Abstract
242514
Introduction: Patients undergoing radionuclide therapy (RNT) may also be scheduled for diagnostic PET imaging, post-infusion, for evaluation of the treatment as well as other clinical findings of significance. Lu-177 is currently the most used isotope for RNT emits a significant amount of low (6.6% 113 keV) medium (11.1% 208 keV) energy photons. Although only 511 keV photons are of concern in PET, detected lower energy emissions from Lu-177 still must be processed by the detector electronics. The presence of high activity concentrations of Lu-177 could therefore interfere with PET imaging performed post-therapy. This interference could produce dead-time, event pileup, random coincidences which could affect image quality and quantification. The purpose of this work is to investigate the effect on image quality in the presence of Lu-177 during PET imaging.
Methods: A phantom study was done using a flangeless ACR PET phantom. All data were acquired on a Siemens mCT PET/CT system with LSO scintillation crystals. A theranostic infused patient during a PET scan were simulated by adding a phantom background with an initial activity of 88.2 µCi/ml of Lu-177. The phantom was the filled with F-18 according to ACR accreditation protocol with a whole-body patient dose of 14 mCi. No additional activity was added outside the scanner field-of-view (FOV).
Scans were repeated at multiple Lu-177 activity concentrations until a final phantom background activity reached 0.05 µCi/ml. Quality as well as quantification analysis methods were performed on reconstructed 10 mm slices. Quantification analysis consisted of measuring average SUV in the phantom background. The contrast, resolution and image quality of the reconstructed images were visually evaluated.
Results: Results from the quantification analysis showed a rapid decrease in SUV accuracy at activity concentrations exceeding 11 µCi/ml. Calculations showed a 5, 10 and 20% reduction in SUV in the uniform background section at activities 2.0, 6.8 and 17 µCi/mlCi respectively.
High photon emission rates lead to event pile-up events and detector mis-positioning and results in visible ring artifacts. The ring artifacts can be seen in throughout the phantom at Lu-177 activity concentrations exceeding 11 µCi/ml.
All rod sectors in the resolution portion of the phantom are visible up to 11 µCi/ml of Lu-177. Exceeding this concentration, a noticeable degradation in resolution and contrast for the smallest 4.8 mm rod pattern was observed. A similar degradation was observed for the 6.5 mm rods at activity concentrations exceeding 25 µCi/ml.
The results of this study may change in the presence of background activity outside the scanner FOV. Furthermore, results may be also different for PET scanners using slower scintillators, such as BGO.
Conclusions: Photons from the Lu-177 background clearly affected both image quality and quantification. Although the photon emissions from Lu-177 have energies outside the energy window used for 511 keV photons, these low energy photons are detected and need to be processed by the detector electronics. At high activity concentrations the Lu-177 background affected both image quality (ring artifacts, loss in contrast and spatial resolution). Quantification was also observed to be affected at background concentrations exceeding 2.0 µCi/ml, which is comparable to the average activity concentrations expected in a patient during the first few hours following an infusion (2.8 µCi/ml immediately after a standard Lu-177 infusion, 200 mCi infusion in a 70 kg patient). Since activity concentration rapidly decreases through excretion and isotope decay, delaying a PET scan at least 24 h post-infusion should minimize the effects of the Lu-177 background.
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