Quantitative X-ray imaging of labelled molecules in tissues and cells

Using cisplatin as a model system, we have been able to demonstrate the feasibility of studying the cellular and subcellular distribution of a labelled molecule containing a single atom of platinum per molecule in bone marrow. An X-ray imaging system consisting of a microcomputer, a 4pi system and a software package was interfaced with an electron microscope enabling the computer to control the beam movements as well as receive signals from the STEM and EDS X-ray detectors. X-ray imaging is useful for both tissue and samples in which the population of cells is not homogeneous. Imaging permits elemental distributions to be measured throughout the sample and not in just randomly selected areas as previously done in X-ray microanalysis. Images are created for not only the element labelling the molecule of interest but also other specified elements present. Three types of maps for imaging labelled molecules are compared and discussed. When the original (collected) data are mapped, the elements of interest are obscured by the continuum. The maps calculated using an internal standard give a concentration distribution on the basis of volume (mmol L(-1) of packed cells). The maps calculated using the continuum normalization method according to Hall produces concentration distribution on the basis of mass (mmolk(-1) dry weight). By recalculating using the 'Peak' or 'Hall' method the continuum problem is removed yielding quantitative images of the intracellular distribution of labelled molecules present in low concentrations.