is no established classification based on maximum velocities for grades of stenosis in common carotid artery stenoses. In addition to the general characteristics of a stenosis (see under internal carotid artery stenoses), the peak velocity ratio (PVR) can be used.
Internal carotid artery
Various angiographic grading methods are available, with differing percentage figures. The residual diameter may refer either to the distal diameter of the internal carotid artery, with the local stenosis grade based on the North American Symptomatic Carotid Endarterectomy Trial (NASCET) criteria; or to the original proximal diameter, with the local stenosis grade based on the European Carotid Surgery Trial (ECST) criteria. An approximate formal conversion can be carried out:
Local stenosis grade (based on ECST): ECST % = 0.6 × NASCET % + 40%
Distal stenosis grade (based on NASCET): NASCET % = (ECST – 40%)/0.6
The traditional duplex ultrasound criteria used by the German Society for Ultrasound in Medicine (Deutsche Gesellschaft für Ultraschall in der Medizin, DEGUM) correlated with the ECST local stenosis grade (although the NASCET method was mainly used in radiography). The DEGUM criteria were revised in 2010 to establish comparability and transferred to NASCET (Table 1.1-3). The method used for duplex ultrasound classification must be clearly stated.
N.B.: Confusion may arise when the old and new stenosis grades for the internal carotid artery are compared or used in parallel. Stenoses in other locations (external carotid artery, vertebral artery, common carotid artery, etc.) are generally continuing to be classified according to the local stenosis grade on the basis of previously customary hemodynamic criteria. Supra-aortic stenoses with the same local stenosis grade in other vascular territories may therefore be given as “higher” percentages than internal carotid artery stenoses, which are classified according to the distal stenosis grade.
Fig. 1.1–6 NASCET–ECST: the principles of stenosis grading based on NASCET and ECST (B flow imaging).
Table 1.1–2 Age-dependent and gender-dependent normal values for intima–media thickness (IMT) in the common carotid artery (CCA), showing means, standard deviations, and 95% confidence intervals (Temelkova-Kurktschiev et al. 2001).
In addition to hemodynamic criteria, new B-image optimization techniques and digital subtraction ultrasonography (B flow) provide effective assistance in demonstrating the morphology. Bifurcation stenoses in particular are often missed or incorrectly classified in angiographic procedures.
Cross-sectional planimetry correlates more with the local stenosis grade and is associated with error due to vascular remodeling of the internal carotid artery. A compensatory increase in the terminal diameter of the vessel occurs with increasing grades of stenosis, so that the percentage grade of stenosis is incorrectly raised. These values therefore need to be treated with caution.
Fig. 1.1–7a, b Bifurcation stenosis of the internal carotid artery. (a) Imaging of a high-grade, eccentric internal carotid artery stenosis with apparently hypoechoic plaque material. (b) The bifurcation stenotic process is detected on the B-image/B-flow.
Table 1.1–3 Stenosis grading in the internal carotid artery based on NASCET (adapted from Arning et al. 2010).
Notes on criteria 1–10 (see text for further explanations): stenosis grade based on NASCET (%): the figures given refer in each case to a 10% range (± 5%). Criterion 2: Evidence of low-grade stenosis (local aliasing effect) distinct from nonstenotic plaque, demonstration of flow direction in moderate to highgrade stenoses and evidence of vascular occlusion. Criterion 3: The criteria apply to stenoses with a length of 1–2 cm and only to a limited extent to processes affecting multiple vessels. Criterion 4: Measurement well distally, outside of the zone with jet stream and flow disturbances. Criterion 5: Possibly only one of the collateral connections may be affected: if only an extracranial examination is carried out, the value of the findings is lower. Criterion 9: Confetti sign can only be recognized when the pulse repetition frequency (PRF) is set low. ACA, anterior cerebral artery; ICA, internal carotid artery; CCA, common carotid artery.
Fig. 1.1–8 Internal carotid artery planimetry. The principle of planimetric stenosis grading. The limitations due to a compensatory increase in the external diameter should be noted.
Additional information on plaque status and on the prognostic assessment is particularly desirable for treatment decision-making in patients with asymptomatic stenoses. The aim is to detect stenoses that are associated with an increased risk of embolism, since at this stage it is usually an embolic source rather than a hemodynamically compromising structure that is removed (otherwise there is a very high number needed to treat in therapy for asymptomatic stenoses). Prognostic significance has not been conclusively evaluated for all of the parameters.
Intracerebral collateralization/autoregulation reserve/CO2 reactivity (see section A 1.2)—limited autoregulation reserve/CO2 reactivity in intracerebral vessels correlates with hemodynamically caused watershed infarction.
There is a risk of overestimating stenoses in the internal carotid artery:
