It had been previously shown that an idealized version of the two-wave-vector extension of the NMR pulsed-field-gradient spin echo diffusion experiment can be used to determine the apparent radius of geometries with restricted diffusion. In the present work, the feasibility of the experiment was demonstrated in an NMR imaging experiment, in which the apparent radius of axons in white matter tissue was determined. Moreover, numerical simulations have been carried out to determine the reliability of the results. For small diffusion times, the radius is systematically underestimated. Larger gradient area, finite length gradient pulses, and a statistical distribution of radii within a voxel all have a minor influence on the estimated radius. Pulsed-field-gradient spin echo (PGSE) NMR experiments have been used to measure apparent diffusion in liquids and biological tissue (1,2). In the narrow gradient pulse limit (i.e., using infinitely short pulses), the signal obtained with the PGSE experiment corresponds to the Fourier transform of the diffusion propagator (3,4). Using this so-called q-space imaging technique, the evolution of the diffusion propagator can be measured by acquiring images or spectra with increasing delays between the pulsed diffusion gradients (5,6) and has been used to characterize the diffusion propagator in liquids (6), red blood cells (7,8), and nerve cells (5). The shape of the diffusion propagator carries information about the microstructure of the sample (e.g., biological tissue) and allows the study of compartments that are much smaller than typical sizes that can be resolved by morphological MRI methods (6,9,10).A two-wave-vector extension to the standard PGSE experiment using two pairs of pulsed gradients in a double spin echo, a so called two-wave-vector exper- iment, has been used to study flow effects in more detail than possible with a single gradient pair (11). In general, two-wave-vector experiments carry more details than obtainable through a one wave vector experiment. For example, a two-wave-vector experiment can distinguish between diffusion in different compartments and diffraction-like behavior which is caused by restricted diffusion (12).In the case of restricted diffusion, theoretical calculations by Mitra have shown that the two-wave-vector experiment can be used to determine the radius of gyration of pores using several approximations (12). The same is true for any geometry with restricted diffusion. Potential applications of this theoretical work include the measurement of the size of microscopic structures in biological tissue, as for example done in (13)(14)(15).In the present work, the theory (12) has been successfully applied to quantify the apparent cell radius in an NMR imaging experiment of biological tissue. Furthermore, it has been investigated how deviations from the idealized assumptions affect the apparent radius. Among the effects studied are finite widths of gradient pulses and radius distributions. The term apparent refers to the fact that the determination...

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Within the frame of lowest-order perturbation theory, the van der Waals potential of a groundstate atom placed within an arbitrary dispersing and absorbing magnetodielectric multilayer system is given. Examples of an atom situated in front of a magnetodielectric plate or between two such plates are studied in detail. Special emphasis is placed on the competing attractive and repulsive force components associated with the electric and magnetic matter properties, respectively, and conditions for the formation of repulsive potential walls are given. Both numerical and analytical results are presented.

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Abstract. A general theory of the Casimir-Polder interaction of single atoms with dispersing and absorbing magnetodielectric bodies is presented, which is based on QED in linear, causal media. Both ground-state and excited atoms are considered. Whereas the Casimir-Polder force acting on a ground-state atom can conveniently be derived from a perturbative calculation of the atom-field coupling energy, an atom in an excited state is subject to transient force components that can only be fully understood by a dynamical treatment based on the body-assisted vacuum Lorentz force. The results show that the Casimir-Polder force can be influenced by the body-induced broadening and shifting of atomic transitions-an effect that is not accounted for within lowest-order perturbation theory. The theory is used to study the Casimir-Polder force of a ground-state atom placed within a magnetodielectric multilayer system, with special emphasis on thick and thin plates as well as a planar cavity consisting of two thick plates. It is shown how the competing attractive and repulsive force components related to the electric and magnetic properties of the medium, respectively, can-for sufficiently strong magnetic properties-lead to the formation of potential walls and wells.PACS. 12.20.-m Quantum electrodynamics -34.50.Dy Interactions of atoms and molecules with surfaces -42.50.Nn Quantum optical phenomena in absorbing, dispersive, and conducting media