Clot Burden Score (CBS)

David Goldemund M.D.
Updated on 21/04/2024, published on 20/12/2022

the extent of intracranial thrombosis predicts clinical outcome, final infarct size, and risk of hemorrhagic transformation in acute ischemic stroke
- clots with proximal localization and increased length are more difficult to treat and have a worse outcome
Clot Burden Score (CBS) is a semiquantitative, CTA-based score that defines the extent of thrombosis in the anterior circulation
10 points are assigned for a normal CTA
- 2 points are subtracted for thrombus in the proximal M1, distal M1, or supraclinoid ICA portion
- 1 point is subtracted for thrombus in M2 branches, A1, and/or infraclinoid ICA portion
the recanalization rate is higher with intravenous tPA in patients with a CBS > 6 [Demchuk, 2009]
a lower CBS is associated with:
- lower ASPECTS [Puetz, 2008]
- higher rates of parenchymal hematoma [Puetz, 2008]
- decreased odds of favorable functional outcome
  - odds ratio 0.09 for CBS ≤ 5, 0.22 for CBS 6-7, and 0.48 for CBS 8-9 [Puetz, 2008]
  - the association between the CBS and functional outcome varies for different collateral scores (Derraz, 2021)

CBS can also be assessed on FLAIR, MRA, or MR-GRE (T2*CBS) [Derraz, 2019]
- susceptibility vessel sign (SVS)
  - diameter and length (thrombus length tends to be smaller in those with early reperfusion)
  - S-shaped or A-shaped
- clot intensity on FLAIR
  - high FLAIR clot intensity may predict successful reperfusion (Fujimoto, 2015)

infraclinoid portion of ICA	– 1
supraclinoid portion of ICA	– 2
proximal M1 segment	– 2
distal M1 segment	– 2
M2 branch	– 1
M2 branch	– 1
ACA	– 1

Clot Burden Score - CBS (Tan, AJNR 2009)

T-occlusion (ICA, MCA, ACA) - Clot Burden Score 5

Collateral circulation assessment

ADD-ONS / ANATOMY

Collateral circulation assessment

David Goldemund M.D.
Updated on 26/12/2023, published on 01/03/2022

[toc]

collateral cerebral circulation plays a critical role in maintaining blood flow to ischemic areas in the acute, subacute, or chronic phases after ischemic stroke
- good collateral circulation is associated with a favorable functional outcome and a lower risk of stroke recurrence
- in occlusions at the level of the circle of Willis and distally, leptomeningeal anastomoses (LMA) are mainly engaged
the efficiency of collateral cerebral circulation can be assessed by evaluating perfusion distal to the occlusion/stenosis
- in acute stroke (⇒ estimation of prognosis and usefulness of thrombectomy, especially when CTP is not available)
- in the management of chronic extracranial steno-occlusive diseases (⇒ may help decide whether to perform revascularization)
imaging methods used to assess cerebral collateral circulation:
- digital subtraction angiography (DSA)
  - gold standard; currently used as a part of endovascular procedure; evaluation by the ASITN/SIR collateral scale helps predict the risk and benefit of acute endovascular treatment
- CT angiography
  - traditional single-phase CTA (more reliable than MRA)
  - CTA source image
  - multiphase CTA (dynamic CTA)
- MR angiography
  - time-of-flight MRA (TOF-MRA)
  - phase-contrast MRA
  - quantitative MRA (QMRA)
- CT perfusion
- MR perfusion
- neurosonology
DSA is considered a gold standard; however, noninvasive imaging modalities are more commonly used
there is no general agreement on the optimal collateral grading system based on noninvasive imaging modalities; further research is needed

Imaging methods to assess the structure of the cerebral collateral circulation	Imaging methods to assess the function of the cerebral collateral circulation
DSA CTA (single phase) TOF-MRA neurosonology	detection of cerebrovascular reserve TCD/TCCD xenon CT single-photon emission CT (SPECT) positron emission tomography (PET) CT perfusion MR perfusion quantitative MRA (QMRA) neurosonology

Collateral circulation assessment on DSA

DSA allows the assessment of collateral circulation dynamics; disadvantages are invasiveness and the need to examine all 4 main arteries
it can be used during the endovascular procedure, e.g., to assess the collateral flow through the ACA in the case of M1 occlusion
it is also necessary to assess the venous phase

