Magnetic resonance image of two causes for dry tap of knee: Lipoma arborescens and a gelatinous mass

MRI showing 2 causes for potential failed arthrocentesis: Chronic synovial hypertrophy with fatty villous hypertrophy (lipoma arborescens, LA), and a nonenhancing gelatinous mass (G). Only placement of the needle into the enhancing fluid area (F, arrow) would successfully yield fluid.