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Biomedical subjects

A V Crewe

Publications and source records attributed to A V Crewe.

7 recordsLinked to original sources
PubMed1979

Dark field imaging of biological macromolecules with the scanning transmission electron microscope.

A scanning transmission electron microscope (STEM) equipped with a field emission gun has been employed for the examination of biological macromolecules at high resolution. The quality of micrographs obtained with the STEM is dependent upon the quality of the substrate used to support biological objects because the image contrast in dark field is proportional to the mass density of the specimen. In order to reduce deleterious effects of the substrates on the image quality, we have developed a method of fabricating substrates consisting of very thin, very clean carbon films supported on very clean fenestrated plastic films. These films are approximately 15 A thick. Well-known biological macromolecules such as glutamine synthetase and tobacco mosaic virus (both stained) and low-density lipoprotein and ferritin (both unstained were placed on these substrates and examined with the STEM by using various modes of contrast. The micrographs obtained by using the dark field mode of contrast employing an annular detector were free from phase contrast, as expected. Using this contrast mode, we have been able to directly observe (in-focus) 2.5- to 4.4-A lattice spacings in the ferritin core. The effect of electron radiation damage on the helical structure of tobacco mosaic virus was also examined. Micrographs as well as corresponding optical diffraction patterns obtained with moderately low doses showed very clear helical structure from both sides of the virus. In addition, the (11.5 A)(-1) layer lines indicated the effective resolution attained on these particles.

Ferritins↗
PubMed1976

The use of backscattered electrons for imaging purposes in a scanning electron microscope.

It is shown that the use of a very large detector for backscattered electrons can provide a signal comparable to that obtained in the secondary emission mode and that the resolution available is also comparable. A technique is described whereby the difference is taken between these two signals, and the use of this difference signal can significantly enhance surface details. It is believed that the use of such a signal should ultimately prove advantageous in increasing resolution.

Animals↗
PubMed1976

Very low voltage electron microscopy.

We conclude that a 150 V scanning microscope with a resolution of 10 A is quite feasible and could have considerable value. It might consist of a field emission source, an electron gun to decelerate the electrons, a condenser lens to produce a parallel beam, a multipole corrector and a short focal length objective lens. Electrons reflected from the specimen surface would pass through a spectrometer whose principal features would be a large collecting power and low (1/200) energy resolution. Finally, we should add that such a microscope presents a considerable challenge and new opportunities for the electron optician in both lens and spectrometer design. We cannot refrain from pointing out that the Scherzer theorem does not necessarily hold for such a lens since the constraints of the theorem do not apply to this case.

Microscopy, Electron, Scanning↗
PubMed1975

High resolution imaging properties of the STEM.

The effect of the finite size of the atom on the resolution of the STEM is investigated. When the probe size becomes comparable to the size of the atom, the quality of the image depends on the scattering properties of the atom as well as the distribution of electrons in the probe. A technique for calculating the image of a single atom is developed by expanding the scattering amplitude. This allows the image of an atom or its spatial frequency to be expanded into various components. The specific case of dark field contrast formed with elastically scattered electrons is considered. The coefficients of the components are evaluated for carbon and thorium using complex scattering amplitudes derived from relativistic Hartree-Fock-Slater wavefunctions. The coefficients are evaluated for a 100 keV microscope using an immersion type objective lens whose aperture is limited to 12 mrad by primary spherical aberration and a 100 keV microscope using the same objective lens in conjunction with a corrector lens for spherical aberration. Secondary spherical aberration limits the objective aperture of the corrected microscope to 30 mrad.

Carbon↗