NEUROIMAGING / COMPUTED TOMOGRAPHY

Computed tomography (CT) in stroke diagnosis

Created 21/03/2021, last revision 10/01/2023

  • computed tomography (CT) is essential for managing patients with acute neurological deficits
  • in most cases, CT is the first imaging method done, primarily to exclude bleeding

CT stroke protocol

In each patient with an acute neurological deficit, we need to know:

  • whether the patient has an acute ischemic stroke (AIS), intracranial bleeding, or another disease (tumor, head trauma, etc.)
  • if there is any arterial or venous occlusion/stenosis
  • what is the extent of the occlusion?
  • what is the actual collateral circulation status in case of occlusion?
  • if recanalization therapy can save the affected area 

Complete CT protocol (including non-contrast CT, CT perfusion, and CT angiography) should help answer the above questions. CT findings in ischemic stroke will be discussed below. The ICH, SAH, and CVT findings are discussed in relevant chapters. 

  • advantages of CT over MRI
    • the examination speed; approx. 10 minutes, including CT angiography and CT perfusion 
    • CT can be performed on restless patients with severe deficits, ventilated patients, or patients with MRI contraindication (such as a pacemaker) 
  • results of NCCT are available immediately; CTA and CTP analysis and processing can be done after the patient leaves for the ICU
CT stroke protocol
NCCT
CT angiography (CTA)
  • maximum intensity projection (MIP) images and 3D reconstructions are suitable for the rapid detection of vascular pathology
  • look for stenosis and/or occlusion extra- and intracranially (if no extracranial CTA is performed, then at least evaluate the ICA content in the carotid canal)  Left ICA occlusion with missing contrast in the carotid canal (red arrow)
  • evaluate possible carotid stenosis (NASCET) 
  • look for dissection, vasculitis, etc.
  • exclude basilar artery occlusion in patients with brainstem symptoms or impaired consciousness 
  • adjust window parameters to facilitate the evaluation of calcified arteries and impaired perfusion (“CTA perfusion” )
  • assess collateral circulation (including the circle of Willis – check hypo-/aplasia of some sections? presence of functional communicants?) 
  •  in ICH, look for vascular malformation and spot sign
  • in SAH, rule out aneurysm as a source of bleeding
CT perfusion
(CTP)
  • keep in mind the limited area of investigation, eliminate motion artifacts 
  • start with the MTT analysis, where the changes are most pronounced. Then continue with the analysis of CBF and CBV maps.
  • use RAPID software if available (best and fastest option) 

Noncontrast CT (NCCT)

  • the main task of NCCT is to rule out bleeding (for the detection of which the CT is highly sensitive) and other causes of neurological deficits (tumor, trauma) 
  • the occurrence and extent of ischemic changes depend on the duration of ischemia, arterial occlusion parameters (localization, thrombus extent), and the state of collateral circulation 
  • often, changes in the parenchyma appear as early as in the first 6 hours, showing ongoing ischemia of the tissue (early CT signs of ischemia
  • hypodensity develops because of cytotoxic edema (an increase of water content by 1% results in a density decrease by 2.5 HU)
  • the earlier these signs develop, the more severe ischemia can be expected
  • note the Prévost (Vulpian) sign –  conjugate ocular deviation (direction depends on stroke localization) Prévost (Vulpian) sign on NCCT - eye deviation toward infarction (left image). A hyperdense MCA on the left side (right image)
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Early CT signs of brain ischemia

  • decreased parenchymal x-ray attenuation (cytotoxic edema)
    • obscuration of the lentiform nucleus
    • loss of distinction between the white and the grey matter in the cortex
    • insular ribbon sign – loss of definition of the gray-white interface in the lateral margin of the insular cortex (insular cortex has the least potential for collateral supply)
    • focal hypodensity
    • visible hypoattenuation (not profound hypodensity) does not always represent core; concurrent CTP can show penumbra in such regions
      • suggested attenuation ratio (swelling/normal tissue) for identifying penumbra is >0.87   (Alzahrani, 2023)
  • tissue swelling (mass effect)
    • loss of sulcal effacement
  • dense artery sign
    • direct proof of thrombosis
Early CT signs of ischemia on NCCT
Early CT signs of ischemia on NCCT
Early CT signs of ischemia in the left MCA territory

  • sensitivity for detection of early signs is about 70% (range 20-87%) and specificity 87% (range 56-100%) – the experience of the evaluating physician is crucial
  • compare the affected area with the contralateral hemisphere
  • adjust window parameters (window/level) for better detection of early ischemic changes  Early CT signs of ischemia before and after adjustment of window parameters (level/window)
  • standardized scales were introduced for MCA territory (ASPECTS) and posterior circulation (PC-ASPECTS)
  • the assessment of early CT signs in the posterior circulation is less reliable ⇒ MR DWI is preferred

ASPECT score (Alberta Stroke Program Early CT Score)

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Dense artery sign (DAS)

  • increased attenuation at the site of artery occlusion (caused by thrombus)
  • DAS has high specificity, but low sensitivity 
  • the site and approximate extent of occlusion can be assessed (longer thrombus ⇒ worse response to thrombolytic therapy ⇒ worse prognosis) 
  • to avoid false positivity, measure the density on both sides
    • thrombus density > 43 HU    Dense artery sign in right MCA occlusion ( (79 HU) Dense artery sign in right MCA occlusion  Dot sign
    • ipsi- and contralateral MCA density ratio > 1.2  [Koo, 2000]
  • false-positive findings: 
    • polycythemia
    • calcifications
  • compare the arteries on both sides; changing the window parameters can be helpful with severe calcifications  CT angio with window parameters changed, showing stenosis bellow 50%
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Computed tomography (CT) in stroke diagnosis
link: https://www.stroke-manual.com/computed-tomography-in-stroke-diagnosis/