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As a polychromatic X-ray beam passes through matter, low energy photons are preferentially absorbed, and the (logarithmic) attenuation is no longer a linear function of absorber thickness. This leads to various artifacts in reconstructive tomography. If a water bag is used, the nonlinear attenuation in bone causes a distortion of the bone values and a spill-over inside the skull, or 'pseudo-cortex' artifact. If no water bag is used, there is an additional effect due to the varying thickness of soft tissue which causes a depression of interior values, or 'cupping'. Both artifacts can be remedied by additional prefiltering of the beam and by applying a linearization correction to the detector outputs.
Brooks et al. (Sat,) studied this question.