Fluorescence Microtomography: External Mapping of Elements Inside Biological Samples

摘要

X-ray fluorescence element microtomography (XFEMT) allows to determine the element specific inner structure of a sample with resolutions in the micron range. It has a wide range of applications in many disciplines and is ideally suited for investigating element distributions inside of biological bulk samples at a cellular level with minimal sample preparation. The high intensity hard x-ray microbeam required for this scanning technique is produced using parabolic compound refractive lenses at a third generation undulator source. The sample is scanned through the microbeam in both translation and rotation and the fluorescence radiation created in the sample is recorded by an energy dispersive detector. From this data, the element distribution on a virtual section through the sample is recovered by tomographic techniques. The excitation of the fluorescence by monochromatic x-rays yields a high signal to background ratio and a low detection limit. As an example, we have investigated the distribution of physiologically relevant ions on a virtual section through a freeze dried root of the mahagony plant (Swietenia macrophylla). Absorption of the fluorescence radiation inside the sample has to be taken into account in tomographic reconstruction and ultimately limits the size of the sample that can be investigated. A self-consistent reconstruction technique not requiring the explicit knowledge of the absorption inside the sample has been developed. Further developments of the technique are discussed.