Biomedical subjects

Thomas Speck

Publications and source records attributed to Thomas Speck.

6 recordsLinked to original sources

PubMed2006

Gradient shimming with spectrum optimisation.

Shimming, i.e. homogenising the unavoidable distortion of the static magnetic field B0 in NMR spectroscopy, is still an annoying, time consuming task. Although compared with conventional manual or computerised search methods gradient shimming initiated a new era in terms of operation and efficiency, there remain aspects that inhibit fully automated shimming with a result of guaranteed quality. The major reason for this limitation is that the judgement of the quality of the B0 homogeneity takes place in the spatial domain, although the actual objects of interest are the lines in the spectral domain. In this work, this restriction is removed by the introduction of a new framework for gradient shimming that enables to directly aim at the spectrum quality. Based on the mapped B0 field shimming is simulated and spectra are calculated for the virtual residual inhomogeneity. Using a suitable criterion to judge the spectrum quality an optimisation is performed, thus providing the predicted optimum spectrum and the corresponding residual B0 field. This target field is then aimed at during the real, iterative shimming procedure. For the widely applicable case of optimising the shape of a single line a powerful quality criterion was developed using an envelope of the calculated lineshape spectrum. The whole procedure is demonstrated for adjusting the on-axis shim functions based on one-dimensional field map data and for both on- and off-axis shimming using three-dimensional data. The results are verified with 1H NMR spectra acquired on standard NMR test samples.

Journal Article↗

PubMed2005

Plant growth forms: an ecological and evolutionary perspective.

Trees, shrubs, lianas and herbs have widely different mechanical architectures, which can also vary phenotypically with the environment. This review investigates how environmental effects, particularly mechanical perturbation, can influence biomechanical development in self-supporting and climbing growth forms. The bifacial vascular cambium is discussed in terms of its significance to growth form variation, ecology and evolution among extant plants, and during its appearance and early evolution. A key aspect of this developmental innovation concerned its potential for architectural and mechanical variation in response to environmental effects as well as optimizing hydraulic supply before the appearance of laminate leaves. Growth form diversity and its importance to past and present ecosystems are discussed in relation to both evolutionary constraints and ecological factors such as climatic change and atmospheric CO2 concentrations. We discuss how widely ranging growth forms such as climbers show a large range of developmental and phenotypic variation that has much to offer in understanding how the environment can modify plant development, particularly in terms of the bifacial vascular cambium. The broad approach we propose would benefit a wide range of studies from research into wood development to long-term ecological censuses of today's potentially changing ecosystems.

Adaptation, Physiological↗

PubMed2004

Distribution of work in isothermal nonequilibrium processes.

Diffusive motion in an externally driven potential is considered. It is shown that the distribution of work required to drive the system from an initial equilibrium state to another is Gaussian for slow but finite driving. Our result is obtained by projection method techniques exploiting a small parameter defined as the switching rate between the two states of the system. The exact solution for a simple model system shows that such an expansion may fail in higher orders, since the mean and the variance following from the exact distribution show nonanalytic behavior.

Journal Article↗

PubMed2003

Modelling primary and secondary growth processes in plants: a summary of the methodology and new data from an early lignophyte.

A mathematical method, based on polar coordinates that allow modelling of primary and secondary growth processes in stems of extant and fossil plants, is summarized and its potential is discussed in comparison with numerical methods using digitizing tablets or electronic image analysing systems. As an example, the modelling of tissue distribution in the internode of an extant sphenopsid (Equisetum hyemale) is presented. In the second half of the paper we present new data of a functional analysis of stem structure and biomechanics of the early lignophyte Tetraxylopteris schmidtii (Middle Devonian) using the polar coordinate method for modelling the tissue distribution in stems of different ontogenetic age. Calculations of the mechanical properties of the stems, based on the modelling of the tissue arrangement, indicate that there is no increase in structural bending modulus throughout the entire development of the plant. The oldest ontogenetic stage has a significantly smaller bending elastic modulus than the intermediate ontogenetic stage, a 'mechanical signal', which is not consistent with a self-supporting growth form. These results, and the ontogenetic variations of the contributions of different stem tissues to the flexural stiffness of the entire stem, are discussed in the evolutionary context of cambial secondary growth.

Biological Evolution↗

PubMed2003

Mechanical, chemical and X-ray analysis of wood in the two tropical lianas Bauhinia guianensis and Condylocarpon guianense: variations during ontogeny.

Mechanical and chemical properties as well as microfibril angles of wood tissues from different ontogenetic stages are determined for the neotropical lianas Bauhinia guianensis and Condylocarpon guianense. The mechanical properties include the elastic moduli under bending and under dynamic torsion. The chemical analyses cover (i) the content of cellulose, lignin and hemicelluloses fractions, (ii) the monomeric composition of the uncondensed lignin, and (iii) the composition of the hemicelluloses with respect to neutral monosaccharides. By comparing the wood properties of these lianas with the corresponding properties of wood from self-supporting deciduous trees, common characteristics and differences are revealed. Additionally, the changes in the lignin and polysaccharides fractions as well as the variations in microfibril orientation that occur during ontogeny of the two liana species are discussed with regard to their implications for the mechanical properties of wood.

Apocynaceae↗

PubMed2003

Growth habit and mechanical architecture of the sand dune-adapted climber Clematis flammula var. maritima L.

Clematis flammula var. maritima is a woody lianoid plant that grows on coastal sand dunes in the Mediterranean region. Older perennial stems are present as extensive underground axes. These generate surface growth of shorter-lived stems producing monospecific trellises above the surface of the sand. Despite its sand dune habitat and shortage of host support plants, this variety of Clematis shows mechanical characteristics during growth that are closely comparable with those of scandent woody lianas. A significant decrease in the value of structural Young's modulus is observed from the aerial trellis-forming shoots (1.619 +/- 0.492 GN m(-2)) to emergent axes (0.855 +/- 0.253 GN m(-2)) and underground woody stems (0.470 +/- 0.113 GN m(-2)). Biomechanical and developmental observations indicate that most emergent branches are optimized geometrically and mechanically in relation to their points of emergence from the sand, with increases in structural Young's modulus and the second moment of area around the surface of the sand. Lianoid plants, physiologically capable of withstanding sand dune environments, might represent acceptable natural or introduced species for dune stabilization and conservation.

Adaptation, Physiological↗