PubMed Health⌕ Search
Home/Archive/PubMed

PubMed · 634228

Linearizing mechanisms in conventional tomographic imaging.

S C Orphanoudakis·J W Strohbehn·C E Metz
View original source ↗Export metadata ↓

Abstract

Implicit in the concept of conventional tomography and in any attempt to characterize the tomographic process by a modulation transfer function is the assumption that the tomographic process is linear. A Fourier decomposition approach and an analysis of nonlinear contributions to the integrated tomographic image intensity are used in this paper to establish the validity of this assumption and to determine the mechanisms by which the tomographic process is effectively linearized.

Explore related subjects

Energy TransferFourier AnalysisModels, TheoreticalTomography, X-RayX-RaysJournal Article

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

S C Orphanoudakis, J W Strohbehn, C E Metz. Linearizing mechanisms in conventional tomographic imaging.. https://doi.org/10.1118/1.594400

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related citations

PubMed1992

Fluorescence of peptide N-terminal 2-oxoacyl and quinoxaline derivatives.

A peptide reacts with glyoxalic acid resulting in transamination of the N-terminal residue to form a 2-oxoacyl group. This further reacts with o-phenylenediamine, leading to a quinoxaline derivative of the original N-terminal amino acid, which is cleavable in mild acid [Dixon & Fields (1972) Methods Enzymol. 25, 409-419]. The 2-oxoacyl peptides are weakly fluorescent with emission maxima around 410 nm and excitation maxima at about 320 nm, depending on the nature and length of the peptide. Formation of the quinoxaline derivative results in a marked increase of fluorescence, with emission maximum of 363 nm when excited at 303 nm. The fluorescence properties of these derivatives change with the nature and length of the peptides and are affected by the presence of organic solvents, NaCl and denaturants. It is suggested that such fluorescent derivatives could be used as probes for the study of the conformation of the N-terminal region of peptides and proteins.

Energy Transfer↗
PubMed1992

Why water-soluble, compact, globular proteins have similar specific enthalpies of unfolding at 110 degrees C.

The changes in free energy, enthalpy, and entropy of unfolding have been measured for many water-soluble, compact, globular proteins by a number of workers. In principle, a wide range in stability could be achieved by proteins, as measured by the free energy of unfolding; in practice, evolution only allows a narrow range in this quantity. Proteins are only marginally stable at room temperature for many possible reasons, including ensuring that folding is reversible and polypeptide chains are not trapped in incorrectly folded structures. Many of these proteins have approximately the same values of enthalpy of unfolding around 110 degrees C. We show here that this arises because the change in entropy of unfolding at room temperature and the change in heat capacity on unfolding, which governs the temperature variation of the enthalpy and entropy, both vary with the magnitude of the hydrophobic effect in the protein. As all these proteins have evolved to achieve similar stabilities at room temperature, the enthalpy of unfolding will also vary with the size of the hydrophobic effect in the protein. A consequence of this is that curves of the specific unfolding enthalpy against temperature for different proteins intersect around 110 degrees C. A similar conclusion, on the basis of similar melting points rather than similar free energies of unfolding, has been reached independently by Baldwin and Muller (R. L. Baldwin, personal communication).

Energy Transfer↗
PubMed1992

Femtosecond spontaneous-emission studies of reaction centers from photosynthetic bacteria.

Spontaneous emission from reaction centers of photosynthetic bacteria has been recorded with a time resolution of 50 fs. Excitation was made directly into both the special-pair band (850 nm) and the Qx band of bacteriochlorophylls (608 nm). Rhodobacter sphaeroides R26, Rhodobacter capsulatus wild type, and four mutants of Rb. capsulatus were studied. In all cases the fluorescence decay was not single exponential and was well fit as a sum of two exponential decay components. The short components are in excellent agreement with the single component detected by measurements of stimulated emission. The origin of the nonexponential decay is discussed in terms of heterogeneity, the kinetic scheme, and the possibility of slow vibrational relaxation.

Energy Transfer↗