Magnetic resonance imaging of gelatinous effusion of knee explaining dry tap

A series of axial T1-weighted images through the midportion of the patella from a patient with rheumatoid arthritis prior to and following the injection of intravenous gadolinium. Preinjection image (panel A) demonstrates a large joint effusion (E). An image taken 5 minutes after the intravenous injection of gadolinium (panel B) shows a rim of enhancing synovial fluid at the periphery of the joint (which is normal), adjacent to the synovium (arrowheads). A third image (panel C) obtained 30 minutes later, after passive exercise and manual massage of the knee joint to diffuse the gadolinium, shows enhancement of most of the effusion (E), with the exception of a small area on the medial side of the joint (arrow). This area presumably represents a semisolid or gelatinous area of the effusion. Placement of an arthrocentesis needle directly into this area could yield a dry tap in spite of the patient's obvious effusion.