1. A method for diagnostic imaging comprising: obtaining a plurality of optimum attenuation coefficients for an energy level at which a monochromatic image generated based upon the plurality of optimum attenuation coefficients has least noise; selecting a desired energy level; obtaining a plurality of desired attenuation coefficients for the desired energy level; computing a scaling factor for a corresponding noise component based on the optimum attenuation coefficients and the desired attenuation coefficients; generating a second monochromatic image at the desired energy level based upon the scaling factor; and adding a scaled noise component to the second monochromatic image to cancel correlated noise in the second monochromatic image.