See slides #18-24 from this presentation by my friend Philippe Desenne at HarvardX:

http://web.mit.edu/xTalks/Phil-Desenne-HarvardX.pdf

And try out the video annotation tool at: http://www.openvideoannotation.org/explore.php

Annotation tools in edX

http://www.equtek.com/clients/edX/annotation_beta/index.php

Do you know about the OpenSeaDragon project? https://openseadragon.github.io/

I’m not sure if this would work for medical images, but it might be worth exploring.