Renal Stones: Imaging

Key points

• Calcium oxalate stones are the commonest urinary stones and are radiopaque on plain X-ray.
• Mixed calcium oxalate/calcium phosphate stones are generally radiopaque; fixed percentage figures vary by population and should not be memorised as universal.
• Struvite (magnesium ammonium phosphate) stones are infection stones associated with urease-producing organisms such as Proteus and usually form in alkaline urine; they are radiopaque/poorly radiopaque depending on composition.
• Calcium phosphate stones are radiopaque and are associated with conditions including distal RTA and primary hyperparathyroidism.
• Uric acid stones are radiolucent on plain X-ray and are strongly associated with persistently acidic urine.
• Cystine stones are poorly radiopaque ; hexagonal cystine crystals are the classic urine finding.
• Staghorn calculi occupy the renal pelvis and extend into multiple calyces. They are classically infection-associated and often contain struvite, but not every staghorn stone is pure struvite.
• MRCP stone table: calcium = radiopaque; uric acid = radiolucent + acidic urine; struvite = urease/infection + alkaline urine; cystine = poorly radiopaque + hexagonal crystals.