Dual time point based quantification of metabolic uptake rates in 18F-FDG PET

Background: Assessment of dual time point (DTP) positron emission tomography was carried out with the aim of a quantitative determination of K-m, the metabolic uptake rate of [18F]fluorodeoxyglucose as a measure of glucose consumption. Methods: Starting from the Patlak equation, it is shown that K-m approximate to mt/c(a)(0) + (V-r) over bar/tau(a), where m(t) is the secant slope of the tissue response function between the dual time point measurements centered at t = t(0). c(a)(0) = c(a)(t(0)) denotes arterial tracer concentration, (V-r) over bar is an estimate of the Patlak intercept, and tau(a) is the time constant of the c(a)(t) decrease. We compared the theoretical predictions with the observed relation between K-s = m(t)/c(a)(0) and K-m in a group of nine patients with liver metastases of colorectal cancer for which dynamic scans were available, and K-m was derived from conventional Patlak analysis. Twenty-two lesion regions of interest (ROIs) were evaluated. c(a)(t) was determined from a three-dimensional ROI in the aorta. Furthermore, the correlation between K-m and late standard uptake value (SUV) as well as retention index was investigated. Additionally, feasibility of the approach was demonstrated in a whole-body investigation. Results: Patlak analysis yielded a mean Vr of (V-r) over bar = 0.53 +/- 0.08 ml/ml. The patient averaged tau(a) was 99 +/- 23 min. Linear regression between Patlak-derived K-m and DTP-derived K-s according to K-s = b . K-m + a yielded b = 0.98 +/- 0.05 and a = -0.0054 +/- 0.0013 ml/min/ml (r = 0.98) in full accordance with the theoretical predictions b = 1 and a approximate to -(V-r) over bar/tau(a). K-s exhibits better correlation with K-m than late SUV and retention index, respectively. K-s((c)) = Ks + (V-r) over bar/tau(a) is proposed as a quantitative estimator of K-m which is independent of patient weight, scan time, and scanner calibration. Conclusion: Quantification of K-m from dual time point measurements compatible with clinical routine is feasible. The proposed approach eliminates the issues of static SUV and conventional DTP imaging regarding influence of chosen scanning times and inter-study variability of the input function. K-s and K-s((c)) exhibit improved stability and better correlation with the true K-m. These properties might prove especially relevant in the context of radiation treatment planning and therapy response control.