1. A method for forming an image of an object, the method comprising: (a) taking magnetic resonance imaging data of the object; (b) estimating a field map from the magnetic resonance imaging data by demodulating the magnetic resonance imaging data at a plurality of demodulation frequencies, wherein the field map has a lower resolution than the magnetic resonance imaging data; (c) fitting a linear map to the field map; and (d) forming the image using the field map.