Inclusion Criteria:  Male or Female Subjects between the ages of 5 to 18 only for Aim 1 and xenon use Subjects 3-18 years of age for Aims 2 and 3 Subjects with persistent moderate or severe OSA after adenotonsillectomy. - -- Persistent moderate or severe OSA will be defined as an oAHI > 5 per hour of sleep. Clinical indication or suspicion of upper-airway obstruction. Examples include but not limited to hypertrophy of the lingual tonsils, disproportionately large tongue, or micrognathia. Subjects who have failed a trial of CPAP. Subjects whose parents elect to pursue surgery without a trial of CPAP. Subjects who require a surgical procedure for OSA based on the clinical assessment of the surgeon (otolaryngologist or plastic surgeon).  Exclusion Criteria:  Children adequately treated with CPAP. Children with braces/metal rods. Children who have a contraindication to sedative. Standard MRI exclusion criteria as set forth by the CCHMC Department of Radiology.