MRI uses a strong magnet to produce a strong magnetic field that pushes protons in the body to align with that field. After that a radio frequency pulse signal is then radiated through the patient stimulating the protons to spin out of equilibrium and pull against the magnetic field. When the radiofrequency field is turned off, the MRI coils are able to detect the energy released from the protons when realigning with the magnetic field. The amount of time taken for the protons to realign with the magnetic field, and the amount released of energy, depending on the environment and the chemical nature of the molecules. Based on these magnetic properties, physicians are able to tell the difference between various types of tissues.