies
  5. 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:

  1. Mesial temporal sclerosis
  2. Cortical dysplasia
  3. Brain tumors
  4. Vascular malformations
  5. Cerebral infarction / hemorrhage
  6. Traumatic brain injury
  7. Infections (encephalitis, cerebral access, granulomas, cysts)

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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

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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

  1. cortical thickening
  2. blurring of gray-white margin
  3. increased signal on FLAIR
  4. subtle tapering bands of gray matter extending from the cortex towards the ventricles

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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

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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:

  1. punctate  foci of T2 signal change in the white matter
  2. many cystic lesions such as arachnoid cysts, choroidal fissure cysts
  3. lacunar strokes
  4. ventricular asymmetry
  5. diffuse atrophy
  6. isolated venous anomalies

MRI changes after seizures

  1. local swelling
  2. increased T2 signal intensity
  3. restricted diffusion
  4. 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>

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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

epilepsy_pet_large

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

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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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