The potential of photon-counting CT for quantitative contrast-enhanced imaging in radiotherapy.

The aim of this study is to use a simulation environment to evaluate the potential of using photon-counting CT (PCCT) against dual-energy CT (DECT) in the context of quantitative contrast-enhanced CT for radiotherapy. An adaptation of Bayesian eigentissue decomposition by Lalonde et al (2017 Med. Phys. 44 5293-302) that incorporates the estimation of contrast agent fractions and virtual non-contrast (VNC) parameters is proposed, and its performance is validated against conventional maximum likelihood material decomposition methods for single and multiple contrast agents. PCCT and DECT are compared using two simulation frameworks: one including ideal CT numbers with image-based Gaussian noise and another defined as a virtual patient with projection-based Poisson noise and beam hardening artifacts, with both scenarios considering spectral distortion for PCCT. The modalities are compared for their accuracy in estimating four key physical parameters: (1) the contrast agent fraction, as well as VNC parameters relevant to radiotherapy such as the (2) electron density, (3) proton stopping power and (4) photon linear attenuation coefficient. Considering both simulation frameworks, a reduction of root mean square (RMS) errors with PCCT is noted for all physical parameters evaluated, with the exception of the error on the contrast agent fraction being about constant through modalities in the virtual patient. Notably, for the virtual patient, RMS errors on VNC electron density and stopping power are respectively reduced from 2.0% to 1.4% and 2.7% to 1.4% when going from DECT to PCCT with four energy bins. The increase in accuracy is comparable to the differences between contrast-enhanced and non-contrast DECT. This study suggests that in a realistic simulation environment, the overall accuracy of radiotherapy-related parameters can be increased when using PCCT with four energy bins instead of DECT. This confirms the potential of PCCT to provide robust and quantitative tissue parameters for contrast-enhanced CT required in radiotherapy applications.