Inclusion Criteria:  WHO group 1 PAH, characterized by mean pulmonary artery pressure ≥25 mmHg, PVR >3 Woods units, and pulmonary capillary wedge pressure or left ventricular end diastolic pressure ≤15 mmHg. Participants must be further classified as idiopathic PAH (IPAH) or connective tissue disease associated PAH (CTD-PAH). New York Heart Association (NYHA) classification I - III criteria of heart failure. Vasodilator therapy naïve, with the intent to initiate pulmonary vasodilator therapy. Age 18 - 75. English speaking and able to provide informed consent.  Exclusion Criteria:  Recent syncope. Baseline 6MWD < 400 feet or NYHA class IV heart failure. Metabolic disorders such as uncontrolled diabetes (A1c > 8%) that may alter cardiac metabolism. Baseline use of oral steroids. FEV1/FVC <60% Contraindications to MRI, including those noted on the UTSW MRI Screening Form such as implants contraindicated at 3T, pacemakers, Implantable Cardioverter Defibrillators (ICD), or significant claustrophobia. Weight >210 lbs (exceeds current IND weight-based dosing guidelines) 8 . Women who are pregnant, lactating or planning on becoming pregnant during the study.  9. Not suitable for study participation due to other reasons at the discretion of the investigators