Paul Kinchesh

The elimination of magnetic susceptibility distortions in the imaging of liquids in solids : the stray field imaging technique

The effects of magnetic susceptibility, X, and its variations within a host lattice on the NMR responses of a contained liquid are well known ( 2 ) . Examples of lattices that one may wish to investigate are wood, rocks and minerals, clays, and soil. The liquid of interest is usually water or oil. Local differences in x produce a spatially dependent BO, i.e., BO( r). The largest NMR effects naturally come from samples in which the range of BO( r) is greatest and most commonly this occurs when the lattice contains paramagnetic centers for which X is large. The effects are expected to be observable even when the lattice contains only diamagnetic centers for which the range of BO( Y) is very much smaller (2). BO( Y) results in a line broadening of the NMR response which is given by

Magnetic susceptibility effects in imaging: Distortion-free images of plant tissue in soil

Magnetic susceptibility effects (MSE) in NMR spectra are well known, and indeed the NMR technique has frequently been applied to measure magnetic susceptibility. In the case of imaging, MSE can lead to image distortion when the sample is heterogeneous. We have performed experiments on a soil sample (iron content approximately 2%) containing plant tissue which gave a NMR signal that was spread over about 15 kHz in the 1H spectrum. We present some results from a 128 x 128 x 128 3D 1H image (voxel size = 150 x 150 x 150 micron3) generated by the stray field imaging (STRAFI) technique in which the use of a 5 kG cm-1 magnetic flux density gradient reduced the magnetic susceptibility distortion to less than 10 microns.

STRAFI-NMR studies of water transport in soil

1-D STRAFI (STRAy FIeld) imaging is used to study water distribution in a sandy loam. The matric potential of the soil can be varied during acquisition of 1-D profiles. Results at a range of potentials are presented showing both the equilibrium distribution and the evolution of the profile following an abrupt change in matric potential. The air breakthrough point and variations in draining behaviour due to differences in soil compaction are identified.

Magnetic Resonance Imaging (MRI) as a Method to Investigate Movement of Water During the Extrusion of Pastes

AbstractPurpose. To assess the potential of Magnetic Resonance Imaging (MRI) as a method of detecting water movement during the extrusion of pastes. Methods. Plug samples were made from mixtures of model materials and microcrystalline cellulose with two water contents at two different ram speeds to simulate ram extrusion. The extrusion process was stopped at different stages and analyzed for water distribution using MRI to assess the influence of water content and the speed of ram on water movement as the extrusion process progresses. Results. Two types of water movement were detected: vertical and radial. When extruding at the faster ram speed, water moved predominantly in the vertical direction, whereas when extruding at a slower ram speed it moved predominantly in the radial direction. At the beginning of the extrusion process a greater water movement in the wetter formulations was observed. Conclusions. MRI appears to be a useful approach to non-invasive water mapping, and is expected to contribute towards a greater understanding of the role of water in the extrusion of pastes.

NMR imaging of 15N at natural abundance

*‘N observation at natural abundance (0.37% of total nitrogen) has become conventional in NMR spectroscopic studies on liquids since the first report in 197 1 ( 1) . This technique has also been useful for solving problems in solids (2). Experiments involving enriched material have proved to be important in labeling studies in chemical applications (3)) in determination of protein structure (4)) and in improving the understanding of plant metabolism (5,6). In the case of plants, crude spatial selection has previously been achieved by physical sectioning of plants which had been fed with “N-labeled nutrients ( 5 ) . No 15N NMR images from samples at either natural abundance or enriched concentration have previously been reported, and only one 14N image (of liquid nitrogen) had been published ( 7) until recently (8). Given the ubiquity of nitrogen in metabolic processes and the interest in the processes of nitrogen fixation and nitrogen turnover in the environment, it seemed sensible to tackle the particular problems associated with ‘*N NMR imaging. These, as in 15N NMR spectroscopic studies, arise from the low isotopic abundance and the low magnetogyric ratio, which give the 15N isotope a receptivity of only 3.85 X 10e6 compared to ‘H. An additional drawback is the long relaxation times frequently encountered ( 9). This renders accumulation difficult but does at least ensure that “N lines are normally narrow. A faster accumulation rate may, however, become beneficial if relaxation times are shortened by the use of paramagnetic reagents or if steady-state techniques are employed. The maximum concentration of 15N spins at natural abundance occurs in liquid nitrogen, for which the molarity of i5N nuclei is 0.2 1 M. Additionally, low temperature (77 K) gives a favorable Boltzmann gain relative to 298 K of about 4 as well as reduced thermal noise to give an overall gain of approximately 7.5. The signal-to-noise ratio was measured to be approximately 10: 1 following a single 90” pulse and the linewidth was less than 70 Hz. The relaxation times are presumably shortened by contamination with oxygen, the amount of which varies from sample to sample. The sample was contained in a two-section, unsilvered Dewar and was bubbling while under investigation so that the signal-to-noise ratio was reduced by removal of magnetized material from the active volume of the RF coil. The upper section functioned as a reservoir and was 8 cm high (od, 6.5 cm) and ended in a finger (id, 1.7 cm and height, 7.5 cm) which was inserted into a lo-turn surface coil (id, 3.0 cm) made of 0.8 mm insulated copper wire. There was sufficient volume of liquid nitrogen in the reservoir to keep the finger full for 3 1 hours.