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Introduction: Microcalcification is a histopathological feature of atheroma at risk of rupture; so-called “vulnerable plaques.” Positron emission tomography (PET) using 18F-sodium fluoride (NaF) has been used to detect microcalcification in ex vivo histology, though its use in vivo has been limited. We investigated the utility of NaF-PET to identify culprit carotid artery atheroma non-invasively in vivo in the setting of acute ischemic stroke.Methods: Symptomatic carotid artery stenosis of ≥50% was imaged using NaF-PET within 14 days of ipsilateral ischemic stroke. Symptomatic arteries were compared to contralateral asymptomatic carotid arteries. NaF dose was 125 MBq with 60 minute uptake on a GE Discovery 690 with 64 slice computed tomography. NaF uptake was measured using standardized uptake values for each artery’s single region of maximum uptake (SUVmax) and the mean of the three slices representing the most diseased segment (MDS-SUVmax). Arteries were compared using Wilcoxon signed-rank testing.Results: Fifty-two carotid arteries were analyzed: 26 symptomatic, 26 asymptomatic. Median SUVmax was higher in symptomatic than asymptomatic arteries: 2.16 (IQR 0.76) and 1.88 (IQR 0.94) respectively (p<0.001). MDS-SUVmax was 2.04 (IQR 0.66) in symptomatic and 1.76 (IQR 0.83) in asymptomatic carotids (p<0.001). Calcium scores did not differ between symptomatic and asymptomatic arteries (p=0.34).Conclusions: The ability to identify culprit carotid atheroma using NaF-PET in vivo represents an important advance in vascular imaging. This has important implications for understanding atherosclerosis pathophysiology, as well as potential clinical applications to identify and risk-stratify vulnerable carotid atheroma.