Intrahepatic fluorine-18-fluorodeoxyglucose kinetics measured by least squares nonlinear computer modelling and Gjedde-Patlak-Rutland graphical analysis.

Abstract

We aimed to use simple physiological equations to show similarities and differences in blood fluorine-18-fluorodeoxyglucose (F-FDG) clearance in the liver (Ki) and distribution volume (V0) in the liver, respectively, generated from nonlinear least squares computer modelling and Gjedde-Patlak-Rutland graphical analysis of dynamic F-FDG PET. We show theoretically that when, as is usually the case, vascular fraction (fraction of liver occupied by blood, Vb) is included as a parameter in modelling but ignored in graphical analysis, the ratio of Ki values, respectively, generated by graphical anlaysis (Ki) and by modelling (Ki) is equal to 1-Vb. This theoretical prediction was then confirmed from dynamic PET data acquired in a clinical population of patients undergoing routine F-FDG PET/computed tomography and from a review of the literature in which it can be seen that Ki/Ki ranges from 0.47 to 0.98. When Vb is not included as a parameter in modelling, Ki is theoretically equal to Ki and to V0·k3, and V0 is equal to V0. There are several attractions to normalising Ki to V0 with respect to liver. Thus, first, there is no need to correct imaging for photon attenuation. Second, it makes no difference whether uptake constant is expressed as blood or plasma clearance. Third, it circumvents the effects of hepatic fat, which, because it accumulates negligible F-FDG, physically dilutes the F-FDG signal and reduces both uptake constant and distribution volume.