Optimized Continuous Application of Hyperpolarized Xenon to Liquids.

In 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-129Xe 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-(OCH2COOH)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 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 44.5 ± 2.7 ms.

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.

Fitting of the beat pattern observed in NMR free-induction decay signals of concentrated carbohydrate-water solutions.

A series of mathematical functions has been used to fit the proton free-induction decays (FIDs) of concentrated carbohydrate-water samples. For the solid protons, these functions included a sinc function, as well as the Fourier transforms of single and multiple Pake functions multiplied by a Gaussian broadening. The NMR signal from the mobile protons is described by an exponential function. It is found that in most cases the sinc function gives a satisfactory result and provides valuable information about the second moment M(2) and the ratio of solid to mobile protons (f(s) / f(m)). A good indication for using the sinc function is the presence of a beat in the FID. For high temperatures this approach breaks down, and a biexponential fit is more appropriate. If a clear dipolar splitting is observable in the NMR spectra, the Pake function (or a multiple Pake fit) should be used. In this case information about M(2) and f(s) / f(m) can also be obtained.

Quantification of water transport in plants with NMR imaging.

A new nuclear magnetic resonance imaging (NMRi) method is described to calculate the characteristics of water transport in plant stems. Here, dynamic NMRi is used as a non-invasive technique to record the distribution of displacements of protons for each pixel in the NMR image. Using the NMR-signal of the stationary water in a reference tube for calibration, the following characteristics can be calculated per pixel without advance knowledge of the flow-profile in that pixel: the amount of stationary water, the amount of flowing water, the cross-sectional area of flow, the average linear flow velocity of the flowing water, and the volume flow. The accuracy of the method is demonstrated with a stem segment of a chrysanthemum flower by comparing the volume flow, measured with NMR, with the actual volumetric uptake, measured with a balance. NMR measurements corresponded to the balance uptake measurements with a rms error of 0.11 mg s(-1) in a range of 0 to 1.8 mg s(-1). Local changes in flow characteristics of individual voxels of a sample (e.g. intact plant) can be studied as a function of time and of any conceivable changes the sample experiences on a time-scale, longer than the measurement time of a complete set of pixel-propagators (17 min).

Microscopic displacement imaging with pulsed field gradient turbo spin-echo NMR.

We present a pulse sequence that enables the accurate and spatially resolved measurements of the displacements of spins in a variety of (biological) systems. The pulse sequence combines pulsed field gradient (PFG) NMR with turbo spin-echo (TSE) imaging. It is shown here that by ensuring that the phase of the echoes within a normal spin-echo train is constant, displacement propagators can be generated on a pixel-by-pixel basis. These propagators accurately describe the distribution of displacements, while imaging time is decreased by using separate phase encoding for every echo in a TSE train. Measurements at 0.47 T on two phantoms and the stem of an intact tomato plant demonstrate the capability of the sequence to measure complete and accurate propagators, encoded with 16 PFG steps, for each pixel in a 128 x 128 image (resolution 117 x 117 x 3,000 microm) within 17 min. Dynamic displacement studies on a physiologically relevant time resolution for plants are now within reach.

Flow and transport studies in (non)consolidated porous (bio)systems consisting of solid or porous beads by PFG NMR.

Displacement imaging pulsed field gradient nuclear magnetic resonance (PFG NMR) is applied to a number of porous model systems, consisting of either solid or porous particles. By pulsed field gradient nuclear magnetic resonance, the molecular displacement can be measured that occurs during a time interval, delta, between two consecutive magnetic field gradient pulses. In contrast to conventional techniques, which cover displacements over distances several times the bead diameter, pulsed field gradient nuclear magnetic resonance covers displacements in the order of subpore to several pore distances. Dimensionless scaling is possible based on the root-mean-square displacement normalised on the bead diameter. In solid particles, v and delta are interchangeable, although different flow regimens are covered. In porous particles, the exchange time between the stagnant mobile phase in the particles and the flowing outside must be taken into account with respect to delta as well as the porosity.

Mass transfer in chromatographic columns studied by PFG NMR.

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) is applied to study convective and diffusional transport in chromatographic columns packed with totally porous support particles. Here stagnant zones exist in the particle pores, and diffusional mass-transfer limitations between fluid molecules diffusing in the intraparticle pore network and flowing in the interparticle void space are detected quantitatively. Axial displacement probability distributions were measured for water over a range of Peclet numbers and observation times, with diffusion lengths between 0.15 and 0.91 times the average support particle diameter. The transition towards complete diffusional exchange is demonstrated, thereby also revealing the development of the classical convective dispersion process in a packed bed of (porous) particles.

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%.

Quantitative T2 imaging of plant tissues by means of multi-echo MRI microscopy.

A method for quantitative T2 imaging is presented which covers the large range of T2 values in plants (5 to 2000 ms) simultaneously. The transverse relaxation is characterized by phase-sensitive measurement of many echo images in a multi-echo magnetic resonance imaging sequence. Up to 1000 signal-containing echo images can be measured with an inter-echo time of 2.5 ms at 0.47 T. Separate images of water density and of T2 are obtained. Results on test samples, on the cherry tomato and on the stem of giant hogweed are presented. The effects of field strength, spatial resolution and echo time on the observed T2 values is discussed. The combination of a relatively low magnetic field strength, short echo time and medium pixel resolution results in excellent T2 contrast and in images hardly affected by susceptibility artifacts. The characterization of transverse relaxation by multi-echo image acquisition opens a new route for studies of water balance in plants.

Modeling of self-diffusion and relaxation time NMR in multi-compartment systems.

The theory of pulsed field gradient (pfg) NMR applied to molecules in cellular systems which contain different subcellular compartments separated by permeable membranes, acting as diffusion barriers, has been extended. A numerical model of restricted diffusion and magnetization relaxation behavior in pfg-CPMG NMR experiments, based on the Fick's second law of diffusion, is presented. This model is applicable to a wide range of systems and allows the exploration of temporal and spatial behavior of the magnetization with and without the influence of gradient pulses. Results of the numerical experiments show their correspondence to the previously observed ones and demonstrate the importance of the inclusion of the time domain data in analyzing diffusion measurements.

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.

Unraveling diffusion constants in biological tissue by combining Carr-Purcell-Meiboom-Gill imaging and pulsed field gradient NMR.

A diffusion-weighted multi-spin-echo pulse sequence is presented, which allows for simultaneous measurement of T2, the fractional amplitude, and the diffusion constant of different fractions. Monte Carlo simulations demonstrate an improvement of this sequence with respect to the accuracy of diffusion constant and fractional amplitude for slow exchange. Examples are shown for a simple phantom containing two fractions. In addition, experiments on cat brain in healthy condition and following occlusion of the middle cerebral artery show that the fractional amplitude and the diffusion constant of cerebral spinal fluid and normal brain tissue can be analyzed within each pixel with acceptable accuracy.