The purpose of this study was to assess the image quality of color K-edge imaging obtained with a spectral photon-counting CT (SPCCT) scanner using a spectral phantom with a mixture of iodine-based and gadolinium-based contrast agents.

A clinical SPCCT scanner prototype was used to scan a spectral phantom. Three dedicated cavities were filled with three contrast agents including iodine alone, gadolinium alone and a mixture of both. Two concentrations of 0.5 and 2 mg/mL were evaluated using nine helical PCCT scans at 120 kVp and 150 mAs. Conventional, color iodine and color K-edge gadolinium images were obtained through a material decomposition algorithm using three basis materials (water, iodine, gadolinium). Attenuation (in Hounsfield unit [HU]), iodine and gadolinium concentrations and task-based transfer function (TTF) were measured on each cavity and image. The noise power spectrum (NPS) was calculated on the phantom's background.

Color K-edge imaging differentiated iodine and gadolinium but underestimated their concentrations. Gadolinium concentrations were underestimated by 9.4 ± 2.2 (standard deviation [SD]) % and 9.2 ± 1.0 (SD) % for gadolinium alone, 14.9 ± 2.3 (SD) % and 11.4 ± 1.2 (SD) % for the mixture, at 0.5 and 2 mg/mL, respectively. Similar TTF values at 50 % were found for color iodine (0.43 ± 0.01 [SD] mm ⁻¹ ) and color K-edge gadolinium (0.45 ± 0.03 (SD) mm ⁻¹ ) images for respective cavities at 2 mg/mL but the lowest values were found for color K-edge gadolinium images (0.43 ± 0.01 [SD] mm ⁻¹ vs . 0.29 ± 0.01 [SD] mm ⁻¹ ) at 0.5 mg/mL. The value of noise magnitude was 24.75 HU, 0.06 mg/mL and 0.03 mg/mL for conventional, color iodine and color K-edge gadolinium images, respectively.

Color K-edge imaging helps distinguish between contrast agents while being associated with lownoise magnitude, high-frequency spatial noise and high spatial resolution.