The American Society of Interventional and Therapeutic Neuroradiology/Society of Interventional Radiology (ASITN/SIR) grading system [Higashida,2003]

Grade	Angiographic collaterals
0	no collaterals visible in the ischemic area
1	slow collaterals to the periphery of the ischemic area with persisting defect
2	fast collaterals to the periphery of the ischemic area with some persisting defect
3	collaterals with slow but complete filling of the ischemic area in the late venous phase
4	complete and rapid blood flow to the entire ischemic territory by retrograde perfusion

Collateral grading score - left ACA occlusion with complete and rapid ACA filling via right ICA and AcoA (grade 4)

Pial Collateral Score [Christoforidis et al,2005]

another DSA-based collateral grading system, that is less frequently used in clinical practice
- grade 1 – collaterals reconstitute the entire distal portion of the occluded vessel segment
- grade 2 – collaterals reconstitute vessels in the proximal portion of the segment adjacent to the occluded vessel
- grade 3 – collaterals reconstitute vessels in the distal portion of the segment adjacent to the occluded vessel
- grade 4 – collaterals reconstitute vessels two segments distal to the occluded vessel
- grade 5 – little or no significant reconstitution of the territory of the occluded vessel
good collateral status (grades 1 and 2) has been correlated with smaller infarct volume, lower risk of hemorrhagic transformation, and lower modified Rankin Scale (mRS) at discharge

Collaterals from the ACA to the occluded MCA territory (Pial collateral score 1)

Collateral circulation assessment on CTA

in addition to the detection of occlusions, CTA also enables the analysis of collateral circulation; the presence of good collateral circulation correlates with smaller infarct size and predicts a better clinical outcome during reperfusion therapy
a simple Collateral Score (CS) may be used for evaluation
- a semi-quantitative rapid comparison of collateral filling in the territory of the occluded artery compared to the contralateral hemisphere
- a single-phase and multiphase CTA (mCTA) can be used
a limitation of conventional (single-phase) CTA is its static presentation; it is acquired during a short interval in the arterial phase, which can lead to an underestimation of delayed collateral circulation
dynamic information is provided by multiphase CTA (MP-CTA / mCTA)
- a total of 3-4 phases of intracranial CTA are performed using a reduced X-ray dose
- mCTA can differentiate between the absence of collaterals and delayed filling [Yang, 2008]
- mCTA can distinguish between minimal anterograde flow and retrograde collateral flow [Fröhlich, 2012]

Multiphase CTA - distinguishing missing collaterasl from slowed flow antero- or retrograde flow

The evaluation of CTA source images (CTA-SI) includes the following steps:

check the circle of Willis for the presence and quality of communicating arteries, hypo/aplasia, etc.
identify arterial occlusions and try to estimate their extent (thrombus length ⇒ Clot Burden Score (CBS)
compare the filling of the arterial branches in both hemispheres
evaluate the degree of the retrograde filling (optimally, the contrast agent should reach the distal end of the thrombus)

Collateral score in the anterior circulation (typically MCA)

Miteff collateral grading on single-phase CTA (Miteff, 2009)
good		major MCA branches are reconstituted distal to the occlusion
moderate		some MCA branches are shown in the Sylvian fissure
poor		only the distal superficial MCA branches are reconstituted
Collateral status is graded in maximum intensity projection reconstructions (MIP) of single-phase CTA in axial, coronal, and sagittal planes in patients with MCA occlusion

Collateral Score (CS) assessed on CT angiography source images

Collateral Score (CS) on single-phase CTA [Tan, 2009] Based on single-phase CTA in patients with unilateral anterior circulation infarct
Score		collaterals on CTA
0		absent collateral supply to the occluded MCA territory
1		collateral supply filling ≤50% but >0% of the occluded MCA territory
2		collateral supply filling >50% but <100% of the occluded MCA territory
3		100% collateral supply of the occluded MCA territory
Higher grades are associated with better CT perfusion parameters (MTT, CBF, and CBV), smaller final infarct volume, smaller thrombus extent, and improved outcome

