Purpose: It remains to be determined whether the impact of plaque characteristics on procedural risks differs between carotid artery stenting (CAS) and endarterectomy (CEA). We studied whether quantitative assessment of carotid plaque echolucency on ultrasound predicts the risk of embolism during CAS or CEA.Materials and Methods: In 50 consecutive patients with symptomatic carotid stenosis randomized to CAS (n = 26) or CEA (n = 24) in the International Carotid Stenting Study (ICSS), semi-automated grayscale measurement of carotid plaques on baseline ultrasound was performed. We determined the grayscale median (GSM), percentage of echolucent plaque area, and a previously defined echographic risk index (ERI) calculated with the echolucent area and degree of stenosis. Brain MRI including diffusion-weighted imaging (DWI) was performed within 7 days before and 3 days after treatment. The primary outcome was the presence of at least 1 new hyperintense DWI lesion (DWI+) after treatment.Results: In the CAS group, DWI+ patients (n = 18) had a significantly higher ERI at baseline (mean 0.11 ± 0.12) than patients without new lesions (n = 8; mean 0.03 ± 0.01; p = 0.012). GSM (mean 26.7 ± 18.7 versus 34.3 ± 8.0, p = 0.16) and echolucent plaque area (mean 42.8 ± 21.1 versus 31.2 ± 8.2, p = 0.054) did not differ significantly. In the CEA group, there were no differences in plaque echogenity measurements between patients with (n = 2) and without DWI lesions (n = 22).Conclusion: Patients with echolucent plaques causing severe narrowing are at increased risk for cerebral embolism during CAS. Quantitative ultrasound plaque analysis, with ERI in particular, may add to clinical variables in identifying patients at risk for procedural stroke with CAS, but larger studies with clinical endpoints are needed.