Evaluating Intracranial Atherosclerosis Rather Than Intracranial Stenosis

Intracranial atherosclerosis (ICAS) is an important cause of ischemic stroke throughout the world, accounting for ≈30% to 50% and 10% of ischemic stroke and transient ischemic attack in Asians and whites, respectively.1 Several imaging modalities, such as transcranial Doppler (TCD), magnetic resonance angiography (MRA), computed tomographic angiography (CTA), and digital subtraction angiography (DSA), are used commonly in routine clinical practice to detect and assess ICAS, as well as in selection criteria of clinical trials.2–4 Although some of these imaging modalities yield flow information, such as TCD revealing velocity data or waveform turbulence and time-of-flight MRA (TOF-MRA) depicting arterial patterns based on blood flow, most attention has been drawn to the maximal percent stenosis of the arterial lumen. The focus on severity of stenosis has been reinforced because severe (70%–99%) atherosclerotic stenosis was demonstrated as an independent predictor for stroke recurrence in the territory of the stenotic artery, with the risk of ≈20% at 1 year, in the Warfarin versus Aspirin for Symptomatic Intracranial Disease (WASID) trial.5 However, those patients with a traditionally considered moderate (50%–69%) atherosclerotic stenosis were also at considerable risk of recurrent stroke, ≈10% at 1 year in the WASID study.5,6 In more recent studies, the role of percent stenosis in predicting subsequent stroke risk has been superseded by collateral flow and hemodynamics in the same patient cohort.7,8 Characterization of the atherosclerotic lesion is also represented poorly by percentage of stenosis measured at the narrowest vessel diameter alone. Beyond the maximal luminal stenosis, many other features may reflect the characteristics of ICAS, such as plaque morphology and components, which might also be promising markers in risk stratification of patients with symptomatic ICAS.9 However, from the view of intracranial stenosis, it could also be attributed to causes …