Collateral Score (CS) on multiphase CTA [Menon, 2015]
Score		Collaterals on CTA
0		no vessels are visible in the affected hemisphere in any phase
1		only a few vessels are visible in the affected hemisphere in any phase
2		a filling delay of two phases in the affected hemisphere with a significantly reduced number of vessels in the ischemic territory, or one phase delay showing regions with no visible vessels
3		a filling delay of two phases in the affected hemisphere or a delay of one phase with a significantly reduced number of vessels in the ischemic territory
4		a filling delay of one phase in the affected hemisphere, but the extent and prominence of pial vessels are the same
5		no filling delay compared to the asymptomatic contralateral hemisphere, normal pial vessels in the affected hemisphere
A score of ≤ 3 indicates a poor prognosis

Case series of mCTA can be seen here

mCTA showing poor collaterals in a patient with left MCA occlusion

mCTA showing good collaterals in patient with left MCA occlusion

Basilar Artery on Computed Tomography Angiography (BATMAN) score

the BATMAN score is a 10-point CTA–based grading system that incorporates thrombus burden and the presence of collaterals
the posterior circulation is divided into 6 segments
- vertebral arteries (VA) – considered as 1 segment = 1 point
- posterior cerebral artery (PCA) – scored separately, 1 point each
- posterior communicant artery (PComA) – scored separately, 2 points each (or 3 points for fetal PCA)
- 3 segments of the basilar artery (BA) – 1 point each
patients with a lower BATMAN score were more likely to have a poor outcome – the absence of PComA (bilateral or unilateral) was the strongest predictor of poor clinical outcome (OR of 6.8) [Alemseged, 2017]

Posterior circulation CTA score

0 – no posterior communicating artery (PComA)
1 – unilateral PComA
2 – bilateral PComA
the presence of bilateral PComA on CTA was associated with more favorable outcomes in patients with BAO undergoing mechanical thrombectomy [Goyal, 2016]

Posterior Circulation Collateral Score (PC-CS)

max. 10 points (normal findings)
AICA, PICA, SCA – assign 1 point to each patent artery (assess bilaterally)
PComA – assign 1 point if PComA is smaller than the P1 segment, 2 points if larger
patients with higher scores have better prognosis [Goyal, 2016]

Time of flight MRA

another noninvasive method commonly used to assess the structure of cerebral collateral circulation
assessment of leptomeningeal collaterals is limited due to relatively low spatial resolution
TOF-MRA is typically used to assess primary collaterals via the circle of Willis
sensitivity and specificity can be increased by combining the TOF-MRA with TCD/TCCD

Neurosonology

TCD/TCCD is a noninvasive, low-cost method reflecting real-time cerebral blood flow velocity, collateral status, and cerebrovascular reactivity
accuracy is highly operator-dependent
TCD/TCCD can asses (directly or indirectly):
- collateral flow through AComA, PComA, ophthalmic artery, and leptomeningeal arteries
- the flow diversion phenomenon – high-velocity and low-resistance flow in the ACA or PCA in the presence of the MCA occlusion/severe stenosis (implies leptomeningeal collateral anastomoses between the ACA/PCA and the distal MCA branches

ASPECTS

Alberta Stroke Program Early CT Score

David Goldemund M.D.
Updated on 25/12/2023, published on 22/03/2021

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 grey and white matter differentiation
ASPECTS can be assessed on:
- noncontrast 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 for automated ASPECTS evaluation are available (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 vary slightly between NCCT and CTP)
- patients with high ASPECTS values are more likely to have favorable outcomes
- an NCCT ASPECTS score of ≤ 7 predicts worse functional outcome at three 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 noncontrast (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]
assessing 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]

CT perfusion (CTP)

