Susceptibility artifacts occur as the result of microscopic gradients or variations in the magnetic field strength that occurs near the interfaces of substance of different magnetic susceptibility. Large susceptibility artifacts are commonly seen surrounding ferromagnetic objects inside of diamagnetic materials (such as the human body). These gradients cause dephasing of spins and frequency shifts of the surrounding tissues. The net result are bright and dark areas with spatial distortion of surrounding anatomy. These artifacts are worst with long echo times and with gradient echo sequences.