A computer-implemented method of analysing Magnetic Resonance Imaging data using the magnitude from acquired multi-echo MRI images to determine the relative signal contributions of at least two species to each voxel of the images, wherein one of the species is fat and another is water, the method comprising the steps of: using the magnitude of the multi-echo MRI images from the subject, where the multi-echo MRI images are acquired at arbitrarily timed echoes and an asymmetric combination of echo times including at least one echo time where water and fat are not substantially in-phase with each other; fitting the magnitude of said acquired multi-echo MRI images to a single signal model using a model fitting algorithm, wherein the single signal model includes a spectral model of the fat species with more than one spectral component to produce a plurality of potential solutions for the relative signal contributions for each of the at least two species from the model, by using a plurality of different starting conditions to generate a particular cost function value for each of the plurality of starting conditions, where said cost function values are independent of a field map term for the muti-echo MRI images; and analysing said cost function values to calculate the relative signal separation contribution for each species at each voxel of the multi-echo MRI images; wherein said analysis of said cost function comprises the step of: comparing the generated cost function values to determine which is the correct solution for said signal separation; wherein the lowest cost function value of said species is determined to be the correct solution for said signal; characterised by using a gradient echo protocol including at least six echoes, and generating each of the cost function values using all of the echoes in a single step when performing the fitting of the magnitude to the single signal model. using the magnitude of the multi-echo MRI images from the subject, where the multi-echo MRI images are acquired at arbitrarily timed echoes and an asymmetric combination of echo times including at least one echo time where water and fat are not substantially in-phase with each other; fitting the magnitude of said acquired multi-echo MRI images to a single signal model using a model fitting algorithm, wherein the single signal model includes a spectral model of the fat species with more than one spectral component to produce a plurality of potential solutions for the relative signal contributions for each of the at least two species from the model, by using a plurality of different starting conditions to generate a particular cost function value for each of the plurality of starting conditions, where said cost function values are independent of a field map term for the muti-echo MRI images; and analysing said cost function values to calculate the relative signal separation contribution for each species at each voxel of the multi-echo MRI images; wherein said analysis of said cost function comprises the step of: comparing the generated cost function values to determine which is the correct solution for said signal separation; wherein the lowest cost function value of said species is determined to be the correct solution for said signal; characterised by using a gradient echo protocol including at least six echoes, and generating each of the cost function values using all of the echoes in a single step when performing the fitting of the magnitude to the single signal model.