X-ray phase imaging is known to enhance contrast, particularly for low atomic number materials, for which absorption contrast is low. However, it requires spatial coherence which is typically achieved with a small (10 to 50 µm) source, or a grating placed in front of the source to essentially break it into multiple small sources. In a previous experiment, polycapillary focusing optics were shown to improve coherence when employed to focus x rays from a large spot rotating anode to a smaller secondary source. Edge-enhancement to noise ratios up to a value of 6.5 were obtained, and sufficiently high quality data was obtained from a single image to allow for phase reconstruction using a phase attenuation duality approach. Alternatively, polycapillary optics might operate in place of a source grating to effectively divide the source into a very large number of small channels. In order to examine the potential use of polycapillary optics to enhance phase imaging, the phase and coherence properties of the optic were modeled by observing the fringe visibility in a simulated Young’s double slit experiment. The optic was modeled using simple ray tracing in a Monte Carlo simulation, with the phase advance associated with each photon path computed from the path length and phase changes upon each reflection through the polycapillary tube. Fringes, which disappeared with a large source, were maintained after the optics, implying that beam coherence was observed for both the collimating and focusing polycapillary optics.
X-ray phase contrast can offer improved contrast in soft tissue imaging at clinical energies. To generate phase contrast in a clinical setting without the need for precisely aligned gratings and multiple exposures has traditionally required the use of specialized sources capable of producing x-ray spots on the order of 10 μm in diameter which necessarily require lengthy exposures due to the low intensity produced. We demonstrate results from two systems capable of overcoming this limitation. In the first, a polycapillary optic is employed to focus a typical clinical source to produce a small secondary source of the size required for phase contrast imaging. In the second, a grid of relatively large pitch is used along with Fourier processing to generate a phase contrast image using a large spot size source.
Monochromatic x-ray beams improve image contrast but suffer from low intensity if produced with a flat
monochromator crystal. A doubly-curved crystal makes more efficient use of the source. However, the beam shape
is not conducive to imaging. A combination of a bent crystal followed by a polycapillary optic can be used to
monochromatize and focus x rays to a small spot to perform monochromatic x-ray imaging with good resolution.
Ray-tracing simulations have been developed which account for defects in both optics types. A comparison was
made to measurements of focal spot sizes, angular divergence, and image quality parameters including resolution
and contrast. Simulations support the experimental results that geometric blur is significantly reduced, and
resolution enhanced, for magnification imaging with this optic combination.