A 45-year-old woman presents with flank pain and microscopic haematuria. Unenhanced CT KUB shows a 6 mm calculus in the left ureter at the level of the pelvic brim with upstream hydronephrosis. The calculus measures 800 Hounsfield Units (HU). Which type of calculus does this density suggest, and does it favour lithotripsy?
- A Calcium oxalate monohydrate; hard, relatively resistant to SWL ✓
- B Uric acid; radiolucent and favourably treated with urinary alkalinisation
- C Struvite; associated with urea-splitting organisms
- D Calcium phosphate (brushite); very high HU, hard and resistant to SWL
Explanation
Unenhanced CT Hounsfield unit (HU) density measurement provides information on calculus composition, which influences management. Uric acid stones are typically 200-400 HU and are radiolucent on plain X-ray; they respond to urinary alkalinisation with potassium citrate. Calcium oxalate dihydrate stones are moderately dense (400-800 HU) and relatively amenable to SWL. Calcium oxalate monohydrate and calcium phosphate (brushite) stones are very dense (800-1600+ HU), making them hard and relatively resistant to shock wave lithotripsy. For a 6 mm stone at 800 HU, the density suggests monohydrate composition and ureteroscopy with laser lithotripsy may be preferred over SWL.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.