Nuclear Magnetic Resonance (NMR) is most commonly known for its use in medical imaging (Magnetic Resonance Imaging, MRI). It is a phenomenon in which the nuclei of material placed in a strong magnetic field will absorb radio waves supplied by a transmitter at particular frequencies. The energy of the radio-frequency photons is used to promote the nucleus from a low-energy state, in which the nuclear spin is aligned parallel to the strong magnetic field, to a higher-energy state in which the spin is opposed to the field. When the source of the radio waves is turned off, many nuclei will revert to the lower state by emitting photons at the characteristic resonance frequencies, providing information about the sample material. Because the single protons in biologically abundant hydrogen atoms react well to the technique, it can produce excellent cross-sectional images of bone marrow and tissue.