The images demonstrate that there is a crossover of the main pancreatic duct and of the distal common bile duct. The main pancreatic duct drains into the the accessory duct (Santorini) which emerges into the duodenum from the proximal minor papilla. The uncinate process of the pancreas drains via the main pancreatic duct (Wirsung) into the main ampulla, also receiving the common bile duct.
These appearances are those of pancreas divisum. This appears due to failure of fusion of the ventral pancreatic bud with the larger dorsal pancreatic bud in the 8th week in utero. The ventral pancreas drains through the Wirsung duct which after the 8th week typically becomes the distal main pancreatic duct emerging into the duodenum at the ampulla. The duct of Santorini which drains the dorsal pancreas distally into the minor ampulla usually becomes atrophic allowing drainage through the more inferior Wirsung duct. Note that confusingly due to rotation of the pancreas in utero the posterior inferior head of the pancreas is actually congenitally the ventral pancreas. When the two duct systems fail to fuse pancreas divisum arises.
Pancreas divisum is present in approximately 10% of patients, depending on how this is evaluated (postmortem 7%, ERCP 10-14%) and the definition of divisum. Although Asian populations have been reported to have lower incidence (1-2%) this has been stated in part in part to relate to less invasive testing. Many of the higher incidences of pancreas divisum relate to a definition that includes predominant flow into the duct of Santorini with small branch duct communication from the pancreatic duct to a small Wirsung duct. Radiologically the term at MRCP is usually confined to cases where there is complete separation with no bridging duct between the pancreatic duct in the neck/head and the Wirsung duct. This results in a crossover appearance of the CBD extending down into the ampulla and of the pancreatic duct extending into the duct of Santorini.
Most cases of pancreas divisum are asymptomatic. In a small minority of cases recurrent pancreatitis may be related to pancreas divisum, due to inadequate drainage from the minor ampulla and the narrowed distal duct of Santorini. This can result in focal ovoid dilatation proximal to the ampulla (Santorinicoele). In these cases treatment can be by sphincterotomy or duct stenting.