Biomedical subjects

M Engelsberg

Publications and source records attributed to M Engelsberg.

At least 19 recordsLinked to original sources

PubMed2006

Enhanced Overhauser contrast in proton-electron double-resonance imaging of the formation of an alginate hydrogel.

Proton-electron double-resonance imaging (PEDRI) was recently employed to monitor the process of formation of a calcium alginate hydrogel at a field of 16mT. Here, under the same experimental conditions, images obtained through this technique are compared to images obtained by conventional T(2)-weighted method. The results confirm that the image contrast obtained using PEDRI, thanks to the Overhauser effect, exhibits an improved sensitivity with respect to changes in water mobility as previously suggested in the literature. Furthermore, by increasing the echo time interval for the T(2)-weighted images, important features of the gelling dynamics obtained via PEDRI could not be reproduced.

Alginates↗

PubMed2006

Water ingress in Y-type zeolite: anomalous moisture-dependent transport diffusivity.

Nuclear magnetic resonance imaging measurements of liquid water ingress in a large number of nonactivated Y-type (Na) zeolite samples prepared under different conditions are reported on. Using an experimental arrangement that permits the application of Boltzmann's transformation of the 1D (one-dimensional) diffusion equation, the spatiotemporal scaling variables required for a collapse of the measured profiles into universal curves revealed subdiffusive behavior in all cases. It is shown that the one-dimensional fractal time diffusion equation constitutes a powerful tool to analyze the data and provides a connection between the moisture dependence of the effective transport diffusivities and the shapes of the universal curves. Thus, even for anomalous diffusion, the relationship between the universal curves and structural characteristics of the system; such as porosity, tortuosity of the pore space and, in some cases, the interplay between mesopores and nanopores can be addressed.

Journal Article↗

PubMed2006

Concentration-dependent diffusivity and anomalous diffusion: a magnetic resonance imaging study of water ingress in porous zeolite.

Magnetic resonance imaging is employed to study water ingress in fine zeolite powders compacted by high pressure. The experimental conditions are chosen such that the applicability of Boltzmann's transformation of the one-dimensional diffusion equation is approximately satisfied. The measured moisture profiles indicate subdiffusive behavior with a spatiotemporal scaling variable eta=x/t(gamma/2) (0<gamma<1). A time-fractional diffusion equation model of anomalous diffusion is adopted to analyze the data, and an expression that yields the moisture dependence of the generalized diffusivity is derived and applied to our measured profiles. In spite of the differences between systems exhibiting different values of gamma a striking similarity in the moisture dependence of the diffusivity is apparent. This suggests that the model addresses the underlying physical processes involved in water transport.

Journal Article↗

PubMed2005

Distant-dipole field in liquids and diffusion: a perturbative approach.

A perturbative approach is employed to solve the Bloch-Torrey equations in the presence of distant-dipole fields in nuclear magnetic resonance. The procedure, although only carried out to first order in the perturbation parameter a=1/k2Dtaud, could, in principle, be generalized to higher orders. Here D is the diffusivity, taud the dipolar demagnetization time, and k is the wave vector of the spatial modulation of magnetization produced by the magnetic field gradient. The results are especially interesting for dilute binary mixtures consisting of molecular species with different diffusivities. In this case the calculated two-dimensional correlation spectroscopy revamped by asymmetric Z-gradient echo detection spectra are shown to be free from some inadequacies resulting from a simplistic application of standard approximations.

Journal Article↗

PubMed2004

Intermolecular double-quantum coherences in two-dimensional spectra of binary mixtures in solution. The role of diffusion.

Proton NMR two-dimensional 2-D spectra of binary mixtures, obtained with the correlation spectroscopy revamped by asymmetric Z gradient echo detection pulse sequence, were employed to test various assumptions usually adopted to describe the role of diffusion in intermolecular double quantum coherences. When two molecular species, with significantly different diffusivities, are considered, the relative amplitudes of the peaks, and their widths, furnish a stringent test that unveils some inadequacies in standard approximations.

Journal Article↗

PubMed2003

Enhanced migration and ionic transport through membranes.

