I had one done on my left shoulder last year before my rotater cuff surgery. It's basically what you said. They numb it with a topical ointment of sorts, then they stick a long needle into your delt and inject it full of dye. They were looking at my rotater cuff, so they injected it into my rear delt. Then they take pictures using some type of MRI machine. It's not too bad. To answer your question about scar tissue, yes, it does show up on an MRI. My doc told me I had a ton of scar tissue in my shoulder but never asked me any questions about it.
Why are you having this done?