Magnetic Resonance Imaging does not use X-Rays (nor any other type of "ionizing" radiation). Instead, it is a technique that combines a large magnetic field and some radio frequency antennas ("coils").  First, the magnetic field causes the protons in the atoms of water within the patient to all "line-up". Then, a high-frequency electro-magnetic pulse knocks many of the protons out of alignment. Next, a very sensitive radio antenna "listens" for the "resonance" signal that each proton gives off, as it goes back into alignment. These minute resonance signals occur in a pattern that a computer uses to create 3D information. The pictures look like "sections" or "cuts" - just like in CT.  Except in the MR, the resulting image primarily reflects the water protons in the patient, as well as their chemical association with proteins, etc.