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R Bowtell

Publications and source records attributed to R Bowtell.

At least 19 recordsLinked to original sources
PubMed2001

Single-shot T2(*) measurement to establish optimum echo time for fMRI: studies of the visual, motor, and auditory cortices at 3.0 T.

The signal change in fMRI is dependent on the echo time and the rate of decay of transverse magnetization. The latter factor may vary across regions of the brain as a result of variations in field homogeneity. Previous measurements of the signal change with echo time have generally employed relatively slow multi-echo techniques, which may be sensitive to movement and habituation effects. Here a fast T(2)(*) measurement technique, involving the generation of six low-resolution echo planar images from a single FID, is described, and its use in the evaluation of the optimum echo time for visual, motor, and auditory fMRI experiments at 3.0 T is outlined.

Auditory Cortex↗
PubMed2001

Imaging the long-range dipolar field in structured liquid state samples.

We describe imaging experiments in which the pattern of the dipolar field generated by spatially modulated nuclear magnetization is directly visualized in simply structured phantoms. Two types of experiment have been carried out at 11.7 T using (1)H NMR signals. In the first, the field from a single spin species is imaged via its own NMR signal. In the second, the NMR signal from one spin species is used to image the field generated by a second species. The field patterns measured in these experiments correspond well with those calculated using simple theoretical expressions for the dipolar field. The results also directly demonstrate the spatial sensitivity of the signal generated using dipolar field effects, indicating that the range of the field depends upon the inverse of the spatial frequency with which the magnetization is modulated.

Journal Article↗
PubMed2001

Cortical responses to single mechanoreceptive afferent microstimulation revealed with fMRI.

The technique of intraneural microneurography/microstimulation has been used extensively to study contributions of single, physiologically characterized mechanoreceptive afferents (MRAs) to properties of somatosensory experience in awake human subjects. Its power as a tool for sensory neurophysiology can be greatly enhanced, however, by combining it with functional neuroimaging techniques that permit simultaneous measurement of the associated CNS responses. Here we report its successful adaptation to the environment of a high-field MR scanner. Eight median-nerve MRAs were isolated and characterized in three subjects and microstimulated in conjunction with fMRI at 3.0 T. Hemodynamic responses were observed in every case, and these responses were robust, focal, and physiologically orderly. The combination of fMRI with microstimulation will enable more detailed studies of the representation of the body surface in human somatosensory cortex and further studies of the relationship of that organization to short-term plasticity in the human SI cortical response to natural tactile stimuli. It can also be used to study many additional topics in sensory neurophysiology, such as CNS responses to additional classes of afferents and the effects of stimulus patterning and unimodal/crossmodal attentional manipulations. Finally, it presents unique opportunities to investigate the basic physiology of the BOLD effect and to compare the operating characteristics of fMRI and EEG as human functional neuroimaging modalities in an unusually specific and well-characterized neurophysiological setting.

Brain Mapping↗
PubMed2001

Representation of pleasant and aversive taste in the human brain.

In this study, the representation of taste in the orbitofrontal cortex was investigated to determine whether or not a pleasant and an aversive taste have distinct or overlapping representations in this region. The pleasant stimulus used was sweet taste (1 M glucose), and the unpleasant stimulus was salt taste (0.1 M NaCl). We used an ON/OFF block design in a 3T fMRI scanner with a tasteless solution delivered in the OFF period to control for somatosensory or swallowing-related effects. It was found that parts of the orbitofrontal cortex were activated (P < 0.005 corrected) by glucose (in 6/7 subjects) and by salt (in 6/7 subjects). In the group analysis, separate areas of the orbitofrontal cortex were found to be activated by pleasant and aversive tastes. The involvement of the amygdala in the representation of pleasant as well as aversive tastes was also investigated. The amygdala was activated (region of interest analysis, P < 0.025 corrected) by the pleasant taste of glucose (5/7 subjects) as well as by the aversive taste of salt (4/7 subjects). Activation by both stimuli was also found in the frontal opercular/insular (primary) taste cortex. We conclude that the orbitofrontal cortex is involved in processing tastes that have both positive and negative affective valence and that different areas of the orbitofrontal cortex may be activated by pleasant and unpleasant tastes. We also conclude that the amygdala is activated not only by an affectively unpleasant taste, but also by a taste that is affectively pleasant, thus providing evidence that the amygdala is involved in effects produced by positively affective as well as by negatively affective stimuli.

Amygdala↗
PubMed2000

Sensory-specific satiety-related olfactory activation of the human orbitofrontal cortex.

