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The detection of thrombus in the left atrial appendage (LAA) is vital in the prevention of stroke. We present a novel technique to detect and characterize LAA thrombus in humans using positron emission tomography (PET) of a fibrin-binding radiotracer, [64Cu]FBP8. Initial testing in healthy volunteers (n = 8) revealed that [64Cu]FBP8 was stable to metabolism and was rapidly eliminated with a blood half-life of one hour. Patients with atrial fibrillation (AF) and recent transesophageal echocardiograms (TEEs) of the LAA (positive n = 12, negative n = 12) were studied. PET, integrated with magnetic resonance (PET-MR), of the thorax was performed one hour after [64Cu]FBP8 injection. The maximum standardized uptake value (SUVMax) in the LAA was significantly higher in the TEE positive than negative subjects, median [interquartile range] of 4.0 [3.0–6.0] vs. 2.3 [2.1–2.5]; p < 0.001. A SUVMax threshold of 2.6 correctly identified 12/12 positive TEEs and 10/12 negative ones, yielding an area-under-the-receiver operating characteristic curve of 0.97. The minimum longitudinal magnetic relaxation time (T1Min) in the LAA was significantly shorter in the TEE positive than TEE negative group 970 [780–1080] vs. 1380 [1120–1620], p < 0.05, with some overlap between the groups. Logistic regression using SUVMax and T1Min allowed all TEE positive and negative subjects to be classified with 100% accuracy. A strong correlation was seen between fibrin (SUVMax) and methemoglobin (T1Min) content in the LAA. In conclusion, the detection of LAA thrombus in humans with PET-MR of [64Cu]FBP8 is highly accurate and provides useful information on the biological properties of cardiac thrombus.One Sentence SummaryFirst-in-human fibrin-targeted PET-MR of thrombus