1. A method of imaging at least two chemical species using magnetic resonance imaging with signal separation for the two chemical species, the method comprising: acquiring first and second echo data at different echo times resulting in a first and second acquired complex dataset, modelling the first and second acquired dataset, said modelling comprising a spectral signal model of at least one of the chemical species, identifying in the first and second acquired dataset the voxels for which the modelling yields a single, unambiguous mathematical solution for the signal separation, and resolving the ambiguity for the voxels for which the modelling yields more than one mathematical solution, if any such voxels remain, further comprising predetermining the different echo times, wherein said predetermination comprises: selecting or calibating an appropriate spectral signal model of at least one of the chemical species, calculating the evolution of the amplitude and phase over echo time of the signal of at least one of the chemical species, described by time variant complex weights, selecting echo times for which these complex weights satisfy predefined constraints, by which the number of voxels for which the modelling yields more than one mathematical solution is minimized in the signal separation, and using the selected echo times for acquiring the first and second echo data resulting in the first and second acquired complex dataset.