Figure 1: In each panel are displayed ultrasound image and corresponding joint aspirate with arrowheads indicating representative lesions. (a) Knee effusion, lateral transverse view, containing fibrin aggregates in a rheumatoid arthritis patient. (b) Posterior longitudinal view of popliteal cyst with hyperechoic specks due to gas bubbles suspended in viscous synovial fluid. (c) Posterior longitudinal view of popliteal cyst with calcium pyrophosphate aggregates. (d) Anterior longitudinal view of knee effusion from a patient with gout and hyperechoic specks due to monosodium urate crystals