1. A method for using magnetic resonance imaging (MRI) to characterize fat, comprising: acquiring a magnetic resonance (MR) image that includes MR data from a target; determining fat characterization parameters based on the acquired MR data, wherein the fat characterization parameters include an amount of fat, an amount of water, an initial phase, a field inhomogeneity, a T2*, and at least one fat spectrum parameter, and wherein the at least one fat spectrum parameter includes a number of double bonds (NDB), a number of double-double bonds (NDDB), or a chain length (CL) of fat from the target in the MR data; and using the determined fat characterization parameters to produce a relationship between regions of fat, regions of water, or regions of fat and water in the MR image, wherein the determining fat characterization parameters comprises: selecting initial values of the fat characterization parameters; estimating values of the fat characterization parameters by iteratively minimizing error between the fat characterization parameters and the acquired MR data; generating an MR signal by simulation using the estimated values of the fat characterization parameters; and matching the simulated MR signal with an actual MR signal based on the acquired MR data, wherein the determining the fat characterization parameters is in accordance with: 
 s ( t )=[ w ·α( t )+ f ·β( t )]·exp( i φ)·exp(− R 2* t )·exp( iΔB 0 t ) 
 α( t )= a water ·exp( iωd water t ) 
 β( t )=Σ a j ( NDB )·exp( iωd j t ), where: a water represents the number of protons in water, a j represents the number of protons in fat, d water represents chemical shifts in water, d j represents chemical shifts in fat, ΔB 0 represents the field inhomogeneity, R2* represents transverse decay (T2*), φ represents the initial phase, w represents the amount of water, and f represents the amount of fat.