| Popis: |
Conventionally, the contrast of X-ray images is due to the attenuation of intensity of X-ray beams after penetrating materials, which is proportional to the imaginary part of the complex refractive index. Subtle density variations within soft tissue or other low-Z materials yield poor attenuation contrast. One method to improve the contrast of X-ray images is to utilize phase information since phase depends depend on the real part of the refractive index, which is typically 1000 times larger than the imaginary part. However, phase imaging relies critically on the spatial coherence of the X-ray beam which traditionally requires synchrotron sources, small-spot, low power laboratory sources, or precisely aligned gratings and multiple exposures. We will discuss two methods to achieve phase imaging with large-spot sources practical for clinical or security screening. The first method relies on using polycapillary optics to focus the beam and achieve the necessary coherence for traditional propagation-based phase imaging methods based on the transport of intensity equation. The second method relies on a coarse wire mesh which structures the illumination to enhance phase signatures and relax the coherence requirement. We will present recent results from both methods, including computational algorithms for phase contrast, phase retrieval and resolution enhancement. |
