An open label, non-comparative study design is appropriate for this Phase 4 study designed to assess whether a core therapy of bosentan, either as monotherapy or with the addition of sildenafil, enables patients with pulmonary arterial hypertension (PAH) to achieve a 6-minute walk distance (6 MWD) of ≥380 meters after 28 weeks of therapy This design is also appropriate to pioneer the utility of cardiac MRI in assessing improved functional capacity in PAH and exploring its correlation with other parameters.