Multi-echo chemical shift based water-fat separation methods are seeing increasing clinical use due to their ability to estimate and correct for field inhomogeneities. Previous chemical-shift based water-fat separation methods have used a relatively simple signal model that assumes both water and fat have a single resonant frequency. However, it is well known that fat has several spectral peaks. This inaccuracy in the signal model results in two undesired effects. First, water and fat are incompletely separated. Second, methods designed to estimate T2* in the presence of fat incorrectly estimate the T2* decay in tissues containing fat. In this work, a more accurate multi-frequency model of fat is included in the IDEAL water-fat separation and simultaneous T2* estimation techniques. The fat spectrum can be assumed to be constant in all subjects and measured a priori using MR spectroscopy. Alternatively, the fat spectrum can be estimated directly from the data using novel spectrum self-calibration algorithms. The improvement in water-fat separation and T2* estimation is demonstrated in a variety of in-vivo applications, including knee, ankle, spine, breast and abdominal scans.
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