Biomedical subjects

J G Seland

Publications and source records attributed to J G Seland.

3 recordsLinked to original sources

PubMed2004

NMR propagator measurements on flow through a random pack of porous glass beads and how they are affected by dispersion, relaxation, and internal field inhomogeneities.

We flow water through a pack of porous glass beads and employ NMR to measure molecular displacement distributions--the propagators--in the Stokes flow regime. Fluid is flowed over several evolution times to attain fixed mean displacements, and the time dependence of the resulting propagators is analyzed for the effects of diffusion, NMR relaxation, and signal loss due to internal fields. We delineate and illustrate the experimental regimes in which these different effects make their presence felt. Simulations on a simple model system reproduce the essential features of our experimental results and provide insight into the mechanisms shaping the propagators.

Journal Article↗

PubMed2000

Improved convection compensating pulsed field gradient spin-echo and stimulated-echo methods.

The need for convection compensating methods in NMR has been manifested through an increasing number of publications related to the subject over the past few years (J. Magn. Reson. 125, 372 (1997); 132, 13 (1998); 131, 126 (1998); 118, 50 (1996); 133, 379 (1998)). When performing measurements at elevated temperature, small convection currents may give rise to erroneous values of the diffusion coefficient. In work with high resolution NMR spectroscopy, the application of magnetic field gradients also introduces an eddy-current magnetic field which may result in errors in phase and baseline in the FFT-spectra. The eddy current field has been greatly suppressed by the application of bipolar magnetic field gradients. However, when introducing bipolar magnetic field gradients, the pulse sequence is lengthened significantly. This has recently been pointed out as a major drawback because of the loss of coherence and of NMR-signal due to transverse relaxation processes. Here we present modified convection compensating pulsed field gradient double spin echo and double stimulated echo sequences which suppress the eddy-current magnetic field without increasing the duration of the pulse sequences.

Magnetic Resonance Spectroscopy↗

PubMedDate not supplied

Measurements of diffusion in porous polyethylene powder using PFGSTE NMR.

Pulsed field gradient stimulated echo (PFGSTE) nuclear magnetic resonance (NMR) has been applied to study the diffusion of toluene in porous semicrystalline polyethylene powder. A pulse sequence with unequal bipolar gradients that reduces effects from internal gradients was used. The effects of different diffusion regimens and restricted diffusion in this system were indicated by use of a simple two-component analysis of the diffusion data.

Crystallization↗