Purpose: To determine the optimal energy level in contrast-enhanced spectral CT imaging for displaying abdominal vessels in pediatric patients. Materials and methods: This retrospective study was institutional review board approved. 15 children (8 males and 7 females, age range, 6-15 years, mean age 10.1 +/- 3.1 years) underwent contrast-enhanced spectral CT imaging for diagnosing solid tumors in abdomen and pelvic areas were included. A single contrast-enhanced scan was performed using a dual energy spectral CT mode with a new split contrast injection scheme (iodixanol at 1-1.5 ml/kg dose. 2/3 first, 1/3 at 7-15 s after the first injection). 101 sets of monochromatic images with photon energies of 40-140 keV with 1 keV interval were reconstructed. Contrast-noise-ratio (CNR) for hepatic portal or vein were generated and compared at every energy level to determine the optimal energy level to maximize CNR. 2 board-certified radiologists interpreted the selected image sets independently for image quality scores. Results: CT values and CNR for the vessels increased as photon energy decreased from 140 to 40 keV: (CT value: 48.29-570.12 HU, CNR: 0.08-14.90) in the abdominal aorta, (58.48-369.73 HU, 0.64-5.87) in the inferior vena cava, and (58.48-369.73 HU, 0.06-6.96) in the portal vein. Monochromatic images at 40-50 keV (average 42.0 +/- 4.67 keV) could display vessels above three levels clearly, and with excellent image quality scores of 3.17 +/- 0.58 (of 4) (k = 0.50). The CNR values at the optimal energy level were significantly higher than those at 70 keV, an average energy corresponding to the conventional 120 kVp for abdominal CT imaging. Conclusion: Spectral CT imaging provides a set of monochromatic images to optimize image quality and enhance vascular visibility, especially in the hepatic portal and vein systems. The best CNR for displaying abdominal vessels in children was obtained at 42 keV photon energy level. (C) 2013 Published by Elsevier Ireland Ltd.