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Magnetic Resonance Imaging (MRI) 
Proton nuclear magnetic resonance (NMR) detects the presence of hydrogens (protons) by subjecting them to a large magnetic field to partially polarize the nuclear spins, then exciting the spins with properly tuned radio frequency (RF) radiation, and then detecting weak radio frequency radiation from them as they "relax" from this magnetic interaction. The frequency of this proton "signal" is proportional to the magnetic field to which they are subjected during this relaxation process. In MRI, an image of a cross-section of tissue can be made by producing a well-calibrated magnetic field gradient across the tissue so that a certain value of magnetic field can be associated with a given location in the tissue. Since the proton signal frequency is proportional to that magnetic field, a given proton signal frequency can be assigned to a location in the tissue. This provides the information to map the tissue in terms of the protons present there. Since the proton density varies with the type of tissue, a certain amount of contrast is achieved to image the organs and other tissue variations in the subject tissue.
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