Evaluation of Pulmonary Perfusion Heterogeneity and Compliance in Patients With Pulmonary Hypertension Using Positron Emission Tomography Imaging

Inclusion Criteria: * General 1. A signed and dated written informed consent is obtained from the subject. 2. The subject is capable of giving informed consent, which includes compliance with the requirements and restrictions listed in the consent form. 3. Available to complete the study. 4. Able to lie flat and able to perform a 30 second breath hold. * Patients with Pulmonary Hypertension 1. Subject is male or female aged = or \> 18 years of age and \< or = 70 years. 2. Subject is diagnosed with pulmonary arterial hypertension (WHO Group I, by right heart catheterization, mean PA pressure \> 25 and PCWP \< 15) or exercise-induced pulmonary hypertension (by right heart catheterization + Level III cardiopulmonary exercise testing, mean PAP \> 30 and PCWP \< 20 during exercise, but normal at rest), with the most recent of above diagnostic tests taking place within 12 months of study entry. 3. If patient has been on PAH-specific therapy, the therapy (agent and dose) has been unchanged for at least 3 months. Exclusion Criteria: * General 1. FEV1 and/or TLC \< 70% predicted 2. PCWP \> 15 mm Hg 3. Inability to perform the study (by primary MD or investigator assessment) 4. Subjects who have a past or present disease, which as judged by the Investigators may affect the outcome of this study 5. The subject has suspected history of drugs or alcohol abuse within the six months prior to the screening visit. 6. The subject has a positive pregnancy test. 7. The subject is unable to perform the respiratory manoeuvres necessary for the exam. 8. The subject has been exposed to a radiation dose over the past year that, when added to the radiation dose expected in this study, would exceed permissible yearly exposure as determined by the MGH radiation safety committee. * Subjects with Pulmonary Hypertension 1. Subject with clinical instability in the judgment of the investigator, or hospitalization for progression of pulmonary hypertension or right heart failure in the three months prior to the study. 2. Subjects in whom PAH therapy has been escalated (additional agent started) within 3 months of enrolment. 3. Subject with diagnosis of active tuberculosis, lung cancer, clinically overt bronchiectasis, allergic rhinitis, asthma, or COPD. 4. Subject that had a respiratory tract infection in the 4 weeks prior to the screening visit and throughout the duration of the study.