Exploiting the x-ray refraction contrast with an analyser: the state of the art

Besides attenuation, x-ray refraction occurs in an x-ray beam passing through an object containing details of different refractive indices or of non-uniform thicknesses. The emerging x-rays present characteristic angular deflections, which are on the microradian scale. Different refraction directions can be resolved by using an angular analyser, for instance a perfect crystal, which can then be revealed by a detector placed behind it. By varying the alignment of the analyser with regard to the incoming x-ray beam, the unrefracted x-rays can either be recorded on a detector or rejected, thus, either contributing or not contributing to the image contrast. The x-ray refraction contrast can greatly exceed the absorption contrast, for instance in imaging low Z materials, where the potentialities of the technique have been exploited from the beginning. An analyser-based imaging technique has been presented in various versions also assuming different names. An important variation, based on an algorithm that differentiates the separation of the refraction contrast from the absorption contrast in an image, has been introduced with the name 'diffraction enhanced imaging' (DEI). The interest in the potential medical application of the diffraction imaging technique has given origin to many projects utilizing analyser-based refraction imaging. Initiated with conventional x-ray generators, it has been rapidly developed in synchrotron radiation sites thanks to the highly intense collimated beams available that permit fast projection and tomographic imaging. Mammographic in vitro protocols and cartilage characterization are the most exploited applications of DEL The potential medical application has refocused the interest towards the utilization of presently available conventional high-power x-ray generators that can open the doors to large-scale utilization. The aim of this paper is to present a review of the technique in its theoretical and practical aspects; an abundant bibliography, highlights the most significant and recent results.