Accuracy and consistency of effective atomic number over object size using deep silicon photon-counting detector CT

Purpose: Photon-counting detector (PCD) CT is the newest generation of CT detector technology. It is critical to characterize its performance in measuring important biomarkers used in quantitative CT including effective atomic number (Zeff). More accurate Zeff measurements could be beneficial in tissue classification and proton therapy tasks. Methods: A phantom of varying water-equivalent diameter (WED) containing clinically relevant inserts was scanned using a prototype deep silicon PCD CT and a dual-energy (DE) energy integrating detector (EID) CT. Zeff maps were generated. Measured Zeff values were compared across WEDs and to theoretical values. Results: The measured Zeff of the polystyrene, solid water, iodine, and bone (50% CaCO3) inserts differed from the theoretical value by a maximum of-14.0%, 4.6%, 8.4% and 13.0% respectively on EID vs 4.5%, 5.2%, 2.3% and 7.2% on PCD. The maximum variation in Zeff over the WED range on EID was 0.71, 0.27, 0.66, and 1.22 vs 0.47, 0.64, 0.1 and 0.22 on PCD for polystyrene, solid water, iodine, and bone (50% CaCO3) respectively. Conclusion: This is the first study to evaluate Zeff measurements made using a prototype whole body PCD CT system. We found that PCD CT outperformed the EID CT in terms of Zeff accuracy and consistency over the WED range on most of the insert materials. Similarly, PCD CT outperformed most previous study's findings using EID CT. The high consistency and accuracy of measured Zeff using deep silicon PCD CT could make quantitative CT increasingly possible over a large range of patient sizes.