Oral appliances, which can maintain patients' jaw forwards during sleep, become more popular in treating obstructive sleep apnea (OSA). However, the exact mechanism of mandible forward on upper airway is not completely known. In this study we will use both static and dynamic MRI to study the 3D morphological changes and dynamic behavior of the upper respiratory tract at different mandible posits. Because in asian society non-obese OSA patients are not uncommonly seen, in this study we focus only on non-obese severe OSA patients.