The lack of a 180° pulse in gradient echo techniques means that T2* effects are not reversed during the sequence, and a greater dephasing of spins than in spin echo techniques results. For this reason, GE techniques possess a greater sensitivity to magnetic susceptibility effects. These are most noticeable at air-tissue interfaces, in the presence of metallic implants or in the presence of haemorrhage.
Magnetic susceptibility refers to the extent to which a substance will become magnetised when placed in a magnetic field. Think of it as how much a substance pulls the magnetic field lines of B0 about. Between two areas with different suceptibility, a small magnetic field gradient will exist. These gradients accelerate the dephasing between the protons on either side of the boundary. This causes signal loss.
Spin echo and gradient echo images. Susceptibility effects reduce the T2* in the vertebrae, causing signal loss in the GE image.