MRI Quantification of Liver Fat in an Obese Population

T. Tuthill, D. Raunig, A. Hickman, B. Peterson, B. Wyman VirtualScopics, Groton, CT, United States, Pfizer, Groton, CT, United States Introduction An elevation in liver fat content (hepatic steatosis) is a common but poorly understood aspect of liver disease and drug toxicity. Investigations into the mechanisms of hepatic steatosis are hampered by the lack of a reliable and non-invasive measure of liver fat content. Standard methods involving collection of liver biopsies are invasive and are prone to sampling errors. Recently, simpler MRI techniques have been developed to provide an estimation of fat fraction in the liver. Comparisons of liver fat content with body weight and circulating liver enzymes may help elucidate mechanisms of hepatic steatosis in obese subjects. Materials and Method In this study, 21 healthy obese subjects with normal hepatic function and body mass indices of 30 to 40 (normal BMI is <25) were imaged using a 1.5T MRI scanner (GE Medical Systems, Milwaukee, WI). Fast gradient echo scans were taken for the two sequences in the modified Dixon technique. In-phase echo time (TE) was 4.2ms with a repetition time (TR) of 9.3ms. Out-of-phase parameters were TE=1.8ms with TR=7.3ms. A 256 by 256 matrix was acquired with a 38 x 38 cm FOV for pixel size of 1.5 x 1.5 mm. Four axial slice pairs were obtained from each patient during two approximately 20second breathholds. Regions of interest (ROI) were selected in homogeneous sections of the liver avoiding hepatic and portal vessels and edges to reduce partial volume effects. One ROI was selected from each of four slices. For each ROI, the mean pixel signal intensity (SI) data was used to calculate the percent fat: Fat fraction = (SIin-phase – SIout-of-phase)/2SIin-phase. For each subject, blood samples were also taken to measure serum alanine aminotransferase (ALT), a common liver enzyme test. Results Figure 1 shows a sample MRI image pair with the selected ROI. For this set of subjects, the mean fat fraction measurements varied from 3 to 18%. Figure 2 shows a scatter plot of ALT levels versus fat fraction and a plot of BMI versus fat fraction.