R Coxon

University of Nottingham, Nottingham, ENG, United Kingdom

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Publications (40)215.91 Total impact

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    ABSTRACT: The new principles of active acoustic control in gradient coil design recently introduced by Mansfield and Haywood (MAGMA 1999;8(Suppl 1):55) are further developed theoretically for the far-field acoustic output for a single sector of a coil system comprising four or more flat rectangular coil sectors. Each sector consists of a split plate arrangement in which are embedded two windings, an outer primary winding and a narrow inner re-entrant loop control winding immediately adjacent to and surrounding the split or air gap. The wire spacing of the control winding is made small so as not to affect substantially the magnetic field created by the primary winding. Experimental results are presented for two sectors each made of a different readily available plastic material and tested over a range of frequencies. They both show substantial average reductions in acoustic output over the full output when the control winding is appropriately driven. New theoretical expressions are derived for particular frequencies based on normal mode expansions for the plate. This new approach is better able to explain the acoustic output difference between the full and reduced output modes. Empirical expressions are also developed which include longitudinal as well as transverse plate characteristics and used to fit the experimental acoustic output data as a function of frequency and indicate good agreement with regard to both the form and amplitude of the acoustic output response.
    Magnetic Resonance in Medicine 11/2001; 46(4):807-18. · 3.27 Impact Factor
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    ABSTRACT: To overcome the problems of gradient strength, large receiver bandwidth and computing time in nuclear magnetic resonance real-time echo-planar imaging (EPI) with large pixel arrays, the concept of zonal image magnification or zoomed EPI (ZEPI) is introduced. The image zone is defined by two selective RF pulses and the receiver bandwidth. For a fixed image array size it is shown that zooming requires less gradient strength to achieve a given spatial resolution than in an equivalent unzoomed image. Two different zoom techniques are described, ZEPI-1 and ZEPI-2, which employ a single and a double pulse sequence respectively. Experimental results obtained with these sequences on a phantom and a live piglet are presented.
    Journal of Physics E Scientific Instruments 11/2000; 21(3):275.
  • P Mansfield, P R Harvey, R J Coxon
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    ABSTRACT: A multi-mode resonant gradient circuit is described for use in ultra high speed imaging techniques like echo-planar and echo-volumar imaging. The circuit behaves overall in a series resonant manner, but at a fixed number of discrete frequencies. By choosing the number of resonant modes, the circuit can be used to generate approximations to a square wave or a trapezoidal waveform. It it shown that trapezoidal waveforms of a given rise time may be described by a more convergent series expansion involving fewer harmonic components. Because the circuit exploits energy conserving principles, much faster current rise times can be achieved with a given driver amplifier and gradient coil than can be achieved by direct coil drive. Expressions are given for the choice of passive circuit components required to accommodate all odd harmonics up to seventh order.
    Measurement Science and Technology 12/1998; 2(11):1051. · 1.44 Impact Factor
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    ABSTRACT: 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.
    Annals of Neurology 11/1997; 42(4):632-7. · 11.19 Impact Factor
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    ABSTRACT: 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.
    Magnetic Resonance in Medicine 03/1995; 33(2):276-81. · 3.27 Impact Factor
  • P Mansfield, R Coxon, J Hykin
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    ABSTRACT: Our goal was to present the first echo-volumar brain images obtained at 3.0 T, together with the first functional imaging results using echo volumar imaging. The results presented were obtained on volunteers using an in-house designed and constructed 3.0 T echo-planar/volumar imager. The results demonstrate the feasibility of obtaining snapshot volumar images comprising up to 64 x 64 x 8 voxels corresponding to a spatial resolution of 3.0 x 3.0 x 2.5 mm3. Results are also presented showing local cortical changes in signal in response to an external visual stimulus for both the left and the right brain hemispheres. The snapshot acquisition of a whole-volume data set has a number of advantages when considering motional effects or functional image changes that may involve time delays or phase effects within different cortical regions. Instantaneous acquisition of the whole data set means accurate phase information may be straightforwardly obtained.
    Journal of Computer Assisted Tomography 01/1995; 19(6):847-52. · 1.58 Impact Factor
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    ABSTRACT: Echo-planar imaging (EPI) is a snapshot technique, which is useful in a wide range of clinical applications, including the study of physiological function. Over recent years, EPI has found a major new use in functional imaging of the brain. Many EPI experiments can benefit from the increased signal-to-noise ratio (S/N) which results from imaging at high magnetic field. Recently, we have built a 3.0-T EPI scanner at Nottingham University. The low-level radiofrequency and control electronics have been constructed in-house. This, coupled with software written specifically for the system, results in a performance and flexibility exceeding that of a commercial system. A quiet head gradient set produces gradients of up to 30 mT m–1. It is driven using a series multiresonant filter circuit, which allows the production of high-strength, trapezoidal- or sinusoidal-switched gradients.Using this scanner it has been possible to obtain images comprising 256256 pixels, with a 2.5-mm slice and 0.75 mm in-plane resolution, in 140 ms. Multislicing allows a volume set of 16,128128 images to be obtained in 1.6 s. A comparison of tests performed at fields of 0.5 T and 3.0 T on the same phantom indicates a better than linear increase in S/N with field strength. EPI images obtained at 3.0 T have been used in studies of brain activation during visual stimulation and execution of a simple motor task.
    MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 09/1994; 2(3):241-245. · 1.86 Impact Factor
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    ABSTRACT: We have used natural abundance13C magnetic resonance spectroscopy (MRS) to measure glycogen content of muscle and liver before and after heavy exercise, and after consumption of different carbohydrate-based drinks. After an overnight fast, five healthy men (meanSEM age 231 years) exercised to exhaustion at 75% of VO2max on two occasions (mean work rate 1658 W for 7814 min) and then, in a single blind random order, consumed either of two drinks containing the same carbohydrate load (177 g). Spectra were recorded over Vastus Lateralis muscle and the liver before and after exercise, and hourly for 5 h after the carbohydrate load. In muscle, glycogen content after exercise was 37% and 31% of basal (preexercise) concentration before consuming the drinks. After carbohydrate loading, glycogen concentration had increased significantly (p
    MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 09/1994; 2(3):429-432. · 1.86 Impact Factor
  • Proceedings of The Nutrition Society 08/1994; 53(2):335-43. · 3.67 Impact Factor
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    ABSTRACT: Functional magnetic resonance imaging (fMRI) is becoming an important tool in the mapping of brain activation. However there are two main concerns that need to be answered before functional imaging can be considered truly useful as a neurophysiological tool. The first is that the detected activation may be derived from large veins and, thus, be spatially separate from the underlying brain activity. The second is the incomplete understanding of the brain transfer function and its relation to brain activity, blood flow, and metabolism. This work contains initial results that will help address these points. Models of the brain vasculature predict that signal changes on SE (spin-echo) images are expected to be much smaller in magnitude but very accurate in localizing true areas of activation than on GE (gradient-echo) images which are susceptable to large veins. By comparing activation from SE and GE EPI at 3 T, we have shown that the regions of activation are spatially very similar, suggesting that GE activation is closely linked to the underlying brain activity. We have identified an experimental impulse response of the brain following 8-s visual stimulation. This impulse response can be used to successfully predict the frequency response obtained experimentally and its shape suggests a resonance phenomenon. This suggests the brain transfer function can be modeled from linear response theory corresponding to the inherent feedback control mechanisms of the brain homeostasis. Continuation of this early work will help to identify the links between fMRI signal change and underlying brain physiology.
    MAGMA Magnetic Resonance Materials in Physics Biology and Medicine 01/1994; 2(3):347-349. · 1.86 Impact Factor
  • P Mansfield, R Coxon, P Glover
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    ABSTRACT: To present the first echo-planar brain images of diagnostic quality obtained at 3.0 T and to point out some of the problems experienced in performing it. The results presented were obtained on volunteers using an in-house designed and constructed 3.0 T EPI imager. The results demonstrate the feasibility of obtaining snapshot imaging comprising up to 256 x 256 pixels and corresponding to a spatial resolution of 0.75 x 0.75 mm2 with a slice thickness of 2.5 mm. Potentially augmented diagnostic information can be obtained with high field EPI of the brain. Some susceptibility artifact is apparent at the bone-air interfaces as expected at 3.0 T.
    Journal of Computer Assisted Tomography 01/1994; 18(3):339-43. · 1.58 Impact Factor
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    ABSTRACT: Upper gastrointestinal motility and transit has been studied in five human volunteers with the ultra high speed, magnetic resonance imaging (MRI) technique MBEST (Modulus Blipped Echo-planar Single pulse Technique), a variant of echo planar imaging. Snapshot images requiring a data acquisition time of only 64-128 msec allowed visualisation of peristalsis in the antrum and duodenum in real time, without motional image degradation, as would normally be seen using conventional MRI. Gastroduodenal flow of the luminal contents was visualised using water as a contrast medium, with appropriate adjustment of the time constant (T2) weighting of the system. Rapid (0.3 Hz) imaging of gastroduodenal motility in both transverse and coronal planes was achieved by respiratory gating to the imaging frequency, allowing repetition rates of up to 20 frames per minute for 2-3 hours. Fast replay of stored images, to produce a 'movie loop', allowed identification of fasting motility patterns of the gastric antrum and proximal small intestine, with depiction of the phases of the migrating motor complex. Images of the fed patterns after the ingestion of a test meal showed good separation of solid and liquid particulate matter and mixing waves in the gastric body. The potential for quantitative measurements with this new imaging modality of the gastrointestinal tract is under development, which will allow us to measure transit and correlate this with motility data.
