Blood clots in lung arteries (pulmonary embolism) are usually detected using a radiological test called computed tomography (CT scan). As technology advances, the CT scans are able to detect smaller and smaller blood clots. Over time, the frequency of blood clots in the pulmonary arteries has increased significantly (CT scan are now detecting very small blood clots that the investigators could not see before). As a result, more and more people are on blood thinners to treat these small blood clots but their true clinical significance is unknown.

The management of blood thinners is costly and also utilizes scarce healthcare resources. These blood thinners need to be monitored with frequent blood work. Furthermore, every year, approximately 3 percent of patients on blood thinners will have a major bleeding event requiring medical attention.

The investigators don't think that treating these small blood clots in the pulmonary arteries detected on CT scan is worth the risk of bleeding from the blood thinners.

The main goal of this study is to find out if it is safe to not treat very small blood clots in the pulmonary arteries.

The investigators plan to follow 300 patients with small blood clots in their lungs for 90 days. These patients will not be treated with blood thinners but will be followed closely with other non-invasive tests to avoid progression or recurrence of blood clots.

Inclusion Criteria:

1. Age greater or equal to 18 years old.

2. Patients with newly diagnosed isolated SSPE* (any number).

   • Isolated SSPE is defined as CTPA demonstrating an intraluminal filling defect in a subsegmental artery with no filling defects visualized at more proximal pulmonary artery levels.

Exclusion Criteria:

1. Proximal lower extremity (popliteal vein or above) or upper extremity (subclavian vein or above) DVT.

2. Need for long term oral anticoagulant therapy for reasons other than VTE.

3. SSPE diagnosed in a hospitalized patient (> 48 hours after hospital admission).

4. Requiring oxygen therapy to maintain an O2 saturation over 92%

5. Previous history of DVT (proximal or distal) of upper or lower extremities, PE, or unusual site thrombosis (e.g. splanchnic or cerebral vein thrombosis).

6. Geographically inaccessible for follow-up

7. Active Malignancy (defined as other than basal-cell or squamous cell carcinoma of the skin; cancer within the past 6 months; any treatment for cancer in the past 6 months; or recurrent or metastatic cancer)

8. Pregnancy

9. Have received more than 48 hours of therapeutic anticoagulation.

   • Prophylactic dose allowed if required for separate indication and acceptable by the investigator.

10. Unable/refuse to sign informed consent

11. Asymptomatic SSPE (e.g. SSPE is an incidental finding on a CT scan conducted for reasons other than suspected PE)

Consecutive patients with symptomatic, isolated SSPE* (any number), that are newly diagnosed by computed tomographic pulmonary angiography will be eligible to participate in the study.