Purpose: To assess the multi-energy performance of a full field-of-view (FOV) photon-counting-detector (PCD) CT system in terms of iodine quantification accuracy.
Methods: Five water phantoms of lateral size 25, 30, 35, 40 and 45 cm mimicking small to large adult body sizes were scanned on a full-FOV PCD-CT system (SOMATOM Count Plus, Siemens Healthcare GmbH) using a multi-energy abdomen protocol (144 x 0.4 mm collimation, 120kV, 0.6 pitch, 0.5s rotation and CAREDose4D with 150 quality reference mAs). The CTDIvol was matched to that of a routine abdomen protocol used in our clinical practice. Vials with iodine solutions at concentrations of 2, 5, 10, 15 and 20 mg/mL were placed inside the water phantoms and scanned. Threshold images at 20-120 keV and 65-120 keV were reconstructed using filtered back projection with a Qr40 kernel at 3 mm slice thickness and 250 mm FOV. Iodine contrast-to-noise ratio (CNR) was measured from the low threshold images for all concentrations and phantom sizes. Iodine and water maps were generated using a prior knowledge aware material decomposition technique. The quantitative accuracy of iodine maps was calculated using root-mean-squared-error (RMSE) for each phantom size.
Results: Material decomposition enabled successful delineation of iodine inserts from the background (water) in the five phantom sizes. The 25 cm phantom exhibited the highest iodine CNR (average CNR = 33.4), and the CNR reduced by 2.6%, 21.5%, 38.1%, 48.9% for 30, 35, 40 and 45 cm, respectively. The iodine RMSE were 0.19, 0.25, 0.6, 1.12 and 0.99 mg/mL for 25, 30, 35, 40 and 45 cm phantoms, respectively. The average percent error for measured concentrations (against reference values) was 7.04% for all phantom sizes.
Conclusion: We evaluated the iodine quantification accuracy in a full-FOV PCD-CT system. Accurate iodine quantification was achieved using a material decomposition technique with built-in noise reduction.
Funding Support, Disclosures, and Conflict of Interest: NIH award numbers R01 EB028590 and C06 RR018898. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of Health. Research support for this work was provided, in part, to Mayo Clinic from Siemens Healthcare GmbH.