Gout
- MSU crystal deposition; 1st MTP (podagra) commonest, then midfoot, ankle, knee, olecranon bursa, hand.
- Radiographs normal for years (except swelling during attacks).
- Chronic tophaceous gout: juxta‑articular or intra‑articular punched‑out erosions with sclerotic margins and overhanging edges (Martel's sign / "rat bite"); dense tophi (may calcify); joint space preserved until late; no periarticular osteopenia; asymmetric polyarticular.
- Ultrasound: double contour sign (urate on the cartilage surface), tophi, "snowstorm" effusion.
- Dual‑energy CT: colour‑coded urate deposits; sensitive and specific; helps when aspiration is not possible.