More than 20 million Americans have gallstones. Gallstones are more common in females and increase in incidence with age. There is also increased incidence in American Indians. Predisposing conditions for gallstone formation include hemolytic anemia, obesity, diabetes, pregnancy, cirrhosis, drugs (cholestyramine and clofibrate), hyperparathyroidism and abnormal enterohepatic circulation in patients with severe small bowel disease such as Crohn's or post-SB resection.

Gallstones are composed of varying proportions of cholesterol, bilirubin, and calcium carbonate. Most gallstones are of mixed composition. Approximately 75% are predominantly cholesterol and 25% are pigment stones. Gallstones may be single or multiple and range in size from 1 mm to 3-4 cm. The shape and surface characteristics vary widely. Although gallstones may be detected by plain films, sonography, OCG and CT, sonography is usually the primary examination done to make the diagnosis.

PLAIN FILMS: 10-15% of gallstones contain enough calcium carbonate to be visible on plain films (Film .3). Although the pattern of RUQ calcification is usually characteristic (rim, central nidus, or laminated) other RUQ calcification may mimic the appearance of gallstones. A unique plain film sign, a lucent stellate appearance (Film .4) resembling the symbol of a Mercedes-Benz (arrow), may permit the diagnosis of gallstones in the absence of calcification. The OCG (Film .5) verifies the fissure in the stone. This fissuring is thought to be secondary to nitrogen gas within clefts in the gallstone and is known as the "Mercedes Benz Sign."

ULTRASOUND: Currently, real time sonography is considered the procedure of choice for the diagnosis of cholelithiasis. The examination is easily and quickly performed on fasting patients, requires no medication and no ionizing radiation and detects calculi with greater than 95% accuracy. The sonographic diagnosis of gallstones is made by demonstrating, mobile, echogenic foci within the gallbladder lumen. Acoustic shadowing is frequently shown and is dependent on the technique. Stones usually occupy a dependent position in the gallbladder.

Cholelithiasis presents on US as one of 3 general patterns (Films .6 - .8): Film .6) shadowing echogenic foci within the gallbladder lumen, Film .7) an echogenic focus within the gallbladder fossa with posterior acoustic shadowing (no visualization of gallbladder itself), and Film .8) non-shadowing echogenic foci within gallbladder, usually <5 mm, which may or may not move. Non-visualization of the gallbladder in a fasting patient usually is secondary to a contracted gallbladder containing stones. An acoustic shadow coming from the gallbladder fossa will frequently be seen. If the ultrasound results are inconclusive, an OCG is done. OCG is also used as a complementary study to US in the assessment of patients for non-surgical therapies such as lithotripsy.

ORAL CHOLECYSTOGRAPHY: A properly performed OCG with adequate visualization of the gallbladder has a 92-95% accuracy in identifying calculi. Eighty-five percent of stones are mainly cholesterol and are not visible on plain films, but will be detected on OCG. On OCG, stones appear as negative filling defects (Film .9, left image) which layer in a dependent position on the upright film (Film .9, right image).

Some stones have a specific gravity less than that of contrast-laden bile and will float or layer within the bile on an upright or decubitus film (Film .10).

COMPUTED TOMOGRAPHY: Because of higher cost and lower sensitivity (approximately 80%) compared to US, CT is not used as a primary method for detecting gallstones. However, both calcified and non-calcified gallstones can be seen with CT. Calcified stones are easily visualized as small masses of high-attenuation in the dependent portion of the gallbladder (arrows) surrounded by low-density bile (Film .11). Non-calcified or cholesterol stones appear as low-attenuation filling defects (arrow) within the surrounding bile (Film .12). Detection of small, layering non-opaque stones is difficult and may require thin sections and careful scrutiny of the scans at low window levels.