Gastric bands are placed for the purposes of inducing weight loss. They cause formation of a small gastric pouch, restricting the volume of food eaten before satiety is achieved. The diameter of the lumen between the pouch and the remainder of the stomach is controlled by inflating or deflating the cuff by accessing the port with a special needle. Adjustment is often undertaken during a fluoroscopic barium study to evaluate the gastric pouch and the caliber of the opening into the remainder of the stomach. 

A complication is band slippage, when the stomach prolapses relative to the cuff. This may be assessed by evaluating the volume of the pouch on a barium study, and by evaluating the position of the cuff. The phi angle is between a line parallel to the vertebral column and the cuff, and should be between 4 and 58 degrees.