Fine‐scale measurement of diffusivity in a microbial mat with nuclear magnetic resonance imaging

Noninvasive 1H‐nuclear magnetic resonance (NMR) imaging was used to investigate the diffusive properties of microbial mats in two dimensions. Pulsed field gradient NMR was used to acquire images of the H2O diffusion coefficient, Ds, and multiecho imaging NMR was used to obtain images of the water density in two structurally different microbial mats sampled from Solar Lake (Egypt). We found a pronounced lateral and vertical variability of both water density and water diffusion coefficient, correlated with the laminated and heterogeneous distribution of microbial cells and exopolymers within the mats. The average water density varied from 0.5 to 0.9, whereas the average water diffusion coefficient ranged from 0.4 to 0.9 relative to the values obtained in the stagnant water above the mat samples. The apparent water diffusivities estimated from NMR imaging compared well to apparent O2 diffusivities measured with a diffusivity microsensor. Analysis of measured O2 concentration profiles with a diffusion‐reaction model showed that both the magnitude of calculated rates and the depth distribution of calculated O2 consumption/production zones changed when the observed variations of diffusivity were taken into account. With NMR imaging, diffusivity can be determined at high spatial resolution, which can resolve inherent lateral and vertical heterogeneities found in most natural benthic systems

MRI in soils: determination of water concent 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 diameter 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 coregistration 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

Nuclear magnetic resonance: a tool for imaging belowground damage caused by Heterodera schachtii and Rhizoctonia solani on sugar beet

Belowground symptoms of sugar beet caused by the beet cyst nematode (BCN) Heterodera schachtii include the development of compensatory secondary roots and beet deformity, which, thus far, could only be assessed by destructively removing the entire root systems from the soil. Similarly, the symptoms of Rhizoctonia crown and root rot (RCRR) caused by infections of the soil-borne basidiomycete Rhizoctonia solani require the same invasive approach for identification. Here nuclear magnetic resonance imaging (MRI) was used for the non-invasive detection of belowground symptoms caused by BCN and/or RCRR on sugar beet. Excessive lateral root development and beet deformation of plants infected by BCN was obvious 28 days after inoculation (dai) on MRI images when compared with non-infected plants. Three-dimensional images recorded at 56 dai showed BCN cysts attached to the roots in the soil. RCRR was visualized by a lower intensity of the MRI signal at sites where rotting occurred. The disease complex of both organisms together resulted in RCRR development at the site of nematode penetration. Damage analysis of sugar beet plants inoculated with both pathogens indicated a synergistic relationship, which may result from direct and indirect interactions. Nuclear MRI of plants may provide valuable, new insight into the development of pathogens infecting plants below- and aboveground because of its non-destructive nature and the sufficiently high spatial resolution of the method

Optimized Continuous Application of Hyperpolarized Xenon to Liquids

International audienceIn recent years, NMR with hyperpolarized (HP) xenon inside functionalized host structures (e.g. cryptophanes) have become a potential candidate for the direct observation of metabolic processes (i.e. molecular imaging). A critical issue for real applications is the dissolution of the HP-gas in the liquid which contains the host. In this work, we present recent developments for an improved and controlled dissolution of HP-Xe in liquids using hollow fiber membranes and different compressor systems. The designed apparatus consists of a compressor and a membrane unit. The compressor provides HP-129 Xe continuously at small adjustable pressures and in a polarization-preserving way. The membrane unit enables a molecular solution of the HP-gas in aqueous liquids, avoiding the formation of bubbles or even foams. Two different types of compressors were tested in terms of function and useful materials. Special emphasis was put on a systematic reduction of transfer losses in the gas and liquid phase. In order to optimize the system parameters, several physical models were developed to describe the transport and the losses of nuclear polarization. Finally, the successful implementation was demonstrated in several experiments. HP-Xe was dissolved in an aqueous cryptophane-A-(OCH 2 COOH) 6 solution, and stable Xe signals could be measured over 35 min, only limited by the size of the gas reservoir. Such long and stable Version 6 21.10.2019 BN experimental conditions enabled the study of chemical exchange of xenon between cryptophane and water environments even for a time-consuming 2D NMR-experiment. The good signal stability over the measurement time allowed an exact determination of the residence time of the Xe-atom inside the cryptophane, resulting in an average residence time of 42.9 ± 3.3 ms

MRI of intact plants

Nuclear magnetic resonance imaging (MRI) is a non-destructive and non-invasive technique that can be used to acquire two- or even three-dimensional images of intact plants. The information within the images can be manipulated and used to study the dynamics of plant water relations and water transport in the stem, e.g., as a function of environmental (stress) conditions. Non-spatially resolved portable NMR is becoming available to study leaf water content and distribution of water in different (sub-cellular) compartments. These parameters directly relate to stomatal water conductance, CO2 uptake, and photosynthesis. MRI applied on plants is not a straight forward extension of the methods discussed for (bio)medical MRI. This educational review explains the basic physical principles of plant MRI, with a focus on the spatial resolution, factors that determine the spatial resolution, and its unique information for applications in plant water relations that directly relate to plant photosynthetic activity

