Hi Diane,
you are getting it,
I think your picture sums it up well.
the term sub-periosteal may be a bit confusing but it just means that the enthesis has a pitted surface. The pitting forms like little caves if you like in the surface of the enthesis. On ultrasound we sound getting “into the bone” and echoes returning from just beneath where we expect the enthesis bone surface to be. The enthesis bone surface looks irregular and pitted.
I will find some examples.
Steve