This is a boy with an atraumatic knee effusion. An US guided arthrocentesis was performed with the probe placed in the transverse position and the needle entering at the superolateral aspect to the patella. Notice the ring-down artifact that comes from metal. You can also see this from metallic foreign bodies.

“The information in these cases has been changed to protect patient identity and confidentiality. The images are only provided for educational purposes and members agree not to download them, share them, or otherwise use them for any other purpose.”