Retained Stones. In the era prior to ercps, stones that left the gallbladder and entered into the common bile duct (cbd) had to be surgically removed by opening the cbd. At the end of the cbd exploration, a t-shaped tube is placed into the bile duct to prevent bile leakage from the duct and to allow a portal of entry into the cbd to retrieve any cbd stones left behind. The t-tube 'gram searches for these stones.
Any day. But usually before your discharge from hospital, when you are fully ambulatory and eating, for a routine post O P. \ T tube cholangiogram.