FIGURE 116.13 Hypertrophic pyloric stenosis Ultrasonography of the abdomen shows a thickened pyloric muscle and elongated pyloric channel If the US study does not show a hypertrophic pylorus and the patient has significant forceful emesis, an upper GI series may be considered in order to eliminate gastroesophageal reflux, malrotation, and antral web as diagnostic possibilities In general, pyloric stenosis can be identified by the presence of a “string sign” in the pyloric channel, seen best on oblique projections on the upper GI series ( Fig 116.14 ) For patients with an initially normal US but progressive emesis, repeat US should be performed 24 to 48 hours later to remeasure the pylorus