A method comprises receiving image data representative of an X-ray image of at least part of a subject comprising tissue, wherein a first part of the X-ray image is directly 5 exposed and a second part of the X-ray image is representative of tissue in a region of the subject; and determining a measure of attenuation in dependence on an energy measure obtained from the first part of the X-ray image and an energy measure obtained from the second part of the X-ray image.