NEUROIMAGING / COMPUTED TOMOGRAPHY
CT angiography
Created 21/03/2021, last revision 30/05/2023
- CT angiography (CTA) is a rapid (takes ∼ 5 min), readily available, and non-invasive imaging modality used to assess extra- and intracranial arteries
- it is based on the spatial reconstruction of the image from a series of axial scans after iodinated contrast agent administration
- the examination usually starts at the level of the aortic arch (or better, LAA) and extends up to the vertex
- approx. 50-60 mL of contrast agent is required
- source images (SI) are essential for the evaluation
- reconstruction images (MIP – maximum intensity projection) may be helpful in certain circumstances
- adjust the window width (WW) and window level (WL) parameters to assess heavily calcified stenoses
- the ideal parameters vary in different scanners
Stenosis and occlusion assessment
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Carotid stenosis evaluation
Evaluation of atherosclerotic plaque characteristics
- CTA is a helpful tool for diagnosing extracranial stenoses
- 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)
- presence of intraluminal thrombi (ILT) or ulcers increases the likelihood of symptomatic stenosis
- smooth or heavily calcified plaques are associated with a low risk of CV event [Eesa, 2010]
- in the presence of extensive calcifications, CTA outperforms the ultrasound, which must rely on the Doppler examination (B mode and color mode are usually inconclusive due to acoustic shadows)
Stenosis diameter measurement (NASCET and ECST)
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Stenosis area measurement on source images
- in addition to diameter, CTA source images allow the measurement of stenosis area
- in the NASCET/ECST equation, exact area can be used instead of diameter [Saba, 2009]
- below is an approximate correlation between diameter and area measurements
- however, all major CEA trials were based on diameter measurements
Assessment of collateral circulation on CTA
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CTA “perfusion”
- in addition to assessing stenosis/occlusion, CTA source images can 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
- many studies have shown that CTA “perfusion” improves the prediction of final infarct volume and clinical outcome
- adjust the window parameters for optimal display
Cardiac CTA
- CTA can be used for the detection of 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 major pulmonary embolism (PE)
- the CTA protocol would cover the area between the left atrium to the vertex
