Objective: To provide evidence for the selection of an optimal cross-sectional reconstruction mode in spectral CT imaging of the abdomen, we compared the monochromatic images with polychromatic images. Methods: Three phase-enhanced CT scans of the abdomen were recorded using the spectral imaging technique on 100 patients. Images were reconstructed using two modes: polychromatic and 70 keV monochromatic. The following variables were then compared: contrast-to-noise ratio (CNR) of the liver, spleen, gallbladder, kidney and pancreas, and the noise. Paired t-tests were used to compare differences between the two sets of images. Three experienced doctors graded the quality of the images with a five-point scale. The image quality scores were compared with a non-parametric rank sum test. Results: Compared with polychromatic images, the 70 keV monochromatic mode images yielded significantly greater tissue-to-fat CNR and lower noise (p,0.001 for all comparisons). The image quality of the 70 keV monochromatic mode showed significantly better results than the polychromatic mode (p,0.001). Conclusions: In abdominal spectral CT imaging, 70 keV monochromatic mode reconstruction images were better than those reconstructed using the polychromatic mode. The monochromatic mode may become the routine reconstruction mode for cross-sectional images. Recently, the advancement of CT technology, such as dual-source technology or fast peak kilovoltage switching technology, has enabled the routine clinical application of dual-energy spectral CT for the abdomen [4]. The results of preliminary investigations suggest that dual-energy techniques may be used to provide information about tissue composition and how tissues behave at different energies in the abdomen [5][6][7][8][9]. New parameters, such as the iodine concentration and the spectral curve, can be acquired using spectral CT. However, because the ability of spectral CT to characterise lesions is still uncertain, no diagnosis is possible, only reference to the pattern of spectral curves or other spectral CT parameters. Therefore the morphological characteristics and enhancement features of lesions in cross-sectional images still remain the basis of diagnosis of abdominal spectral CT images. In addition, the reconstruction of cross-sectional images remains a routine procedure for spectral CT imaging of the abdomen. In the present spectral CT system based on fast peak kilovoltage switching technology, there are two cross-sectional image reconstruction modes available: monochromatic and polychromatic modes. To our knowledge, there has not been a study designed to demonstrate which routine reconstruction mode should be used after spectral CT imaging. To answer this question, our study seeks to compare the quality of CT images of the abdomen reconstructed in monochromatic mode with those from a polychromatic mode, and therefore provide evidence for the selection of the optimal reconstruction mode.