Studies magnetic field gradient diffusion

When pulsed magnetic field gradients are applied to study diffusive processes, the MR technique is often referred to as pulsed field gradient or pulsed gradient spin echo (PGSE) MR. Application of PGSE MR techniques to quantify molecular diffusion was pioneered by Stejskal and Tanner 17,18), and the techniques typically probe molecular displacements of 10 -10 m over time scales of the order 10 M s. 

Tanner [49] measured diffusion coefficients of water in three different types of frog muscle cells. He used a variety of magnetic field gradient techniques so as to cover a wide range of diffusion times A= 1 ms to 1 s. The time dependence of the diffusion coefficient was analyzed to obtain the intracellular diffusion coefficients and estimates of the permeability of the cell membranes. In restricted diffusion studies three 90 degree r.f. pulse sequences (stimulated echo) are often used which provides PG-NMR experiments with long diffusion times to explore the dependence of diffusion time on the echo attenuation [49]. 

Stejskal, E.O. Use of spin echoes in a pulsed magnetic-field gradient to study anisotropic restricted diffusion and flow, /. Chem. Phys., 43, 3597, 1965. 

Besides imaging, pulsed magnetic field gradients are also used to study self-diffusion of solutes and solvents. Basically, the pulsed field gradients encode a structure on the spin system, and the evolution of this structure yields the rate of trans-... 

Combinations of applied magnetic field gradients and EM radiation pulses can be used to give ID profiles or two-dimensional (2D) maps showing the spatial distribution of a particular species. NMR imaging has been used extensively in several fields of research to probe stmcture or to study dynamic processes such as diffusion and transport (Gladden, 1994 Mande and Sederman, 2003). 

In earlier studies on diffusion of PDMS, the Carr-Purcell technique was used with magnetic field gradients present at all times (SFG NMR). The measurement of small diffusion coefficients (e.g., in polymers) requires strong gradients which means that as gradient strength is increased wider resonance lines and narrower echo envelopes will be... 

See also Contrast Mechanisms in MRI Diffusion Studied Using NMR Spectroscopy EPR Imaging Fourier Transformation and Sampling Theory Magnetic Field Gradients in High Resolution NMR MRI Applications, Biological MRI Instrumentation MRI of Oil/Water in Rocks MRI Theory NMR Principles NMR Pulse Sequences NMR Relaxation Rates. 

The T2 relaxation is the main limiting factor of the magnetic field gradient experiment, especially when studying slow diffusion. Unfortunately there is an intrinsic correlation between short T2 values and low D values in a given system. For this reason, the PGSE technique is not normally applicable to solids, and it is often also inapplicable to most heteronuclei. The overwhelming majority of studies are made on protons in solution. 

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