Detrimental Effects of Recent Radionuclide Therapy and SPECT Radionuclide Activities on Subsequent PET Imaging due to Detector Pulse Pile-Up and Dead Time

Abstract

This study investigated the detrimental effects of non-PET radionuclides, specifically, Tc-99m, Lu-177, and I-131 on subsequent PET imaging. This is relevant to clinical protocols utilizing both SPECT and PET/CT imaging, or combined radionuclide therapy with PET/CT imaging post-treatment. The single photons from the non-PET radionuclides can degrade PET performance through detector dead time and pile-up, affecting image quality and quantitative accuracy. To investigate these effects, four experimental sessions were conducted using a large torso phantom containing a plastic bottle with either F-18, Tc-99m, Lu-177, or I-131 to simulate patient background activity. Each phantom was scanned (6 min) on three different PET/CT systems: the LYSO-based GE Discovery 690 (D690); PSPMT LYSO-based GE Discovery MI (DMI); and BGO-based GE Discovery IQ (DIQ) (all from GE Healthcare, Milwaukee, WI), both with and without background activity. Spatial resolution (represented by full-width-at-half-maximum, FWHM), count rates, dead time correction factors, and sphere activity recovery were analyzed. Spatial resolution on the D690 and DMI remained stable, while the BGO-based DIQ exhibited noticeable degradation for all radionuclides examined. Single-photon backgrounds consistently reduced uncorrected count rates and increased dead-time correction factors across all systems. Furthermore, the results showed that small biases remain among each of the scanners in how they handle dead-time correction (correction was not perfect). Sphere quantitation revealed systematic underestimation (3 to 47 %), worst for I-131 on the DIQ. These findings demonstrate that non-PET radionuclides can significantly perturb PET performance, with effects dependent on detector design and radionuclide type. BGO-based scanners are particularly susceptible. The results emphasize the need for scanner and radionuclide-specific activity limits when planning PET/CT after previous scans or therapies.