Gallstone. Doppler ultrasound scan of the gallbladder of a 81 year old male patient showing a gallstone measuring 5 centimetres in the neck of the gallbladder. Gallstones (choleliths) are most commonly hard deposits of cholesterol that form due to an imbalance between bile and cholesterol levels. Stones can also arise from bilirubin and calcium salts found in bile, which is produced by the liver and then stored by the gallbladder until it is needed. Doppler ultrasound scans detect the speed and direction of moving fluids.