The effect of a uniform magnetic field gradient in the z-direction on spin I, is considered to be a rotation around z-axis by an angle = γGz; where Gz is the gradient magnitude (along the z-direction) and γ is the gyromagnetic ratio of spin I. It introduces a phase factor to the magnetizations:
Φ (z,τ) = (γ)(Gz)(τ)
The time duration τ is in the order of milliseconds.