Chronic thromboembolic pulmonary hypertension \[CTEPH\] is caused by pulmonary emboli that have enlarged in pulmonary arteries and have become organized into the vessel wall. Many patients with CTEPH are treated with balloon pulmonary angioplasty \[BPA\] which mechanically opens the narrow pulmonary arteries. It is unclear how much downstream functional pulmonary capillary surface area \[FCSA\] is recovered during BPA. The investigators plan to measure FCSAIn CTEPH patients, before and after a session of BPA.