Is Patlak y -intercept a relevant metrics?
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EDITORIAL
Is Patlak y-intercept a relevant metrics? Eric Laffon 1,2,3,4
&
Roger Marthan 1,2,3
# Springer-Verlag GmbH Germany, part of Springer Nature 2020
In daily positron emission tomography (PET) imaging, whole-body (WB) static acquisitions allow the nuclear physicians to quantitatively report on the tracer uptake in tissues by using the standardized uptake value (SUV), which is the tissue activity concentration normalized to the injected dose divided by patient’s weight [1]. However, this semi-quantitative uptake index has limitations, e.g., SUV in [ 18 F]-fluoro-deoxy-glucose (FDG) PET imaging is only a surrogate for assessing the increased glycolysis of cancer cells. Kinetic model analyses have been implemented to more specifically assess tracer uptake, of which Patlak graphical analysis (PGA) is considered a gold standard, assuming irreversible trapping of the tracer [2–4]. PGA requires dynamic acquisition, knowledge of the blood time-activity curve (i.e., the so-called input function (IF)), and steady state of the system. It then provides an estimate of the tracer uptake-rate constant (i.e., the net influx constant: Ki) in the tissue of interest, which is the slope of a linear fit. However, this linear fit also provides a y-intercept (i.e., Y0) that, potentially, might be considered as a metrics, independent of Ki, to further characterize the tissue of interest [5, 6]. Performing PGA at the voxel level can provide PGAbased WB parametric images in routine practice whose clinical interest may be decisive for the patient care, This article is part of the Topical Collection on Editorial * Eric Laffon [email protected] 1
CHU de Bordeaux, 33000 Bordeaux, France
2
Centre de Recherche Cardio-Thoracique de Bordeaux, University of Bordeaux, 33000 Bordeaux, France
3
Centre de Recherche Cardio-Thoracique de Bordeaux, INSERM U-1045, 33000 Bordeaux, France
4
Service de Médecine Nucléaire, Hôpital du Haut-Lévèque, Avenue de Magellan, 33604 Pessac, France
especially since dynamic WB PET imaging and 194cm-long PET/CT scanners are now available [6, 7]. However, whereas Ki-parametric i-imaging has been extensively investigated, the potential ability of PGA to complementarily provide Y0-parametric imaging remains to be clarified. Therefore, the current theoretical study aims at investigating whether Y0 may actually be a relevant metrics. For this, its different contributions have been identified along with their relative magnitude and their sources of variability in FDG PET imaging.
Theory A two-tissue compartment model is considered involving trapped-tracer and free-tracer compartments, where the rate constants K1 and k2 account for forward and reverse transport between blood and reversible compartment, and k3 and k4 account for forward and reversed transport between reversible and trapping compartment, respectively (Fig. 1). PGA assumes irreversible trapping of the tracer, that is, k4 is considered negligible. The activity at time t per tissue volume unit (in kBq mL−1) from free and trapping compartment, wi
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