Hi Diane,
Yes this would be MSU I think !
Fat pad impingement is most commonly not diagnosable with ultrasound but rather MRI,
The impingement occurs under the acoustic shadow of the bony patella so hence we don’t see it well,
In some cases the impingement causes a large enough area of fat pad oedema that we see it protruding from beneath the patella as an echogenic blush in the fat pad.
Most cases I scan we have an MRI already performed and you can easily see the increased signal in the fat pad and then the ultrasound is normal. We sometimes are asked to inject the area and we simply guide the needle under the patella in the area where the MRI shows the abnormality.
I have never sen it protruding into the patellar tendon or anything cool like that.
Steve