Some implementations provide a system that includes: a housing having a bore in which a subject to be image is placed; a main magnet configured to generate a volume of magnetic field within the bore, the volume of magnetic field having inhomogeneity below a defined threshold; one or more gradient coils configured to linearly vary the volume of magnetic field as a function of spatial location; one or more pulse generating coils configured to generate and apply radio frequency (RF) pulses to the volume of magnetic field in sequence to scan the portion of the subject; one or more shim gradient coils configured to perturb a spatial distribution of the linearly varying volume of magnetic field; and a control unit configured to operate the gradient coils, pulse generating coils, and shim gradient coils such that only the user-defined region within the volume of magnetic field is imaged.