David Goldemund M.D.
Updated on 09/10/2023, published on 21/03/2021

[toc]

brain infarction is the consequence of a localized decrease in cerebral blood flow (CBF)
CT perfusion (CTP) can be used to assess cerebral blood flow and differentiate the penumbra (salvageable brain tissue) from the core (area of damaged brain tissue)
CTP does not detect necrotic tissue; it demonstrates the decrease in blood flow often associated with necrosis
- however, necrosis may not necessarily be present in the core area – if the altered CBF corresponding to the core is detected shortly after stroke onset and rapid recanalization occurs, a portion of this zone may be salvageable
- conversely, in necrotic tissue, flow improvement may be achieved after late recanalization, and the core and penumbra may disappear or be reduced
- similarly, late recruitment of collaterals can improve flow in the periphery of the “core,” potentially reducing its size on CTP, even though it is a zone of complete ischemia
the CTP mismatch evaluated by the RAPID system became the basis of breakthrough studies with recanalization therapy beyond the standard time windows (> 4.5 h for intravenous thrombolysis, > 6h for mechanical thrombectomy)
- the DAWN study (2017) showed an effect in the 6-24h window in a strictly selected group (based on CTP findings)
- in January 2018, the DEFUSE study) presented positive results in the 6-16 hours window (Odds Ratio 2.77!!)
- EXTEND trial (2019) showed that the use of tissue plasminogen activator (tPA) between 4.5 – 9.0 hours after stroke onset in patients with hypoperfused but salvageable brain regions (detected by CTP) resulted in a higher percentage of patients with no or minor neurological deficit compared to placebo

→ Ischemic penumbra

Technical comments

after the intravenous administration of an iodinated contrast agent, a transient increase occurs in parenchymal density, which is proportional to the amount of contrast agent in the area
- administration rate: 5-6 mL/s; the green cannula is required (18G, 1.3 mm)
the software calculates CTP parameters from time-attenuation curves, which are based on the differences between arterial inflow and venous outflow
software processing of the measured brain tissue density during the passage of a contrast agent yields 4 parameters:
- Cerebral Blood Volume (CBV) – amount of blood in a given volume of tissue (mL per 100 mg of tissue)
- Cerebral Blood Flow (CBF) – blood flow (mL/100g of tissue/minute)
- Mean Transit Time (MTT) – average time of arteriovenous blood passage by a given volume of tissue (in seconds)
- Time To Peak (TTP) – average time to maximum density in the scanned area (in seconds)
the relationship between the above parameters is expressed by the equation: CBF = CBV / MTT; based on the measured and calculated values, colored perfusion maps are generated for each parameter, thereby facilitating the distinction and comparison of areas with varying blood flow

Evaluation of CTP maps

Mean Transit Time (MTT)

start with assessing the MTT (TTP) parameter, which is prolonged in ischemia (>145% of contralateral normal tissue) and serves as a marker of regional blood flow abnormalities
- normal MTT/TTP effectively rules out arterial occlusion
- however, MTT is not suitable for assessing viability; it only identifies areas of slower contrast-filling
the MTT area includes:
- benign oligemia (persistent occlusion usually doesn’t result in infarction in this area)
- infarct core (irreversibly damaged tissue)
- penumbra (tissue that can be rescued by prompt recanalization therapy)

Cerebral Blood Flow and Volume (CBF and CBV)

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Penumbra on CTP with normal or increased CBV

CTP mismatch and final infarction after succesfull recanalisation of dominant M2 occlusion

Left MCA occlusion with CBF deficit (B) and areas with low CBV (C) showing core. IV thrombolysis led to rapid recanalisation, so NCCT and MRI control show infarction in regions of presumed core.

Stroke in PICA territory due to AV occlusion

Clinical-core mismatch

a mismatch between age-adjusted NIHSS and CTP core (defined as stated above) (Chen, 2021)
- NIHSS ≥10 + core volume <31 mL (age <80)
- NIHSS ≥20 + core volume 31–51 mL (age <80)
- NIHSS ≥10 + core volume <21 mL (age ≥80)

Hypoperfusion intensity ratio (HIR)

Hypoperfusion Intensity Ratio (HIR) or hypoperfusion index : defined as Tmax > 10s / Tmax > 6s
an index ≥ 50% correlates with poor collaterals on angiography; such patients had an 83% probability of significant core growth [Guenego, 2018]
poorer (higher) HIR is associated with the occurrence of parenchymal hematoma (PH) after EVT (HIR might reflect tissue vulnerability to reperfusion hemorrhage) (Winkelmeier, 2022)
high HIR on baseline imaging may warrant repeat brain imaging upon transfer to a comprehensive stroke center to avoid futile endovascular procedure

Relatively favourable hypoperfusion index (40%)

Unfavourable hypoperfusion index (70%), with poor collaterals on multi-phase CTA (mCTA)

