A method adapted to diagnose airway obstruction in a subject is disclosed. Plural cross-sectional ultrasound images of a region of the respiratory tract are obtained during the subject's normal breathing, wherein each pixel in the ultrasound image has a color scale value. A user selects a region of interest (ROI) in each ultrasound image, and a first statistic value of the pixels therein is calculated as a sum of a measure of location of the color scale values plus a constant multiplied by a measure of dispersion. In each ROI the pixels having color scale values larger than or equal to the respective first statistic value are identified to define a respective airspace region, and a respective width of the respiratory tract is calculated. The status about an airway obstruction in the subject can be classified according to a second statistic value obtained by the subject being asked to breathe in a specific manner.
