Second, the investigators propose to exploit the power of Xe129 MRI as a diagnostic tool to monitor therapeutic responses of a combination inhaler, Advair, which contains a long-acting beta-adrenoceptor agonist (LABA) and an inhaled corticosteroid (ICS) - two major classes of the current COPD therapeutics. The investigators will characterize the functional changes of the lungs with COPD at baseline, and investigate the responses of the lungs to the treatment after a three-month trial. Also the investigators will compare corresponding results obtained by Xe129 dissolved phase (DP) MRI to the results obtained by gadolinium-based dynamic contrast-enhanced perfusion MRI (perfusion MRI) and high resolution computed tomography (HRCT) of the lung to indirectly validate the Xe129 DP MRI technique. The investigators anticipate that the results from this project will greatly improve the investigators understanding of the lung functional responses of COPD subjects to current therapeutics. Also, the investigators expect that this project will provide evidence to consider Xe129 MRI as a diagnostic strategy to assess and monitor therapeutic responses of existing and new pharmaceuticals, and thus Xe129 MRI will stimulate development of novel therapies for COPD in the future