Paul T. Callaghan

NMR microscopy of dynamic displacements: k-space and q-space imaging

The superposition of pulsed gradient spin-echo (PGSE) and NMR imaging experiments results in a spin density image which is phase and amplitude modulated according to the local self-correlation function for nuclear spin displacements over the time between the PGSE gradient pulses. It is shown that such an experiment can image both static and dynamic spin displacements, the reciprocal space for these image spaces being termed k-space and q-space respectively. Simultaneous imaging of diffusion and flow at microscopic resolution is demonstrated and the Poiseuille velocity distribution agrees well with the velocity map obtained for the motion of water in a 0.7 mm capillary tube.

Velocity and diffusion imaging in dynamic NMR microscopy

Abstract The imposition of resolution gradients in a pulsed-gradient spin-echo (PGSE) NMR sequence induces motionally dependent phase and amplitude modulation in the image, a technique which we have termed dynamic NMR microscopy. Fourier analysis of this modulation gives a dynamic displacement profile for each pixel which can then be analyzed to obtain velocity and diffusion maps. The application of this method at high spatial resolution is motivated by a desire to measure vascular flow in living plants and variations in molecular self-diffusion under the influence of velocity shear in narrow capillaries. The theory of dynamic NMR microscopy is presented and potential artifacts discussed, including the effect of slice selection gradients, PGSE gradient nonuniformity, and specific problems associated with the measurement of self-diffusion in the presence of velocity gradients. It is demonstrated that a double-echo PGSE pulse sequence can be used to restore coherent phase shifts associated with steady-state flow, and examples of self-diffusion maps and signed velocity maps from sequences of phase-encoded images obtained by projection reconstruction are given. This method has been applied at 20,um transverse resolution in laminar capillary flow.

A non-invasive measurement of phloem and xylem water flow in castor bean seedlings by nuclear magnetic resonance microimaging

A flow-sensitive nuclear magnetic resonance (NMR) microimaging technique was applied to measure directly the in-vivo water flow in 6-d-old castor bean seedlings. The achieved in-plane resolution of the technique allowed discrimination between xylem and phloem water flow. Both the xylem- and the phloem-average flow velocities in the intact seedling could be quantified. Furthermore, the total conductive cross-sectional area of the xylem vessels and the phloem sieve elements could be determined using the non-invasive and non-destructive NMR microimaging technique. Hence, it was possible to calculate the in-vivo volume flow rates for both xylem and phloem water flow. Our non-destructive technique showed that previously used methods to measure phloem water flow affected the flow rate itself. In the intact seedlings we found values of 16.6 μl·h−1, two fold lower than those previously estimated from phloem exudation rates. Finally, our results demonstrate for the first time that water is internally circulated between phloem and xylem, and that water flow within the xylem is maintained by this internally circulated water, even in the absence of any significant transpiration or evaporation.

Diffusion in porous systems and the influence of pore morphology in pulsed gradient spin‐echo nuclear magnetic resonance studies

We report a combined experimental, theoretical, and simulation study of pulsed gradient spin‐echo (PGSE) nuclear magnetic resonance (NMR) for fluid saturated porous media. A simple pore hopping theory is developed on the basis of the assumption that diffusion within pores is very much faster than diffusion between pores. For suitable periodic media, the theoretical results are found to be in good agreement with random‐walk simulations. The theory for glasslike media is then used to analyze experimental PGSE NMR data for a water‐saturated random loose pack of nearly monodisperse polystyrene spheres. The structural parameters extracted by this method are consistent with the known geometry of such packings. An important observation from the simulations is that the long‐time effective diffusion constant is already accessed at times so short that a single spin will only have diffused across one pore width.

Rheo-NMR: nuclear magnetic resonance and the rheology of complex fluids

The application of nuclear magnetic resonance methods to the study of complex fluids under shearing and extensional flows is reviewed. Both NMR velocimetry and spectroscopy approaches are discussed while specific systems studied include polymer melts, rigid rod and random coil polymers in solution, lyotropic and thermotropic liquid crystals and liquid crystalline polymers, and wormlike micelles. Reference is made to food systems.

Diffusion of fat and water in cheese as studied by pulsed field gradient nuclear magnetic resonance

Abstract Pulsed field gradient nuclear magnetic resonance was used to measure water and fat self-diffusion coefficients in Cheddar and Swiss cheeses. The water diffusion coefficients are about one-sixth that of bulk water at the same temperature and there is strong evidence to suggest that water diffusion is confined to surfaces within the protein matrix. The echo attenuation for the fat indicates restricted diffusion consistent with the fat being present in the form of small droplets within the cheese. The theory of restricted diffusion has been extended to allow for a spread of droplet size. The data conform to a gaussian distribution of molecular numbers (sphere volume) over droplet radius. A small attenuation approximation is used in the theory but consistent values of droplet radii and standard deviations in droplet size are obtained over a large echo attenuation range.

Diffusion of water in the endosperm tissue of wheat grains as studied by pulsed field gradient nuclear magnetic resonance.

Pulsed field gradient nuclear magnetic resonance has been used to measure water self-diffusion coefficients in the endosperm tissue of wheat grains as a function of the tissue water content. A model that confines the water molecules to a randomly oriented array of capillaries with both transverse dimension less than 100 nm has been used to fit the data and give a unique diffusion coefficient at each water content. The diffusion rates vary from 1.8 x 10(-10) m2s-1 at the lowest to 1.2 x 10(-9) m2s-1 at the highest moisture content. This variation can be explained in terms of an increase in water film thickness from approximately 0.5 to approximately 2.5 nm over the moisture range investigated (200-360 mg g-1).

