NEUROIMAGING / COMPUTED TOMOGRAPHY
Computed tomography angiography (CTA)
Created 21/03/2021, last revision 06/11/2023
- 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 needed, 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)
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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
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- 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) - check the entire extracranial portion of the CCA and ICA
- carotid stenosis assessment 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 with proximal blood stagnation (the situation is often clarified on DSA during embolectomy)
- occlusion of the proximal ICA due to atherothrombosis combined with distal occlusion
- ICA aplasia
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)
- 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
- location and extent of occlusion (thrombus length)
- 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)
- CTA (as well as neurosonology) is a reliable non-invasive tool for long-term monitoring of intracranial vasculopathies
Carotid stenosis evaluation
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)
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%
- ECST – estimated normal lumen diameter at the site of the lesion (outer-to-outer)
- 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%
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 in the cases of dense calcifications to decrease beam-hardening artifact
- 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)
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
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- and multiphase CTA (mCTA) can be used
- a limitation of conventional (single-phase) CTA is its static presentation, acquired during a short interval in the arterial phase, which can lead to an underestimation of the collateral circulation due to its delayed filling
- 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]
- a total of 3-4 phases of intracranial CTA are performed using a reduced X-ray dose
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, 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)
| Collateral Score (CS) on single-phase CTA [Tan, 2009] Based on single-phase CTA in patients with unilateral anterior circulation infarct |
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| 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] |
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| 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
- vertebral arteries (VA) – considered as 1 segment = 1 point
- patients with a lower BATMAN score were more likely to have a poor outcome – the absence of PCoA (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]
CTA “perfusion”
- in addition to evaluating stenosis/occlusion, CTA source images may also be used for a rough assessment of perfusion deficit (especially if CTP is not part of the standard examination protocol or is not available) [Coutts, 2004]
- the contrast agent fills the capillaries in the normally perfused tissue but is absent in the ischemic area, which will appear hypodense
- numerous studies have shown that CTA-based “perfusion” improves the prediction of final infarct volume and clinical outcome
- adjust the window parameters for optimal visualization
Cardiac CTA
- CTA may be used to detect left atrial thrombus
- both high sensitivity and specificity were reported compared to TEE (Hur, 2009)
- LAA thrombus is a potential radiologic marker of AFib (Senadeera, 2020)
- the examination might also rule out significant pulmonary embolism (PE)
- the CTA protocol would cover the area from the left atrium to the vertex
CTA and diagnosing brain death
- CTA is the official method for confirming cerebral circulatory arrest in many countries
- 4- and 7-point scales are used
