Hi Xue,
This one is a puzzle and I would be keen to hear the opinion of anyone else lucky enough to see these images.
The things that come to mind:
It looks like it is within the extensor digitorum communus tendon sheath (extensor compartment 4).
It is bilateral,
It is clinically swollen but not painful.
what happens inside tendon sheaths in the wrist?
Synovitis (chronic in this case). It is usually less echogenic than this but may be echogenic on occasions. It is usually vascular like you are showing. It is usually more painful than this patient. It would explain why it is bilateral as this would be part of a systemic rheumatoid condition.
What about Lipoma Arborescens in this unusual location. It would explain the echogenicity, the chronic nature and the lack of pain???
PVNS / giant cell tumour. Usually less echogenic
Synovial haemangiona??? unlikely
I can’t think of anything else.
If you get some follow up please keep us informed,
Great case Xue,
Steve