Sharper Images of Biological Samples

To mark AIP's 75th anniversary, PNU will run a series of occasional comparisons between noteworthy physics topics from 1931 and 2006. This is the first article of the series.

Physicists in Germany have taken a crucial step towards achieving sharper electron microscope images of biological samples and other "weak-contrast" objects.

Typically, microscope images of samples made of low-weight elements like hydrogen, carbon, nitrogen, and oxygen, are characterized by poor contrast. In the new approach, contrast will be improved for a transmission electron microscope (TEM) by imposing a large relative phase shift to the electron waves scattered from samples.

The use of a beam of electrons as an illumination source for microscopy was pioneered in the early 1930s by Ernst Ruska, who won a Nobel Prize for the effort half a century later. Since then, electron microscopes have been a workhorse for imaging small things, often with a spatial resolution superior to that available with light microscopes. Nevertheless, even electron microscopes have resolution problems.

In a TEM device most of the electrons pass through the thin electron-transparent sample without scattering. Scattering of electron waves, when it does happen, occurs not because of absorption -- the amplitude of the electron beam is largely undiminished -- but through the shifting of the electron phase. Scattered and unscattered waves are focused and recombine downstream of the sample in a recording medium, typically a charged coupled device (CCD).

Unfortunately, in weak-phase objects the phase shifting is slight, resulting in poor contrast. What scientists at the University of Karlsruhe and the Max-Planck Institute for Biophysics in Frankfurt have done to remedy this situation is to interpose a special free-suspended, micro-scaled electrostatic lens beyond the sample; this electrostatic lens has the effect of shifting the phase of the unscattered waves by a further 90 degrees but leaving the scattered waves unshifted (see figures at Physics News Graphics).

This dramatically improves the contrast in the resultant images. This electrostatic lens is called a Boersch phase plate in honor of Hans Boersch, who proposed the technique in 1947. It has not been achieved until now because of its demanding size specifications.

Schultheiss et al., Review of Scientific Instruments, March 2006
Contact Fabian Perez-Willard, perez@lem.uni-karlsruhe.de
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