A pulsed field gradient is a short, timed pulse with spatial-dependent field intensity. Any gradient is identified by four characteristics: axis, strength, shape and duration.

Pulsed field gradient (PFG) techniques are key to magnetic resonance imaging

Magnetic resonance imaging

Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...

, spatially selective spectroscopy and studies of diffusion

Diffusion

Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 via NMR. PFG techniques are widely used as an alternative to phase cycling in modern NMR spectroscopy.

Common field gradients in NMR

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 = γ_IGz; where G is the gradient magnitude and γ_I is the gyromagnetic ratio of spin I. It introduces a phase factor to the magnetizations:

Φ (z,τ) = γ_IzGτ

The time duration τ is of order ms.