Nonuniformity in the RF field is a very significant factor in image uniformity and depends strongly on the design and geometry of the RF coils (transmit and receive coils) in use. Flat surface coils produce (e.g. spine) images with severe fall-off of signal with distance from the coil, and therefore it is not appropriate to evaluate uniformity with such coils. Good uniformity should be expected for coils with cylindrical geometry, but only in the xy plane. Uniformity is often of a lesser standard in the z-direction (head-feet), depending on the length of the coil.
Uniformity can be affected by a poor main magnetic field homogeneity. A badly shimmed magnet is an example of this, although this is very unlikely to occur in a modern commercial system. Artefacts can be obvious when using frequency selective fat suppression. Inhomogeneities can also be introduced if the patient has a ferrous implant. Spin echo techniques avoid the problems this can present.
Particular pulse sequences depend on RF pulses being delivered uniformly across the slice. Any significant variation in the transmit field will lead for example in a STIR pulse sequence to non-uniform fat suppression.