  A method for extracting the dark-field signal in propagation-based
phase-contrast imaging is proposed. In the case of objects consisting
predominantly of a single material, or several different materials with similar
ratios of the real decrement to the imaginary part of the complex refractive
index, the proposed method requires a single image for extraction of the
dark-field signal in two-dimensional projection imaging. In the case of
three-dimensional tomographic imaging, the method needs only one image to be
collected at each projection angle. A preliminary example demonstrates that
this method can improve the visualization of microcalcifications in
propagation-based X-ray breast cancer imaging. It is suggested that the
proposed approach may be useful in other forms of biomedical imaging, where it
can help one to obtain additional small-angle scattering information without
increasing the radiation dose to the sample.
