Nuclear magnetic resonance is a non invasive technique of providing detailed images of sections through the body. It is especially useful for detecting brain tumours.
The nuclei of atoms have a property called spin which allows them to act like tiny magnets and align themselves with an applied magnetic field. They align imperfectly though, and oscillate about the magnetic field. This oscillation has a frequency in the radio part of the electromagnetic spectrum.
The particular frequency depends of the strength of the applied field and the particular nucleus, and is called the Larmor frequency. If a photon at the Larmor frequency hits the nucleus, it can be absorbed by the nucleus. The photon is re - emitted as the nucleus relaxes, after a time called the relaxation time. The emitted radio waves can be processed to produce an nuclear magnetic resonance scan.
The images produced are very detailed. One set is shown below.
