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.
