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- Waterton, John C., et al.
(författare)
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Repeatability and reproducibility of longitudinal relaxation rate in 12 small-animal MRI systems
- 2019
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Ingår i: Magnetic Resonance Imaging. - : Elsevier BV. - 1873-5894 .- 0730-725X. ; 59, s. 121-129
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Tidskriftsartikel (refereegranskat)abstract
- Background: Many translational MR biomarkers derive from measurements of the water proton longitudinal relaxation rate R 1 , but evidence for between-site reproducibility of R 1 in small-animal MRI is lacking. Objective: To assess R 1 repeatability and multi-site reproducibility in phantoms for preclinical MRI. Methods: R 1 was measured by saturation recovery in 2% agarose phantoms with five nickel chloride concentrations in 12 magnets at 5 field strengths in 11 centres on two different occasions within 1–13 days. R 1 was analysed in three different regions of interest, giving 360 measurements in total. Root-mean-square repeatability and reproducibility coefficients of variation (CoV) were calculated. Propagation of reproducibility errors into 21 translational MR measurements and biomarkers was estimated. Relaxivities were calculated. Dynamic signal stability was also measured. Results: CoV for day-to-day repeatability (N = 180 regions of interest) was 2.34% and for between-centre reproducibility (N = 9 centres) was 1.43%. Mostly, these do not propagate to biologically significant between-centre error, although a few R 1 -based MR biomarkers were found to be quite sensitive even to such small errors in R 1 , notably in myocardial fibrosis, in white matter, and in oxygen-enhanced MRI. The relaxivity of aqueous Ni 2+ in 2% agarose varied between 0.66 s −1 mM −1 at 3 T and 0.94 s −1 mM −1 at 11.7T. Interpretation: While several factors affect the reproducibility of R 1 -based MR biomarkers measured preclinically, between-centre propagation of errors arising from intrinsic equipment irreproducibility should in most cases be small. However, in a few specific cases exceptional efforts might be required to ensure R 1 -reproducibility.
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- Bargmann, Swantje, 1980, et al.
(författare)
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On configurational forces within Green–Naghdi thermo-hyperelasticity
- 2008
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Ingår i: Solid Mechanics and its Applications. - Dordrecht : Springer Netherlands. - 1875-3507. - 9789048134465 ; 17, s. 203-214
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Konferensbidrag (refereegranskat)abstract
- The contribution's main objective is the examination of configurational forces in non-classical nonlinear thermoelasticity based on the approach of Green and Naghdi. In the early 1990s, Green and Naghdi introduced a theory attracting interest as heat propagates as thermal waves at finite speed, does not necessarily involve energy dissipation and fully integrates the classical theory. A wide range of heat flow problems can be modeled and the classical theory is fully embedded. As configurational forces have proven to be well suited for the examination of defect mechanics, a numerical example from that research area is discussed. The numerical realization is based on Galerkin finite elements in space as well as in time.
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- Kaessmair, Stefan, et al.
(författare)
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Variationally consistent computational homogenization of chemomechanical problems with stabilized weakly periodic boundary conditions
- 2021
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Ingår i: International Journal for Numerical Methods in Engineering. - : Wiley. - 0029-5981 .- 1097-0207. ; 122:22, s. 6429-6454
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Tidskriftsartikel (refereegranskat)abstract
- A variationally consistent model-based computational homogenization approach for transient chemomechanically coupled problems is developed based on the classical assumption of first-order prolongation of the displacement, chemical potential, and (ion) concentration fields within a representative volume element (RVE). The presence of the chemical potential and the concentration as primary global fields represents a mixed formulation, which has definite advantages. Nonstandard diffusion, governed by a Cahn–Hilliard type of gradient model, is considered under the restriction of miscibility. Weakly periodic boundary conditions on the pertinent fields provide the general variational setting for the uniquely solvable RVE-problem(s). These boundary conditions are introduced with a novel approach in order to control the stability of the boundary discretization, thereby circumventing the need to satisfy the LBB-condition: the penalty stabilized Lagrange multiplier formulation, which enforces stability at the cost of an additional Lagrange multiplier for each weakly periodic field (three fields for the current problem). In particular, a neat result is that the classical Neumann boundary condition is obtained when the penalty becomes very large. In the numerical examples, we investigate the following characteristics: the mesh convergence for different boundary approximations, the sensitivity for the choice of penalty parameter, and the influence of RVE-size on the macroscopic response.
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