Phase-contrast Imaging - X-ray Imaging

X-ray Imaging

There are four main techniques for x-ray phase-contrast imaging, which use different principles to convert phase variations in the x-rays emerging from the object into intensity variations at an x-ray detector. Propagation-based phase contrast uses free-space propagation to get edge enhancement, talbot interferometry uses a set of diffraction gratings to measure the derivative of the phase, refraction-enhanced imaging uses an analyzer crystal also for differential measurement, and x-ray interferometry uses a crystal interferometer to measure the phase directly. The advantage of these methods compared to normal absorption-contrast x-ray imaging is higher contrast that makes it possible to see smaller details. One disadvantage is that these methods require more sophisticated equipment, such as synchrotron or microfocus x-ray sources, x-ray optics, and high resolution x-ray detectors. This sophisticated equipment provides the sensitivity required to differentiate between small variations in the refractive index of x-rays passing through different media. The refractive index is normally smaller than 1 with a difference from 1 between 10−7 and 10−6.

All of these methods produce images that can be used to calculate the projections (integrals) of the refractive index in the imaging direction. For propagation-based phase contrast there are phase-retrieval algorithms, for talbot interferometry and refraction-enhanced imaging the image is integrated in the proper direction, and for x-ray interferometry phase unwrapping is performed. For this reason they are well suited for tomography, i.e. reconstruction of a 3D-map of the refractive index of the object from many images at slightly different angles. For x-ray radiation the difference from 1 of the refractive index is essentially proportional to the density of the material.

Synchrotron X-ray tomography can employ phase contrast imaging to enable imaging of the interior surfaces of objects. In this context, phase contrast imaging is used to enhance the contrast that would normally be possible from conventional radiographic imaging. A difference in the refractive index between a detail and its surroundings causes a phase shift between the light wave that travels through the detail and that which travels outside the detail. An interference pattern results, marking out the detail.

This method has been used to image Precambrian metazoan embryos from the Doushantuo Formation in China, allowing the internal structure of delicate microfossils to be imaged without destroying the original specimen.