ABSTRACT
Figure 11.3 The partially coherent X-ray beam forms coherent images depending on the object-detector distance y, sample size d, and X-ray wavelength l [9].
11.3 Phase-Contrast TomographyPhase-contrast tomography methods are used for samples with less or no absorption contrast or for samples composed of matter with almost the same absorption contrast. The precondition for phase contrast is (partially) coherent X-ray radiation. Modern insertion devices (such as undulators) in the synchrotron storage rings produce partially coherent X-rays. Because for phasecontrast imaging moderate coherence requirements are sufficient, this measurement method can be carried out at third-generation synchrotrons without additional X-ray optics for producing a coherent beam. Therefore, several phase-contrast X-ray imaging methods have become standard methods for tomography at third-generation synchrotrons. Coherent X-rays, which interact with a sample, are attenuated and also shifted in phase. These can vary independently of each other. This behavior can be described by the complex index of refractionn = 1-d + ib where the real part d corresponds to the phase shift due to refraction and the imaginary part b belongs to absorption. Then, the key for phase-contrast imaging is to convert phase shifts into variations in the intensity.
