Are units refer to a diagnostic modality in which the magnetic nuclei (especially protons) of a patient are aligned in a strong, uniform magnetic field, absorb energy from tuned radio frequency pulses, and emit radio frequency signals as their excitation decays. These signals, which vary in intensity according to nuclear abundance and molecular chemical environment, are converted into sets of tomographic images by using field gradients in the magnetic field, which permit 3-D localisation of the point sources of the signals. Unlike conventional radiography or CT, MRI does not expose patients to ionising radiation.