Biomedical subjects
Publications and source records attributed to B P Hills.
We explore the implications of motional relativity in NMR and show that sample translation can be used to acquire NMR signals without the need for pulsed RF excitation or pulsed magnetic field gradients. Novel single-shot, on-line NMR acquisition protocols for samples being conveyed at high speed are discussed and preliminary results using a low-cost, on-line prototype NMR sensor are presented.
A working prototype of a novel low-cost Halbach-array-based NMR system is described. The new design provides open access to the sample relative to conventional NMR magnet designs and this facilitates the simultaneous use of multi-sensor techniques on the same sample, in which NMR/MRI can potentially be combined with other spectroscopies such as impedance spectroscopy, laser scattering and rheological experiments.
Evolving microstructure in a model dextran solution is shown to exert a major influence on the survival of Escherichia coli K-12 frag 1 and Salmonella typhimurium LT2. The microstructure results from microscopic phase separation, which develops over several hours resulting in hardening of the solution into a glassy state. The microstructure is characterized by an array of physical methods including image analysis, electron spin resonance and bulk rheology, and it is shown that bacterial survival depends on the formation of microscopic. water-rich domains and not primarily on bulk water activity or hardness.
The survival of Salmonella typhimurium LT2, Escherichia coli K-12 and Pseudomonas putida in several model porous media poised at a water activity of 0.94 is shown to depend critically on the microstructure of the particulate matrix and the microscopic water distribution. The porous media were made by randomly dispersing a liquid inoculum containing ca. 10(7) cells/ml throughout the pores and interparticle spaces of packed beds of silica particles and Sephadex microspheres. The purely "microstructural stress" effects were isolated by comparison with a homogeneous liquid growth medium having the same water activity. The possibility of exploiting similar microstructural stress effects in food preservation is discussed.
The pulsed field gradient stimulated echo technique with selective excitation is used to probe the diffusion of water in the continuous phase of concentrated oil-in-water emulsions. The dependence of the echo amplitude, S(q,Delta), on wavevector, q, and diffusion time, Delta, shows that the water diffusion propagator is sensitive to emulsion microstructure. This is analyzed using a multiple exponential time series expansion of S(q,Delta) in Delta, with wavevector dependent expansion coefficients. These coefficients are compared with predictions from several theoretical models for three types of stable emulsion, each differing in microstructure. The relationship between the nuclear magnetic resonance q-space measurements and bulk rheology for all three types of emulsion is also explored.
A combination of 1H and 23Na NMR is used to probe the dynamic state of water in gelatine gels as the water content is lowered from 70% to dryness. A sharp increase in the proton and sodium transverse relaxation rates is observed as the water content falls from 20 to 15% while the proton longitudinal and dipolar cross relaxation rates show a maximum at ca. 15%. We show that these observations can be understood if monolayer coverage occurs at 15% and multilayers of less strongly interacting hydration water are formed between 15 and 20%. Above 20% the water appears to behave as an unperturbed bulk phase.
A method for measuring noninvasively the viscosity of a polysaccharide model meal using the known relationship between relaxation times and polysaccharide concentration in solution in vitro is presented. The aim is to develop a method for monitoring digesta viscosity in vivo, using EPI to capture the motion of the gastrointestinal lumen. The transverse relaxation rate T-12 of locust bean gum solutions was calibrated against the zero-shear viscosity at 37 degreesC. Differences in viscosity were distinguished significantly using T-12 measurements. T-12 and viscosity were insensitive to exposure to gastric juice and changes in pH, and the model meal was well received by volunteers and provided good contrast in vivo in EPI images. Therefore it would be possible to use this method to monitor the changes in meal viscosity within the gastric lumen in vivo.
The survival of Salmonella Typhimurium LT2 in randomly packed beds of glass beads, microporous silica particles and Sephadex microspheres is examined. It is shown that the decrease in the percentage cell recovery in these porous materials at reduced water content is not correlated with the global water activity as determined by conventional vapour pressure measurements but rather with the osmotic shock induced by the sudden redistribution of water and air among the microscopic pores in the matrix surrounding the cells. For this reason the bacterial survival and growth data correlates best with physical measurements, such as NMR and electrical conductivity, which are sensitive to the microscopic air-water distribution. The implications of this observation in food safety and preservation are discussed.
