Don't assume that. Think of wherever you work. Think about how things get lost. Think about how mistakes are made there. Think about that time someone forgot something important and it messed up a bunch of other stuff.

Now think of your doctor's office. They're just a bunch of office workers with a couple of specialists. They fall prey to all the same mistakes as offices everywhere. Normally they should call you back to tell you the scan was negative. They didn't. Why? Because it was negative? Or did your result get picked up with someone else's and put in the wrong folder, unseen?

Call back and follow up. It's probably nothing, but why risk it?