Clinical photon-counting CT increases CT number precision and reduces patient size dependence compared to single- and dual- energy CT.

Objectives: To study whether photon-counting computed tomography (PCCT) can improve CT number accuracy, precision and reduce patient size dependence compared to dual-energy CT (DECT) virtual monoenergetic imaging (VMI) and single-energy CT (SECT).

Methods: Clinical PCCT, DECT and SECT scanners were used to image a multi-energy quality assurance phantom and tissue-equivalent inserts with/without an outer nested annulus, representing two object sizes (18 and 33 cm). CT numbers were converted to linear attenuation coefficients (LAC) and regions-of-interest applied. Theoretical monoenergetic LAC were calculated from known elemental compositions as a ground truth. Percent difference in mean LAC between phantom sizes, between mean and theoretical LAC, and its coefficient of variation (COV) were calculated.

Results: Mean LAC percent differences between small and larger phantoms were highest in DECT (within -3% to 9%) and SECT (within 1% to 5%), particularly at higher calcium and iodine concentrations, while being relatively constant in PCCT over material concentrations and VMI energies (within ±2%). The COV in mean LAC was consistently lower (about 2 to 5 times) in PCCT relative to DECT and SECT for calcium in the large phantom. With consideration of the theoretical uncertainties of 2%, both PCCT and DECT showed comparable agreement to theoretical LAC.

Conclusions: PCCT VMI produces CT numbers with less dependence on patient size and increased precision in large object sizes than DECT VMI and SECT.

Advances In Knowledge: Clinical PCCT provides less variable CT numbers than DECT and SECT with less sensitivity to the imaged object size.