Inclusion Criteria of Cohort 1  Subjects must meet all of the following inclusion criteria to be eligible for enrollment into the trial:  Outpatients of either gender, Age 18-75 Awaiting a lung transplant Diagnosis of Group 1 PH using the criteria from previous clinical trials (right heart catheterization demonstrating hemodynamic criteria of a mean pulmonary artery pressure (mPAP) ≥ 25 mmHg, pulmonary vascular resistance ≥ 3 WU, pulmonary capillary wedge pressure ≤ 15 mmHg) Willing and giving informed consent and adhere to visit/protocol schedules (consent must be given before any study procedures are performed). Women of childbearing potential must have a negative urine pregnancy test before MRI  Exclusion Criteria of Cohort 1  Subjects presenting with any of the following will not be included in the trials:  Pulmonary Veno-occlusive disease or Porto-pulmonary hypertension Moderate to severe heart disease (LVEF <45%, Severe LV hypertrophy, Moderate to severe valvular disease) PH due to schistosomiasis Sarcoidosis Active cancer Sickle cell anemia Liver disease (Childs-Pugh class C) Prisoners and pregnant women will not be approached for the study Conditions that will prohibit MRI scanning (metal in eye, claustrophobia, inability to lie supine) Medical or psychological conditions which, in the opinion of the investigator, might create undue risk to the subject or interfere with the subject's ability to comply with the protocol requirements  Inclusion Criteria of Cohort 2  Subjects must meet all of the following inclusion criteria to be eligible for enrollment into the trial:  Treatment naïve or treatment started within the last 3 months Outpatients of either gender, Age 18-75 WHO functional class (FC) 2-3 symptoms with a diagnosis of group 1 PH (mean pulmonary artery pressures (mPAP) > 20 mmHg, pulmonary capillary wedge pressure (PCWP) ≤ 15mmHg and pulmonary vascular resistance (PVR) ≥3 WU) Willing and able to give informed consent and adhere to visit/protocol schedules (consent must be given before any study procedures are performed). Women of childbearing potential must have a negative urine pregnancy test before MRI  Exclusion Criteria of Cohort 2  Subjects presenting with any of the following will not be included in the trials:  Sarcoidosis Active cancer Sickle cell anemia Liver disease (Childs-Pugh class C) Any conditions that prevent the performance of 129Xe MRI scans. Prisoners and pregnant women will not be approached for the study. Conditions that will prohibit MRI scanning (metal in eye, claustrophobia, inability to lie supine). Medical or psychological conditions which, in the opinion of the investigator, might create undue risk to the subject or interfere with the subject's ability to comply with the protocol requirements