Comparison of fat-water MRI and single-voxel MRS in the assessment of hepatic and pancreatic fat fractions in humans

Abstract

The ability to accurately and noninvasively quantify fatty infiltration in organs such as the liver and the pancreas remains a critical component in understanding the link between obesity and its comorbidities such as type 2 diabetes and fatty liver disease. Single-voxel ((1)H) proton magnetic resonance spectroscopy (MRS) has long been regarded as the gold-standard noninvasive technique for such measurements. Recent advances in three-dimensional fat-water magnetic resonance imaging (MRI) methods have led to the development of rapid, robust, and quantitative approaches that can accurately characterize the proportion of fat and water content in underlying tissues across the full imaging volume, and hence entire organs of interest. One such technique is called IDEAL (Iterative Decomposition with Echo Asymmetry and Least squares estimation). This article prospectively compares three-dimensional (3D) IDEAL-MRI and single-voxel MRS in the assessment of hepatic (HFF) and pancreatic fat fraction (PFF) in 16 healthy subjects. MRS acquisitions took 3-4 min to complete whereas IDEAL acquisitions were completed in 20-s breath-holds. In the liver, there was a strong correlation (slope = 0.90, r(2) = 0.95, P < 0.001) between IDEAL and MRS-based fat fractions. In the pancreas, a poorer agreement between IDEAL and MRS was observed (slope = 0.32, r(2) = 0.51, P < 0.02). The discrepancy of PFF is likely explained by MRS signal contamination from surrounding visceral fat, presumably during respiratory motion. We conclude that IDEAL is equally accurate in characterizing hepatic fat content as MRS, and is potentially better suited for fat quantification in smaller organs such as the pancreas.

Figure 1

Results from oil–water emulsion experiment. Reconstructed (a) water, (b) fat, and (c) fat fraction images demonstrate excellent fat and water separation and show clear differences in the amounts of fat and water in each vial. (d) Correlation of true fat volume fraction against the mean fat fraction computed from IDEAL (circle) and MRS (open square) is strong across the full fat fraction range. The best-fit-lines are not shown because they nearly overlap the identity line (gray). Note that water in the bowl is correctly reconstructed to the water image. IDEAL, Iterative Decomposition with Echo Asymmetry and Least squares estimation; MRS, magnetic resonance spectroscopy.

Figure 2

Two examples of hepatic fat fraction (HFF) results from IDEAL and MRS. IDEAL-reconstructed (a) water, (b) fat, (c) fat fraction images are shown, along with (d) MRS plots. White boxes in (c) indicate location of the MRS voxel. IDEAL and MRS fat fractions agree closely in both subjects. Same color map is used from Figure 1. IDEAL, Iterative Decomposition with Echo Asymmetry and Least squares estimation; MRS, magnetic resonance spectroscopy.

Figure 3

(a) Correlation of hepatic fat fraction (HFF) between IDEAL and MRS in 16 studies. (b) Bland–Altman plot, with dashed lines representing the 95% confidence interval. (c) Correlation of pancreatic fat fraction (PFF) between IDEAL and MRS in 8 studies. (d) Corresponding Bland–Altman plot with dashed lines representing the 95% confidence interval. IDEAL, Iterative Decomposition with Echo Asymmetry and Least squares estimation; MRS, magnetic resonance spectroscopy.

Figure 4

Single reformatted coronal slice (fat fraction map) from five sets of axial acquisitions, showing contiguous coverage of the whole abdomen. Two source axial images are shown, one at the level of the liver and pancreas, and one at the level of the kidneys. Each axial acquisition was obtained in a 20-s breath-hold, acquiring a small slab as shown along the right. Profile along the dashed white line illustrates nonuniform fat fraction across the liver, highlighting the potential for IDEAL to detect heterogeneous organ fat infiltrations. The profile does not cross any hepatic vascular structures. Note the clear delineation of subcutaneous and visceral adipose tissue depots, along with fat fraction representation of major abdominal organs. HFF, hepatic fat fraction; IDEAL, Iterative Decomposition with Echo Asymmetry and Least squares estimation.