Biomedical subjects

K G Hollingsworth

Publications and source records attributed to K G Hollingsworth.

9 recordsLinked to original sources

PubMed2006

Influence of perfusion on hepatic MR diffusion measurement.

Percutaneous liver biopsy is the current gold standard for the diagnosis and assessment of hepatic fibrosis: it is an uncomfortable procedure with associated risks of infection and internal bleeding. Diffusion-weighted MRI may be important as a non-invasive measure of the grade of hepatic fibrosis, owing to the morphological and physiological changes in hepatic tissues as fibrosis develops. The factors that influence hepatic diffusion measurements are complex and are thought to include hepatic perfusion. This study assesses whether hepatic diffusion measurements are influenced by hepatic perfusion changes as this could be an important variable when investigating the relationship between diffusion measurements and grade of hepatic fibrosis. Hepatic apparent diffusion coefficients (ADCs) were measured in 10 healthy volunteers after an 8-h fast and 1-h after a standard meal (which increases portal vein flow and hepatic sinusoidal perfusion) using a breath-hold technique at b-values of 750, 500 and 200 s/mm(2). ADC measurements using b = 750 and 500 s/mm(2) in the posterior right lobe were unaffected by the perfusion changes (P = 0.15 and 0.14, respectively) whereas the results from the anterior right lobe significantly increased post-prandially (P = 0.03 and 0.008, respectively): inter-subject variability was low ( approximately 5%) for these b-values. Portal perfusion had a significant effect in both the posterior and anterior segments of the right lobe when using a b-value of 200 s/mm(2) (P = 0.01 and 0.00001 respectively): inter-subject variability was found to be high ( approximately 25%). The most reproducible measurements of the ADC can be made in the posterior right lobe and the effect of perfusion is minimised at the higher b-values (b = 500 and 750 s/mm(2)).

Adult↗

PubMed2006

Low-carbohydrate diet induced reduction of hepatic lipid content observed with a rapid non-invasive MRI technique.

Low carbohydrate diets are currently fashionable for inducing weight loss, but the metabolic effects at organ level are not well understood, especially the effect on liver fat storage. Such studies require serial hepatic fat measurements, for which liver biopsy is impractical. In 10 healthy volunteers we demonstrate the use of rapid (total 2 min acquisition time, 10 min magnet room time), non-invasive, quantitative MRI to serially measure hepatic fat changes induced by following a low carbohydrate diet for 10 days. A significant (p<0.01) reduction in hepatic fat after 3 days of dieting was observed in 5 subjects. All subjects demonstrated significant (p<0.01) reductions in hepatic fat by day 10. A strong correlation (kappa = 0.81) existed between the initial fat content and the percentage fat content reduction in the first 3 days of the diet. All subjects lost weight (average 1.7 kg at day 3 and 3.0 kg at day 10), but this was not correlated with hepatic fat loss after 3 days or 10 days of dieting. The results presented illustrate the potential value of MR hepatic fat quantification in longitudinal studies of hepatic fat content.

Adipose Tissue↗

PubMed2005

Droplet migration in emulsion systems measured using MR methods.

The migration of emulsion droplets under shear flow remains a largely unexplored area of study, despite the existence of an extensive literature on the analogous problem of solid particle migration. A novel methodology is presented to track the shear-induced migration of emulsion droplets based on magnetic resonance imaging (MRI). The work is in three parts: first, single droplets of one Newtonian fluid are suspended in a second Newtonian fluid (water in silicone oil (PDMS)) and are tracked as they migrate within a Couette cell; second, the migration of emulsion droplets in Poiseuille flow is considered; third, water-in-silicone oil emulsions are sheared in a Couette cell. The effect of (a) rotational speed of the Couette, (b) the continuous phase viscosity, and (c) the droplet phase concentration are considered. The equilibrium extent of migration and rate of migration increase with rotational speed for two different emulsion systems and increased continuous phase viscosity, leads to a greater equilibrium extent of migration. The relationship between the droplet phase concentration and migration is however complex. These results for semi-concentrated emulsion systems and wide-gap Couette cells are not well described by existing models of emulsion droplet migration.

Journal Article↗

PubMed2005

Spatially resolved emulsion droplet sizing using inverse Abel transforms.

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) is well established as a tool for determining emulsion droplet-size distributions via measurement of restricted self-diffusion. Most measurements made to date have not been spatially resolved, but have measured an average size distribution for a certain volume of emulsion. This paper demonstrates a rapid method of performing spatially resolved, restricted diffusion measurements, which enables emulsion droplet sizing to be spatially resolved as a function of radius in cylindrical geometries or pipes. This is achieved by the use of an Abel transform. The technique is demonstrated in various annular systems containing two emulsions, with different droplet-size distributions, and/or a pure fluid. It is also shown that by modifying the pulse sequence by the inclusion of flow-compensating magnetic field gradients, the technique can measure spatially resolved droplet-size distributions in flowing emulsions, with potential applications in spatially resolved on-line droplet-size analysis.

Journal Article↗

PubMed2004

An NMR study of the freezing of emulsion-containing drops.