Computed tomography angiography (CTA)

David Goldemund M.D.
Updated on 10/04/2024, published on 21/03/2021

[toc]

CT angiography (CTA) is a fast (∼ 5 minutes), readily available, and non-invasive imaging modality used to assess both extra- and intracranial arteries
it is based on the spatial reconstruction of images from a series of axial scans taken after the administration of iodinated contrast agent
the examination usually starts at the level of the aortic arch (or better, the left atrial appendage) and extends up to the vertex
approx. 50-60 mL of the contrast agent is required
source images (SI) are essential for evaluation
reconstructed images (MIP – maximum intensity projection) may be helpful in certain circumstances
adjust the window width (WW) and window level (WL) parameters when assessing heavily calcified stenoses
- the ideal parameters may vary from scanner to scanner

Stenosis and occlusion assessment

systematically evaluate both source images (SI) and reconstructions (MIP)
adjust window parameters (width and level) if necessary, especially in the presence of heavy calcifications

I. Extracranial cerebral arteries and aorta

check the condition of the aorta (plaques, dissection, thrombi) and major supraaortic arteries (subclavian arteries, CCAs, vertebral arteries, brachiocephalic trunk)
- exclude dissection or thrombus
- assess the extent of atherosclerosis → aortic arch atherosclerosis
- look for signs of inflammation (such as diffuse wall thickening in Takayasu’s arteritis)
- evaluate the significance of potential anatomical variants, particularly if an endovascular procedure is planned
if visible, inspect the pulmonary arteries for possible pulmonary embolism (PE)
consider extending emergency CTA to the level of the heart to detect the left atrial appendage (LAA) thrombus or pulmonary embolism (Popkirov, 2019)
examine the entire extracranial portion of the CCA and ICA
- assessment of carotid stenosis is discussed below (NASCET x ECST)
- in younger patients, exclude carotid web
- rule out hypoplasia/aplasia or other anatomical variants
- if the entire ICA is not visible on CTA, consider the following scenarios:
  - thrombosis of the entire ICA (or even CCA) versus terminal ICA occlusion (thrombus reaches bellow OA ostium) with stagnation of contrast agent proximally (the situation is often clarified on DSA during embolectomy)
  - occlusion of the proximal ICA due to atherothrombosis combined with distal occlusion (typically uneven and calcified contour of the artery stump is found)
  - occlusion due to dissection (younger patient, string sign, absence of AS changes in other arteries)
- differentiation of acute vs. chronic occlusion
  - via ultrasound
  - carotid ring sign can be found in the acute total occlusion (intraluminal hypodense thrombus and/or the ring enhancement of the vasa vasorum in the arterial wall (Michel, 2011) (Yi, 2024)
  - multiphasic CTA may also help

II. Intracranial cerebral arteries

look for intracranial occlusion or stenosis and consider etiology (atherothrombosis, dissection, spasm, vasculitis)
- check all segments, including the terminal ICA and the ICA-MCA junction (atherosclerosis is typically located in the carotid siphon)
if occlusion is present, assess the following:
- location and extent of occlusion (thrombus length)
  - anterior circulation – Clot Burden Score (CBS)
  - posterior circulation – BATMAN score
- presence of residual flow through the thrombus (indicating increased surface area for fibrinolysis and better prognosis)
- nature of the underlying process (atherosclerosis with/without thrombus, dissection or other vasculopathies, and inflammation)
- condition of collateral circulation
  - good collaterals are associated with smaller infarct volumes and better clinical outcome
carefully examine the peripheral sections of the intracranial arteries (finding a “dot sign” on the NCCT may be helpful)

sensitivity and specificity of CTA, compared with digital subtraction angiography (DSA) and MR angiography (MRA), for detecting stenoses and occlusions of the main cerebral arteries are 89-99%
- MRA slightly overestimates the severity of stenosis compared to CTA
- CTA is useful for detecting pathology in the posterior circulation, including basilar artery occlusion (BAO)
neurosonology is a reliable non-invasive tool for long-term monitoring of intracranial vasculopathies

specialized software, such as eCTA or Rapid CTA, enables the automated evaluation of the presence of large vessel occlusion (LVO)
- quickly identifies suspected occlusions by automatically processing CT scans and delivering easy-to-interpret CTA images
- helps to speed up triage or/and transfer decisions

