The overall goal of this study is to evaluate lung, large vessel, and right ventricular glucose uptake rate using 18FDG PET and distribution of pulmonary perfusion using 13NN PET in patients with pulmonary hypertension before and after treatment with ambrisentan. The investigators propose a pilot study of patients with Group I PAH. The investigators plan to complete this pilot study with 5 patients (projected enrollment of 8 to allow for drop-out rate). Taken together, the studies above demonstrate that FDG-PET can be used to detect changes associated with PAH, and to monitor response to therapy. FDG-PET is a widely available technology, and is firmly established in the field of clinical oncology. Ambrisentan is an endothelin receptor antagonist that is FDA-approved to treat patients with pulmonary arterial hypertension. Patients will be eligible for participation in this study if they have already made a decision to start treatment with ambrisentan for pulmonary hypertension. The investigators propose a physiologic study of patients who will already have made a decision to start ambrisentan therapy. Participation in the study will not alter or affect participants decisions regarding treatment.