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Biomedical subjects

R Bowtell

Publications and source records attributed to R Bowtell.

31 records · Page 2Linked to original sources
PubMed1999

Detecting activations in event-related fMRI using analysis of variance.

The most common design of a functional MRI (fMRI) experiment is a block design. The use of rapid imaging, however, and carefully designed paradigms makes the separation of cognitive events possible. Such experiments make use of event-related paradigms, in which a task involving several cognitive processes is repeated. In analyzing data from such experiments, existing methods often prove inadequate, because the prediction of the exact shape or timing of the time course is difficult. Here we present an analysis of variance (ANOVA) method for analyzing fMRI data that does not require any assumptions about the shape of the activation time course. Consequently, this method can simultaneously detect brain areas showing a variety of stimulus-locked time courses in the same experiment. The utility of this technique is demonstrated by the analysis of data from two event-related paradigms in which regions of activation are detected that correspond to a variety of distinct neural processes, yielding significantly different temporal signal changes. Magn Reson Med 42:1117-1122, 1999.

Analysis of Variance↗
PubMed1999

Resolution in high field echo planar microscopy.

The application of echo planar imaging to NMR microscopy offers a temporal resolution unparalleled by other techniques. However, a major difficulty in imaging at the high field strengths used for microscopy is the effect of local field inhomogeneities caused by magnetic susceptibility effects. This can give rise to both image distortion and signal loss. In addition, the effect of diffusion in the presence of the large imaging gradients gives rise to a broadening of the point spread function and hence loss of true resolution. We compare the sensitivity of two techniques, MBEST and PEPI, to both of these effects. Analytic expressions for the signal in each echo of the two sequences are developed, and the point spread functions for the two techniques are calculated. Using PEPI, we have been able to produce images with an in-plane resolution of 50 micrometer from a single free induction decay. This technique has been extended to three dimensions allowing the generation of 64(3) images with an isotropic resolution of 80 micrometer.

Diffusion↗
PubMed1998

Multilayer Gradient Coil Design

In standard cylindrical gradient coils consisting of wires wound in a single layer, the rapid increase in coil resistance with efficiency is the limiting factor in achieving very large magnetic field gradients. This behavior results from the decrease in the maximum usable wire diameter as the number of turns is increased. By adopting a multilayer design in which the coil wires are allowed to spread out into multiple layers wound at increasing radii, a more favorable scaling of resistance with efficiency is achieved, thus allowing the design of more powerful gradient coils with acceptable resistance values. By extending the theory used to design standard cylindrical gradient coils, we have developed mathematical expressions which allow the design of multilayer coils, and the evaluation of their performance. These expressions have been used to design a four-layer, z-gradient coil of 8 mm inner diameter, which has an efficiency of 1.73 Tm-1 A-1, a resistance of 1.8 Omega, and an inductance of 50 µH. This coil produces a gradient which deviates from linearity by less than 5% within a central cylindrical region of 4.5 mm length and 4.5 mm diameter. A coil has been constructed from this design and tested in simple imaging and pulsed gradient spin echo experiments. The resulting data verify the predicted coil performance, thus demonstrating the advantages of using multilayer coils for experiments requiring very large magnetic field gradients. Copyright 1998 Academic Press.

Journal Article↗
PubMed1997

Functional magnetic resonance imaging of single motor events reveals human presupplementary motor area.

Conventional functional imaging paradigms use periods of repetitive task performance to generate sustained functional signal changes. We have developed a technique of imaging the small, transient signal changes that occur after single cognitive events. The technique uses echo-planar imaging at 3 T to generate functional images of the whole brain with a temporal resolution of 3 seconds. It uses a signal averaging technique to create time sweeps of functional activity. After a single cognitive event, widely distributed patterns of brain activation can be detected and their time course measured. This technique enables the individual cognitive tasks that constitute a paradigm to be analyzed separately and compared. We describe the application of this new technique to separate the cognitive elements in a simple "go/no-go" motor paradigm. Comparison of activation patterns during "go" and "no-go" responses reveals hierarchical subdivision of the medial premotor cortex into an anterior region (presupplementary motor area) involved in movement decision making and a posterior region (supplementary motor area proper) directly involved in motor execution.

Adult↗
PubMed1995

Active acoustic screening: reduction of noise in gradient coils by Lorentz force balancing.

We have designed and constructed a quiet gradient set with restricted access for the combined purposes of evaluating the principles of active acoustic screening, recently introduced by Mansfield, Glover, and Bowtell, and for EPI studies of the head at 3.0 T. The design utilizes the return paths of the conductors in a closed arc loop arrangement to eliminate net Lorentz forces thereby attenuating acoustic noise especially at low frequency. This design should significantly reduce the dangers to patients of high noise levels, especially in high field magnetic resonance imaging systems.

