An auto-tuning method for focusing and astigmatism correction in HAADF-STEM, based on the image contrast transfer function

Measurement of large low-order aberrations by using a series of through-focus Ronchigrams

A method for measuring large aberrations up to second order (defocus, 2-fold astigmatism and axial coma), which uses a through-focus series of Ronchigrams, is proposed. The method is based on the principle that line-focus conditions in Ronchigrams can be locally detected and low-order aberrations can thereby be measured. The proposed method provides auto-tuning of large low-order aberration; in particular, iterative aberration measurement and correction reduce low-order aberrations from several thousand nanometers to less than a few hundred nanometers, which can be handled by conventional fine-aberration tuning methods.

Three-dimensional optical transfer functions in the aberration-corrected scanning transmission electron microscope

In the scanning transmission electron microscope, hardware aberration correctors can now correct for the positive spherical aberration of round electron lenses. These correctors make use of nonround optics such as hexapoles or octupoles, leading to the limiting aberrations often being of a nonround type. Here we explore the effect of a number of potential limiting aberrations on the imaging performance of the scanning transmission electron microscope through their resulting optical transfer functions. In particular, the response of the optical transfer function to changes in defocus are examined, given that this is the final aberration to be tuned just before image acquisition. The resulting three-dimensional optical transfer functions also allow an assessment of the performance of a system for focal-series experiments or optical sectioning applications.

Low-dosage Maximum-A-Posteriori Focusing and Stigmation

Radiation damage is often an issue during high-resolution imaging, making low-dose focusing and stigmation essential, in particular when no part of the sample can be "sacrificed" for this. An example is serial block-face electron microscopy, where the imaging resolution must be kept optimal during automated acquisition that can last months. Here, we present an algorithm, which we call "Maximum-A-Posteriori Focusing and Stigmation (MAPFoSt)," that was designed to make optimal use of the available signal. We show that MAPFoSt outperforms the built-in focusing algorithm of a commercial scanning electron microscope even at a tenfold reduced total dose. MAPFoSt estimates multiple aberration modes (focus and the two astigmatism coefficients) using just two test images taken at different focus settings. Using an incident electron dose density of 2,500 electrons/pixel and a signal-to-noise ratio of about one, all three coefficients could be estimated to within <7% of the depth of focus, using 19 detected secondary electrons per pixel. A generalization to higher-order aberrations and to other forms of imaging in both two and three dimensions appears possible.

Three-dimensional localization of ultrasmall immuno-gold labels by HAADF-STEM tomography

The localization of scarce antigens in thin sections of biological material can be accomplished by pre-embedment labeling with ultrasmall immuno-gold labels. Moreover, with this method, labeling is not restricted to the section surface but occurs throughout the section volume. Thus, when combined with electron tomography, antigens can be localized in three dimensions in relation to the 3D (three-dimensional) ultrastructure of the cell. However, for visualization in a transmission electron microscope, these labels need to be enlarged by silver or gold enhancement. The increase in particle size reduces the resolution of the antigen detection and the large particles obscure ultrastructural details in the tomogram. In this paper we show for the first time that these problems can be avoided and that ultrasmall gold labels can be localized in three dimensions without the need for gold or silver enhancement by using HAADF-STEM (high angular annular dark-field-scanning transmission electron microscopy) tomography. This method allowed us to three-dimensionally localize Aurion ultrasmall goat anti-rabbit immuno-gold labels on sections of Epon-embedded, osmium-uranium-lead-stained biological material. Calculations show that a 3D reconstruction obtained from HAADF-STEM projection images can be spatially aligned to one obtained from transmission electron microscopy (TEM) projections with subpixel accuracy. We conclude that it is possible to combine the high-fidelity structural information of TEM tomograms with the ultrasmall label localization ability of HAADF-STEM tomograms.