RecruitingCombination productDownstream Effects of Airway Mucus Plugs on 129Xenon MRI in Severe AsthmaIn this study, xenon MRI will be used to evaluate regional functional consequences of mucus plugs in the lungs of patients with severe asthma. Mucus plugs will be identified using CT imaging, and xenon MRI will be used to evaluate ventilation and gas exchange impairments in regions of the lungs corresponding to the airways downstream of mucus plugs.