Inclusion Criteria:

• * Age ≥ 18 years of age
• * Documented diagnostic right heart catheterization (RHC) at any time prior to screening confirming the diagnosis of WHO PAH Group 1 in any of the following subtypes:
• * Idiopathic PAH
• * Heritable PAH
• * Drug/toxin-induced PAH
• * PAH associated with CTD
• * Symptomatic PAH classified as WHO FC I, II, or III.
• * Documented Historical RHC with a mPAP ≥ 20 mmHg, PCWP ≤15 mmHg, and PVR ≥ 3 Wood Units (WU)
• * Ability to adhere to the study visit schedule, adhere, and comply with all protocol requirements.
• * Ability to understand and provide written informed consent.
• For patients in the Initiating Therapy Arm:
• * Initiation of pulmonary vasodilator therapy within ±3 weeks of baseline imaging timepoint.
• For patients in the Stable Arm:
• * On stable doses of background PAH therapy and diuretics (i.e., patient-specific dose goal for each therapy already achieved) for at least 90 days prior to screening; for infusion prostacyclins, dose adjustment within 10% of optimal dose is allowed per medical practice

Exclusion Criteria:

• * Subject unable to undergo MRI based on MRI safety screening
• * Diagnosis of pulmonary hypertension WHO Groups 2, 3, 4, or 5
• * Pregnant or breastfeeding female subjects
• * Prisoners or incarcerated individuals
• * Any major surgical procedure within 90 days prior to study enrollment or planned surgical procedure during the study period.
• * Concomitant medical disorder, condition, or history, that in the opinion of the Investigator would impair the subject's ability to participate in or complete the requirements of the study
• * Other medical or psychiatric condition which, in the opinion of the Investigator, would place the subject at increased risk or would preclude obtaining voluntary consent or would confound the objectives of the study