NEUROIMAGING / COMPUTED TOMOGRAPHY

Computed tomography (CT) in stroke diagnosis

Created 21/03/2021, last revision 08/05/2023

computed tomography (CT) is essential for managing patients with acute neurological deficits
in most cases, CT is the first diagnostic test performed to exclude bleeding in the first place

CT stroke protocol

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

is it an acute ischemic stroke (AIS), intracranial bleeding, or another obvious medical condition (tumor, head trauma, etc.)?
is there any arterial or venous occlusion/stenosis?
what is the extent of the occlusion?
what is the actual status of the collateral circulation in case of occlusion?
can recanalization therapy 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 contraindications (e.g., pacemaker)
NCCT results are available immediately; CTA and CTP analysis and processing can be performed after the patient is admitted to 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 appropriate 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 useful for the rapid detection of vascular pathology look for stenosis and/or occlusion extra- and intracranially (if extracranial CTA is not available, at least evaluate the ICA within the carotid canal) 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 for hypo-/aplasia of some sections and presence of functional communicants? look for vascular malformation and spot sign in ICH rule out aneurysm as a source of bleeding in SAH
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 proceed 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, parenchymal changes occur within the first 6 hours, indicating ongoing tissue ischemia (early CT signs of ischemia)
hypodensity develops as a consequence of cytotoxic edema (a 1% increase in water content results in a 2.5HU decrease in density)
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)

0-? hours (individual)		normal subtle early CT signs of ischemia (visible in <3h in 30-60% of patients) dense artery sign (DAS) – indicative of a thrombus
?-12 hours (individual)		early signs of ischemia (see below) with evolving hypodensity due to cytotoxic edema early signs of ischemia are detectable in 32–82% of patients during the first 6–12 h after the stroke symptoms onset
12-24 hours		marked hypodensity progression of cytotoxic edema
3.-7. day		since day 5, vasogenic edema occurs
1-3 weeks		gradual regression of edema and HEB dysfunction transient disappearance of ischemia on NCCT (“fogging effect”) infarct lesion exhibits postcontrast enhancement (luxury perfusion)
> 1 month		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 white and 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 the core; concurrent CTP may show penumbra in such regions
  - suggested attenuation ratio (swelling/normal tissue) to identify penumbra is >0.87 (Alzahrani, 2023)
tissue swelling (mass effect)
- loss of sulcal effacement
dense artery sign (DAS)
- direct evidence of thrombosis

Early CT signs of ischemia in the left MCA territory

sensitivity for detecting early signs is approx. 70% (range 20-87%), and specificity is 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
standardized scales were introduced for MCA territory (ASPECTS) and posterior circulation (PC-ASPECTS)
assessment of early CT signs in the posterior circulation is less reliable ⇒ MR DWI is preferred

ASPECT score (Alberta Stroke Program Early CT Score)

the Alberta Stroke Program Early CT Score (ASPECTS) is used to standardize and increase the reliability of early signs of ischemia detection
ASPECTS can be assessed on:
- non-contrast CT (NCCT) – width/level adjustment can be helpful
- CT perfusion (CTP) [Aviv, 2007]
- CTA source images (CTA-SI) [Puetz, 2009]
ASPECTS primarily evaluates the MCA territory
PC-ASPECTS was designed to evaluate changes in the posterior circulation
there are commercial programs available for the automatic evaluation of ASPECTS (e.g., BRAINOMIX)

MCA territory

a 10-point quantitative CT scan score used for patients with stroke in MCA territory
10 points are normal; 1 point is subtracted from the initial score of 10 for each region with early signs of ischemia
- C – caudate nucleus
- L – lentiform nucleus
- IC – internal capsule (any portion)
- I – the insular cortex
- M1-3 are at the level of the basal ganglia
  - M1 – anterior MCA cortex corresponding to the frontal operculum
  - M2 – MCA cortex lateral to insular ribbon corresponding to the anterior temporal lobe
  - M3 – posterior MCA cortex corresponding to the posterior temporal lobe
- M4-5 are above the basal ganglia at the level of the ventricles
  - M4 – anterior MCA territory immediately superior to M1
  - M5 – lateral MCA territory immediately superior to M2
  - M6 – posterior MCA territory immediately superior to M3
ASPECTS is a valuable technique for the evaluation of the prognosis in acute ischemic stroke (thresholds are slightly different for NCCT a CTP)
- patients with high ASPECTS values are more likely to have favorable outcomes
- an NCCT ASPECTS score of ≤ 7 predicts a worse functional outcome at three months [Esmael, 2021]
- patients with CTP ASPECTS score of < 8 treated with thrombolysis mainly did not achieve a good clinical outcome [Aviv, 2007]

Ischemic changes in regions I, M2 and M5

Posterior circulation

PC-ASPECTS (The posterior circulation Acute Stroke Prognosis Early CT score)

normal brain scores 10 points; points are subtracted for each affected region
- thalami (1 point each)
- occipital lobes (1 point each)
- midbrain (2 points – uni- and bilateral)
- pons (2 points – uni- and bilateral)
- cerebellar hemispheres (1 point each)
pc-ASPECTS < 8 points ⇒ poor prognosis [Puetz, 2009]

pc-ASPECT score predicts prognosis [Puetz, 2009]

Dense artery sign (DAS)

increased attenuation at the site of arterial occlusion (caused by thrombus)
DAS has high specificity but low sensitivity
the localization and approximate extent of the occlusion can be assessed (longer thrombus ⇒ worse response to thrombolytic therapy ⇒ worse prognosis)
to avoid false-positive results, measure the density on both sides
- thrombus density > 43 HU
- 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 may be helpful in severe calcifications

try to confirm occlusion on CT angiography
look for DAS in the peripheral sections (“dot sign“)
follow the whole course of the ACA, including the A2 segment
exclude basilar artery occlusion (BAO) in a patient with an altered level of consciousness

Some authors are engaged in the quantitative and qualitative evaluation of thrombi on the NCCT. Thrombus size predicts the likelihood of recanalization. In a cohort of 138 thrombolyzed patients with ACM occlusion, none with a thrombus > 8 mm recanalized. Qualitative evaluation of the thrombus with measurement of its density seems helpful in estimating the prognosis and determining stroke etiology. In Hounsfield units (HU), we measure the artery’s density on the affected and healthy side and determine their mutual ratio (rHU). White thrombi, associated with large artery involvement, are composed of platelets, atheromatous masses, and relatively fewer erythrocytes. Therefore, they have a lower density than cardioembolic thrombi (in which erythrocytes and fibrin predominate). A lower rHU value predicts a poor response to thrombolytic therapy, as white thrombi are more resistant to fibrinolysis.