New exponential functions based on CT density to estimate the percentage of liver that is fat

In fluorine-18 fludeoxyglucose positron emission tomography/CT, hepatic standardized uptake value (SUV) is reduced through "signal dilution" by hepatic fat. The maximum SUV (SUV_max) is less affected than the mean SUV (SUV_mean), therefore SUV_max/SUV_mean correlates with hepatic fat. The SUV can be corrected for signal dilution using an equation relating CT density (CTD) to %fat. The objective was to exploit the relationship between SUV indices and CTD to assess the validity of two previously published equations (one linear and one sigmoid) for estimating %fat from CTD and two new exponential equations.

Study population comprised 465 patients having routine fluorine-18 fludeoxyglucose positron emission tomography/CT. The SUV_max, SUV_mean and CTD were measured from a 3-cm-diameter region of interest over the liver. The exponential equations assumed that 100% fat corresponds to CTD of -50 or -100 HU. The proportion of liver occupied by fat (P_F) was estimated from all four equations. Then fat-corrected SUV_mean is SUV_mean/(1 - P_F). The ideal equation should give SUV_mean approaching but not exceeding SUV_max and give fat-corrected SUV_mean/SUV_max that shows no correlation with CTD.

The linear equation failed at CTD values exceeding 55.8 HU because it gave negative P_F values. Moreover, fat-corrected SUV_mean/SUV_max still correlated with CTD. The sigmoid equation grossly overcorrected SUV_mean at low CTD. The exponential equations abolished the correlation between fat-corrected SUV_mean/SUV_max and CTD.

The sigmoid equation is unsuitable for estimating %fat from CTD. The linear equation performed well, but the exponential equation assuming that 100% fat corresponds to -50 HU performed best. Advances in knowledge: Improved (exponential) equations to estimate hepatic fat from hepatic CTD.