D. van Dusschoten

Characterization of the diffusive properties of biofilms using pulsed field gradient-nuclear magnetic resonance

The mobility of water in intact biofilms was measured with pulsed field gradient nuclear magnetic resonance (PFG-NMR) and used to characterise their diffusive properties. The results obtained with several well-defined systems, viz. pure water, agar, and agar containing inert particles or active bacteria were compared to glucose diffusion coefficients measured with micro-electrodes and those calculated utilising theoretical diffusion models. A good correspondence was observed indicating that PFG-NMR should also enable the measurement of diffusion coefficients in heterogeneous biological systems. Diffusion coefficients of several types of natural biofilms were measured as well and these results were related to the physical biofilm characteristics. The values had a high accuracy and reflected the properties of a sample of ca. 100 biofilms, while non-uniformity or non-geometrical shapes did not negatively influence the results. The monitored PFG-NMR signal contains supplementary information on e.g. cell fraction or spatial organisation but quantitative analysis was not yet possible. Copyright 1998 John Wiley & Sons, Inc.

NMR methods for imaging of transport processes in micro-porous systems.

Abstract Magnetic resonance imaging (MRI) allows non-destructive and non-invasive measurement and visualisation of both static and dynamic water phenomena. Flow and transport processes can either be measured by following the local intensity in time-controlled sequential images, by mapping the effect of contrast agents or labelled molecules, or by mapping the (proton) displacement in a well known time interval directly. By a proper choice of methods, a time window ranging from milliseconds to weeks (or even longer) can be covered. Combining transport measurements with relaxation time information allows the discrimination of transport processes in different environments or of different fluids, even within a single picture element within an image. Here we present an overview of the principles of NMR imaging techniques to visualise and unravel complex, heterogeneous transport processes in porous systems. Applications and limitations will be discussed, based on results obtained in model and artificial soil systems.

Spatially resolved transport properties in radially compressed bead packings studied by PFG NMR.

Pulsed field gradient (PFG) multi-echo (ME) and turbo spin-echo (TSE) imaging is used to study dispersive flow in radially compressed chromatographic columns packed with porous silica beads. By using the pulsed field gradient turbo spin-echo sequence spatially resolved displacement imaging can be accelerated by a factor of 16. The positive effect of homogeneous radial packing on flow velocity and dispersion is demonstrated. Small heterogeneities of only a few percent are shown to cause changes of the dispersion coefficient of up to 50%.

MRI in Soils: Determination of Water Content Changes Due to Root Water Uptake by Means of a Multi-Slice-Multi-Echo Sequence (MSME)

Root water uptake by ricinus communis (castor bean) in fine sand was investigated using MRI with multiecho sampling. Before starting the experiments the plants germinated and grew for 3 weeks in a cylindrical container with a di- ameter of 9 cm. Immediately before the MRI experiments started, the containers were water-saturated and sealed, so water content changes were only caused by root water uptake. In continuation of a preceding work, where we applied SPRITE we tested a multi-echo multi-slice sequence (MSME). In this approach, the water content was imaged by setting TE = 6.76 ms and nE = 128 with an isotropic resolution of 3.1mm. We calculated the water content maps by biexponential fitting of the multi-slice echo train data and normalisation on reference cuvettes filled with glass beads and 1 mM NiCl2 solution. The water content determination was validated by comparing to mean gravimetric water content measurements. By co- registration with the root architecture, visualised by a 3D fast spin echo sequence (RARE), we conclude that the largest water content changes occurred in the neighbourhood of the roots and in the upper layers of the soil.

Visualising the water flow in a breathing carp using NMRi

The study of water flow inside the fish mouth and opercular cavities has been hampered by the lack of a non-invasive measuring device. Measurements of the water flow during breathing of a carp are presented here, using a non-invasive technique, Nuclear Magnetic Resonance imaging (NMRi). Using NMRi, velocity profiles in a whole slice of the fish can be measured at once. The present results, which represent the average flow pattern during a breathing cycle, confirm literature data of an elongated volume of moving water in front of the mouth of the fish. Furthermore, it is observed that in the narrow spaces between the hemibranchs the water flow remains fast. Suggestions arc given for improvement of the experimental set-up, such that more detailed and complete data of the flow in the fish mouth and opercular cavities may be obtained.