A method includes receiving a monochromatic image comprising a head of a subject from a Computed Tomography (CT) scanner and detecting a petrous bone of the head in the monochromatic image. The method further includes determining a linear attenuation coefficient of at least one petrous voxel representing the petrous bone and calculating a mass attenuation coefficient of the petrous voxel based on the linear attenuation coefficient and a density of the petrous bone. The method also includes computing a monochromatic energy level of the monochromatic image based on the mass attenuation coefficient of the petrous voxel and recalibrating the monochromatic image corresponding to the computed monochromatic energy level to the desired monochromatic energy level.