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

Background: Partition modeling allows personalized activity calculation for holmium-166 ( ¹⁶⁶ Ho) 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 ¹⁶⁶ Ho-scout, for treatment simulation, and technetium-99m ( ^99m Tc) stannous phytate for healthy-liver delineation was proposed. This study assessed the accuracy of automatic healthy-liver segmentation using ^99m Tc images derived from a phantom experiment. In addition, together with data from a patient study, the effect of different ^99m Tc activities on the ¹⁶⁶ Ho-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 ¹⁶⁶ Ho-chloride, with a tumor to non-tumorous liver activity concentration ratio of 10. Eight SPECT/CT scans were acquired, with varying levels of ^99m Tc added to the non-tumorous liver compartment (ranging from 25 to 126 MBq). For comparison, forty-two scans were performed in presence of only ^99m Tc from 8 to 240 MBq. ^99m Tc image quality was assessed by cold-sphere (tumor) contrast recovery coefficients. Automatic healthy-liver segmentation, obtained by thresholding ^99m Tc images, was evaluated by recovered volume and Sørensen-Dice index. The impact of ^99m Tc on ¹⁶⁶ Ho images and the role of the downscatter correction were evaluated on phantom scans and twenty-six patients' scans by considering the reconstructed ¹⁶⁶ Ho count density in the healthy-liver.
Results: All ^99m Tc image reconstructions were found to be independent of the ¹⁶⁶ Ho activity present during the acquisition. In addition, cold-sphere contrast recovery coefficients were independent of ^99m Tc activity. The segmented healthy-liver volume was recovered fully, independent of ^99m Tc activity as well. The reconstructed ¹⁶⁶ Ho count density was not influenced by ^99m Tc activity, as long as an adequate downscatter correction was applied.
Conclusion: The ^99m Tc 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, ^99m Tc could be injected up to at least 126 MBq without compromising ¹⁶⁶ Ho image quality. Clinical trials The clinical study mentioned is registered with Clinicaltrials.gov (NCT02067988) on February 20, 2014.
(© 2022. The Author(s).)