When a food is eaten to satiety, its reward value decreases. This decrease is usually greater for the food eaten to satiety than for other foods, an effect termed sensory-specific satiety. In an fMRI investigation it was shown that for a region of the orbitofrontal cortex the activation produced by the odour of the food eaten to satiety decreased, whereas there was no similar decrease for the odour of a food not eaten in the meal. This effect was shown both by a voxel-wise SPM contrast (p <0.05 corrected) and an ANOVA performed on the mean percentage change in BOLD signal in the identified clusters of voxels (p <0.006). These results show that activation of a region of the human orbitofrontal cortex is related to olfactory sensory-specific satiety.

Eating↗
PubMed2000

Sensory-specific satiety-related olfactory activation of the human orbitofrontal cortex.

When a food is eaten to satiety, its reward value decreases. This decrease is usually greater for the food eaten to satiety than for other foods, an effect termed sensory-specific satiety. In an fMRI investigation it was shown that for a region of the orbitofrontal cortex the activation produced by the odour of the food eaten to satiety decreased, whereas there was no similar decrease for the odour of a food not eaten in the meal. This effect was shown both by a voxel-wise SPM contrast (p<0.05 corrected) and an ANOVA performed on the mean percentage change in BOLD signal in the identified clusters of voxels (p<0.006). These results show that activation of a region of the human orbitofrontal cortex is related to olfactory sensory-specific satiety.

Brain Mapping↗
PubMed2000

Analytic calculations of the E-fields induced by time-varying magnetic fields generated by cylindrical gradient coils.

Analytic expressions which allow the direct calculation of the electric field generated inside an infinite conducting cylinder by varying the current through the wires of any cylindrical coil are presented. These expressions provide some general insight into the spatial characteristics of the electric field generated inside the body by switched gradients and can be used to evaluate the locations where nerve stimulation by rapid gradient switching is likely to occur. They may also be employed at the design stage to produce gradient coils which can provide higher gradient switching rates without causing nerve stimulation. Using these expressions the electric field patterns produced by transverse and longitudinal, whole-body gradient coils were calculated. Example data are presented along with the associated magnetic field patterns. The effect on the induced electric field pattern of varying the body size and the size of the region of gradient linearity was explored.

Body Constitution↗
PubMed2000

fMRI of the responses to vibratory stimulation of digit tips.

Three studies were carried out to assess the applicability of fMRI at 3.0 T to analysis of vibrotaction in humans. A novel piezoelectric device provided clean sinusoidal stimulation at 80 Hz, which was initially applied in separate runs within a scanning session to digits 2 and 5 of the left hand in eight subjects, using a birdcage RF (volume) coil. Significant clusters of activation were found in the primary somatosensory cortex (SI), the secondary somatosensory cortex (SII), subcentral gyrus, the precentral gyrus, posterior insula, posterior parietal regions (area 5), and the posterior cingulate. Digit separation in SI was possible in all subjects and the activation sites reflected the known lateral position of the representation of digit 2 relative to that of digit 5. A second study carried out in six additional subjects using a surface coil, replicated the main contralateral activation patterns detected in study one and further improved the discrimination of the digits in SI. Significant digit separation was also found in SII and in the posterior insula. A third study to investigate the frequency dependence of the response focused on the effect of an increase in vibrotactile frequency from 30 to 80 Hz, with both frequencies applied to digit 2 during the same scanning session in four new subjects. A significant increase in the number of pixels activated within both SII and the posterior insula was found, while the number of pixels activated in SI declined. No significant change in signal intensity with frequencies was found in any of the activated areas.

Adult↗
PubMed2000

Spatial and temporal distribution of solutes in the developing carrot taproot measured at single-cell resolution.

The time-course and spatial distribution of sugars and ions in carrot (Daucus carota L.) was studied at fine resolution using single cell (SiCSA) and tissue analysis. Four phases of osmolyte accumulation in the taproot were identified: an amino acid (germination) phase, when internal sources of amino acids provide seedlings with osmotica; an ion phase, when inorganic and organic ions were the main solutes; a hexose phase, when concentrations of glucose and fructose sharply increased and reached their maximum; and a sucrose phase, when sucrose became the major solute. Spatial distribution of sugar in taproot cells showed a general trend of highest concentration on both sides of the vascular cambium (some 200 mM sucrose, 150 mM glucose) and a minimum in the pith (some 100 mM sucrose, 60 mM glucose) and in periderm. Electrolytes (e.g. potassium) followed a distribution generally reciprocal to that of sugars; minimum in the tissue adjacent to the cambium (some 10 mM) and maximum in the pith and periderm (some 60-100 mM). The cambial cells contained unexpectedly low concentrations of sugars and potassium. These spatial and temporal patterns indicate that amino acids, other electrolytes and sugars are interchangeable in the tissue osmotic balance. The nature of the solute is developmentally determined both temporally and spatially. During the accumulation of electrolytes following the initial amino acid phase, osmotic pressure to 420 mosmol kg-1 rises and then remains constant despite large changes in the concentration of individual solutes. This indicates that osmotic pressure is regulated independently of the individual concentrations of solutes.

