Optimal energy level of virtual monoenergetic images for iron detectability in dual-energy CT: a Phantom study

Different energy levels of virtual monoenergetic images (VMI) characterize the attenuation properties of the image material. This study aims to investigate the optimal energy level of VMI for accurately determining the CT numbers of iron-containing materials in blood at different locations across varying conditions. A phantom containing blood-iron materials with relative electron densities of 1.03, 1.07, and 1.10 was scanned using a dual-energy CT across monoenergetic levels ranging from 40 to 140 keV at the noise indices of 15, 20, and 25. Images were reconstructed using the adaptive statistical iterative reconstruction-V (ASIR-V) technique at different strength levels. The CT numbers and relative percentage error (RPE) were analyzed for rods positioned at inner and outer locations. The RPEs of the CT numbers on VMIs at 80 keV were within ±5% under all examined conditions. At lower and higher VMI energy levels, the RPEs were greater than ±5%. The selection of VMI energy that yields accurate CT numbers should be considered in conjunction with the iron accumulation. The location of rods has a minimal effect on the CT number accuracy. Higher noise indices degraded accurate CT numbers, while higher strength levels of the ASIR-V algorithm suppressed noise, especially at low keV levels. VMI at 80 keV provides the CT number that most accurately reflects the theoretical value throughout all study settings. These findings support the optimization of clinical practices that require precise iron quantification under a low-dose protocol and in a larger patient size.