[18F]FES PET/CT in PAH

Inclusion Criteria: * 1\. Women who are post-menopausal, defined as one of the following 1. \> 50 years old and a) have not menstruated during the preceding 12 months per medical record review or self-report or b) have follicle-stimulating hormone levels \> 40 IU/L at screening OR 2. \< 50 years and follicle-stimulating hormone levels \> 40 IU/L at screening OR 3. history of bilateral oophorectomy per medical record review or self-report. 2\. Diagnosis of Pulmonary Arterial Hypertension (PAH) per medical record review. 3\. Patients must be candidates to receive treatment on the companion therapeutic trial "Estrogen Receptor-α Inhibitor in Patients with Pulmonary Arterial Hypertension (ERA PAH)" (IRB# 824861 "ERA PAH") 4\. Participants must be informed of the investigational nature of this study and be willing to provide written informed consent and participate in this study in accordance with institutional and federal guidelines prior to study-specific procedures. Exclusion Criteria: * 1\. Inability to tolerate imaging procedures in the opinion of an investigator or treating physician 2\. Serious or unstable medical or psychological conditions that, in the opinion of the investigator, would compromise the subject's safety or successful participation in the study. 3\. Ineligible for the therapeutic trial IRB# 824861 "ERA PAH" entitled "Estrogen Receptor Antagonist in Patients with Pulmonary Arterial Hypertension (ERA PAH)" (IRB# 824861 "ERA PAH")