Pyloric stenosis is idiopathic pyloric thickening resulting in projectile vomiting, dehydration and hypochloraemic acidosis. Patients usually present between the ages of one week and three months with a male to female ratio of five to one. The first two images demonstrate a normal pylorus. The remaining images demonstrate a thickened, abnormal pylorus. The diagnosis is often made using ultrasound, but clinical examination also has a high sensitivity and specificity.

I use the measurements 5, 10 and 15 mm as an aid to diagnosis, where 5 mm is the muscle wall thickness, 10 mm is the total thickness and 15 mm is the length. These numbers are easy to remember as abnormal, but should not be thought of as cut off measurements - the values vary in the literature. It is important to assess the sonographic appearance of the muscle and to ensure that the pyloric canal does not open throughout the examination.

See here for more information: Hernanz-Schulman M. Infantile Hypertrophic Pyloric Stenosis1. Radiology. 2003;227(2):319–331.