Clinical X-ray imaging has always been based on the biological tissue's differences in X-ray attenuation ever since Roentgen discovered X-ray over 100 years ago. However X-ray-tissue interaction causes X-ray phase changes as well. We have identified the four clinically important factors that affect the X-ray phase visibility in clinical imaging. These factors are: body part attenuation, the spatial coherence of incident X-rays from an X-ray tube, the polychromatic nature of the X-ray source, and radiation dose to patients for clinical applications. A Wigner-distribution-based theory of phase-contrast imaging is presented to account for the effects of these four factors. Numerical simulations for X-ray phase-contrast mammography for phase reconstruction are described