We have developed further a new technique based on energy-filtered electron tomography for imaging the three-dimensional distribution of specific chemical elements in cells. The technique has been implemented in a 300 kV field-emission transmission electron microscope equipped with an advanced imaging filter including a 2048 x 2048 pixel CCD camera with high detective quantum efficiency and fast read-out. Acquisition is controlled by means of flexible computer scripts, which enable correction for specimen drift and defocus between successive tilt angles and collection of energy-filtered images at defined energy for each tilt angle. Projected elemental distributions are obtained by acquiring images above and below characteristic core-edges in the energy-loss spectrum and by subtracting the extrapolated background intensity at each pixel. We have implemented a dual-axis simultaneous iterative reconstruction technique (SIRT) to improve the signal-to-noise ratio compared with that obtainable by a conventional weighted Fourier back-projection reconstruction. By applying a thickness correction algorithm that takes into account plural inelastic scattering, and by incorporating scattering cross sections for excitation of core-shell electrons, we have shown that it is possible to quantify the elemental distributions in terms of the number of atoms per voxel. Experiments are in progress to image phosphorus in three dimensions and hence to determine the distribution of DNA in the chromatin of the cell nucleus.
| Aronova, M A; Kim, Y C; Zhang, G et al. (2007) Quantification and thickness correction of EFTEM phosphorus maps. Ultramicroscopy 107:232-44 |
