Hi Dave,
This makes perfect sense and it is all about ultrasound physics and specifically anisotropy.
When you scan short axis only the bit of the pulley at the top of the arch is specular to the ultrasound beam and hence this part will look more echogenic and “collagen” like. The components of the A1 pulley that are running down the side of the tendons has a collagen direction which will ensure anisotropy and hence they look black. Any part of the pulley will look echogenic and “collagen” if you can hit the fibre direction at close to 90 degrees.
This also explains why it looks more echogenic in the long axis as you are only seeing the “apex” of the pulley and you are perfectly specular to the collagen lines.
It is also interesting that the finger triggered when the pulley looked thin.
I have seen this many times where a thin but tight pulley causes triggering. It is often in the setting of a degree of tenosynovitis which makes the tendons larger and no longer able to fit beneath the pulley apparatus.
Steve