NEUROIMAGING / COMPUTED TOMOGRAPHY
CT angiography
Created 21/03/2021, last revision 27/06/2022
- CT angiography (CTA) is a rapid (takes ∼ 5 min), easily available, and non-invasive imaging method 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 Â
- 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 are needed
- source images (SI) are essential for 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 stenosesÂ
- the ideal parameters vary in different scanners
Stenosis and occlusion assessment
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Carotid stenosis evaluation
Evaluation of atherosclerotic plaques characteristics
- CTA is a helpful tool to diagnose extracranial stenoses
- it shows not only the grade of stenosis, but it yields information about carotid plaque characteristics (which can also be assessed by ultrasound or MRI → see here)
- 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 thrombi (ILT)
- presence of intraluminal thrombi (ILT) increases the likelihood of symptomatic stenosis
- smooth or heavily calcified plaques pose a low risk of CV event [Eesa, 2010]
- in case of extensive calcifications, CTA outperforms the ultrasound, which must rely on the Doppler examinationÂ
Stenosis diameter measurement (NASCET and ECST)
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Stenosis area measurement on source images
- apart from the diameter, CTA source images allow measuring the area of stenosis as well
- in the NASCET/ECST equation, exact area instead of diameter can be used [Saba, 2009]
- below is an approximate correlation between diameter and area measurements
- however, all major CEA trials were based on diameter measurement
Assessment of collateral circulation on CTA
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CTA “perfusion”
- besides the evaluation of 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
- for optimal display, adjust the window parameters
Cardiac CTA
- CTA can be used for the detection of left atrial thrombus
- both high sensitivity and specificity were reported in comparison with TEEÂ (Hur, 2009]
- the examination might rule out major pulmonary embolism (PE) as well
- the protocol would entail imaging starting at the level of the left atrium up to the vertex
CTA and the brain death diagnosis
→ see here
