Complex sentence: The values in the phase images are constrained from-π to π so if the value goes above π it wraps to-π, inhomogeneities in the magnetic field cause low frequency background gradients.
Split into simpler sentences: The values in the phase images are constrained from-π to π so if the value goes above π it wraps to-π. Inhomogeneities in the magnetic field cause low frequency background gradients.