3d phase Contrast Imaging

3D Phase Contrast Imaging
P. Cloetens¹, W. Ludwig², E. Boller¹, R. Mokso¹, O. Hignette¹, J. Lambert³, and S. Bohic^1
1European Synchrotron Radiation Facility, Grenoble, France

²GEMPPM, INSA Lyon, Villeurbanne, France

³GMCM, University Rennes I, Rennes, France
Keywords: micro-tomography, phase retrieval, Kirckpatrick-Baez optics, fluorescence
The large depth of focus, the element and chemistry specific interaction near absorption edges and the simple analytical description of the wave-matter interaction make hard X-rays an invaluable imaging tool. They are particularly suited for quantitative three-dimensional imaging in absorption, phase contrast and fluorescence mode. The high brilliance of modern synchrotron sources has opened new possibilities in 3D X-ray microscopy.

The partial coherence of the beams allows for a simple way of phase contrast imaging through propagation, i.e. Fresnel diffraction or defocusing. Holotomography (see Figure a), combines a numerical phase retrieval procedure based on 2D images recorded at different distances for one orientation of the sample, with the tomographic rotation. The result is a 3D map of the variations of the electron density. Holotomography was applied to several fields, including polymer foams, semisolid alloys, composite materials, human hair, plant tissue.

Because tomography is non-destructive, it is possible to follow the evolution of the microstructure and the composition as a function of time, temperature or under the effect of an external load. This can be done either by sequential tomography, interrupting the process during the time needed for the 3D acquisition, or by fast, real-time tomography, if the acquisition time (as short as a few seconds) is small compared to the evolution time of the system.

While the resolution is determined by the detector system in the parallel beam geometry, it is of interest to obtain magnified images, either by magnifying X-ray optics or in a projection geometry by focusing the beam. Using a mirror device (Kirkpatrick-Baez) a focused spot size as small as 90 nm x 90 nm was obtained at high energy (20 keV) with a very high flux (up to 10¹² ph/s). Such a point source can be used in a single setup (see Figure b) for full-field projection microscopy and fluorescence mapping. Putting the object a small distance downstream (or upstream) of the focus a magnified Fresnel diffraction pattern is obtained with a resolution limited by the KB focused spot. The requirements on the surface quality of the mirrors are very demanding in order to avoid distortions of the images. Phase contrast tomographic images are sensitive to the electron density and yield essential micro-structural information. Putting the object in the focus and through a scanning procedure micro-fluorescence maps of selected portions of the specimen are obtained. This gives element specific information at a very fine scale and/or on trace elements.

Figure Principle of 3D imaging using (a) a parallel beam geometry and (b) a projection geometry with a focused beam. Fluorescence maps can be obtained by scanning the sample in the plane of focus.