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. This results in multiple reflected echos of variable intensity, and so the liver looks a medium shade of gray on US. Organs are slightly different from each other in their shade of gray on ultrasound based on differences in their tissue composition.

Fluid filled structures (gallbladder or urinary bladder) have no internal tissue interfaces and therefore do not 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.