NEUROIMAGING / COMPUTED TOMOGRAPHY
Computed tomography angiography (CTA)
David Goldemund M.D.
Updated on 10/04/2024, published on 21/03/2021
Updated on 10/04/2024, published on 21/03/2021
- 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
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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) - 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
- via ultrasound
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)
- 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 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)
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-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]
- 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 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 |
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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 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]
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
