1. A medical image processing apparatus comprising: processing circuitry that separates projection data into pieces of line-integrated data each of which corresponds to a different one of a plurality of basis materials that are set in advance, reconstructs pieces of basis material image data from the pieces of line-integrated data each of which corresponds to a different one of the plurality of basis materials, the pieces of basis material image data being configured so that each pixel value of each of pixels indicates an abundance ratio of corresponding each of the basis materials that is present at each of the pixel, extracts an artifact region, on a basis of attenuation coefficients of each of the pixels calculated from the pieces of basis material image data each of which corresponds to a different one of the plurality of basis materials, and corrects the attenuation coefficients at the artifact region, wherein the processing circuitry generates a monochromatic X-ray image by using the corrected attenuation coefficients.