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Figure 4: Ultrasound double contour sign. Transverse ultrasound image of the suprapatellar knee joint demonstrates two parallel hyperechoic contours on either side of the hypoechoic hyaline cartilage (HC). The deep echogenic contour (long arrows) represents the femoral cortex, while the superficial echogenic contour (arrowheads) represents uric acid crystals accumulating on the surface of the hypoechoic hyaline cartilage (HC).