Acoustics↗
PubMed1995

A modified imaging sequence for accurate T2 measurements using NMR microscopy.

A modified spin-echo pulse sequence is described that enables accurate T2 measurements to be made in NMR microimaging experiments. The modified sequence eliminates cumulative diffusion losses that lead to an underestimation of the T2 relaxation time using conventional spin-echo pulse sequences. The approach is theoretically justified and confirmed in comparative experiments on phantoms.

Humans↗
PubMed1994

NMR microscopy of hydrating hydrophilic matrix pharmaceutical tablets.

NMR microscopy has been used to monitor the formation of the gel layer in hydrating hydrophilic polymer tablets. Such tablets are used in the controlled delivery of drugs, where it has been found that the rate and extent of the swelling of the outer gel layer critically influences the kinetics of drug release. Tablets were hydrated in distilled water at 37 degrees C and then imaged at discrete time intervals using a 500 MHz microscope. The growth of the gel layer was clearly observed in time sequences of radial and axial sections. Axial images showed some interesting dimensional changes, with the gel at the flat surface of the tablet developing a concave shape. This is probably a reflection of the occurrence of uni-axial stress relaxation as hydration proceeds. Diffusion- and T2-weighted images provided evidence that the water in the gel layer is more strongly bound close to the dry core of the tablet than at the more fully hydrated outer surface. In images of tablets containing diclofenac, disruption of the gel layer was shown to occur primarily from the flat surfaces of the tablet, whilst the distribution of particles could be seen in tablets doped with insoluble calcium phosphate.

Diclofenac↗
PubMed1992

Magnetic resonance imaging: applications of novel methods in studies of porous media.

NMR imaging is finding broad applications in nonbiological areas including the study of fluid flow and fluid ingress in porous media. The porous media include at the one end mineral rocks and various building materials through various solid plastic materials to foodstuffs at the other end of the spectrum. The fluids within these various media range from crude oil and water mixtures, and water itself, to a range of organic solvents in plastic materials. This paper is concerned with the flow and ingress of water through Bentheimer sandstone and Ninian reservoir specimens, and also in solid nylon blocks.

Echo-Planar Imaging↗
PubMed1991

Gradient coil design using active magnetic screening.

Active magnetic screening allows the production of gradient coils which are magnetically decoupled from their surroundings. Using such coils the eddy current problems associated with the generation of large, rapidly switched gradients within the confines of a superconductive magnet can be eliminated. In this paper the principles of active screening are outlined and some of the methods of designing screened gradient coils are described. Problems relating to the construction of screened gradient coils are also addressed.

Equipment Design↗
PubMedDate not supplied

Self-diffusion and molecular mobility in PVA-based dissolution-controlled systems for drug delivery.

Nuclear magnetic resonance (NMR) microscopy has been used to monitor the hydration of poly(vinyl alcohol) (PVA) samples of varying molecular weight. One-dimensional profiles weighted to predominantly show the variation of water concentration were acquired every 3 min during the first 30 min of hydration and subsequently at 1 and 2 h. Diffusion-weighted profiles obtained after 30 min and 1 and 2 h were used to calculate the spatial variation of the water self-diffusion coefficient. The resulting data provide supporting evidence for the hypothesis that phenomena such as reptation are important near the glassy/rubbery interface of polymers during dissolution, while the diffusion gradually changes to Zimm type near the rubbery/solvent interface.

Diffusion↗
PubMedDate not supplied

Strong gradients for spatially resolved diffusion measurements.

A new multilayer approach to gradient coil design, which allows the production of very strong gradient coils with reasonable resistance and consequent power dissipation, has been developed. Using this approach we have designed and built a strong z-gradient coil that will accommodate vertically mounted samples contained in 5-mm nuclear magnetic resonance tubes. The coil has an efficiency of 1.73 Tm-1A-1, an inductance of 49 microH, and a resistance of 1.8 omega, with a homogeneous volume consisting of a central cylinder of 4.5-mm length and diameter. This coil has been used to monitor the diffusion of water in Nylon 6.6 at room temperature, during desorption. This system is difficult to monitor via nuclear magnetic resonance (NMR), because the diffusion coefficients are typically less than 10(-13) m2s-1, while the T2 relaxation time is less than 1 ms even when the sample is fully saturated. The resulting measurements show a strong concentration dependence of the T2 relaxation time and self-diffusion coefficient of the absorbed water. The measured concentration profiles are consistent with a Fickian diffusion process with a concentration-dependent diffusion coefficient. The measured self-diffusion values are in reasonable agreement with those inferred from the variation of the concentration profiles as a function of time, using the one-dimensional Fickian diffusion equation.

Absorption↗