Classification Gastric outlet obstruction: only one/two air-fluid levels Homogeneous mass displacing transverse colon Duodenal obstruction: double-bubble sign; Frequently normal due to absence of gas from vomiting Classification Gastric outlet obstruction: – only one/two air-fluid levels – Homogeneous mass displacing transverse colon Duodenal obstruction: – double-bubble sign; – Frequently normal due to absence of gas from vomiting