Simplified quantification of FDG metabolism in tumors using the autoradiographic method is less dependent on the acquisition time than SUV

The standard uptake value (SUV) is the most often used semi-quantitative measure of (18)F-fluorodeoxyglucose (FDG) uptake. We tested the hypothesis that the autoradiographic method with a population-based input curve yields an approximation of FDG metabolism represented by the flux value Ki, which is less dependent on the acquisition time point than SUV.

Methods: We analyzed 20 patients with chest tumors (16 males, age 65±10 years). After injection of 350 MBq FDG using the INTEGO PET infusion system, a series of 35 scans of 10- to 300-s duration were acquired until 45 min. FDG flux was calculated using the Patlak method (Ki(patlak)) and also quantified with the autoradiographic method using the last acquisition only and the individual image-derived input function (Ki(autoreal)), as well as with a population-based input function (Ki(autonorm)). In a simulation study, the time courses of tumor SUV, tumor-to-blood ratio and tumor Ki values were calculated from 30 to 90 min.

Results: The FDG flux values (Ki) of the different tumors, obtained with the autoradiographic methods and the Patlak method, showed a high correlation. The simulation study showed a 16.8±3.3% increase in the SUV values from 50 to 70 min, but only a 1.3±2.8% change in the Ki values calculated with the autoradiographic method.

Conclusion: Compared to the SUV, the autoradiographic Ki values are advantageous for various reasons. First, they are much less dependent on the time of acquisition than the SUV. Second, their calculation does not require the knowledge of the body weight or the injected activity. Furthermore, the values are comparable to the ones obtained with the widely accepted Patlak method. The method can be easily implemented in a clinical setting, as it uses only one static scan.