Nuclei that have an odd number of protons or neutrons (eg 1H, 31P, 17O) spin. Imaging using the proton 1H is more practical as it is the most abundant. The normal random orientation of the nuclei become parallel to the field when subjected to a very strong magnetic field. A radio frequency pulse is applied displacement of the nuclei in proportion to the strength of the radio frequency pulse. The magnetisation induces a signal in a receiver coil. After the radio frequency pulse nuclei regress to their original equilibrium position signal decreases
T1 weighted image – the fat image – tissues that have a shorter relaxation rate have a greater signal intensity.
T2 weighted image – the water image – tissues that contain higher amounts of free hydrogen produce a brighter signal