Nice example Xue,
Personally I don’t think you can tell the difference with ultrasound.
With synovitis you have synovial proliferation.
With lipoma arborescens you have synovial proliferation just the same but containing more fat within it.
So pretty similar.
Both will have some Doppler signal.
The lipomatous type will be a little more echogenic than the plain synovitis and your example is certainly echogenic.
Lipoma arborescens is common in the knee so you have the correct location so I think it is likely to be the lipomatous type.
Can I be 100% sure? not really.
A CT or MRI will settle the debate gif it is warranted.
Either way it is a benign condition
Steve