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The Standard Uptake Value (SUV) approach in oncological PET has known shortcomings all of which affect the reliability of the SUV as a surrogate of the targeted quantity, the metabolic rate of FDG ([18F]fluorodeoxyglucose), Km. Among the shortcomings are time dependence, susceptibility to errors in scanner and dose calibration, insufficient correlation between systemic distribution volume and body weight, and, consequentially, residual inter-study variability of the arterial input function (AIF) despite SUV normalization. Especially the latter turns out to be a crucial factor adversely affecting the correlation between SUV and Km and causing inter-study variations of tumor SUVs that do not reflect actual changes of the metabolic uptake rate. In this work, we propose to replace tumor SUV by the tumor to blood standard uptake ratio (SUR) in order to distinctly improve the linear correlation with Km. Methods: Assuming irreversible FDG kinetics, SUR can be expected to exhibit a much better linear correlation to Km than SUV. The theoretical derivation for this prediction is given and evaluated in a group of 9 patients with liver metastases of colorectal cancer for which 15 fully dynamic investigations were available and Km could thus be derived from conventional Patlak analysis. Results: For any fixed time point T at sufficiently late times p.i. the Patlak equation predicts a linear correlation between SUR and Km under the following assumptions: 1.) approximate shapeinvariance (but arbitrary scale) of the AIF across scans/patients and 2.) low variability of the apparent distribution volume Vr (the intercept of the Patlak Plot). This prediction – and validity of the underlying assumptions – has been verified in the investigated patient group. Replacing tumor SUVs by SURs does improve the linear correlation of the respective parameter with Km from r = 0:61 to r = 0:98. Conclusion: SUR is an easily measurable parameter that is highly correlated to Km. In this respect it is clearly superior to SUV. Therefore, SUR should be seriously considered as a drop-in replacement for SUV-based approaches.