medicare insurance question

I have been on medicare part a for quite a while, but I don't go to doctors.

Recently I am thinking of going for a round of medical tests and also possibly having my gall bladder out. I'm pretty sure when they do a nuclear imaging test they will find it is not working and they usually recommend surgery in those cases.

Is it possible to have this all done through part a, or would I have to re-sign up for part b?

Or is there some other insurance option available. I haven't kept up with all the changes in health care laws.