An imaging system includes an x-ray source, a detector, a data acquisition system (DAS) operably connected to the detector, and a computer operably connected to the DAS. The computer is programmed to obtain CT scan data with two or more incident energy spectra, decompose the obtained CT scan data into projection CT data of a first basis material and a second basis material, generate a first basis material image and a second basis material image using the decomposed projection CT data, generate a first monochromatic image from the first basis material image and the second basis material image at a first energy that is selected based on an amount of correlated noise at the first energy, noise-reduce the first monochromatic image to generate a noise-reduced first monochromatic image, and generate a final monochromatic image based at least on the noise-reduced first monochromatic image.