Andrew G Webb

Leiden University Medical Centre, Leyden, South Holland, Netherlands

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Publications (338)1089.24 Total impact

  • Sebastian Aussenhofer · A.G Webb ·
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    ABSTRACT: In this work a new concept for designing transmit coils for magnetic resonance using a plasma is introduced. Unlike conventional coils, a plasma can be turned on and off, eliminating electrical interactions between coils, and enabling radiofrequency-invisibility when desired. A surfatron has been designed to produce a surface-mode wave which propagates along the inner surface of a commercial fluorescent lighting tube. NMR spectra and images have been produced using the plasma as the transmit coil and a copper-based monopole to receive the signal. The transmit efficiency of the plasma tube was estimated, and is currently much lower than that of an equivalently-sized metal-based structure: however, there are many potential methods for increasing the efficiency using a custom-built plasma tube.
    Journal of Magnetic Resonance 10/2015; 261. DOI:10.1016/j.jmr.2015.09.012 · 2.51 Impact Factor
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    ABSTRACT: Diffusion tensor imaging (DTI) is a popular method to assess differences in fiber organization in diseased and healthy muscle tissue. Previous work has shown that muscle DTI measurements depend on signal-to-noise ratio (SNR), %fat, and tissue T2 . The goal of this study was to evaluate the potential biasing effects of these factors on skeletal muscle DTI data in patients with Duchenne Muscular Dystrophy (DMD). MR images were obtained of the right lower leg of 21 DMD patients and 12 healthy controls on a Philips 3T system. DTI measurements were combined with quantitative in-vivo measures of mean water T2 , %fat and SNR to evaluate their effect on DTI parameter estimation. All outcome measures were determined within ROIs drawn for six lower leg muscles. Between group analysis, using all ROIs, revealed a significantly elevated FA in the GCL, SOL and PER muscles (p<0.05) and an increased mean diffusivity (p<0.05) and λ3 (p<0.05) in the TA muscle of DMD patients. In-vivo evaluation of the individual confounders showed behaviour in line with predictions from previous simulation work. To account for these confounders, subsequent analysis used only ROIs with SNR greater than 20. With this criterion we found significantly greater MD in the TA muscle of DMD patient (p<0.009) and λ3 in the TA and GCL muscles (p<0.001) of DMD patients, but no differences in FA. As both increased %fat and lower SNR are expected to reduce the apparent MD and λ3 , these between-group differences are likely due to pathophysiology. However, the increased FA, observed when using all ROIs, likely reflects the effect of low SNR and %fat on the DTI parameter estimation. These findings suggest that measuring mean water T2 , %fat and SNR is essential to ascribe changes in DTI measures to intrinsic diffusion changes or to confounding influences. Copyright © 2015 John Wiley & Sons, Ltd.
    NMR in Biomedicine 10/2015; 28(11). DOI:10.1002/nbm.3427 · 3.04 Impact Factor
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    ABSTRACT: Objective: The aim of this study was to determine whether high permittivity (HP) pads can be used to increase the signal-to-noise ratio (SNR) of cardiac proton magnetic resonance spectroscopy at 3 T, allowing faster data acquisition. Materials and methods: The institutional review board approved the study protocol, and written informed consent was obtained from all participants. In 22 healthy volunteers, water-suppressed localized spectra were acquired in the interventricular septum without and with HP pads. The SNR and myocardial triglyceride content (MTGC) were measured without and with the HP pads, and the results were compared with a paired sample Student t test. Results: Application of HP pads increased mean (SD) SNR from 27.9 (15.6) to 42.3 (24.4) (P < 0.0001), a mean gain of 60%. The acquisition time can thereby be reduced from just under 5 minutes to just under 2 minutes while maintaining the same SNR. The mean (SD) MTGC was 0.39% (0.17%) without pads and 0.38% (0.15%) with pads (P = 0.83) for the healthy volunteers, showing that no bias is introduced by using the pads. No difference in spectral linewidth was measured (P = 0.80), the values being 17.4 (4.9) Hz without and 17.1 (3.2) Hz with pads. Both transmit and receive maps showed increases in sensitivity due to the presence of the HP pads. Conclusions: High permittivity pads improve cardiac proton magnetic resonance spectroscopy at 3 T by increasing the SNR on average by 60%, which can be used to reduce data acquisition time significantly, allowing fast assessment of MTGC without compromising spectral quality. The SNR increase arises primarily from the increase in receive sensitivity of the phased array, which is more closely coupled to the body via the HP pads. In addition, the transmit efficiency is also increased, allowing shorter or lower power radiofrequency pulses.
    Investigative radiology 10/2015; DOI:10.1097/RLI.0000000000000214 · 4.44 Impact Factor