The effect of Coulomb forces upon transport enhancement of mobile ions, in the presence of slowly migrating charged polymeric chains, is investigated in a nonequilibrium regime brought about by a semipermeable membrane and a chemical reaction. By means of a numerical solution of the Nernst-Planck-Poisson equations, we predict the size of the effect, the conditions for positive or negative enhancement, and the dependence upon all relevant parameters. The limitations of the description of migration enhancement by an effective diffusion coefficient are also established.

Journal Article↗

PubMed2003

Low field intermolecular double-quantum coherence imaging via the Overhauser effect.

Intermolecular double-quantum coherence (i-DQC) signals in liquids are usually associated with high magnetic fields. We demonstrate that, in a magnetic field of only 16mT, i-DQC imaging of water protons is feasible thanks to the nuclear magnetization enhancement produced by the Overhauser effect. i-DQC images of a phantom containing an aqueous solution of a trityl free radical, with phase encoding in the DQC evolution period or in the acquisition period, are presented. Possible applications of low field i-MQC images are proposed.

Feasibility Studies↗

PubMed2001

Flow sensitivity and coherence in steady-state free spin precession.

Steady-state free precession (SSFP) of nuclear spins in the presence of a magnetic field gradient is known to be very sensitive to flow. We present a theoretical and experimental study of flow sensitivity in a regime where the spacing of the radio-frequency pulses is extremely short compared with the free induction decay time and the relaxation times. Under these rather drastic conditions, a truly continuous wave free precession (CWFP) regime is established, in which, unlike standard SSFP, a large degree of coherence is preserved. This leads to a quite different flow sensitivity, which is significant even when very small magnetic field gradients are present. The unspoiled coherence is predicted to cause different flow effects which we confirmed experimentally. Tailored flow sensitivity can be achieved by adjusting the frequency offset from resonance, which plays a dominant role in the CWFP regime.

Journal Article↗

PubMed2000

Quantitative analysis using steady-state free precession nuclear magnetic resonance

The use of steady-state free precession nuclear magnetic resonance (NMR) for quantitative analysis in low magnetic field is investigated and shown to exhibit substantial advantages compared to more conventional NMR methods. With only minor additional requirements, the technique permits a considerable increase in signal-to-noise ratio for a given acquisition time. The experimental conditions needed for implementation and optimization of the acquisition parameters are explored and shown to be easily accessible with unsophisticated equipment. The applicability of the technique for quantitative analysis of samples with a range of relaxation rates is tested using various examples of practical interest. Highly reproducible results can be obtained much faster and without any special sample-dependent adjustments.

Journal Article↗

PubMed1999

Monte Carlo simulations of non-Fickian water transport in a saturated porous gel.

A Monte Carlo algorithm, which incorporates tensile effects as well as diffusion, is proposed. It provides a description of water transport in a drying porous gel close to saturation permitting an interpretation of magnetic resonance imaging profiles. Boltzmann's transformation of the one-dimensional diffusion equation is employed to examine the onset of a non-Fickian transport regime caused by the collective motion of diffusers associated with tensile forces.

Journal Article↗

PubMed1999

Tensile water transport in a porous gel.

A Monte Carlo simulation model, which incorporates the effect of tensile forces as well as diffusion, is proposed to explain the behavior of water transport in a saturated porous gel. The algorithm is able to account for the puzzling moisture profiles, which were first observed by conventional magnetic resonance imaging and, more recently, by Overhauser imaging.

Journal Article↗

PubMed1998

Mobility and free radical concentration effects in proton-electron double-resonance imaging.

The dependence of the enhancement of proton-electron double-resonance images upon the mobility of the proton bearing molecules, of the concentration of free radicals, and of the pulsed saturating RF power is studied in a magnetic field of 16 mT. The data exhibit a behavior which, in the potentially interesting region of small free radical concentration, may differ substantially from the high-concentration regime depending upon experimental conditions. The results permit a clearer understanding of the factors determining enhancement and contrast in images obtained by dynamic nuclear polarization.

Electron Spin Resonance Spectroscopy↗

PubMed1996