Probing water motion in heterogenous systems : a multi-parameter NMR approach

In this Thesis a practical approach is presented to study water mobility in heterogeneous systems by a number of novel NMR sequences. The major part of this Thesis describes how the reliability of diffusion measurements can be improved using some of the novel NMR sequences. The reliability of the data can be further enhanced by combining different NMR characteristics in a single fit routine. In addition, a fast NMR sequence for flow measurements is shown. A wide variety of samples is used to demonstrate how the NMR sequences and the subsequent analysis work.Throughout this Thesis the term heterogeneous system is used whenever a sample contains different physical or chemical environments or compositions, each of which influence the NMR signal in a distinguishable way. One of the effects of such a heterogeneous system is the variation of the magnetic susceptibility within the sample causing so-called in situ magnetic field gradients. These gradients lower the NMR signal amplitude and may cause a substantial deviation of the real diffusion constant from the one measured by NMR using pulsed field gradient spin echo or stimulated echo sequences. In the second half of Chapter 2 an improved version of the PFG multiple spin echo sequence is introduced which minimises the degrading effects of in situ field gradients. While using additional r.f. pulses between the pulsed field gradients to reach the desired reliability no compromises with regard to flexibility were necessary.This flexibility is important to study the change of the apparent diffusion constant with the echo time. This phenomenon, superficially appearing as an artefact, arises because compartments, as for example a vacuole, not only have a characteristic diffusion constant but also a T 2 which may significantly differ from other T 2 's in the sample. In Chapter 3 it is shown how a T 2 difference can be used to separate diffusion constants (D) even if these have a similar magnitude. Using Diffusion Analysis by Relaxation Time Separation the diffusion constant for water in the vacuole, the cytoplasm and the extra cellular space, respectively, can be distinguished in apple parenchyma tissue. In the second half of Chapter 3 a fast implementation of DARTS PFG NMR is presented which is combined with NMR imaging. Here, spatially localised T 2 measurements are preceded by pulsed field gradients for diffusion weighting. In this way the diffusion constant and fractional amplitude of cerebral spinal fluid (CSF) and white and grey matter in cat brain can be measured. The validity of these measurements are supported by Monte Carlo simulations of computer generated 2D data sets.Chapter 4 deals primarily with improving the DARTS technique without imaging (DARTS PFG MSE Carr Purcell Meiboom Gill) and determining the best resolution (for discriminating diffusion constants) which can be obtained, using Monte Carlo simulations on 2D data sets. The use of the 2D fitting routine is extensively studied. Furthermore, existing theoretical models are mutually compared and confronted with the results obtained by the PFG MSE CPMG sequence for different samples. Four heterogeneous samples with different complexity are studied, i.e.:a- A sample consisting of two tubes with different fluids, between which exchange can be ignored.b- Whole Blood where diffusive exchange between the red blood cells (RBC's) and the plasma causes the apparent D of the RBC's to increase with increasing observation time.c-Apple parenchyma tissue where the membrane between the vacuole and the cytoplasm (the tonoplast) is shown to severely restrict, but not prevent, diffusive exchange between these compartments.d- A column with Sephadex beads (porous beads) where flow is introduced. The effect of this flow on the water displacement outside and inside the beads is described. Experiments demonstrated exchange between the flowing fraction and the stationary water in the beads.In all instances it is profitable to combine the diffusion and T 2 measurements and analyse the resulting 2D data set as a whole. In this way an intuitive understanding is obtained of diffusion in complex systems measured by NMR. By using the T 2 as a label, the resolving power of NMR to distinguish diffusion constants is greatly improved and a difference between the diffusion constants as small as 30 % is demonstrated to be resolvable. None of the presented theories can be used to quantitatively describe the data.In Chapter 5 the subject of flow in heterogeneous systems is studied in further detail. In the first half of this Chapter novel flow measurements in and around the buccal cavity of a Carp are described. These measurements are performed on a standard medical imager without special, fast NMR sequences. The described data can therefore only be used in a qualitative manner. In the second half the line scan flow measurement is introduced. A temporal resolution of 16 ms can be obtained with this sequence allowing accurate, real time flow measurements. The combination of this line scan sequence with displacement imaging yields NMR images which picture the distribution of flow velocities over a line. Demonstrations of displacement imaging are performed in a tube with glass beads and in a pipe with a glass bead filter. The presented data can be used quantitatively.The diffusion and flow measurements described in the Thesis all employ pulsed field gradients to encode for motion. Despite the obvious similarities between the measurements, the optimisation of the NMR sequences results in sequences which can be rather distinct. By careful tuning of the NMR sequences the range of displacements which can thus be measured lies between 5 μm and about 5 cm. This range and the fact that small differences in flow velocities and diffusion constants can be resolved, if necessary using other NMR characteristics, makes NMR a powerful tool to study water mobility in heterogeneous systems