Reliability of uptake estimates in FDG PET as a function of acquisition and processing protocols using the CPET

Standardized uptake values (SUV) are commonly used in FDG PET to characterize suspicious high uptakes. To better understand the reliability and the limits of SUV, we studied the accuracy of uptake estimates as a function of a number of parameters using phantom data acquired on the CPET. Methods: Using the Data Spectrum thorax phantom in which spheres were inserted, we studied the effect of the sphere sizes, of out-of-the-field-of-view activity (OFOVA), of the emission scan duration and of attenuation and partial volume effect (PVE) corrections upon biases in tumor-to-normal tissue uptake ratio (TNR) estimates. Considering a specific acquisition and processing protocol as a reference, we determined the changes in TNR when modifying this protocol. Results: For a true TNR of 8, estimated TNR were strongly dependent on lesion size, but also on the acquisition and processing protocol. Depending on the method used to derive the attenuation map, estimated TNR could change by more than 50% for lung spheres less than 2 cm in diameter. Using PVE correction, TNR increased by a factor greater than 2 for spheres less than 2 cm in diameter. The very method used to estimate sphere activity from the reconstructed images could also change the TNR by more than 50%. Even with CT-based attenuation correction and PVE correction, TNR estimates remained underestimated by more than 20% in lesions less than 2 cm in diameter. Conclusion: Biases in uptake estimates strongly depend on the acquisition and processing protocols. This suggests that comparing SUV between studies make sense only if the scanner and the acquisition and processing protocols are strictly identical.