1. An imaging system comprising: an x-ray source that emits a beam of x-rays toward an object to be imaged; a detector that receives the x-rays attenuated by the object; a data acquisition system (DAS) operably connected to the detector; and a computer operably connected to the DAS and 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.