Electron tomography (ET) is a

tomography Tomography is imaging by sections or sectioning that uses any kind of penetrating wave. The method is used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, astrophysics, quantu ...

technique for obtaining detailed 3D structures of sub-cellular, macro-molecular, or materials specimens. Electron tomography is an extension of traditional

transmission electron microscopy Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...

and uses a

transmission electron microscope Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a gr ...

to collect the data. In the process, a beam of

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...

s is passed through the sample at incremental degrees of rotation around the center of the target sample. This information is collected and used to assemble a three-dimensional image of the target. For biological applications, the typical resolution of ET systems are in the 5–20 nm range, suitable for examining supra-molecular multi-protein structures, although not the secondary and

tertiary structure Protein tertiary structure is the three dimensional shape of a protein. The tertiary structure will have a single polypeptide chain "backbone" with one or more protein secondary structures, the protein domains. Amino acid side chains may int ...

of an individual

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

polypeptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A p ...

. Recently, atomic resolution in 3D electron tomography reconstructions has been demonstrated.

BF-TEM and ADF-STEM tomography

In the field of biology, bright-field

(BF-TEM) and high-resolution TEM (

HRTEM High-resolution transmission electron microscopy is an imaging mode of specialized transmission electron microscopes that allows for direct imaging of the atomic structure of samples. It is a powerful tool to study properties of materials on the a ...

) are the primary imaging methods for tomography tilt series acquisition. However, there are two issues associated with BF-TEM and HRTEM. First, acquiring an interpretable 3-D tomogram requires that the projected image intensities vary monotonically with material thickness. This condition is difficult to guarantee in BF/HRTEM, where image intensities are dominated by phase-contrast with the potential for multiple contrast reversals with thickness, making it difficult to distinguish voids from high-density inclusions. Second, the contrast transfer function of BF-TEM is essentially a

high-pass filter A high-pass filter (HPF) is an electronic filter that passes signals with a frequency higher than a certain cutoff frequency and attenuates signals with frequencies lower than the cutoff frequency. The amount of attenuation for each frequency d ...

– information at low spatial frequencies is significantly suppressed – resulting in an exaggeration of sharp features. However, the technique of annular dark-field

scanning transmission electron microscopy A scanning transmission electron microscope (STEM) is a type of transmission electron microscope (TEM). Pronunciation is tɛmor �sti:i:ɛm As with a conventional transmission electron microscope (CTEM), images are formed by electrons passing ...

(ADF-STEM), which is typically used on material specimens, more effectively suppresses phase and diffraction contrast, providing image intensities that vary with the projected mass-thickness of samples up to micrometres thick for materials with low

atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...

. ADF-STEM also acts as a

low-pass filter A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter des ...

, eliminating the edge-enhancing artifacts common in BF/HRTEM. Thus, provided that the features can be resolved, ADF-STEM tomography can yield a reliable reconstruction of the underlying specimen which is extremely important for its application in materials science. For 3D imaging, the resolution is traditionally described by the

Crowther criterion The conventional method to evaluate the resolution of a tomography reconstruction is determined by the Crowther criterion. The minimum number of views, ''m'', to reconstruct a particle of diameter In geometry, a diameter of a circle is any str ...

. In 2010, a 3D resolution of 0.5±0.1×0.5±0.1×0.7±0.2 nm was achieved with a single-axis ADF-STEM tomography.

Atomic Electron Tomography (AET)

Atomic level resolution in 3D electron tomography reconstructions has been demonstrated. Reconstructions of crystal defects such as stacking faults,

grain boundaries In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional crystallographic defect, defects in the crystal structure, and tend to decrease the ...

dislocations In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to sl ...

, and twinning in structures have been achieved. This method is relevant to the physical sciences, where

cryo-EM Cryogenic electron microscopy (cryo-EM) is a cryomicroscopy technique applied on samples cooled to cryogenic temperatures. For biological specimens, the structure is preserved by embedding in an environment of vitreous ice. An aqueous sample sol ...

techniques cannot always be used to locate the coordinates of individual atoms in disordered materials. AET reconstructions are achieved using the combination of an ADF-STEM tomographic tilt series and

iterative Iteration is the repetition of a process in order to generate a (possibly unbounded) sequence of outcomes. Each repetition of the process is a single iteration, and the outcome of each iteration is then the starting point of the next iteration. ...

algorithms In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing c ...

for

reconstruction Reconstruction may refer to: Politics, history, and sociology *Reconstruction (law), the transfer of a company's (or several companies') business to a new company *'' Perestroika'' (Russian for "reconstruction"), a late 20th century Soviet Unio ...

. Currently, algorithms such as the real-space algebraic reconstruction technique (ART) and the

fast Fourier transform A fast Fourier transform (FFT) is an algorithm that computes the discrete Fourier transform (DFT) of a sequence, or its inverse (IDFT). Fourier analysis converts a signal from its original domain (often time or space) to a representation in th ...

equal slope tomography (EST) are used to address issues such as image noise, sample drift, and limited data. ADF-STEM tomography has recently been used to directly visualize the atomic structure of screw dislocations in nanoparticles. AET has also been used to find the 3D coordinates of 3,769 atoms in a tungsten needle with 19 pm precision and 20,000 atoms in a multiply twinned palladium nanoparticle. The combination of AET with

electron energy loss spectroscopy In electron energy loss spectroscopy (EELS) a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their pa ...

(EELS) allows for investigation of electronic states in addition to 3D reconstruction. Challenges to atomic level resolution from electron tomography include the need for better reconstruction algorithms and increased precision of tilt angle required to image defects in non-crystalline samples.

Different tilting methods

The most popular tilting methods are the single-axis and the dual-axis tilting methods. The geometry of most specimen holders and electron microscopes normally precludes tilting the specimen through a full 180° range, which can lead to artifacts in the 3D reconstruction of the target. Standard single-tilt sample holders have a limited rotation of ±80°, leading to a missing wedge in the reconstruction. A solution is to use needle shaped-samples to allow for full rotation. By using dual-axis tilting, the reconstruction artifacts are reduced by a factor of

\sqrt

compared to single-axis tilting. However, twice as many images need to be taken. Another method of obtaining a tilt-series is the so-called conical tomography method, in which the sample is tilted, and then rotated a complete turn.

References

{{reflist Electron microscopy Multidimensional signal processing Condensed matter physics

BF-TEM and ADF-STEM tomography

Atomic Electron Tomography (AET)

Different tilting methods

See also

References