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Decay Models (decay + models)

Distribution by Scientific Domains

Earth and Environmental Science	50%

Selected Abstracts

Reproducibility and dependence on diffusion weighting of line scan diffusion in the lumbar intervertebral discs

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 4 2005
David C. Newitt PhD
Abstract Purpose To investigate the dependence of line scan diffusion imaging (LSDI) in the lumbar vertebral discs on diffusion weighting, fat suppression (FS), and postprocessing noise correction. Materials and Methods Eleven normal volunteers were scanned using 4 b-value and 12 b-value LSDI protocols, with and without FS. Three repeated four b-value scans were performed for evaluation of the reproducibility of apparent diffusion coefficient (ADC) values calculated with mono- and biexponential decay models. Two-point ADC analysis for 12 b-value scans was performed with and without noise correction to evaluate the ADC dependence on diffusion weighting. Correlations between different ADC calculation and acquisition methods were evaluated. Results Monoexponential ADC measures had a coefficient of variation (CV) under 3%, while use of a constrained biexponential increased the CV to 6% to 9%. Strong dependence on b-value was seen from chemically shifted marrow fat signal and noise. These systematic variations in ADC were eliminated using noise correction and FS. ADC values from 4 and 12 b-value FS scans correlated strongly (R2 = 0.91), while biexponentially derived ADC values correlated moderately well with the FS ADC (R2 = 0.51). Conclusion LSDI gives reproducible ADC measurements in the lumbar discs, largely independent of b-value and signal-to-noise ratio (SNR) when used with noise correction and FS. J. Magn. Reson. Imaging 2005;21:482,488. © 2005 Wiley-Liss, Inc. [source]

Planning-level source decay models to evaluate impact of source depletion on remediation time frame

REMEDIATION, Issue 4 2005
Charles J. Newell
A recent United States Environmental Protection Agency (US EPA) Expert Panel on Dense Nonaqueous Phase Liquid (DNAPL) Source Remediation concluded that the decision-making process for implementing source depletion is hampered by quantitative uncertainties and that few useful predictive tools are currently available for evaluating the benefits. This article provides a new planning-level approach to aid the process. Four simple mass balance models were used to provide estimates of the reduction in the remediation time frame (RTF) for a given amount of source depletion: step function, linear decay, first-order decay, and compound. As a shared framework for assessment, all models use the time required to remediate groundwater concentrations below a particular threshold (e.g., goal concentration or mass discharge rate) as a metric. This value is of interest in terms of providing (1) absolute RTF estimates in years as a function of current mass discharge rate, current source mass, the remediation goal, and the source- reduction factor, and (2) relative RTF estimates as a fraction of the remediation time frame for monitored natural attenuation (MNA). Because the latter is a function of the remediation goal and the remaining fraction (RF) of mass following remediation, the relative RTF can be a valuable aid in the decision to proceed with source depletion or to use a long-term containment or MNA approach. Design curves and examples illustrate the nonlinear relationship between the fraction of mass remaining following source depletion and the reduction in the RTF in the three decay-based models. For an example case where 70 percent of the mass was removed by source depletion and the remediation goal (Cg/C0) was input as 0.01, the improvement in the RTF (relative to MNA) ranged from a 70 percent reduction (step function model) to a 21 percent reduction (compound model). Because empirical and process knowledge support the appropriateness of decay-based models, the efficiency of source depletion in reducing the RTF is likely to be low at most sites (i.e., the percentage reduction in RTF will be much lower than the percentage of the mass that is removed by a source-depletion project). Overall, the anticipated use of this planning model is in guiding the decision-making process by quantifying the relative relationship between RTF and source depletion using commonly available site data. © 2005 Wiley Periodicals, Inc. [source]

Accounting for periods of wetness in displacement of Fusarium pseudograminearum from cereal straw

ANNALS OF APPLIED BIOLOGY, Issue 1 2010
D.P.S. Lakhesar
Displacement of pathogenic Fusarium species from cereal residues by other fungi is an important mechanism for the effectiveness of fallows and crop rotations on disease management, as well as in potential biological control. The effect of fluctuating environmental conditions on the rate of displacement was assessed using two different approaches. In the first, wetness durations between 4 and 10 h were simulated by spraying water onto straw inoculated with Fusarium pseudograminearum and antagonists in a greenhouse. For a given cumulative period of wetness, displacement of F. pseudograminearum was generally higher for short (4 h) than longer (10 h) wetting durations, indicating that it was the number of wetting events, rather than their individual durations, that determined the rate of displacement. In the second approach, exponential decay models using thermal time adjusted for rainfall were fitted to published data on survival of Fusarium species in residues. Heat sums calculated from the mean temperature of days on which rain fell, or rainday-degrees (RDD), gave good fits to data from short-term experiments on displacement of F. pseudograminearum by antagonists under natural conditions. RDD and two other indices, decomposition days (DCD) and corrected degree-days (CDD), were equally satisfactory for modelling straw decomposition and mortality of Fusarium in longer term data sets. Such models could be useful for predicting the effects of environmental variation on rotations and biocontrol for Fusarium management in cereals. [source]

Real-time measurement of protein leaching from micro-particulate larval fish feeds

AQUACULTURE RESEARCH, Issue 16 2008
Peter M Nicklason
Abstract The small size and high surface area to volume ratio of larval fish feed presents challenges for nutrient retention in micro-particulate diets. A method for the accurate and rapid measurement of nutrient retention or loss from micro-particulate feed in water is needed to help develop micro-particulate feeds with good nutrient retention characteristics. The present study developed and validated an instrument method using fibre optic technology that measures protein leaching in real time. Larval fish feed particles of different sizes (100,500 ,m) and formulations were measured. Under consistent experimental conditions, a feed could be assayed for the rate of mass loss and the half-life or time of 50% total soluble mass loss. The results closely approximated natural decay models with coefficients of determination (r2) >0.95. The end result is a fast and accurate method to quantify and provide solid reference data for a feed formulation or particle size. Using this method allows different feeds to be compared and conclusions drawn for relative performance. [source]