What are the types of gallstones, and what proportion are radio-opaque?

Clinical relevance. Knowing most stones are radiolucent explains why ultrasound, not plain X-ray, is used to detect them.

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Types of gallstones

Gallstones are classified by composition, which relates to their cause and radiological appearance.

Types

• Cholesterol stones: from cholesterol supersaturation (often associated with the classic risk factors).
• Pigment stones: from excess bilirubin, e.g. in chronic haemolysis.
• Mixed stones: the commonest, containing cholesterol and pigment.

Radiological appearance

• Only about 10% are radio-opaque (those with enough calcium), so most are radiolucent and not seen on plain X-ray.
• Ultrasound is the imaging of choice for detecting gallstones.