Deadtime effects in quantification of 177Lu activity for radionuclide therapy

Carlos F Uribe,Jesse Tanguay,Wei Zhao,Anna Celler,Pedro L Esquinas,Marjorie Gonzalez

doi:10.1186/s40658-017-0202-7

Carlos F Uribe, Jesse Tanguay + Show 4 more

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https://doi.org/10.1186/s40658-017-0202-7

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Abstract

BackgroundThe aim of this study was to investigate the deadtime (DT) effects that are present in 177Lu images acquired after radionuclide therapy injection, assess differences in DT based on the full spectrum and the photopeak-only measurements, and design a method to correct for the deadtime losses.A Siemens SymbiaT SPECT/CT camera with a medium energy collimator was used. A 295-mL bottle was placed off-center inside a large cylinder filled with water, and 177Lu activity was sequentially added up to a maximum of 9.12 GBq. The true count rates vs. observed count rates were plotted and fitted to the DT paralyzable model. This analysis was performed using counts recorded in the full spectrum and in other energy windows. The DT correction factors were calculated using the percentage difference between the true and the observed count rates.ResultsThe DT values of 5.99 ± 0.02 μs, 4.60 ± 0.052 μs, and 0.19 ± 0.18 μs were obtained for the primary photons (PP) recorded in the 113- and 208-keV photopeaks and for the full spectrum, respectively. For the investigated range of count rates, the DT correction factors of up to 23% were observed for PP corresponding to the 113-keV photopeak, while for the 208-keV photopeak values of up to 20% were obtained. These values were almost three times higher than the deadtime correction factors derived from the full spectrum.ConclusionsThe paralyzable model showed to be appropriate for the investigated range of counts, which were five to six times higher than those observed in the patient post-therapy imaging. Our results suggest that the deadtime corrections should be based on count losses in the scatter-corrected photopeak window and not on the deadtime determined from the full spectrum. Finally, a general procedure that can be followed to correct patient images for deadtime is presented.

Highlights

The aim of this study was to investigate the deadtime (DT) effects that are present in 177Lu images acquired after radionuclide therapy injection, assess differences in DT based on the full spectrum and the photopeak-only measurements, and design a method to correct for the deadtime losses
Plots of observed count rates vs. true count rates for the lower scatter window (LSW), upper scatter window (USW), and photopeak energy windows (PW) and for the primary photons corresponding to the 113 and 208 keV photopeaks of 177Lu are shown in Figs. 5 and 6, respectively
Deadtime measurements for the Siemens SymbiaT camera have been performed, and the resulting data were fitted to the paralyzable model

Summary

Introduction

The aim of this study was to investigate the deadtime (DT) effects that are present in 177Lu images acquired after radionuclide therapy injection, assess differences in DT based on the full spectrum and the photopeak-only measurements, and design a method to correct for the deadtime losses. The true count rates vs observed count rates were plotted and fitted to the DT paralyzable model This analysis was performed using counts recorded in the full spectrum and in other energy windows. In typical diagnostic imaging scans, the administered activities are low, resulting in low count rates in the SPECT camera with no deadtime (DT). In radionuclide therapy procedures, as those performed with 177Lu, patients are injected with high activities (of the order of GBq), which result in high photon flux and may cause camera DT when imaging studies are performed. In order to accurately quantify patient’s activity in these situations, correcting images for DT losses might be necessary

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