What can and can't you image/scan and why?

[mrphys]

Is it only the protons of water that can be (are) excited in MRI?

Why can't I scan a book? What determines what can and can't be scanned?

[spinecho]

You can tune MR to image anything, it's just that the quality of images of organic stuff is best when protons are imaged - best signal to noise can be achieved. There are plenty of protons around. The first MR image was an image of tube filled with water... Organic stuff has a bunch of CH- type molecules and a bunch of water H2O - plenty of protons.