Simultaneous emission and transmission scanning in PET oncology: the effect on parameter estimation

The authors investigated potential sources of bias due to simultaneous emission and transmission (SET) scanning and their effect on parameter estimation in dynamic positron emission tomography (PET) oncology studies. The sources of bias considered include: i) variation in transmission spillover (into the emission window) throughout the field of view, ii) increased scatter arising from rod sources, and iii) inaccurate deadtime correction. Net bias was calculated as a function of the emission count rate and used to predict distortion in [¹⁸F]2-fluoro-2-deoxy-D-glucose (FDG) and [¹¹C]thymidine tissue curves simulating the normal liver and metastatic involvement of the liver. The effect on parameter estimates was assessed by spectral analysis and compartmental modeling. The various sources of bias approximately cancel during the early part of the study when count rate is maximal. Scatter dominates in the latter part of the study, causing apparently decreased tracer clearance which is more marked for thymidine than for FDG. The irreversible disposal rate constant, K_i, was overestimated by <10% for FDG and >30% for thymidine. The authors conclude that SET has a potential role in dynamic FDG PET but is not suitable for ¹¹C-labeled compounds