X-Ray Absorption Spectroscopy Imaging of Biological Tissues

X-ray absorption spectroscopy (XAS) is proving invaluable in determining the average chemical form of metals or metalloids in intact biological tissues. As most tissues have spatial structure, there is great additional interest in visualizing the spatial location of the metal(loid) as well as its chemical forms. XAS imaging gives the opportunity of producing maps of specific chemical types of elements in vivo in dilute biological systems. X-ray fluorescence microprobe techniques are routinely used to study samples with spatial heterogeneity. Microprobe produces elemental maps, with chemical sensitivity obtained by recording micro-XAS spectra at selected point locations on the map. Unfortunately, using these procedures spatial detail may be lost as the number of point spectra recorded generally is limited. A powerful extension of microprobe is XAS imaging or chemically specific imaging. Here, the incident energy is tuned to features in the near-edge which are characteristic of the expected chemical forms of the element. With a few simple assumptions, these XAS images can then be converted to quantitative images of specific chemical form, yielding considerable clarity in the distributions.