Inhaled Treprostinil (Tyvaso Nebulizer) For COPD Patients With Hypoxemia

The goal of this clinical trial is to evaluate whether inhaled Treprostinil (Tyvaso) can improve oxygen delivery and blood flow in the lungs in adults (age ≥40) with chronic obstructive pulmonary disease (COPD) and hypoxemia who have less severe reduction in lung blood volume (diffusing capacity of the lungs for carbon monoxide \[DLCO\] ≥45%). The main questions it aims to answer are: 1. Does inhaled Treprostinil increase pulmonary capillary blood volume in ventilated lung regions, as measured by hyperpolarized xenon-129 magnetic resonance imaging (HP129Xe MRI)? 2. Does inhaled Treprostinil improve oxygen delivery (measured as red blood cell \[RBC\] chemical shift) and maintain or only slightly change pulmonary vascular resistance (measured by RBC oscillation amplitude)? 3. Can pre-treatment MRI parameters (RBC transfer and RBC oscillation amplitude) predict who will respond to inhaled Treprostinil? Participants will: * Use the Tyvaso nebulizer (inhaled Treprostinil) 4 times daily for 4 weeks, starting at 3 breaths per session and increasing to a maximum of 6 breaths per session as tolerated. * Undergo HP129Xe MRI before and after treatment to assess regional lung function and oxygen exchange. * Complete pulmonary function tests (PFTs), 6-minute walk tests (6MWT), and echocardiograms at the beginning and end of the study. * Be monitored for adverse events, with a phone check-in midway through and after the treatment period.