M1 segment occlusion on CTA and NCCT (with a dense artery sign)

Internal carotid artery dissection on CTA

Carotid stenosis evaluation

Stenosis of the left carotid artery on CTA

Left interal carotid artery (ICA) stenosis on CTA

Evaluation of atherosclerotic plaque characteristics

CTA is a useful tool in the diagnosis of extracranial stenosis
it not only shows the degree of stenosis but also provides information about carotid plaque characteristics, which can also be assessed by ultrasound or MRI → see Classification of atherosclerotic plaques)

each plaque can be characterized by:
- size (length and width)
- shape (circular, semicircular, eccentric)
- surface (smooth, rough, exulcerated)
- density (hypodense, isodense, hyperdense)
- homogeneity (homogeneous x heterogeneous)
- presence of calcifications and intraluminal thrombi (ILT)
the presence of ILT or ulcers increases the likelihood of symptomatic stenosis
smooth or heavily calcified plaques are associated with a relatively low risk of cardiovascular events [Eesa, 2010]
in the case of extensive calcifications, CTA (with an adjusted window) outperforms ultrasound, which must rely on Doppler examination (B-mode and color mode are usually inconclusive due to acoustic shadows)

Smooth, irregular and exulcerated plaque on CTA

Hypodense, isodense and hyperdense plaque on CTA

→ Carotid stenosis evaluation on CTA

Stenosis diameter measurement (NASCET and ECST)

accurate quantification of stenosis is crucial for selecting an appropriate therapeutic strategy
the predominant measurement techniques are based on the NASCET and ECST trials (originally using DSA as a diagnostic method)
in both trials, the lumen diameter was measured at the site of maximal stenosis (inner-to-inner lumen); the denominators in the equations differed:
- ECST – estimated normal lumen diameter at the site of the lesion (outer-to-outer)
- NASCET – normal distal lumen diameter; with the distal lumen collapsed, stenosis was classified as 95%
the NASCET method is preferred (it has better agreement with ultrasound findings)

stenosis according to NASCET (%)	stenosis according to ECST (%)
30	50
40	70
50	75
60	80
70	85
80	91
90	97

a typical example where both methods produce completely different stenosis values is circular stenosis (e.g., r = 3 mm, R = 6mm)
NASCET = no lumen reduction at the site of the stenosis compared to the distal ICA ⇒ no calculated stenosis is present
ECST = an approx. 50% lumen reduction is calculated
area reduction measurement = stenosis quantified as 75%

For CT angiography to be consistent with NASCET measurement, proceed as follows:

use source images (CTA-SI)
adjust window level and width, especially in the presence of significant calcifications (adjustment mitigates blooming artifact from heavily calcified plaques, enabling more accurate stenosis quantification)
select a scan with maximal stenosis and measure the diameter of the residual lumen
measure the diameter of the first normal distal arterial segment located above the carotid bulb
stenosis, according to NASCET, is calculated from these two variables
- if the residual lumen in the stenosis is 2 mm and the diameter of the artery distal to the bulb is 8 mm, the stenosis is calculated as (1- (2/8)) x 100 = 75%

ICA stenosis 58% according to NASCET (1-2.44.01/5.77 x 100)

ICA stenosis 71% according to NASCET (1-1.45/5.03 x 100)

Direct millimeter measures

direct CTA millimeter stenosis values provide an excellent method to classify moderate and severe stenosis (Bartlett, 2008)
- measurements should be obtained from axial images
- window settings (width/level) should be adjusted forf dense calcifications to decrease beam-hardening artifacts
cut-off for stenosis 50-70% (NASCET): 1.9-2.4 mm
cut-off for stenosis >70% (NSACET): 1.1-1.4 (specificity 98.6% for 1.1 mm)

Gender-specific estimated percentage stenosis values (presented as 95% predictive intervals) corresponding to specific millimeter stenosis measures, calculated from the respective gender-specific linear regression models. The shaded areas represent the millimeter stenosis ranges that correspond to the estimated percentage stenosis ranges that include the 70% and 50% stenosis cut-off values (severe and moderate stenosis, respectivel

