FDG PET/CT of the non-malignant liver in an increasingly obese world population

Because of obesity, non-alcoholic fatty liver disease (NAFLD) is becoming increasingly important. 10% of NAFLD patients develop non-alcoholic steatohepatitis (NASH), which may progress to cirrhosis and is now the leading indication for liver transplantation in the Western world. Prefibrotic NASH can only be reliably diagnosed by biopsy. However, given its success in other inflammatory diseases, PET/CT with ¹⁸ F-fluorodeoxyglucose (FDG), although non-specific, may offer a promising approach to diagnosing not only NASH but also other inflammatory liver conditions. In addition, FDG PET has generated pathophysiological information on hepatic glucose metabolism and, diagnostically, used liver for quantification of tumour FDG accumulation (e.g. Deauville scoring). A review of hepatic FDG PET is therefore timely. There are two general approaches to the quantification of hepatic FDG accumulation: firstly, standard uptake value (SUV) and secondly dynamic PET. SUV is a poor index of hepatic metabolic function because most hepatic FDG (~75%) is un-phosphorylated 60-min postinjection. Hepatic fat is increased in NAFLD but accumulates negligible FDG. Because fat distribution is heterogeneous, maximum pixel SUV is therefore preferred to mean pixel SUV. Computer modelling of dynamic PET dissects the transport constants governing hepatic FDG kinetics but is challenged by the liver's dual blood supply. Graphical analysis is less informative but more robust and will be the preferred clinical approach to measurement of hepatic FDG phosphorylation. Previous dynamic PET studies have ignored hepatic fat and therefore potentially underestimated glucose accumulation in patients with hepatic steatosis. Future work should use graphical analysis of dynamic PET and correction for hepatic fat.