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

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 "αi" 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.

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.

Trapping of injected tracer molecules affects each αi 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).

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.