Various nuclear magnetic resonance (NMR) techniques were used to monitor the freezing behaviour of suspended 2-mm-diameter drops. The drops were composed of hydrocarbon oils emulsified in either water or water/sucrose mixtures. As such they were good model systems for the study of spray freezing, sharing structural similarities with potential products such as ice cream. In particular, simple 1H NMR spectroscopy was used to monitor and individually quantify the freezing or solidification behaviour of the various constituent species of the drops. In addition, the effect of freezing on the emulsion droplet size distribution (and hence emulsion stability) was also measured based on NMR self-diffusion measurements. The effect of freeze/thaw cycling was also similarly studied. The nucleation temperature of the emulsion droplets was found to depend on the emulsion droplet size distribution: the smaller the droplets, the lower the nucleation temperature. Emulsion droplet sizing indicated that oil-in-sucrose-solution emulsions were more stable, showing minimal coalescence, whereas oil-in-water emulsions showed significant coalescence during freezing and freeze/thaw cycling.

Journal Article↗

PubMed2004

Fast emulsion droplet sizing using NMR self-diffusion measurements.

Emulsion droplet sizing using pulsed field gradient (PFG) nuclear magnetic resonance (NMR) is a well-established technique. Traditionally these measurements require total acquisition times of typically 5-20 min per sample, which severely limits our ability to use this method to study dynamic processes. Here we present the application and verification of an NMR pulse sequence, Difftrain, which enables emulsion droplet size distributions to be measured in 3-10 s. We have previously introduced applications of Difftrain (C. Buckley, K.G. Hollingsworth, A.J. Sederman, D.J. Holland, M.L. Johns, L.F. Gladden, J. Magn. Reson. 161 (2003) 112-117), including the droplet sizing of a single unimodal emulsion sample. In this paper, several model emulsions containing different oils are measured and the results compared directly with sizing provided by laser scattering. In this manner, the Difftrain method is verified and its possibilities and limitations are explored. Guidelines are proposed for the range of droplet sizes for which accurate results can be produced. The Difftrain technique opens up the possibility of studying non-equilibrium emulsions; a study of the in situ emulsification of a 21% v/v water-in-silicone oil emulsion is presented.

Journal Article↗

PubMed2004

Development and application of rotationally compensated RARE.

In this paper we describe a new extension to the RARE rapid imaging technique--ROTACOR--which compensates for constant sample rotational motion by rotating the direction of the gradient coordinate system (read and phase directions) between each refocusing RF pulse and therefore between each acquisition of a line of k-space in the read direction. In the laboratory frame this corresponds to an irregularly sampled k-space raster; for a sample rotating at the predefined rotation rate this will correspond to the sampling of a rectilinear k-space raster. This technique is applied to two rotating systems. First, to demonstrate the technique, a rotating mixing paddle in water is imaged using conventional RARE and then using the ROTACOR sequence, demonstrating the improvement of image quality produced by ROTACOR. Second, ROTACOR is used to image the deformation of water droplets in silicone oil, being sheared in a wide-gap Couette cell. Accurate imaging of the droplet shape as a function of shear rate, permits determination of the interfacial tension between the two fluids concerned; the results compare favourably with reported literature values.

Journal Article↗

PubMed2003

Measurement of emulsion droplet sizes using PFG NMR and regularization methods.

The droplet size distributions of emulsions have been measured using pulsed field gradient (PFG) nuclear magnetic resonance (NMR) for many years. This technique finds particular application with emulsions that are concentrated and/or opaque, since such emulsion systems are difficult to characterize by other methods. Most studies employing PFG techniques assume a lognormal form when extracting the droplet size distribution from the experimental data. It is clearly desirable to retrieve a droplet size distribution from the experimental data without assuming such a functional form. This is achieved for the first time using regularization techniques. Regularization based on the distribution area and on its second derivative are compared and assessed along with the following techniques for selecting the optimal regularization parameter: the L-curve method, generalized cross validation (GCV), and the discrepancy principle. Regularization is applied to both simulated data sets and experimental data. It is found that when the experimental error can be estimated accurately, the discrepancy principle with area regularization is the best approach. When the error is not known the GCV method, with second derivative regularization and allowing only nonnegative values, is most effective.

Journal Article↗

PubMed2003

Applications of fast diffusion measurement using Difftrain.

Novel applications of fast self-diffusion measurement are presented. Difftrain (Diffusion train), which uses successive stimulated echoes from a single excitation pulse where a portion of the available magnetisation is recovered for each echo, is used to measure self-diffusion by varying the observation time. It is applied to produce the droplet size distribution of an oil-in-water emulsion in less than 4s. This is verified by comparison with the droplet size distribution produced by a standard pulsed field gradient (PFG) technique. Difftrain is also extended to enable the application of incremental gradients, in addition to varying the observation time. This is used to produce propagators or displacement probabilities of water flowing through a packed bed for a range of 16 observation times in under 10 min. Again verification is provided by acquisition of the same propagators using a conventional PFG technique.

Journal Article↗