CT and MRI features of gallbladder and biliary tree calculi
What are the CT and MR imaging features of gallbladder and biliary tree calculi?
Calculi appear as nonenhancing round, oval, or polygonal filling defects in the gallbladder or biliary tree and are usually mobile and seen to be layering along the dependent luminal surface.
On CT, calculi may sometimes have attenuation similar to bile, in which case they are not visible. This accounts for the suboptimal sensitivity of CT (≈75% to 80%) for the detection of calculi in the gallbladder and biliary tree. In other cases, they may either have low attenuation (related to cholesterol content) or high attenuation (related to calcium content) components relative to bile and are then visible. Occasionally, very low attenuation stellate foci of nitrogen gas may be seen within central fissures of the calculi, leading to the “Mercedes Benz” sign
On MRI, calculi have low signal intensity on T2-weighted images relative to surrounding high signal intensity bile and may either have low T1-weighted signal intensity relative to bile when composed of cholesterol or high T1-weighted signal intensity relative to bile when composed of biliary pigments. The sensitivity of MRI for detection of calculi in the gallbladder and biliary tree is >95%. Associated upstream bile duct dilation may be visualized when there is biliary obstruction.