Design: U.S.-based, single-center, proof-of-concept study Brief Description: A standard clinical contrast-enhanced chest CT scan performed 48 hours after clinically-indicated standard anticoagulation will be compared with a standard clinically-indicated baseline contrast-enhanced chest CT scan using a previously-studied and previously-validated 3-dimensional reconstruction technique to assess changes in the pulmonary vasculature in patients with acute pulmonary embolism (PE). This previously-studied and previously-validated 3-dimensional reconstruction technique has been used to assess the response of the pulmonary vasculature to catheter-based fibrinolysis in acute PE as well as to assess the pulmonary vasculature in a number of chronic lung diseases. However, the pulmonary vascular response to standard anticoagulation for acute PE has not been assessed previously. Purpose: To compare the pulmonary vasculature before and after standard clinically-indicated anticoagulation for acute PE using a previously-studied and previously-validated 3-dimensional reconstruction technique applied with a standard clinically-indicated baseline contrast-enhanced chest CT scan (used to diagnose the acute PE) and a standard clinical contrast-enhanced chest CT scan performed 48 hours later as indicated by the study protocol. Population: Inpatients diagnosed with acute PE, in whom clinical providers have prescribed standard anticoagulation alone for treatment based on clinical grounds at Brigham and Women's Hospital. Enrollment: 10 subjects with acute PE Clinical Site Location: Single-center, Brigham and Women's Hospital Study Duration: 12 months Primary Imaging Outcome: CT-determined percent change in perfusion of the pulmonary vasculature from baseline to 48 hours in inpatients diagnosed with acute PE, in whom clinical providers have prescribed standard anticoagulation alone for treatment based on clinical grounds at Brigham and Women's Hospital. Secondary Imaging Outcome: CT-determined percent change in right ventricular (RV) volume from baseline to 48 hours in inpatients diagnosed with acute PE, in whom clinical providers have prescribed standard anticoagulation alone for treatment based on clinical grounds at Brigham and Women's Hospital.