NEUROIMAGING / COMPUTED TOMOGRAPHY

Computed tomography (CT) in stroke diagnosis

David Goldemund M.D.
Updated on 02/01/2024, published on 21/03/2021

computed tomography (CT) is essential in the management of patients with acute neurological deficits
because of its rapid acquisition and immediate availability of results, it is usually the primary imaging modality used to rule out bleeding and stroke mimics
magnetic resonance imaging (MRI) may be used in some cases to provide additional data on tissue viability and to assess the underlying cause of the stroke

CT stroke protocol

The following questions should be answered in every patient presenting with an acute neurologic deficit:

is there an acute ischemic stroke (AIS), intracranial bleeding, or another obvious medical condition (tumor, contusion, etc.)?
is there any arterial or venous occlusion/stenosis present?
what is the degree of stenosis or extent of the occlusion?
what is the current status of the collateral circulation in the presence of an occlusion?
can recanalization therapy save the affected area?
a complete CT protocol includes non-contrast CT (NCCT), CT perfusion (CTP), and CT angiography (CTA) and should help answer the above questions to assist in planning recanalization procedures
- NCCT helps identify early signs of ischemia
- CTP provides information about cerebral perfusion, identifying regions of reduced blood flow or ischemia
- CTA visualizes the cerebral vasculature and detects vascular abnormalities, such as arterial occlusions, stenosis, or aneurysms
advantages of CT over MRI
- CT is faster than MRI; approx. duration of 10 minutes, including CT angiography and CT perfusion
- CT can be performed on agitated patients with severe neurological deficits, ventilated patients, or patients with MRI contraindications (such as those with pacemakers or other metallic implants)
- NCCT results are immediately available; CTA and CTP analysis and processing can be performed after the patient is admitted to the Intensive Care Unit (ICU)
CT findings specific to ischemic stroke will be discussed below; findings related to intracranial hemorrhage (ICH), subarachnoid hemorrhage (SAH), and cerebral venous thrombosis (CVT) are discussed in relevant chapters

CT stroke protocol
NCCT	rule out other causes of neurological deficit (e.g., tumor, trauma) 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 of SAH CT evaluation of cerebral venous thrombosis CT evaluation of ICH
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 (BAO) 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 communicating arteries? look for vascular malformation and the Spot sign in ICH rule out aneurysm as a source of bleeding in SAH
CTP	keep in mind the limited area of study, eliminate motion artifacts start with the Mean Transit Time (MTT) analysis, as changes in MTT are often the most pronounced and can help identify regions of delayed blood flow then proceed to the analysis of CBF and CBV maps – these parameters provide additional information about tissue perfusion and viability. use RAPID software if available (widely used tool for automated CT perfusion analysis)

Noncontrast CT (NCCT)

the primary function of NCCT is to rule out bleeding (for which CT is highly sensitive) and to identify other causes of neurological deficits (such as tumors and trauma)
the occurrence and extent of ischemic changes depend on the duration of ischemia, arterial occlusion parameters (location, extent of the thrombosis), and the status of the collateral circulation
typically, 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 are observed, the more severe ischemia can be expected
note the Prévost (Vulpian) sign – conjugate ocular deviation (direction depends on whether cortex or brainstem is involved)

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 hours following the onset of stroke symptoms
12-24 hours		marked hypodensity progression of cytotoxic edema
day 3-7		vasogenic edema occurs
1-3 weeks		gradual regression of edema and HEB dysfunction transient disappearance of ischemia on NCCT (“fogging effect”) infarct lesion shows 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 reveal 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) to better detect 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 detecting early signs of ischemia
- early ischemic changes are defined as incipient parenchymal hypodensity or loss of gray and white matter differentiation
ASPECTS can be assessed using:
- non-contrast CT (NCCT) – adjust width/level
- 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
commercial software programs are available for automated ASPECTS evaluation (e.g., BRAINOMIX)

MCA territory

a 10-point quantitative CT scan score used for evaluating patients with stroke in the MCA territory
a score of 10 points indicates a normal finding; 1 point is subtracted from the initial score of 10 for each region exhibiting early signs of ischemia
- C – caudate nucleus
- L – lentiform nucleus
- IC – internal capsule (any portion)
- I – insular cortex
- C, L, IC, I, and M1-3 are assessed on axial scans at the basal ganglia level
  - M1 – anterior MCA cortex, corresponding to the frontal operculum
  - M2 – MCA cortex lateral to the 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 lateral ventricles (supraganglionic level)
  - 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 prognostic evaluation in acute ischemic stroke (thresholds may differ slightly between NCCT and CTP)
- patients with high ASPECTS values are more likely to have a favorable outcome
- an NCCT ASPECTS score of ≤ 7 predicts worse functional outcome at 3 months [Esmael, 2021]
- patients with CTP ASPECTS score of < 8 treated with thrombolysis mainly did not achieve favorable clinical outcomes [Aviv, 2007]
- the threshold for thrombectomy is gradually decreasing (probably ASPECTS 3 based on SELECT2, ANGEL ASPECT trials results) ⇒ ASPECT score will lose some of its importance because most of the patients will be treated anyway

Ischemic changes in I, M2 and M5 regions

ASPECTS 8 - hypodensities in C and L regions

Posterior circulation

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

helps to assess early ischemic changes on non-contrast (NCCT) and optionally on CTA source images (CTA-SI)
normal brain scores 10; 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 is associated with poor prognosis [Puetz, 2009]
assessment can be inaccurate in the following situations:
- recent ischemia superimposed on an older lesion
- extensive leukoencephalopathy
- poor image quality

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 location and approximate extent of the occlusion can be assessed (longer thrombus ⇒ worse response to thrombolytic therapy ⇒ worse prognosis)
to avoid false-positive results, density should be measured 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 if severe calcifications are present

Dense artery sign in a patient with ICA-MCA occlusion

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

Some authors focus on the quantitative and qualitative assessment of thrombus on the NCCT. Thrombus size is predictive of the likelihood of recanalization. In a cohort of 138 thrombolyzed patients with MCA occlusion, none with a thrombus > 8 mm recanalized. Qualitative evaluation of the thrombus with measurement of its density seems to help estimate prognosis and determine stroke etiology. Arterial density is measured on the affected and healthy sides, and their ratio (rHU) is calculated. 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 because white thrombi are more resistant to fibrinolysis