Comparison of Hounsfield Unit variability between two same model photon counting CT detector systems. A phantom study applied to lung CT

Accurate quantification of lung density, in Hounsfield Units (HU), is of high importance to monitor progression of diseases such as emphysema using chest CT imaging. Reproducibility of HU quantification on independent photon counting detector CT (PCD-CT) systems with a focus on lung imaging have not yet been evaluated. We thus aimed to evaluate HU reproducibility on 2 independent PCD-CT systems using a repeatable phantom setup with identical acquisition and image reconstruction settings. A COPDGene phantom comprising densities of air, water and lung was scanned on 2 independent PCCT systems using 3 different radiation exposures, 2 medium-sharpness reconstruction kernels (Br40 and Qr36), with and without iterative reconstruction (levels 0 vs 3). Our results demonstrate that acquisitions performed with full dose (3.2 mGy), half dose (1.6 mGy), and one-eighth dose (0.4 mGy) had minimal influence on HU accuracy (<6 HU) when using Br40 and Qr36 kernels. The level of iterative reconstruction also has a minimal impact (<6 HU) with the same kernels. Between the 2 PCD-CT systems evaluated, reproducible HU quantification was achieved for changes to CTDI_vol, iterative reconstruction level and reconstruction kernel.