Infant with projectile vomiting, weight loss and severe dehydration AXR (above) shows an extremely distended stomach with peristaltic waves (caterpillar sign). Sagittal (below left) and transverse (below right) US of the pylorus shows the pylorus muscle to be thickened and elongated in length, measuring 3.9 mm thick and 20 mm in length. The diagnosis was hypertrophic pyloric stenosis.