Simple synthesis and functionalization of iron nanoparticles for magnetic resonance imaging.

Magnetic nanoparticles (NPs) are increasingly important in many biomedical applications, such as drug delivery, hyperthermia, and magnetic resonance imaging (MRI) contrast enhancement. For MRI, iron oxide NPs are the only commercial T2 or negative contrast agents, due to their biocompatibility and ease of synthesis and research in the area is highly active. The efficacy of these contrast agents depends mainly on the surface chemistry and magnetic properties of the NPs. Materials with larger magnetization could induce more efficient transverse (T2) relaxation of protons and result in greater contrast enhancement. As iron has the highest saturation magnetization at room temperature among all elements, and is biocompatible, it is an ideal candidate for MRI contrast enhancement. Nevertheless, the development of using iron NPs for magnetic applications has been challenging and limited compared to those of its oxides, due to the difficulty in preparing stable iron NPs with simple synthesis methods and precursors. 6] Under ambient conditions, iron NPs of 8 nm or smaller oxidize completely upon exposure to air. For larger NPs, an oxide shell of 3–4 nm forms instantly on the surface, forming iron/iron oxide core/shell NPs. Groundbreaking studies for the synthesis of iron NPs of larger than 8 nm has largely been achieved by decomposition of iron pentacarbonyl, [Fe(CO)5]. [6,8] Additional reports include the use of other precursors in forming iron nanocubes. However, all of these processes are limited in terms of ease of synthesis and scalability; [Fe(CO)5] is volatile and highly toxic, [5] and other processes involve precursors that are expensive and airsensitive, or require high decomposition temperatures. Here, we chose an easy to handle iron organometallic sandwich compound as the precursor and prepared singlecrystal iron NPs using a simple, low-temperature synthesis method. The iron nanocrystals oxidized naturally to form highly crystalline iron/iron oxide core/shell NPs. The ease of this synthesis facilitates the larger-scale application of stabilized iron NPs. To enable the use of these NPs in biological applications, the NP surface was modified to make the NPs water soluble. The strongly magnetic core/shell NPs are shown to be more effective T2 contrast agents for in vivo MRI and small tumor detection, compared to pure iron oxides. The successful detection of small tumors in vivo demonstrated here holds a great promise for accurate detection of early metastases in human lymph nodes, which has a large impact on the treatment and prognosis of a range of cancers. The iron/iron oxide core/shell NPs were prepared by first synthesizing iron nanocrystals by decomposition of the iron precursor [Fe(C5H5)(C6H7)], in the presence of oleylamine (OLA) stabilizing molecules. The non-carbonyl, sandwich compound was chosen for its simple preparation and ease of decomposition compared to other more stable sandwich compounds such as ferrocene. The synthesis was carried out in a closed reaction vessel under a mild hydrogen atmosphere, at 130 8C. The temperature required was lower than the usual temperature range (150–300 8C) needed for decomposition of other iron precursors in previous studies. Once [*] Dr. K. W. Feindel, Prof. P. T. Callaghan, Prof. R. D. Tilley School of Chemical and Physical Sciences and The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Wellington 6012 (New Zealand) Fax: (+ 64)4-463-5237 E-mail: richard.tilley@vuw.ac.nz Dr. S. Cheong, Dr. B. Ingham Industrial Research Limited and The MacDiarmid Institute for Advanced Materials and Nanotechnology P. O. Box 31-310, Lower Hutt 5040 (New Zealand) Dr. P. Ferguson, Dr. I. F. Hermans Malaghan Institute of Medical Research P. O. Box 7060, Wellington 6012 (New Zealand)

High-resolution imaging. The NMR microscope

Nuclear magnetic resonance imaging has traditionally been applied to macroscopic objects with dimensions in excess of 1 cm. The submillimeter regime may be termed microscopic and to date has been largely unexplored. NMR microscopy is inherently difficult to perform due to the smaller signal which arises from each volume element as the resolution is enhanced. Indeed the problem is dramatically indicated by the dependence of the imaging time on the sixth power of the resolution (I). Some of the early imaging experiments utilized the limited sample space of NMR spectrometers and were performed on smalI scale objects. Transverse resolutions of between 0.2 and 0.3 mm have been reported (2,3). Since that time major developments in imaging techniques have centered on “scaling up” and the NMR imaging literature reports few, if any, examples in which microscopy is the principal objective. Hall et al. (4) have recently coined the term “chemical microscopy” to refer to the localization of chemical-shift information in small-scale samples. Using phantoms consisting of 1.6 mm i.d. capillaries they indicate a transverse resolution of 0.1 mm on long samples without slice selection. In this communication we report microscopic imaging studies carried out at 60 MHz on both phantom and plant stem samples in which the transverse resolution is 25 pm. A slice thickness of 1.5 mm takes advantage of the existing longitudinal symmetry. Plant stem microscopy is of course ideally suited to high resolution in two dimensions. Signal-to-noise considerations are central to understanding limiting resolution. The time-domain signal to noise available in the free induction decay is simply expressed (5) in SI units as

Diffusion-diffusion correlation and exchange as a signature for local order and dynamics

We demonstrate the use of new two-dimensional nuclear magnetic resonance experiments in the examination of local diffusional anisotropy under conditions of global isotropy. The methods, known as diffusion-diffusion correlation spectroscopy and diffusion exchange spectroscopy, employ successive pairs of magnetic field gradient pulses, with signal analysis using two-dimensional inverse Laplace transformation. Diffusional anisotropy is measured for water molecules in a polydomain lamellar phase lyotropic liquid crystal, 40 wt % nonionic surfactant C10E3 (C10H21O(CH2CH2O)6H) in H2O.