Impact of patient habitus and acquisition protocol on iodine quantification in dual source photon-counting CT.

Objective Evaluation of iodine quantification accuracy with varying iterative reconstruction level, patient habitus, and acquisition mode on a first-generation dual-source photon-counting computed tomography (PCCT) system. Methods A multi-energy CT phantom (20 cm diameter/small) was imaged with and without an extension ring (30 by 40 cm/large). It was equipped with various iodine inserts (0.2, 0.5, 1.0, 2.0, 5.0, 10.0, 15.0 mg/ml) and scanned over a range of radiation dose levels (CTDIvol 0.5, 0.8, 1.2, 1.6, 2.0, 4.0, 6.0, 10.0, 15.0 mGy) using four different acquisition modes: single source 120 kVp (SS120), 140 kVp (SS140) and dual-source 120 kVp (DS120), 140 kVp (DS140). Iodine density maps were produced with different levels of iterative reconstruction (QIR 0, 2, 4). To assess the agreement between nominal and measured iodine concentrations, root mean square error (RMSE) and Bland-Altman plots were generated by grouping different radiation dose levels (ultra-low: < 1.5 mGy; low: 1.5 - 5 mGy; medium: 5 - 15 mGy) and iodine concentrations (low: < 5 mg/ml; high: 5 - 15 mg/ml). Results Overall, quantification of iodine concentrations was accurate and reliable even at ultra-low radiation dose levels. With low and high iodine concentrations, RMSE ranged from 0.25 to 0.37, 0.20 to 0.38, and 0.25 to 0.37 mg/ml for ultra-low, low, and medium radiation dose levels, respectively. Similarly, for the three acquisition modes (SS120, SS140, DS 120, DS140), RMSE was stable at 0.31, 0.28, 0.33 and 0.30 mg/ml, respectively. Considering all levels of radiation dose, acquisition mode, and iodine concentration, the accuracy of iodine quantification was higher for the phantom without extension ring (RMSE 0.21 mg/ml) and did not vary across different levels of iterative reconstruction. Conclusions The first-generation PCCT allows for accurate iodine quantification over a wide range of iodine concentrations and radiation dose levels. Even very small concentrations of iodine can be quantified accurately at different simulated patient sizes. Stable accuracy across iterative reconstruction levels may allow further radiation exposure reductions without affecting quantitative results.