Water consists of hydrogen and oxygen atoms. At each hydrogen atom’s center is a single proton; a small particle with a positive charge that is very sensitive to magnetic fields. As the patient lies under the scanner magnet, all protons in the body line up in the same direction, similar to the way a magnet directs the needle of a compass.
Radio waves knock protons out of alignment. When the radio waves stop, the protons realign and send out small radio signals that MRI receivers are able to pick up. Protons in different types of tissue realign at different speeds and thus produce distinct signals, allowing the machine to create an accurate view of the body. Signals from millions of protons create a detailed image, comparable to the pixels of a television screen.