Stenosis area measurement on source images

in addition to diameter, CTA source images allow measurement of stenosis area
- in the NASCET/ECST equation, the exact area can be used instead of the diameter [Saba, 2009]
- an approximate correlation between diameter and area measurements is shown below
however, all major CEA trials were based on diameter measurements

carotid stenosis - linear measurement and area

Assessment of collateral circulation on CTA

in addition to the detection of occlusions, CTA also enables the analysis of collateral circulation; the presence of good collateral circulation correlates with smaller infarct size and predicts a better clinical outcome during reperfusion therapy
a simple Collateral Score (CS) may be used for evaluation
- a semi-quantitative rapid comparison of collateral filling in the territory of the occluded artery compared to the contralateral hemisphere
- a single-phase and multiphase CTA (mCTA) can be used
a limitation of conventional (single-phase) CTA is its static presentation; it is acquired during a short interval in the arterial phase, which can lead to an underestimation of delayed collateral circulation
dynamic information is provided by multiphase CTA (MP-CTA / mCTA)
- a total of 3-4 phases of intracranial CTA are performed using a reduced X-ray dose
- mCTA can differentiate between the absence of collaterals and delayed filling [Yang, 2008]
- mCTA can distinguish between minimal anterograde flow and retrograde collateral flow [Fröhlich, 2012]

The evaluation of CTA source images (CTA-SI) includes the following steps:

check the circle of Willis for the presence and quality of communicating arteries, hypo/aplasia, etc.
identify arterial occlusions and try to estimate their extent (thrombus length ⇒ Clot Burden Score (CBS)
compare the filling of the arterial branches in both hemispheres
evaluate the degree of the retrograde filling (optimally, the contrast agent should reach the distal end of the thrombus)

Collateral score in the anterior circulation (typically MCA)

Miteff collateral grading on single-phase CTA (Miteff, 2009)
good		major MCA branches are reconstituted distal to the occlusion
moderate		some MCA branches are shown in the Sylvian fissure
poor		only the distal superficial MCA branches are reconstituted
Collateral status is graded in maximum intensity projection reconstructions (MIP) of single-phase CTA in axial, coronal, and sagittal planes in patients with MCA occlusion

Collateral Score (CS) on single-phase CTA [Tan, 2009] Based on single-phase CTA in patients with unilateral anterior circulation infarct
Score		collaterals on CTA
0		absent collateral supply to the occluded MCA territory
1		collateral supply filling ≤50% but >0% of the occluded MCA territory
2		collateral supply filling >50% but <100% of the occluded MCA territory
3		100% collateral supply of the occluded MCA territory
Higher grades are associated with better CT perfusion parameters (MTT, CBF, and CBV), smaller final infarct volume, smaller thrombus extent, and improved outcome

Collateral Score (CS) on multiphase CTA [Menon, 2015]
Score		Collaterals on CTA
0		no vessels are visible in the affected hemisphere in any phase
1		only a few vessels are visible in the affected hemisphere in any phase
2		a filling delay of two phases in the affected hemisphere with a significantly reduced number of vessels in the ischemic territory, or one phase delay showing regions with no visible vessels
3		a filling delay of two phases in the affected hemisphere or a delay of one phase with a significantly reduced number of vessels in the ischemic territory
4		a filling delay of one phase in the affected hemisphere, but the extent and prominence of pial vessels are the same
5		no filling delay compared to the asymptomatic contralateral hemisphere, normal pial vessels in the affected hemisphere
A score of ≤ 3 indicates a poor prognosis

Case series of mCTA can be seen here

Basilar Artery on Computed Tomography Angiography (BATMAN) score

the BATMAN score is a 10-point CTA–based grading system that incorporates thrombus burden and the presence of collaterals
the posterior circulation is divided into 6 segments
- vertebral arteries (VA) – considered as 1 segment = 1 point
- posterior cerebral artery (PCA) – scored separately, 1 point each
- posterior communicant artery (PComA) – scored separately, 2 points each (or 3 points for fetal PCA)
- 3 segments of the basilar artery (BA) – 1 point each
patients with a lower BATMAN score were more likely to have a poor outcome – the absence of PComA (bilateral or unilateral) was the strongest predictor of poor clinical outcome (OR of 6.8) [Alemseged, 2017]