• For Atrial Fibrillation Patient subjects:
• * History of atrial fibrillation or paroxysmal atrial fibrillation;
• * Retrospective enrollment: TEE to evaluate LAA within the last 14 days, provided the anticoagulation regimen the patient is on is not changed after the TEE. If a patient has a negative TEE and continues the same stable anti-coagulation regimen he/she is on, then it is extremely unlikely that a new thrombus will develop in the left atrial appendage within the next two weeks. Likewise, if a patient not taking any anti-coagulation has a thrombus in the left atrial appendage, then it is extremely unlikely that this thrombus will resolve spontaneously in the next 14 days if the patient continues not to take any anticoagulation. If the TEE leads to a change being made in the anticoagulation regimen (started/stopped/dose modified), then a time window of 72 hours from the TEE to PET imaging will be used. This scheme will ensure that the TEE can serve as an accurate gold standard;
• * Prospective enrollment: TEE to evaluate LAA scheduled in upcoming 14 days;
• For COVID-19 Patient subjects:
• * Positive test for SARS-CoV-2 RNA detected by RT-PCR collected from the upper or lower respiratory tract analyzed by a certified lab with an FDA approved assay within the last month;
• * Patient not requiring mechanical ventilation;
• For Cancer Patient subjects:
• • Patient is diagnosed with cancer;
• For Other Patient subjects:
• * Ultrasound- or CT-confirmed or high likelihood of thrombus (e.g., elevated D-dimer)
• * Has not received thrombolytics

Exclusion Criteria:

• * Subjects less than 18 years of age;
• * Electrical implants such as cardiac pacemaker or perfusion pump;
• * Pregnant or breastfeeding (a negative STAT quantitative serum hCG pregnancy test is required for females having child-bearing potential before the subject can participate);
• * Claustrophobic reactions;
• * Subjects will be excluded if research-related radiation exposure exceeds current Radiology Department guidelines (i.e. 50 mSv in the prior 12 months);
• * Unable to lie comfortably on a bed inside the PET scanner;
• * Subjects under direct or indirect (i.e. same department as PIs) supervision of the principal investigator;
• * Body weight over the weight limit for the moving table (\> 300 lbs for the MRI table and \>441 lbs for the CT table);
• * Metallic or electric implants contraindicated for MR-PET scanning when applicable;
• * Does not have the ability to give written informed consent.
• * Determined by the investigator(s) to be clinically unsuitable for the study (e.g. based on screening visit and/or during study procedures);
• Additional exclusion criteria for Atrial Fibrillation Patient subjects:
• * Stroke within the last 3 months;
• * Myocardial infarction within the last 3 months;
• * Cardiac or major surgery within the last 3 months;
• * History of chest pain within the last 6 weeks unless followed by a subsequent stress test or coronary angiography;
• * History of syncope within the last 6 weeks;
• * Heart rate persistently \>120 bpm or persistently \< 50 bpm;
• * Presence of daytime pauses \> 3s