(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.
Plaque echogenicity: hypoechoic plaques are prognostic for a 4–5-fold increase in the risk of stroke (Mathiesen et al. 2001).
Plaque perfusion: the presence and extent of neovascularization of a plaque—demonstrated using ultrasound contrast enhancement with a low mechanical index (MI) technique—correlates positively with the risk of stroke and the general rate of cardiovascular events.
Rate of spontaneous cerebral embolism (high-intensity transitory signals, HITS; see section A 1.2): for example, when there is evidence of HITS in asymptomatic 60% internal carotid artery stenosis, there is an approximately 15-fold increase in the risk of stroke in comparison with negative HITS (Spence et al. 2005).
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:
When there is contralateral internal carotid artery occlusion or very high-grade stenosis with collateral function in the ipsilateral ICA (only with collateralization via the anterior communicating branch (transcranial Doppler examination required)
When there are collaterals in the ICA via the posterior communicating branch in the flow area of the posterior cerebral circulation
In cases of distal arteriovenous malformation
Possibly in cases of elongation and kinking at the site of the stenosis
Measurement of the relevant parameters during cardiac irregularities—e.g., after extrasystoles with a compensatory pause, or in absolute arrhythmia after a longer RR interval, or with an increased ejection volume in cases of aortic insufficiency or marked bradycardia
There is a risk of underestimating stenoses in the internal carotid artery:
In cases of tandem stenoses of the internal carotid artery or highgrade flow obstructions in the area of the carotid “T” or main trunk of the middle cerebral artery
With hyperventilation (intracerebral vasoconstriction)
In cases of marked cerebral microangiopathy or raised intracranial pressure with disturbed distal outflow
When measurements are carried out during hemodynamically compromising tachycardic heart action
In cases of upstream high-grade flow obstructions
Occlusion can be differentiated from “pseudo-occlusion” or filiform stenosis by:
Low flow–optimized setting of the device (PRF low, line thickness increased, etc.)
Increasing the color enhancement appropriately (caution: artifacts)
Using alternative color scales (mixed mode), color procedures (power mode), and imaging modes (B flow, color B flow)
Optimizing the transducer head position and beam position
Using ultrasound contrast enhancement
Including the intracranial findings
Check-up examinations postoperatively or after stent angioplasty in the internal carotid artery:
In both procedures: residual stenosis, recurrent stenosis, intimal hyperplasia in the internal carotid artery, intraluminal thrombi, flow conditions at the external carotid artery orifice?
After surgery: intimal flap, proximal/distal step formation, aneurysmal dilation in the patch area (TEA)
After internal carotid artery stenting: is the stent well opposed or is there flow behind it; is the stenotic area completely covered?
