Various embodiments are described herein for an apparatus and a method for measuring and characterizing geometric distortions for a region of interest in images obtained using magnetic resonance. The method comprises deriving a computed set of 3D distortion vectors for a set of points within a region of interest covered by a phantom by using harmonic analysis to solve an associated boundary value problem based on boundary conditions derived from a measured set of 3D distortion vectors. The characterized image distortions may be used for various purposes such as for image correction or for shimming, for example.