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Fluorescence microtomography is a hard x-ray scanning microscopy technique that has been developed at synchrotron radiation sources in recent years. It allows one to reconstruct non-destructively the element distribution on a virtual section inside a sample. The spatial resolution of this microbeam technique is limited by the lateral size of the microbeam. Since recently, nanofocusing refractive x-ray lenses (NFLs) are under development that were shown to produce hard x-ray microbeams with lateral resolution in the range of 100nm. Future improvements of these optics might reduce the microbeam size down to below 20nm. Using nanofocusing lenses, fluorescence microtomography with sub-micrometer resolution was performed. As an example, the element distribution inside a small cosmic dust particle is given. Tomographic reconstruction was done using a refined model including absorption effects inside the sample.

Since 1998 we have developed X-Ray fluorescence tomography for microanalysis. All aspects were tackled starting with the reconstruction performed by FBP or ART methods. Self-absorption corrections were added and combined with Compton, transmission and fluorescence tomographies to obtain fully quantitative results. Automatic "smart scans" minimized overhead time scanning/aligning non-cylindrical objects. The scans were performed step-by-step or continuously with no overhead time. Focusing went from 5 to 1 micron range, using FZP or CRL lenses, and finally KB bent mirrors which yield sub-micron high intensity beams. Recently, we have performed the first quantitative 3D fluo-tomography by helical scanning. We are now studying energy dependent fluo-tomography for chemically-sensitive imaging of the internal structure of samples. This chronology yielded the present level of sophistication for both experiments and data treatment and finally a method ready for wide dissemination among scientists.

In various scientific fields -- such as materials sciences, biology or even astrophysics -- the relation between morphology and the chemical composition is a key for the understanding of structures and their function. Hard x-ray tomography is a suitable tool for structural analyzes on the micrometer scale and can give additional chemical information by combining this imaging technique with spectroscopic methods. In chemistry, X-ray absorption near-edge spectroscopy (XANES) is a well-known and established technique. By scanning the X-ray energy in the vicinity (50-100 eV) of the absorption edge of an element, information can be obtained about the oxidation state of the probed atoms. We used a fast read-out and low noise detector for XANES imaging and were able to distinguish different oxidation states in three dimension performing tomographic scans at different characteristic energies of the probed atom.

We present recent results of fluorescence tomography experiments obtained on a variety of samples originating from the fields of Mineralogy, Space Sciences or Botany. The ID22 hard X-ray microanalysis beamline of the ESRF was used in scanning beam mode to record fluorescence spectra in pencil-beam collection mode for energies of 14 to 22.5 keV and micron-sized beamspots. Trace element concentrations of a few hundred ppm were successfully imaged in inhomogenous samples of less than 500 microns and resolutions up to 2 microns.

In chemistry, x-ray absorption near-edge spectroscopy (XANES) is a well-known and established technique. By scanning the x-ray energy in the vicinity (50-100 eV) of the absorption edge of an element, information can be obtained about the oxidation state of the probed atoms. The (conventional) technique mainly employed until now applies for homogeneous, specifically prepared flat samples where the measured signal can be considered as the average over the whole irradiated volume. This restriction for samples is partially released when the XANES method is combined with imaging techniques. Two-D resolved data is acquired using area detectors or by scanning with a focused beam. X-ray absorption tomography is a method of choice for investigating the 3D structure of objects and its dual energy version is used for getting information about the 3D distribution of a given element within the sample. Although the combination of XANES and tomography seems to be a natural extension of dual-energy tomography, in practice several experimental problems have to be overcome in order to obtain useable data. In the following we describe the results of XANES imaging and tomography obtained measuring a phantom sample of pure molybdenum compounds using a FreLoN 2000 camera system at the ESRF undulator beamline ID22. This system allowed making volume resolved distinctions between different oxidation states with spatial resolution in the micrometer range.

Parabolic compound refractive lenses (PCRLs) are high quality hard x-ray imaging optics that can be used to image a synchrotron source onto a sample in a strongly demagnifying setup. This allows to produce an intensive microbeam with lateral extensions in the (sub-)micrometer range. Aluminium PCRLs can be operated in an energy range from about 10keV to 60keV and withstand the high heat load of the white beam of an ESRF undulator source. The microbeam properties using monochromatic and single undulator harmonic (pink) radiation are discussed, focusing on beam size, depth of field, background, flux, and gain. The large depth of focus allows to scan fairly large samples (a few millimeters in thickness) with a beam of constant lateral extension. This makes tomographic scanning techniques, such as fluorescence microtomography possible. As applications, fluorescence microtomography of plant samples with sub-cellular resolution and the mapping of trace elements in single cancer cells is shown.

First experimental results of fluorescence microtomography with 6 micrometer resolution obtained at the ESRF are described. The set-up comprises high quality optics (monochromator, mirrors, focusing lenses) coupled to the high energy/brilliance/coherence of the ID 22 undulator beamline. The tomographic set-up allows precise measurements in the 'pencil-beam' geometry. The image reconstruction is based either on the filtered back-projection (FBT) method or on a modification of the algebraic reconstruction method (ART) but includes simplifications of the model. The quality and precision of the 2D reconstructed elemental images of two phantom sample are encouraging. The method will be further refined and applied for the analysis of more complex inhomogeneous samples.