The identification of gallstones with ultrasound has been highly accurate. Sonography has largely replaced oral cholecystography as the first line study for diagnosing gallstones. This case represents a typical, easily diagnosable gallstone. It 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, the structure is seen to move freely within the gallbladder when the patient's position is changed, demonstrating that it is not a polyp attached to the gallbladder wall. Note that the patient was examined in two different positions to demonstrate this finding. 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.
The acoustic shadow is not clearly identified on all images of the gallstone. It is important to attempt to demonstrate this finding, but it may not be seen in every view. In this case, the shadow is obscured by bowel gas lying immediately adjacent to the stone on Scan 2. 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.
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.
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