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

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 (Z_eff). More accurate Z_eff measurements could be beneficial in tissue classification and proton therapy tasks.

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. Z_eff maps were generated. Measured Z_eff values were compared across WEDs and to theoretical values.

The measured Z_eff 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 Z_eff 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.

This is the first study to evaluate Z_eff measurements made using a prototype whole body PCD CT system. We found that PCD CT outperformed the EID CT in terms of Z_eff 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 Z_eff using deep silicon PCD CT could make quantitative CT increasingly possible over a large range of patient sizes.