oa Biliary Secretion

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Bile is produced by the liver and stored in the gallbladder during fasting. With eating, bile is discharged into the small intestine. As a digestive secretion, bile acts as a detergent by forming mixed micelles with lipid nutrients, thereby enhancing their absorption. As an excretory secretion, it delivers lipid waste products to the intestine from which they are poorly absorbed. The main component of bile is conjugated bile acids, which are amphipathic steroids formed in the liver as the end-products of cholesterol metabolism. Bile acids are recycled between liver and small intestine (i.e. the enterohepatic circulation) with low plasma concentrations. In addition to providing the detergent property of bile and being a major driving force of bile flow, bile acids are signalling molecules that carry signals from the intestine to the liver. Homeostasis of bile acid synthesis and ileal conservation is essential not only to achieve the physiological functions of bile acids, but also to avoid pathological effects caused by their amphipathic property. Bile acid retention in the hepatocyte due to bile duct disease (e.g. PBC, PSC) leads to cell death. In terms of functional molecular imaging, dynamic planar cholescintigraphy is currently the modality of choice for the evaluation of the biliary system often with Tc-99m-mebrofenin as the radiotracer. Hepatic bile acid synthesis and handling, as well as intestinal malabsorption of bile acids, have been assessed by dynamic planar cholescintigraphy using the labelled bile acid analogue 75SeHCAT. Positron emission tomography (PET) using radiolabelled bile acid analogues has, to date, only been applied in preliminary studies regarding biliary secretion.