Fluid levels in medical imaging.

Fluid levels are commonly observed on a range of imaging methods in both normal and abnormal circumstances. Radiologists must be familiar with the appearances and significance of fluid levels, but more fundamentally, require an understanding of the mechanisms by which fluid levels occur and the principles necessary for the demonstration of fluid levels. These are the prerequisites of a cavity and at least two types of immiscible fluid, and most importantly, the requirement of an image that is orientated in the vertical plane.