2600 Fat content Steatosis, the accumulation of fat in the liver, can lead to fatty liver disease (steatohepatitis and cirrhosis) [56, 57]. The prevalence of steatosis (liver fat greater than 5% [58]) ranges from 30 to 34% [59–62] and is an early marker of cardiovascular disease [62]. Liver fat contents under 20% are required for donor livers [63]. Among current MRI methods for quantifying liver fat content [64], single-voxel MR spectroscopy (MRS) has become the gold standard owing to the demonstration of high correlations with pathological analysis of liver samples [65, 66]. Localized MRS is based on spin-echo sequences and must therefore be corrected for the T2 relaxation of the water and fat signals (which may vary amongst different tissues), and also for T1 relaxation if a short TR is used. Even properly performed single-voxel MRS yields information about one volume only, rendering the method nearly as irreproducible as analysis of biopsies. Recently, we adapted multivoxel MRS for the determination of reproducible hepatic fat contents in volunteers and patients [67, 68]. This possibility of determining the water-to-fat contents for an array of voxels, yielding the average liver fat content with the standard deviation being a measure of heterogeneity, is illustrated by Fig. 5. Although MRS measurements of liver fat content are performed without suppression of the water signal, an additional water-suppressed measurement can be used to assess the composition of the fatty acid chains by focusing on the minor signals of the methene, diallylic, methylene to carboxyl, and allylic groups [69]. This can lead to improved understanding and management of fatty liver disorders. MRI methods for quantification of liver fat content are faster, though, and therefore recommendable for use in clinical practice. Those in most widespread use are based on Dixon’s two-point technique [70], for instance, a breath- hold gradient echo dual flip angle adaptation to generate intermediate-weighted and T1-weighted opposed-phase and in-phase images from which tissue fat contents can be calculated [71]. Even after taking precautions to prevent occasional mix-ups of water and fat signals, the MRI fat determinations tend to overestimate liver fat content by up to 4%, especially at the lower end of the range encountered in most healthy individuals (fat content less than 10%) [68]. The correlation between MRI and MRS data is strong, however, so the systematic error in MRI measurements can be corrected for [68]. Others have proposed variations in the Dixon method using either three or more acquisitions rather than two. In one of these studies, using seven echoes, excellent correspondence was found between liver fat contents according to MRI and MRS [72]; however, as pointed out elsewhere [73], at the TR of 2,500 ms—the only one used—the spectral water signals were saturated relative to the fat (T1water ≫ T1fat). The resulting uncorrected systematic overestimation of liver fat in the MRS data implies that the closely matched MRI fat contents reported Fig. 5 The volume of interest (a), spectral map (b) and a curve fitted spectrum (c) of a healthy (obese) volunteer. The water and fat peaks in the 32 quantified voxels yield a liver fat content, i.e. the ratio of fat/(water + fat), of 21.3±5.5%. Unlike single-voxel MRS giving a single value, multivoxel MRS provides a mean with a standard deviation, the latter being a measure of tissue heterogeneity (unpublished material from a study by Irwan et al. [68])