When generating a 3D image of a subject or patient, a cone beam X-ray source (20a, 20b) is mounted to a rotatable gantry (14) opposite an offset flat panel X-ray detector (22a, 22b). A wedge-shaped attenuation filter (24a, 24b) of suitable material (e.g., aluminum or the like) is adjustably positioned in the cone beam to selectively attenuate the beam as a function of the shape, size, and density of a volume of interest (18) through which X-rays pass in order to maintain X-ray intensity or gain at a relatively constant level within a range of acceptable levels.