**4. Transcranian Doppler (TCD) and transcranian color—coded sonography (TCCS)—diagnostic tests for intracranial arterial stenosis (ICAS)**

Intracranial circulation can be examined by transcranial Doppler ultrasonography (TCD) or transcranial color-coded duplex sonography (TCCS) through different bone windows (transtemporal, transforaminal, and transorbital). The signal can be enhanced by using ultrasound contrast agents [33–38]. TCD combines in real-time intracranial blood flow patterns and velocities modifications with arterial diameter in the stenotic vessels. The most important data are: depth, blood flow direction, different velocities (peak systolic-PSV, end diastolic-EDV, and mean blood flow velocity-MFV), pulsatility index-PI, and resistance index-RI. The physiological data assessed from TCD are complementary to the anatomical data analyzed from other neuroimaging techniques (DSA, CTA, and MRA) [33–38].

TCD has some advantages: inexpensive, noninvasive, portable test than can be performed bedside, serial examination, emboli detection, and vasomotor reactivity testing. TCD has high specificity, sensitivity, and negative predictive value (NPV) [33–38]. In the same time, TCD has some disadvantages: low reliability, technical limits (inadequate or absent windows, the tortuous course of the basilar artery, etc.), and operator-dependent results. TCD presents a modest positive predictive value (PPV) (36–75%). Therefore, it is useful to exclude significant ICAS with high certainty but requires confirmation by other imaging methods when stenosis is suggested [33–41]. The circle of Willis is complete in only 20% of cases; in other cases, one or several vascular segments may be hypoplastic or aplastic. Visualization of the intracranial vessels and assessment of cerebral hemodynamics are only possible with TCCS, but this technique still requires further certification in larger studies [33–41].

Prabhakaran and coworkers suggested that TCD can specify the mechanisms of stroke in symptomatic ICAS by using surrogate imaging markers of stroke risk: for the mechanism of decreased antegrade flow—the surrogate imaging marker of flow velocity; for the progression of stenosis—the flow velocity; for the poor collateral flow—the circle of Willis collaterals; for the artery-to-artery embolism—the microembolic signal; and for the impaired vasomotor reactivity—the cerebrovascular reactivity [33–41]. Serial monitoring of flow velocities by TCD can detect the evolution of ICAS and therapeutic effects [22, 41].

Transcranial ultrasound is used for multiple aims: TCD/TCCS can detect, localize, and grade the severity of ICAS; can detect and localize the intracranial arterial occlusion; can realize the real-time monitoring of recanalization in patients treated with systemic thrombolysis and of rescue reperfusion techniques (identification of reocclusion, hyper-perfusion syndrome, etc.); can detect clinically silent emboli: microembolic signals (MES), which recognizes patients at higher risk of embolic stroke; can recognize patients with extracranial internal carotid artery (ICA) stenosis at a higher stroke risk; can assess both collateral pathways and the vasomotor reactivity (VMR), which detects the risk stratification of hemodynamic stroke [22, 33, 37, 41]; and can identify intracranial arterial blood flow steals.
