[Image differentiation procedures in electron microscopy]

Combined approaches to study virus structures

A virus particle must work as a safe box for protecting its genome, but at the same time it has to undergo dramatic conformational changes in order to preserve itself by propagating in a cell infection. Thus, viruses are miniaturized wonders whose structural complexity requires them to be investigated by a combination of different techniques that can tackle both static and dynamic processes. In this chapter we will illustrate how major structural techniques such as X-ray crystallography and electron microscopy have been and can be combined with other techniques to determine the structure of complex viruses. The power of these hybrid method approaches are revealed through the various examples provided.

First experimental test of a new monochromated and aberration-corrected 200kV field-emission scanning transmission electron microscope

The first 200 kV scanning transmission electron microscope (STEM) with an imaging energy filter, a monochromator and a corrector for the spherical aberration (Cs-corrector) of the illumination system has been built and tested. The STEM/TEM concept with Koehler illumination allows to switch easily between STEM mode for analytical and TEM mode for high-resolution or in situ studies. The Cs-corrector allows the use of large illumination angles for retaining a sufficiently high beam current despite the intensity loss in the monochromator. With the monochromator on and a 3 microm slit in the dispersion plane that gives 0.26 eV full-width at half-maximum (FWHM) energy resolution we have obtained so far an electron beam smaller than 0.20 nm in diameter (FWHM as measured by scanning the spot quickly over the CCD) which contains 7 pA current and, according to simulations, should be around 0.12 nm in true size. A high-angle annular dark field (ADF) image with isotropic resolution better than 0.28 nm has been recorded with the monochromator in the above configuration and the Cs-corrector on. The beam current is still somewhat low for electron energy-loss spectroscopy (EELS) but is expected to increase substantially by optimising the condenser set-up and using a somewhat larger condenser aperture.

3-D repositioning and differential images of volumetric CT measurements

In quantitative computed tomography (QCT), time serial measurements are performed to detect a global bone density loss or to identify localized bone density changes. A prerequisite for an unambiguous analysis is the comparison of identical bone volumes. Usually, manual repositioning is too coarse. A mathematical procedure that allows matching two three-dimensional image volumes is presented. The algorithm is based on correlation techniques. The procedure has been optimized and applied to computer-tomographic 3-D images of the human knee. It has been tested with both artificially created and in vivo measured image data. Typical differential images calculated from real bone measurements are presented.

Prospects of three-dimensional high resolution electron microscopy of non-periodic structures

In the first part of the paper the general features of three-dimensional electron microscopy of non-periodic structures at high resolution are discussed. Such an analysis requires series of 20 or more exposures of mechanically moved specimens. It is fundamentally important that the integral radiation dose, however, need not be increased--three-dimensional analysis yields extra information without additional radiation loading. It is demonstrated that the constraints characteristic for atomic resolution can in future facilitate many difficulties like restricted tilting range (conical tilting), inaccuracies in the refinement of the origin determination etc. Data collection up to atomic resolution is possible with existing image reconstruction methods if the chromatic spread of the cathodes can be partially corrected. The relatively great radiation induced changes of "stable" specimens will require additional averaging in the experiment (quasi-simultaneous registration). The advantage of a time dependent analysis of radiation sensitive structures--especially the possibility of using redundancies in the analysis (trace structure analysis)--is discussed. The experimental part (studies of radiation damage and three-dimensional work on carbon foils) presents examples of such analyses. The main experimental difficulty consists in the collection of many exposures with no extra load of the specimen in the intervals between exposures.