166Holmium-99mTechnetium dual-isotope imaging: scatter compensation and automatic healthy-liver segmentation for 166Holmium radioembolization dosimetry

Background Partition modeling allows personalized activity calculation for holmium-166 (Ho-166) radioembolization. However, it requires the definition of tumor and non-tumorous liver, by segmentation and registration of a separately acquired CT, which is time-consuming and prone to error. A protocol including Ho-166-scout, for treatment simulation, and technetium-99m (Tc-99m) stannous phytate for healthy-liver delineation was proposed. This study assessed the accuracy of automatic healthy-liver segmentation using Tc-99m images derived from a phantom experiment. In addition, together with data from a patient study, the effect of different Tc-99m activities on the Ho-166-scout images was investigated. To reproduce a typical scout procedure, the liver compartment, including two tumors, of an anthropomorphic phantom was filled with 250 MBq of Ho-166-chloride, with a tumor to non-tumorous liver activity concentration ratio of 10. Eight SPECT/CT scans were acquired, with varying levels of Tc-99m added to the non-tumorous liver compartment (ranging from 25 to 126 MBq). For comparison, forty-two scans were performed in presence of only Tc-99m from 8 to 240 MBq. Tc-99m image quality was assessed by cold-sphere (tumor) contrast recovery coefficients. Automatic healthy-liver segmentation, obtained by thresholding Tc-99m images, was evaluated by recovered volume and Sorensen-Dice index. The impact of Tc-99m on Ho-166 images and the role of the downscatter correction were evaluated on phantom scans and twenty-six patients' scans by considering the reconstructed Ho-166 count density in the healthy-liver. Results All Tc-99m image reconstructions were found to be independent of the Ho-166 activity present during the acquisition. In addition, cold-sphere contrast recovery coefficients were independent of Tc-99m activity. The segmented healthy-liver volume was recovered fully, independent of Tc-99m activity as well. The reconstructed Ho-166 count density was not influenced by Tc-99m activity, as long as an adequate downscatter correction was applied. Conclusion The Tc-99m image reconstructions of the phantom scans all performed equally well for the purpose of automatic healthy-liver segmentation, for activities down to 8 MBq. Furthermore, Tc-99m could be injected up to at least 126 MBq without compromising Ho-166 image quality. Clinical trials The clinical study mentioned is registered with Clinicaltrials.gov (NCT02067988) on February 20, 2014.

Automated summary

This study assessed the accuracy of automatic healthy-liver segmentation using Tc-99m images and investigated the impact of different Tc-99m activities on Ho-166-scout images. The results showed that Tc-99m image reconstructions and automatic healthy-liver segmentation were independent of Tc-99m activity, while the reconstructed Ho-166 count density was influenced by downscatter correction. Tc-99m could be injected up to at least 126 MBq without compromising Ho-166 image quality.