A high field strength, constant gradient stimulated-echo pulse sequence is applied to a model heterogeneous system consisting of randomly packed beds of glass microspheres. A multiple exponential analysis of the dependence of the stimulated echo amplitude on diffusion time, delta, yields coefficients that depend explicitly on both the wavevector, q and on the time delay, delta. The wavevector and delta- dependence of the coefficients is analyzed both theoretically and experimentally and shown to be sensitive to the effects of coupled relaxation and diffusion. It is proposed that these effects could be exploited as a new probe of microstructure.
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.
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.
A general theory of spatially dependent bacterial growth in heterogeneous systems is developed by combining a structured-cell kinetic model of bacterial metabolism with reaction-diffusion equations describing the transport of nutrients in the growth medium. Rate constants in the theory are determined for Listeria monocytogenes by fitting the viable count batch growth data acquired in homogeneous, shake-flask cultures. The rate constants are then used to calculate the form of bacterial growth when it occurs as localized microcolonies in structured systems. The same theory is used to analyse the effects of transient variations in environmental variables.
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.
Radial imaging protocols suitable for monitoring water transport in biopolymer and food materials during processes such as drying and rehydration are developed and tested on a well-characterized model sample. This model consisted of a randomly packed bed of Sephadex beads with varying water content. The results are interpreted with theoretical models for the dependence of the initial water magnetization, transverse relaxation, and diffusive attenuation on water content for two slice-selective radial imaging pulse sequences. It is shown that volume shrinkage and changes in packing density complicate the dependence of the initial magnetization on water content, so that the transverse relaxation rate provides the most reliable monitor of water content. Radial imaging is shown to offer many advantages over conventional two-dimensional imaging whenever the sample can be made with cylindrical symmetry.
NMR water proton relaxation times are reported for suspensions of silica powder of varying silica/water ratios. Pore size distributions and pore connectivities are derived from the relaxation time distributions for the water-saturated suspension. Capillary theory appears to explain the relaxation behaviour of the unsaturated, packed suspensions. The relaxation data in suspensions that have lower solid/liquid ratios than the saturated, packed suspension are sensitive to the particular radial distribution function. This is analysed with a simple cluster model.
A quantitative analysis of the dependence of water proton relaxation on water content in randomly packed beds of Sephadex is undertaken. A combination of Osmotic and Capillary theory is used to describe the morphological changes occurring in the packed beds as the water content is lowered. At each water content the water proton relaxation is calculated using a "proton exchange-diffusion" model which takes into account fast chemical exchange between water and dextran hydroxyl protons and the diffusion of water molecules between the water compartments inside and outside the Sephadex beads. The relaxation time distribution are shown to provide a sensitive probe of the air-water distribution in the bed and of the shrinkage of the Sephadex beads at lower water contents. The theoretical models provide an accurate, quantitative description of the relaxation behavior except for the largest beads at high water contents when there is slow diffusion between the water compartments. In this case, a more realistic three-dimensional description of the air-water distribution in a randomly packed bed is required.
A novel microimaging method is described for measuring the effective diffusion coefficient (D infinity) of water in saturated, microporous materials. A simple multicompartment computer simulation was used to show how the value of D infinity, when combined with measurements of water proton relaxation times and pulsed field gradient spin-echo amplitudes, allows the pore size distribution and pore connectivity to be characterized.
The effects that the spatial distribution of water protons and their transverse relaxation times have on the image contrast of spin echo images of courgette was investigated. The T2-weighted image of courgette contains the most anatomical information. The image contrast was explained using a phenomenological theory based on the Bloch equations, which gave an insight into the morphology and microdynamics of water in the plant tissue. The perceived contrast in the spin echo images of courgette, glucose and Sephadex bead solutions can be dramatically altered by keeping all the imaging acquisition parameters constant, such as the recycle and echo time, but reducing the interpulse spacing by introducing a CPMG train of 180 degrees pulses into the middle of the sequence. These changes were interpreted by considering the microenvironment of the water. This work demonstrates that the origin of image contrast in T2-weighted images of plant tissue can be understood using the water proton transverse relaxation theory developed by Hills et al.