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

Quantitative permeability imaging of plant tissues

A method for mapping tissue permeability based on time-dependent diffusion measurements is presented. A pulsed field gradient sequence to measure the diffusion encoding time dependence of the diffusion coefficients based on the detection of stimulated spin echoes to enable long diffusion times is combined with a turbo spin echo sequence for fast NMR imaging (MRI). A fitting function is suggested to describe the time dependence of the apparent diffusion constant in porous (bio-)materials, even if the time range of the apparent diffusion coefficient is limited due to relaxation of the magnetization. The method is demonstrated by characterizing anisotropic cell dimensions and permeability on a subpixel level of different tissues of a carrot (Daucus carota) taproot in the radial and axial directions

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

Flow MRI teaches us some lessons on hydraulic conductivity in trees

Hydraulic conductivity of long-distance xylem and phloem transport in plants and trees is key information to validate biophysical structure-function plant models. Such models are in use to address water stress-induced effects and growth limitations. In addition, such models are used to quantify the contribution of plant evapotranspiration and carbon exchange within global atmospheric circulation models [1]. In xylem and phloem, the effective flow conducting area and the resistance within the vessel or tracheid connections determine hydraulics. Using conventional methods, it is very difficult to determine the active flow conducting area and to study the dynamics (e.g., day–night, stress responses) therein.A very promising and attractive method for providing detailed quantitative information on effective flow-conducting area in intact plants is flow magnetic resonance imaging (MRI) based on PFG methods. Because the diameter of flow-conducting vessels and tracheids are small in comparison to the pixel size, a crucial step in quantification of the flow and effective flow conducting area is to discriminate stationary and flowing water within a single pixel. The fact that the propagator for stationary water is symmetrical around zero is used to separate the stationary from the flowing water. The signal in the nonflow direction is mirrored around zero displacement and subtracted from the signal in the flow direction to produce the displacement distribution of the flowing and the stationary water. Using this approach, for each pixel in the image, the following flow characteristics are extracted in a model-free fashion: total amount of water, amount of stationary water, amount of flowing water (or flow conducting area), average velocity (including the direction of flow) and volume flow per pixel. A propagator flow imaging method was developed that allowed the flow profile of every pixel in an image to be recorded quantitatively, with a relatively high spatial resolution, while keeping measurement times down to 15–30 minutes [2].Phloem transport in plants is particularly difficult to measure quantitatively. The slow flow velocities and the very small flowing volumes in the presence of large amounts of stationary water make it difficult to distinguish the slowly flowing phloem sap from freely diffusing water. We have optimized the MRI hardware (3-T vertical-bore, intact-plant MRI) and the propagator-fast imaging method to quantitatively measure, for the first time, detailed flow profiles of phloem flow in large and fully developed plants, including trees [3]. In this way, the dynamics in phloem and xylem flow and flow conducting area are studied. The observed differences for day and night in flow-conducting area, which directly relate to xylem and phloem hydraulics, are one of the most striking observations, which demonstrates the potential of the method to study hydraulics in intact plants under normal and stress conditions.Here, we discuss the accuracy of the method to determine the effective flow-conducting area and present results of dynamics in effective flow-conductive area under different conditions. Some striking lessons emerge from the results, which demonstrate that trees are dynamic, unsaturated porous media

0.7 and 3 T MRI and Sap Flow in Intact Trees: Xylem and Phloem in Action

Dedicated magnetic resonance imaging (MRI) hardware is described that allows imaging of sap flow in intact trees with a maximal trunk diameter of 4 cm and height of several meters. This setup is used to investigate xylem and phloem flow in an intact tree quantitatively. Due to the fragile gradients in pressure present in both xylem and phloem, methods to study xylem and phloem transport must be minimally invasive. MRI flow imaging by means of this hardware certainly fulfils this condition. Flow is quantified in terms of (averaged) velocity, volume flow (flux) and flow conducting area, either in imaging mode or as a nonspatially resolved total. Results obtained for one tree, imaged at two different field strengths (0.7 and 3 T), are compared. An overall shortening of observed T 2 values is manifest going from 0.7 to 3 T. Although some susceptibility artefacts may be present at 3 T, the results are still reliable and the gain in sensitivity results in shorter measurement time (or higher signal-to-noise ratio) with respect to the 0.7 T system. The results demonstrate that by use of dedicated hardware, xylem and phloem flow and its mutual interaction, can be studied in intact trees in relation to the water balance and in response to environmental (stress) conditions

Suspension flow in microfluidic devices - a review of experimental techniques focussing on concentration and velocity gradients

Microfluidic devices are an emerging technology for processing suspensions in e.g. medical applications, pharmaceutics and food. Compared to larger scales, particles will be more influenced by migration in microfluidic devices, and this may even be used to facilitate segregation and separation. In order to get most out of these completely new technologies, methods to experimentally measure (or compute) particle migration are needed to gain sufficient insights for rational design. However, the currently available methods only allow limited access to particle behaviour. In this review we compare experimental methods to investigate migration phenomena that can occur in microfluidic systems when operated with natural suspensions, having typical particle diameters of 0.1 to 10 µm. The methods are used to monitor concentration and velocity profiles of bidisperse and polydisperse suspensions, which are notoriously difficult to measure due to the small dimensions of channels and particles. Various methods have been proposed in literature: tomography, ultrasound, and optical analysis, and here we review and evaluate them on general dimensionless numbers related to process conditions and channel dimensions. Besides, eleven practical criteria chosen such that they can also be used for various applications, are used to evaluate the performance of the methods. We found that NMR and CSLM, although expensive, are the most promising techniques to investigate flowing suspensions in microfluidic devices, where one may be preferred over the other depending on the size, concentration and nature of the suspension, the dimensions of the channel, and the information that has to be obtained. The paper concludes with an outlook on future developments of measurement techniques