Determination of Pb concentration in human bone and tissue

This interdisciplinary project combines expertise of internal/occupational medicine and modern X-ray physics to synthesize analytical results of trace element determination by energy dispersive X-ray fluorescence and health of occupational heavy metal exposed people. Lead (Pb) is known to be a toxic element which even at low concentrations induce adverse health effects like cognitive deficits. Measuring Pb concentrations in whole blood only reflects roughly the actual exposure, since the main storage site of Pb is the bone. The in vivo determination of Pb concentration in bone is therefore of greatest interest. Energy dispersive X-ray fluorescence analysis is a convenient method for measuring trace element concentrations in reasonable time and at low dose. Within this project a polarizer-monochromator unit coupled to a high power X-ray tube with a polycapillary X-ray optics will be realized. This new optimized radiation source produces monochromatic linear polarized Mo-Ka X- rays ideally for strong excitation of the Pb L-shells. In combination with a large area high resolution X-ray detector in vivo routine investigations from bone will be performed. Due to the properties of the radiation used the radiation risk should be minimized. After optimization of excitation and detection geometry in vivo time for measuring trace elements is expected to be reduced. Due to the complex structure of human bone trace elements are not homogenously distributed. But the knowledge of the distribution is essential when only in vivo measurements are available. Micro X-ray Fluorescence Analysis and Synchrotron radiation with focusing X-ray optics offers a spatial resolution down to 15m. Pre-experiments at HASYLAB, Hamburg made on bone slices showed the different distribution of Pb and Ca measuring profiles between substantia compacta and substantia spongiosa. These results stimulate further investigations resulting in correlations between in vivo and in vitro experiments in human bone. Since cognitive deficits in Pb exposed subjects are suspected to occur in the pre-frontal cortex different brain tissues should be investigated. These investigations will be performed with the brightest microbeam available at present (European Synchrotron Radiation Source (ESRF) in Grenoble, ID 18F). Aim of the project is to develop an in vivo system for the determination of Pb in human bone and to correlate these results with in vitro measurements besides the localisation of Pb in different brain areas.

The distribution of various chemical elements in human tissue (bone and brain) has been studied by synchrotron radiation induced micro X-ray fluorescence analysis (SR-micro XRF) in scanning and in tomographic mode. Experiments have been performed at three different synchrotron radiation facilities, HASYLAB, Hamburg, ANKA, Karlsruhe, ESRF, Grenoble where dedicated micro-focus beamlines, offering sufficient photon flux and adequate X-ray optics for the different analytical tasks, were used. In case of bone the matching of elemental maps from spatially resolved micro-XRF measurements with backscattered electron images, providing information on the bone structure, allows to assign fluorescence intensities to the features of the calcified tissue and therefore the identification of zones of increased accumulation for the different elements. Especially for lead (Pb) -a toxic metal -, which resorbs into circulation when an increased amount of Ca is needed or in times of metabolic changes of the tissue due to bone diseases, the knowledge of the Pb storage sites in bones are of great medical importance. Starting from conventional micro-XRF measurements on 4mm thick samples the spatial resolution and therefore the accuracy of the method has been steadily enhanced by reduction of the sample thickness to 200m and finally employing micro-XRF in the confocal mode. From comparison of the determined elemental maps with backscattered electron images from the analyzed bone areas (prepared by a group from the Ludwig Boltzmann- Institut of Osteology, Vienna, Austria), it could be shown that the accumulation of Pb in bone is mostly restricted to the so called tidemark which divides noncalcified from calcified tissue and is considered as a metabolically active zone. Furthermore a strong correlation between Zn and Pb was found on this calcification front. Results from confocal micro-XRF on bones with duplicated tidemarks exhibit the eventuality of a time delay in the metabolism of Pb compared to Zn. When applying combined X-ray absorption and fluorescence tomography, similar results were obtained showing also the restriction of highest Zn and Pb intensities to small zones of the analyzed bone. Elemental mapping has been performed on various slices from different areas of human brain at ESRF, ID-22. Whereas the assignment of the maps to different structures of the brain is much more difficult than it is for bone tissue, an inhomogeneous distribution of Pb was found in all analyzed samples from frontal cortex, hippocampus and thalamus. A setup for the determination of Pb in bone for invivo analysis was developed based on compact portable components. The detection limits achieved were 3 g/g at an applied dose of 60 mGy, which is acceptable.