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The identification of gallstones with ultrasound has been highly accurate. Sonography has largely replaced oral cholecystography as the first line study for diagnosing gallstones. Gallstones presents as a sharp discontinuity to the smooth posterior wall of the gallbladder. A discrete acoustic shadow is caused by the absorption of sound within the stone and by reflection of sound by the stone.

Additionally, gallstones are seen to move freely within the gallbladder when the patient's position is changed, demonstrating that they are not a polyp attached to the gallbladder wall. Examining the patient in at least two positions is also important to detect stones which may be hidden and sometimes impacted within the gallbladder neck. Upon rotating the patient, these stones may fall into the gallbladder fundus. The common bile duct also should be included in the gallbladder examination since choledocholithiasis is one of the significant complications of gallstones.

In other situations, the stone may not be properly placed within the focal zone of the ultrasound beam, or the stone may be relatively \"soft,\" thereby failing to absorb enough sound to clearly demonstrate a shadow. Very small stones may also not demonstrate shadows because a portion of the sound beam passes around the stone, permitting the detection of echoes deep to the stone. Conversely, if the gallbladder has multiple stones, there may be nonvisualization of the body or fundus. This may result in visualization of the anterior gallbladder wall, an echogenic line representing numerous stones and then a large posterior shadow. This is the classic Wall-Echo-Shadow sign, indicative of a gallbladder full of stones.

One should also be aware that small stones are often the most clinically significant. It is such stones that have the opportunity to lodge in the gallbladder neck, cystic duct, or common bile duct. Such small stones are also most amenable to therapy, such as dissolution by oral medication or biliary lithotripsy.