|Factors affecting the appearance|
COMPOSITION OF TISSUE
Solid organs, like the liver, have many internal tissue interfaces such as the bile ducts, portal vein branches, hepatic artery branches, hepatic veins, peri-portal fat, etc. The numerous interfaces cause many echos to be reflected whic are variable in intensity. This is why solid organs appear as a medium shade of gray on US. However, each organ is slightly different in its shade of gray than the others.
Fluid filled structures, as for example the gallbladder or urinary bladder, do not have internal tissue interfaces and therefore do not reflect sound waves. The walls of these structures do reflect sound waves. As a result, the fluid filled portion of the structure will appear black and the walls gray to white. The US image of the gallbladder on the right illustrates this.