Hi Mike,
There are a few options here,
It is certainly an inflammatory condition.
The sub-cutaneous fat is really echogenic and you report that the knee is swollen, red and warm.
I agree there is increased vascularity n the quadriceps tendon and the tendon itself is more echogenic than I would expect.
The knee joint does NOT appear to have any effusion and it might be worth putting a power Doppler box into the supra patellar pouch to see if there is any knee joint synovitis. The absence of a knee joint effusion makes my think primary knee synovitis is unlikely.
So that leaves us with things that cause inflammation in the sub-cutaneous tissues superficial to the knee joint.
Pre patellar bursitis
Cellulitis
Gout
Anything else you can think of?
I include gout as this would explain the involvement of the quad tendon.
You could do some bloods
I can’t see anything that is aspiratable at this stage.
Steve.