Overview: MRI uses magnetic fields to align subatomic particles in tissue and then uses a radio frequency to perturb them. The time the particles take to relax to their original state is used in conjunction with locational information to generate a 3D image.

MRI uses three magnetic fields. The static field is constant and determined by the strength of the main electromagnet. The patient is placed inside the center of this magnet. This static field causes all the subatomic particles composing the tissue to align their spin states. This alignment gives the system a base state to measure from.

Once the particles are aligned, another magnetic field, "pulse sequence", is released from a radio antenna. The pulse sequence is set based on the resonant frequency of the substance the scientist wishes to study. The most common substances are water or fat. The pulse sequence causes the particles responsive to the resonant frequency to relax from the aligned state to their original state. A receiver coil measures the relaxation time. The intensity of the signal recorded by the coil indicates the concentration of the substance in question.

Locational information is provided by a gradient field.

The combination of locational and intensity information is used to generate the three dimensional image of the tissue.