NEUROIMAGING / COMPUTED TOMOGRAPHY

Computed tomography (CT) in stroke diagnosis

Created 24.11.2019, last update 04.04.2022

  • computed tomography (CT) is essential for managing patients with an acute ischemic stroke (AIS)
  • 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 

CT is the primary imaging method for patients with acute neurological deficits. A complete CT protocol (including non-contrast CT, CT perfusion, and CT angiography) should help answer the above questions. CT findings in AIS will be discussed below. The ICH, SAH, and CVT findings are discussed in relevant chapters. 

  • advantages of CT over MRI
    • the examination speed – 10 minutes, including CT angiography and CT perfusion 
    • CT can be performed on restless patients with severe deficits, ventilated patients, or patients with any 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
  • rule out another etiology of neurological deficit (tumor, trauma) 
  • in AIS, look for early CT signs of ischemia (see below), always choose suitable window parameters (width/level)
  •  look for a dense artery sign (DAS), including a “dot sign” in the peripheral arteries
  •  evaluate the extent of thrombus (CBS)
  • CT evaluation in SAH
  • CT evaluation in cerebral venous thrombosis
  • CT evaluation in ICH
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 (↑ water content by 1% results in ↓ 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)
0-? hours (individual)
  • normal
  • subtle early CT signs of ischemia (in < 3 hours visible in 30-60% of patients)
  • dense artery sign (DAS) – indicating thrombus presence
?-12 hours (individual) Hyperacute ischemia on NCCT
  • early signs of ischemia (see below) with cytotoxic edema
  • early signs of ischemia are detectable in 32–82% of patients during the first 6–12 hours after the stroke onset 
12-24 hours Acute ischemia (12-24 hours)
  • marked hypodensity
  • progression of cytotoxic edema
3.-7. day Subacute ischemia (day 3-7)
  • since day 5, the progression of vasogenic edema
1-3 weeks Subacute ischemia (1-3 weeks)
  • gradual regression of edema and HEB disorder
  • transient disappearance of ischemia on NCCT (“fogging effect”)  Fogging effect on NCCT
  • infarct lesion exhibits postcontrast enhancement (luxury perfusion)
> 1 month Chronic ischemia (months)
  • atrophy, retraction, pseudocyst formation
  • loss of enhancement

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
  • tissue swelling (mass effect)
    • loss of sulcal effacement
  • dense artery sign
    • 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 and level) for better detection of early 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
  • during the evaluation, compare the arteries on both sides; changing the window parameters can be helpful in the case of severe calcifications  CT angio with window parameters changed, showing stenosis bellow 50%
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