    Gut 07/1993; 34(6):848-52. · 10.73 Impact Factor
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    ABSTRACT: Using echo-planar (EP) magnetic resonance imaging (MRI), cerebrospinal fluid (CSF) flow patterns have been demonstrated in the normal subject and patients with pathological conditions including communicating hydrocephalus, aqueduct stenosis and syringohydromyelia. Snap-shot imaging times of 128 ms allow detailed demonstration of transient intraventricular CSF flow patterns, which is not possible with conventional MRI. The potential of EPI as a method for qualitative and quantitative assessment of CSF dynamics is illustrated.
    British Journal of Radiology 03/1991; 64(758):89-97. · 1.22 Impact Factor
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    ABSTRACT: Ultra-high-speed echo-planar imaging (EPI) allows acquisition of a complete two-dimensional image in 64 to 128 ms devoid of movement artefact and without sacrifice of contrast due to relaxation time effects. In conventional whole-body MRI, however, obtrusive movement artefact and extended imaging time, resulting from the need to apply multiple sequences to facilitate lesion detection and pathological characterization, remain limitations. Reduced total examination time increases patient tolerance and throughput; furthermore optimization of contrast to achieve maximal conspicuity of particular features in liver or brain pathology is achieved simply and interactively by real time adjustment of the imaging parameters. The method provides the opportunity to study in real time dynamic events such as flow phenomena in the vascular and cerebrospinal fluid compartments of the brain as well as the kinetics of administered contrast agents. EPI is the only means of capturing the irregular motion of aperiodic cardiac events and bowel peristalsis.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 12/1990; 333(1632):507-514. · 2.89 Impact Factor
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    ABSTRACT: The snap-shot capability of the echo-planar imaging technique is used to freeze motion effectively in human fetal studies in utero. These first results obtained at 0.5 T demonstrate diagnostic quality images without the need for averaging. Although averaging improves the image signal to noise ratio, it is shown that significant image blurring is produced even when only eight separate images are averaged over a period of a few seconds. Results are presented showing anatomical detail of the internal organs of the fetus. Some pathology is also demonstrated. These results were obtained using the modulus blipped echo-planar single-pulse technique (MBEST). Running at 10 frames/second, the modulus version of the fast low-angle excitation echo-planar technique (FLEET) is used to produce ungated fetal cardiac movies.
    British Journal of Radiology 12/1990; 63(755):833-41. · 1.22 Impact Factor
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    ABSTRACT: The ultrafast echo-planar magnetic resonance imaging technology, developed and built in Nottingham, has been used to produce the first snapshot images of the human fetus in utero. The imager, operating at a proton resonance frequency of 22 MHz, produces transaxial views in 64 or 128 milliseconds. These images comprise either 64 x 128 or 128 x 128 pixels with an in-plane resolution of 3 x 3 mm2. The slice thickness is 10 mm. Fetal scans of up to 32 contiguous slices are produced in a few minutes. These have been used to study the internal structure of the uterus and the fetus in a range of cases with gestations ranging from 26 weeks to term. Echo-planar imaging seems particularly suitable as an imaging modality since its high speed obviates image blurring arising from fetal motion.
    American Journal of Obstetrics and Gynecology 09/1990; 163(2):601-7. · 3.88 Impact Factor
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    ABSTRACT: Echo planar imaging has enabled us to image safely and without sedation the thorax of an infant with pectus excavatum deformity. The heart was displaced into the left side of the thorax, and the right lung was calculated to be 1.6 times larger than the left lung.
    European Journal of Pediatrics 08/1990; 149(10):698-9. · 1.98 Impact Factor
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    ABSTRACT: Echo-planar imaging (EPI) is a magnetic resonance imaging (MRI) technique which provides MR images in, typically, 50-100 ms. The potential of EPI as an imaging modality for the liver has been investigated in volunteers and patients with liver disease. Images with improved quality are presented. Obtained at a field strength of 0.52 Tesla, these true unaveraged snap-shot images have larger data arrays, comprising 128 X 128 pixels.
    British Journal of Radiology 07/1990; 63(750):430-7. · 1.22 Impact Factor
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    ABSTRACT: Echo-planar imaging is used in combination with a spin preparation phase to produce a T1-weighted image. The small additional time penalty in this procedure does not detract significantly from the ultrahigh-speed imaging capability of EPI, allowing a rapid real-time optimization of tissue parameters in the image display. Results obtained on a head demonstrate this technique.
    Magnetic Resonance in Medicine 04/1990; 13(3):514-7. · 3.27 Impact Factor
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    ABSTRACT: We have demonstrated that echo-planar imaging can be used to produce high-quality snapshot images of the human fetus in utero in a fraction of a second. These images are free of motional artifact or blurring and allow detailed depiction of fetal structures in normal and pathological pregnancies.
    Magnetic Resonance in Medicine 03/1990; 13(2):314-8. · 3.27 Impact Factor