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MRI stands for Magnetic Resonance Imaging. The general principal - constructing a computer calculated virtual image of a slice of the body - is the same as that for the CT scan. However, in this case, no xrays are involved. The patient is placed in a very strong but biologically harmless magnetic field, and a microwave radio signal is beamed through the body at a a specific frequ/EncyMaster/ which interacts with the water molecules in the body, causing them to absorb a tiny amount of energy. These molecules then produce their own little radio signals as they re-emit the energy they absorbed from the exitation beam. Computer software calculates the density of the body tissues at each point in the slice and constructs an image. As with the CT scanner, the software can manipulate the images to differentiate tissue types as well as produce three-dimensional views, longitudinal slices, and so forth. MRI, since it is basically imaging water densities, is best for soft tissues and least useful for bony structures (for which CT scanning is best).
We used to call MRI "Nuclear Magnetic Resonance", but the terminally uninformed got nervous about the Nuclear part of the name. There is nothing nuclear or concerning xradiation involved, believe me.