1. A method for recalibrating a monochromatic image, the method comprising: receiving a monochromatic image comprising a head of a subject from a computed tomography (CT) scanner; detecting a petrous bone of the head in the monochromatic image; determining a linear attenuation coefficient of at least one petrous voxel representing the petrous bone; calculating a mass attenuation coefficient of the petrous voxel based on the linear attenuation coefficient and a density of the petrous bone; 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.