Hi Jonathan,
Absolutely I have seen this before.
This is an appearance that you get sometimes when you have an acute hydroxyapatite deposition within a tendon.
I have seen this exact appearance in the supraspinatus and the subscapularis before.
It is one of the appearances you can get as the hydroxyapatite matures.
It is a type of liquificateon of the hydroxyapatite deposit.
If you scanned it a week ago it would have just looked like a regular calcific tendinosis, but now the deposit has liquified and you can see the central cavity.
If you stick a needle into it you usually get an aspiration that is somewhat orange in colour with fluid / blood and hydroxyapatite crystals making up the contents.
You can see a similar appearance when we perform a barbotage / lavage procedure on a hydroxyapatite deposit. As we irrigate and aspirate the deposit it reveals this central cavity which looks just like your image.
Steve