1. A method of obtaining magnetic resonance signals with signal separation for at least two chemical species in a heterogeneous magnetic field, comprising the steps of: a) obtaining first magnetic resonance signals from pixels in an object having at least two chemical species using a first repetition time and a first echo time, b) obtaining at least second and third magnetic resonant signals from the pixels using second and third echo times, c) determining a signal estimate for each pixel by combining all signals for the pixel using a linear least squares fitting to decompose the chemical species, assuming a first value of field heterogeneity (ψ 0 ), d) calculating a first error to the field heterogeneity, e) repeating step c) using the first value of field heterogeneity and the error from step d), f) repeating step d) to calculate a second error to the field heterogeneity, and g) updating the value of field heterogeneity and repeating steps c) and d) until an acceptable error is calculated.