Amino Acids↗
PubMed2000

Lip-reading ability and patterns of cortical activation studied using fMRI.

Lip-reading is a complex cognitive skill with large individual differences in performance. The basis of these individual differences remains poorly understood. Functional magnetic resonance imaging (fMRI) techniques allows brain activation accompanying complex cognitive activities to be studied noninvasively. In the present paper, fMRI was used to study the patterns of cortical activation that occur during the silent lip-reading of connected speech and to investigate whether there are detectable differences in activation between subjects with widely differing lip-reading abilities. From a cohort of 26 volunteers, nine subjects who fell into three distinct lip-reading ability groups were selected. Brain activation was measured in two conditions: an experimental condition where subjects attempted to lip-read sentences; and a baseline condition where subjects passively viewed a static image of a talker's face. Relative to the baseline condition, lip-reading induced activation in several cortical areas, including the auditory cortices, despite the lack of an auditory component to the task. In comparison to the better two groups of lip-readers, subjects in the poorest group displayed significantly less activation in superior and middle temporal gyrus, but not inferior temporal gyrus. These preliminary results justify more extensive investigations of the cortical basis of individual differences in lip-reading.

Adult↗
PubMed1999

The representation of pleasant touch in the brain and its relationship with taste and olfactory areas.

Although there has been much investigation of brain pathways involved in pain, little is known about the brain mechanisms involved in processing somatosensory stimuli which feel pleasant. Employing fMRI it was shown that pleasant touch to the hand with velvet produced stronger activation of the orbitofrontal cortex than affectively neutral touch of the hand with wood. In contrast, the affectively neutral but more intense touch produced more activation of the primary somatosensory cortex than the pleasant stimulus. This indicates that part of the orbitofrontal cortex is concerned with representing the positively affective aspects of somatosensory stimuli, and in further experiments it was shown that this orbitofrontal area is different from that activated by taste and smell. The finding that three different primary or unlearned types of reinforcer (touch, taste, and smell) are represented in the orbitofrontal cortex helps to provide a firm foundation for understanding the neural basis of emotions, which can be understood in terms of states elicited by stimuli which are rewarding or punishing.

Brain↗
PubMed1999

Automatic compensation of motion artifacts in MRI.

Patient motion during the acquisition of a magnetic resonance image can cause blurring and ghosting artifacts in the image. This paper presents a new post-processing strategy that can reduce artifacts due to in-plane, rigid-body motion in times comparable to that required to re-scan a patient. The algorithm iteratively determines unknown patient motion such that corrections for this motion provide the best image quality, as measured by an entropy-related focus criterion. The new optimization strategy features a multi-resolution approach in the phase-encode direction, separate successive one-dimensional searches for rotations and translations, and a novel method requiring only one re-gridding calculation for each rotation angle considered. Applicability to general rigid-body in-plane rotational and translational motion and to a range of differently weighted images and k-space trajectories is demonstrated. Motion artifact reduction is observed for data from a phantom, volunteers, and patients.

Algorithms↗
PubMed1999

Analytic approach to the design of transverse gradient coils with co-axial return paths.

Transverse gradient coils with co-axial return paths offer reduced acoustic noise compared with standard cylindrical gradient coils, due to local force balancing, and can also easily be made to have a length to diameter ratio that is less than one. Analytic expressions for the magnetic field and vector potential generated by this type of coil are described here, along with a formula for calculating the coil inductance. It is shown that these expressions allow the implementation of powerful analytic methods of coil design, as well as the incorporation of active magnetic screening. It is also demonstrated how the mathematics specifies the best parameters to use when designing coils with small numbers of elements. A head gradient coil for use at 3.0 T has been designed using the analytic approach described here. The process of coil design and construction is outlined and the performance of the coil in comparison with a similar standard cylindrical coil is described.

Brain↗
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↗