Double contour sign, pyrophosphate deposition disease or anisotropy

What’s the double contour sign in ultrasound (US)?
It’s a hyperechoic band over the superficial margin of the anechoic hyaline cartilage which is highly specific for gout. 

Pitfall double contour sign?
Assuming that you’re doing a great job with scanning the hyaline cartilage in normal joints perfectly perpendicular: leading to a cartilage interface artifact. This normal hyperechoic superficial margin of hyaline cartilage is thinner than the double contour sign and does not persist with dynamic movements because the double contour sign is not dependent on the angle of the US beam (not anisotropy related). 

Differential Diagnosis?
The double contour sign should be distinguished from the cartilage appearance in calcium pyrophosphate deposition disease, where a hyperechoic band from calcium deposits is seen within the middle of the hyaline cartilage, not at the margin.