Reliability of the Iodine Signal in Dual-energy CT on Two Scanner Types: A Phantom Study Featuring Beam Hardening

PURPOSE A dual-energy (DE) phantom study was performed on two scanner types in order to evaluate (1) the stability and accuracy of CT numbers and iodine concentrations in function of object size; (2) the effect of beam hardening. METHOD AND MATERIALS Iodine concentrations of 3 and 6 mgI/ml were poured into two respective rows of cylindrical cavities with diameters ranging from 3 to 25.5 mm in an AAPM CT performance phantom. Besides this, a 16 mm diameter tube filled with 6 mgI/ml was placed in the main water-filled compartment, partly covered by a Teflon beam hardening ring. The phantom was scanned in standard 120 kVp and DE mode on a GE Healthcare Discovery HD750 and Siemens SOMATOM Definition Flash scanner at 3 CTDIvol's (9, 19 and 29 mGy). Iodine concentrations were determined for all cavities using the scanner's DE mode, and compared to their true concentrations. CT number stability was determined accordingly and a comparison was made between the DE acquisition and standard 120 kV scanning. RESULTS (1) Relatively constant (max VAR 14%) HU values for both scanners as a function of the object diameters were observed. The Siemens scanner showed a consistent overestimate of approximately 0.5 mgI/ml for both contrast concentrations. A significant decrease in concentration accuracy for object diameters smaller than 6 mm was present for both scanners. (2) A step-like increase of 10 HU upon transitioning out of the Teflon part was observed for both scanners at single energy 120 kV and the Siemens blended DE image, less for the GE DE image (AV 148 HU, max VAR 5 HU). CONCLUSION (1) Measured and true iodine concentrations were consistent down to 6 mm object size, notwithstanding an observed offset with one scanner type. (2) A step-like increase in HU highlights the effect of beam hardening, and is more prominent for classical scans compared to DE. CLINICAL RELEVANCE/APPLICATION DE CT techniques are being increasingly used in tumor imaging. Since iodine is a contrast agent commonly used in CT imaging, determining its detectable lower limit is a crucial factor. Iodine concentration estimations should be made with appropriate consideration.