Ultrasound Signs of Gout: Double Contour, Tophi, and 'Snowstorm' in Differential Diagnosis with CPPD
Double Contour Sign
The double contour sign ('urate icing') is a thin echogenic line on the surface of hyaline cartilage due to the deposition of monosodium urate crystals. It is the most sensitive sign of gouty arthritis and a specific sign of gouty arthropathy (Griffith, 2025).
Difference from the normal hyperechoic cartilage interface: the latter is visible only when the beam is perpendicular to the cartilage surface and is uniform, not dotted (Jacobson, 2026). The double contour sign disappears when serum urate levels drop below 6 mg/dL (Jacobson, 2026).
Important: although the double contour sign most often indicates gout, it can also be observed with calcium pyrophosphate dihydrate deposition in the presence of chondrocalcinosis (Jacobson, 2026).
Tophi
Tophi have a characteristic sonographic appearance: an amorphous but well-defined echogenic area with internal punctate hyperechoic foci, surrounded by a hypoechoic inflammatory rim; the internal dotted hyperechoic foci are particularly characteristic (Jacobson, 2026). Griffith (2025) describes tophi as formations with irregular edges and a variable hypoechoic rim — 'wet clump of sugar', with multiple grouped tophi being common.
| Type of Tophi | Ultrasound Characteristic |
|---|---|
| Soft Tophi | Hyperechoic without acoustic shadow |
| Hard Tophi | Hyperechoic with intense acoustic shadow |
Tendons and Synovial Effusion
Tendons and ligaments are likely the most common sites of crystal deposition in gout: from microdeposits (small hyperechoic foci) to large hyperechoic foci with intense acoustic shadow. The fibrillar pattern is disrupted, the tendon is stretched and displaced; acoustic shadow in formations >5 mm, hypervascularization in the active phase, rarely — tendon rupture (Griffith, 2025).
In gout effusion, there are hyperechoic aggregates: fine dotted echogenicity ('urate sand') and large echogenic clusters (snowstorm appearance) (Griffith, 2025).
Differential Diagnosis with CPPD (Calcium Pyrophosphate Deposition Disease)
The key difference in the localization of crystals relative to cartilage:
| Sign | Gout | CPPD / Calcium Pyrophosphate Deposition Disease |
|---|---|---|
| Hyaline Cartilage | Thin echogenic line on the cartilage surface (double contour, 'urate icing') | Echogenic signals inside the cartilage (chondrocalcinosis); echogenic line in the middle zone of the articular cartilage |
| Effusion | Multiple echogenic aggregates, 'snowstorm' | Fewer echogenic aggregates in joint fluid |
In CPPD, an echogenic focus in the meniscus (e.g., lateral) due to CPPD deposition is also described; such a pattern of crystal deposition inside the cartilage is quite specific for CPPD disease (Griffith, 2025).
Additional Imaging Methods for Tophi
Dual-energy CT (DECT) is highly specific for tophi, allowing automatic differentiation and quantitative assessment of the total volume of crystals, but requires crystal accumulation for visualization (Griffith, 2025). On MRI, tophi are isointense to muscle on T1WI, with variable intensity on T2WI; with contrast, they show heterogeneous 'honeycomb' enhancement (avascular urate surrounded by vascularized granulation tissue) (Griffith, 2025).
Frequently asked questions
How does the double contour sign in gout differ from the normal cartilage interface?
The normal hyperechoic interface is visible only when the beam is perpendicular and is uniform; the double contour in gout is dotted and persists regardless of angle (Jacobson, 2026).
How to differentiate gout from CPPD based on crystal location?
In gout, urate crystals deposit on the cartilage surface (double contour), while in CPPD, echo signals are located inside the cartilage (chondrocalcinosis), including in the middle zone of the articular cartilage and menisci (Griffith, 2025).
Is the double contour sign specific only to gout?
It most often indicates gout and is the most sensitive sign, but it can occur with calcium pyrophosphate dihydrate deposition in the presence of chondrocalcinosis (Jacobson, 2026).
What is the 'snowstorm' sign and when does it occur?
It refers to large echogenic clusters in synovial fluid (snowstorm appearance) in gout; in pseudogout, there are fewer echogenic aggregates in the fluid (Griffith, 2025).
Does the urate level affect the visualization of the double contour?
Yes, it has been shown that the double contour sign disappears when serum urate levels drop below 6 mg/dL (Jacobson, 2026).