Kevin Michael Wright

T1−T2 NMR correlation studies of high-pressure-processed starch and potato tissue

Two-dimensionalT1−T2 correlation spectroscopy is used to monitor the effect of high-pressure and microwave processing on the microscopic water distribution and starch chain dynamics in water-saturated packed beds of native A and B type starches. B type starches are shown to be more resistant to pressure treatment than A type starches. High-pressure-induced A type starch gels are also shown to be radically different from the corresponding thermally induced gel. Although the cell walls of raw potato are resistant to high-pressure damage, the tonoplast and plasmalemma membranes are disrupted by high pressure.

Microstructural characterization of starch systems by NMR relaxation and Q-space microscopy.

Nuclear magnetic resonance (NMR) relaxation and q-space diffusion measurements have been used to probe the microscopic water distribution in a variety of starch-based systems, including packed beds of native starch granules with varying water contents, starch gels, and freeze-dried starch gels. The q-space data for the granular beds is compared with a variety of theoretical models and conforms best to unbounded diffusion in a lower dimensional space. In contrast to some earlier reports, the data for the gelatinized samples are not anomalous and conform to simple unrestricted diffusion in a three-dimensional space. The paper concludes with a novel method for probing pore size distribution in freeze-dried starch gels by infusion of acetone.

Dynamic NMR Q-space studies of microstructure with the multigrade CPMG sequence

A new approach to q-space studies of microstructure is proposed, which exploits the combined information contained in the water proton transverse relaxation time distribution and the frequency dependence of the apparent water diffusivity in heterogeneous systems. Using an automated two-dimensional multigrade CPMG sequence, both the pulse spacing and the amplitude of the applied field gradient are varied systematically and used to measure the frequency and wave vector dependence of the multiple exponential echo decay constants and amplitudes. Undesirable crossterms in the applied and background field gradients are eliminated by a simple procedure involving a sign reversal in the applied gradient. Nonlinear, local susceptibility-induced field gradients are shown to lead to enhanced, frequency-dependent apparent water diffusivities that are sensitive to the local microstructure.

NMR protocol for on-line Brix determination

Water suppression by diffusive attenuation was used to measure Brix in intact cellular tissue of apple and strawberry. Given the signal-to-noise ratio, the correlation for apple was established without repeated acquisition, so this protocol should also be useful for rapid, on-line measurements at low spectrometer frequencies. Water suppression by theT1-Null method fails with cellular tissue because of the considerable variation in the longitudinal relaxation times of vacuolar and cytoplasmic water.

An MRI study of drying in granular beds of nonporous particles.

MRI is used to study the role of capillary and gravitational forces in controlling mass transport of water during isothermal drying of granular beds of nonporous particles. A new model is presented that shows how capillary and gravitational forces relate to Fickian diffusion driven by a chemical potential gradient. In granular beds, where capillary and gravitational forces dominate, the image profiles give direct information on the dependence of the degree of saturation on capillary suction pressure. The effects of changing particle size and surface tension on the profiles is investigated quantitatively, and it is shown that high capillary pressures create anomalies in the drying profiles.

The characterisation of mammalian tissue with 2D relaxation methods

The potential of two-dimensional (2D) relaxometry for characterising mammalian tissue is explored on samples of liver, kidney (cortex, medulla and ureter) as well as cartilage. Significant differences are found between the T(1)-T(2) spectra of healthy and diseased human cartilage which suggests that 2D relaxometry could have potential use in clinical diagnosis. The effect of reducing the recovery delay on the T(1)-T(2) spectrum is explored to try to identify the optimum balance between speed and accuracy.

Combined relaxation and diffusion studies of porous media using the multigrade CPMG sequence

A new approach to q-space NMR studies of microstructure is proposed that exploits the combined information contained in the water proton transverse relaxation time distribution and the frequency dependence of the apparent water diffusivity. A simple protocol is used to eliminate undesirable crossterms in background susceptibility gradients. Local, microscopic nonlinearity in the applied field gradients is shown to lead to enhancement of the apparent water diffusivity in small pores at high frequencies. The relationships between NMR relaxation studies of pore emptying with sorption isotherms, electrical conductivity, and microbiological survival are indicated.

Ultrafast T1–T2 relaxometry using FLOP sequences

By periodically flipping the longitudinal magnetisation with a chain of 180 degrees pulses it is possible to establish a steady-state of longitudinal polarisation that effectively stores the information of T(1) relaxation. The pulse sequence for achieving this, called steady-state Flipped LOngitudinal Polarisation (FLOP) can be used for the fast acquisition of a two-dimensional T(1)-T(2) relaxation time spectrum in both periodic and a-periodic modes. We have therefore called this new class of sequences periodic or a-periodic FLOP-T(1)-T(2).

Chapter 2 Maximum entropy methods in nmr data processing

Publisher Summary This chapter is an introduction to maximum entropy, reviewing the basic principles and algorithms and the difficulties associated with it. Maximum entropy method (MEM) is based on a very general principle, which has been applied in many areas ranging from nuclear magnetic resonance (NMR) to X-ray crystallography, tomography, radio astronomy, and general image processing. The reason for its widespread applicability is that it is not a physical theory of anything in particular; rather, it is an extension of the principles of rational inference in science. Maximum entropy provides a powerful and general method of inferring the form of a probability distribution, given only the expectation values or moments of quantities drawn from that distribution. MEM has been claimed to be the only correct and consistent method of inference from incomplete and noisy data. It has its roots deep in information theory and statistical mechanics. MEM is not a panacea, but it has taken its place as a useful data processing technique in many areas of NMR spectroscopy. MEM must be used with care; it cannot rescue bad data, but it is sometimes capable of achieving quite remarkable results.