Gallstones typically display characteristic features on a CT scan. Depending on the calcium content and the chemical composition of the stones, they can appear as high-density, isodense, or low-density stones. High-density stones present as multiple areas of increased density within the gallbladder, though they can also be solitary. A solitary high-density stone generally appears as a concentric circle with low central density surrounded by high density. Isodense stones have the same density as bile, which sometimes makes them difficult to distinguish. Low-density stones, which are primarily cholesterol stones, can sometimes contain gas, appearing as low-density shadows. CT is not the first choice for diagnosing gallstones; abdominal ultrasound is generally preferred.