The total amount of uptake may affect the input function: a theoretic approach about 18F-FDG PET imaging

Introduction: The aim of this theoretic approach is to establish an analytical relationship between the total amount of uptake (TAU), either pathological or physiological, and the time constants 'α_i' that describe the time decay of the tracer input function (IF). The proposed approach uses parameters of a (population-based) arterial plasma IF for 18 F-FDG PET imaging. Methods: A previously published formula provides an estimate of the ratio 'p' of 18 F-FDG molecules that are irreversibly trapped in an 18 F-FDG-positive tissue during a whole PET examination, to the number of injected molecules. Then, the change in the magnitude of the IF time constants is derived, involving a corrective procedure. Results: Trapping of injected tracer molecules affects each α_i that is increased by a factor of 1/(1-p), with p ranging between zero and, theoretically, less than 1. This result is illustrated in a patient showing an intense uptake in the mediastinum at initial staging of a diffuse large B-cell lymphoma (DLBCL). Conclusions: An analytical relationship between TAU and the IF time constants is available in 18 F-FDG PET imaging, showing that TAU may actually affect the IF