Indications
Vomiting usually occurring occasionally at first, then progressing to occur after every feeding and becoming projectile, which is considered pathognomic for pyloric stenosis.
Delayed diagnosis can lead to failure to thrive, dehydration and electrolyte disturbances. It is the most common cause of non bilious surgical vomiting in infancy.

Use a high frequency linear transducer 9-15 with the patient in a supine position. 

For fussy babies use sugar water and pacifier.

Begin at midline epigastrium use the pancreas, left lobe of the liver and gallbladder as landmarks. If gassy give sips of liquid also turn right lateral decubitus so the gas and fluid will shift.
The pylorus like the appendix can vary in its orientation but is usually in a longitudinal lie with your transducer transverse over the epigastrium.

                                        Normal Pylorus

Pathology
Hypertrophic Pyloric Stensosis is caused by the thickening of the pyloric muscle resulting in a mechanical obstruction.
Typical presentation 4-8 weeks

                                       Pyloric Stenosis

Sonographic measurement parameters

• greater that 0.3 cm muscle thickness
• greater than 1.5 cm length

Beware pylorospasm, some patients may have a pylorus that looks thickened and positive when they are in fact having a spasm of the pylorus muscle. This is transient and should subside on it’s own or after feeding.

Sources:

http://pubs.rsna.org/doi/full/10.1148/radiol.2272011329

Mack H. A history of hypertrophic pyloric stenosis and its treatment. Bull Hist Med 1942; XII:465-689.

Teele RL, Smith EH. Ultrasound in the diagnosis of idiopathic hypertrophic pyloric stenosis. N Engl J Med 1977; 296:1149-1150.