stroke imaging basics - intermountainphysician...goals •introduce the basic physical properties of...
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Stroke Imaging Basics
Jeremy Hopkin M.D.
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Goals
• Introduce the basic physical properties of imaging used in stroke.
• Understand why each modality is used in the setting of stroke.
• Understand some strengths and weaknesses of each modality.
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Modalities Commonly Used For Imaging Stroke
• Non contrast CT
• CT Angiography (CTA)
• CT Perfusion (CTP)
• MRI
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X –ray basics
• Energy beam (photons) from a source is passed through the body exposing film/detectors behind the body.
• Different tissues in the body absorb the energy at different rates.
• This creates contrast between tissues and a resulting image.
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CT scan
• Used the same basic principles as x-ray
• Images are acquired around the entire surface of the body
• Complex mathematics generates a cross sectional image based on the different absorption of x rays in different tissues
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What is CT good for?
• Very good at detecting blood in the brain or around the brain.
• Very good for looking at bone.
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What is CT not good at
• OK but not great at looking at brain tissue -Not good at detecting any small or subtle abnormalities in the brain tissue
• OK but not great at looking at soft tissues
• Not good at looking at cranial nerves, arteries or veins
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Blood in the brain and surrounding the brain detected by CT. Blood is brightduring the acute phase and CT is very good at detection.
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As stroke becomes older and if it is large, it can be detected by non contrast CT.
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What do we use non contrast CT for?
• Initial imaging for stroke patients
– Exclude hemorrhage
– Evaluate for large/older stroke
– Evaluate for other gross pathology (large tumors, big ventricles etc.)
• Follow up of stroke
– Evaluate for hemorrhage into stroke
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DON’T use non contrast CT for this!
• EXCLUDE or R/O stroke
• Evaluate for non hemorrhagic stroke or stroke mimics.
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CT Angiography
• Same basic principles as CT
• Contrast is injected into a vein and tracked until it is in the arteries.
• CT scan is performed while contrast is in the arteries
• Arteries are dense with contrast and project well on the CT
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What is CTA good at
• Looking at arteries
– Occlusion
– Atherosclerosis
– Ulcerated plaque
• Looking at veins
– Clot in dural sinuses
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What is CTA not good at
• Essentially the same things as non contrast CT
• Slightly improved evaluation of stroke (but still relatively poor)
• Worse than non contrast head CT when looking for blood (dense contrast can hide dense blood)
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What do we use CTA for in the setting of stroke?
• Evaluate for occlusion/narrowing of a large cranial or neck artery by clot or dissection(ICA, MCA and sometimes posterior circulation)
• Limited evaluation of collateral circulation –helps in possible treatment planning.
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DON’T use CTA for this!
• EXCLUDE or R/O stroke. It cannot perform this task.
• Evaluate for stroke mimics
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CT Perfusion (CTP)
• Similar properties as CT and CTA.
• Multiple scans of the brain are obtained during the passage of contrast from arteries through tissue of the brain and into veins
• Mathematic models are used to generate “maps” of the brain showing different aspects of how the contrast moves from arteries to veins.
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What is CTP good at?
• Evaluating flow patterns with a large vessel occlusion.
• Providing information about collateral pathways of blood flow
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What do we use CTP for?
• Help endovascular doctor decide whether or not to go into the brain arteries with a catheter and pull out a clot that is occluding an artery.
• Help decide whether or not to give TPA (very rare)
• Evaluate for large stroke in a patient who can’t have an MRI (rare)
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DON’T use CTP for this!
• EXCLUDE or R/O stroke. It can not do this with a high degree of accuracy. A normal CTP does NOT exclude stroke.
• Evaluate for stroke mimics
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Why is CTP not ideal for detecting stroke?
• Complex physiology contributes to stroke– Flow– Oxygen extraction fraction– Oxygen requirements of the tissue– Temperature– Blood pressure
• CTP measures only one of these factors– Flow
• There is a correlation between perfusion abnormalities and stroke, but it is not consistent or have a high accuracy
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MRI
• Although the circular apparatus looks similar to CT, MRI uses completely different physical properties of image acquisition
• Does not use ionizing radiation
• Utilizes magnetic fields and radiofrequency waves to gather information about tissues in the body.
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All protons in the body function as tiny magnets. They align with the magnetic field of the scanner and also spin around this axis like a spinning top.
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-Radiofrequency pulse sent into the patient “harmonizes” with spinning protons and flips them into ahigher energy state.- As the proton “relaxes” to its original state it releases its own radiofrequency waves that are read by a receiver.
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What is MRI good at?
• The best imaging evaluation of brain tissue. Improved visualization of cranial nerves, contrast between grey and white matter and evaluation of wide range of brain pathology.
• Detecting stroke – Gold standard.
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What do we use MRI for?
• EXCLUDE or R/O stroke. If MRI is normal, there is no stroke.
• Evaluate stroke volume with high degree of accuracy
• Provide some information about timing of stroke
• Evaluate for stroke mimics.
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Problems with MRI
• Can not be performed in patients with certain medical devices – most commonly pacemakers
• More time consuming than CT
• Loud
• Issues with claustrophobia
• Does not evaluate bone very well
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DON’T use MRI for this!
• Emergent imaging to exclude blood in the brain. This should be done with NON CONTRAST CT
• Emergent imaging to evaluate large vessels of neck/brain during work up of acute stroke. This should be done with CT ANGIOGRAPHY.