The trial is an open-label, randomized, trial examining novel biomarkers of thrombosis in patients managed with rivaroxaban vs. standard care following treatment of pulmonary embolism (PE) with catheter-guided alteplase. Patients >18 years old who present with PE and are managed with catheter-guided alteplase will be screened for study inclusion. Patient's meeting inclusion/exclusion criteria will undergo informed consent. Immediately following completion of alteplase infusion, patients will be randomized to receipt of rivaroxaban 15 mg oral bid for 21 days followed by 20mg oral daily or continuation on unfractioned heparin or low-molecular weight heparin with initiation of warfarin adjusted to INR of 2-3. Blood samples will be taken within 2 hours of CDT completion prior to receipt of study treatment (study day 1), at 8h-12h, 24h, 48h, 5d (or prior to hospital discharge), and at 30 day follow-up. Clinical endpoints, including bleeding, evidence of thrombosis progression, and death will be tracked during index hospitalization and at follow-up 30 days post-discharge.