Gallstones. Coloured X-ray of the abdomen showing three large gallstones (blue, left) in the gall bladder. Gallstones are hard masses composed of varying amounts of bile pigments, cholesterol and calcium salts. They develop when an upset occurs in the chemical composition of bile. They are slow growing, forming a series of layers. Only those gallstones with a relatively high calcium content become visible on X-ray film. Gallstones with a low calcium content can only be seen using cholecystography, a special X-ray technique in which the gall bladder is outlined with a contrast medium, such as iodine.
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