  • Neuromuscular Disorders 10/2015; 25:S250-S251. DOI:10.1016/j.nmd.2015.06.239 · 2.64 Impact Factor
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    Wyger M Brink · Johan S van den Brink · Andrew G Webb ·
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    ABSTRACT: Background: Dual-channel transmit technology improves the image quality in cardiovascular magnetic resonance (CMR) at 3 T by reducing the degree of radiofrequency (RF) shading over the heart by using RF shimming. Further improvements in image quality have been shown on a dual-transmit system using high permittivity pads. The aim of this study is to investigate the transmit field (B 1 (+)) homogeneity and the specific absorption rate (SAR) using high permittivity pads as a function of the complete range of possible RF-shim settings in order to gauge the efficacy and safety of this approach. Methods: Electromagnetic (EM) simulations were performed in five different body models using a dual-transmit RF coil, with and without high permittivity pads. The RF shimming behaviour in terms of B 1 (+) homogeneity and local SAR were determined as a function of different RF-shim settings. Comparative experimental data were obtained in healthy volunteers (n = 33) on either a standard-bore (60 cm diameter) or wide-bore (70 cm diameter) 3 T CMR system. Results: EM simulations and experimental data showed higher (B 1 (+)) homogeneity and lower SAR for optimized RF-shim settings when using the high permittivity pads. The power distribution between the two channels was also much closer to being equal using the pads. EM simulations showed that for all five body models studied, optimized RF-shim settings corresponded to reduced local SAR using high permittivity pads. However, there are also specific, non-optimal RF-shim settings for which the actual SAR using the pads would be higher (up to ~20 %) than that calculated by the CMR system. Conclusions: The combination of active (dual transmit) and passive (high permittivity pads) RF shimming shows great promise for increasing image quality for cardiac imaging at 3 T. Optimized RF-shim settings result in increased B 1 (+) homogeneity and reduced SAR with the high permittivity pads: however, there are non-optimal cases in which SAR might be underestimated, and these merit further investigation.
    Journal of Cardiovascular Magnetic Resonance 09/2015; 17(1):82. DOI:10.1186/s12968-015-0188-z · 4.56 Impact Factor
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    ABSTRACT: The aim of this study was to implement and evaluate a flexible and time-efficient interleaved imaging approach for the acquisition of proton and sodium images of the human knee at 7 T within a clinically relevant timescale. A flexible software framework was established which allowed the interleaving of multiple, different, fully specific absorption ratio (SAR)-validated scans. The system was able to switch between these different scans at flexible time points. The practical example presented consists of interleaved proton (Dixon imaging and T2 * mapping) and sodium (mapping the sodium content and fluid-suppressed component separately) sequences with the key idea to perform proton MRI whilst the sodium nuclei relax towards thermal equilibrium, and vice versa. Comparisons were made between these four scans being acquired sequentially in the normal mode of scanner operation and those acquired in an interleaved fashion. Images acquired in the interleaved mode were very similar to those acquired in sequential scans with no image artifacts produced by the slight intra-sequence variation in steady-state magnetization. A reduction in scanning time of almost a factor of two was established using the interleaved scans, allowing such a protocol to be completed within 30 min. Phantom experiments and in vivo scans performed in healthy volunteers and in one patient proved the basic feasibility of this approach. This approach for the interleaving of multiple proton and sodium scans, each with different contrasts, is an efficient method for the design of new practical clinical protocols for sodium MRI. Copyright © 2015 John Wiley & Sons, Ltd. Copyright © 2015 John Wiley & Sons, Ltd.
    NMR in Biomedicine 08/2015; 28(10). DOI:10.1002/nbm.3368 · 3.04 Impact Factor
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    ABSTRACT: To study diffusion-prepared neurography optimized for a large field-of-view (FOV) to include the neck and both shoulders. In a large FOV poor homogeneity of the magnetic field (B0 ) often leads to poor image quality and possibly to poor diagnostic accuracy. The aim was therefore to find an optimal (combination of) shimming method(s) for diffusion-prepared neurography in a large FOV. A 3D diffusion-prepared sequence with a large FOV was tested with and without the use of a susceptibility-matched pillow combined with image-based (IB) or standard shimming in six healthy volunteers on a 3T system. B0 , B1 , signal to noise ratio (SNR), and contrast to noise ratio (CNR) were compared between all protocols. Additionally, nerve visibility, fat suppression, artifacts, and overall image quality were ordinally (5-point scale) assessed by two readers. Furthermore, correlations between B0 and B1 (offset and variation) and SNR, CNR, and image quality were explored. The use of the susceptibility-matched pillow led to a 43% reduction of B0 variation over the brachial plexus compared to the situation without a pillow (P < 0.05). The combination of the pillow with IB-shimming and the optimized diffusion-prepared sequence resulted in good nerve visibility, good fat suppression, no artifacts that would hinder clinical diagnosis, and a good overall quality (median scores ≥4). Reducing B0 variation was associated with SNR, CNR, and the above-mentioned scored features (P < 0.05). The use of a susceptibility-matched pillow in combination with IB-shimming enables robust and high-quality neurography of the complete brachial plexus. J. Magn. Reson. Imaging 2015. © 2015 Wiley Periodicals, Inc.
    Journal of Magnetic Resonance Imaging 08/2015; DOI:10.1002/jmri.25025 · 3.21 Impact Factor

  • Ontology & Neurotology 07/2015; 36(8):1. DOI:10.1097/MAO.0000000000000826 · 1.79 Impact Factor
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    ABSTRACT: Whole-brain territory mapping using planning-free vessel-encoded pseudocontinuous arterial-spin-labeling (VE-pCASL) takes approximately 5 min, which is frequently considered too long for standard clinical protocols. In this study, vessel-encoded dynamic-ASL (VE-DASL) is optimized to achieve fast (< 30 s) cerebral flow territory mapping, especially aimed for the acute setting. VE-DASL is based on the creation of a continuous stream of magnetically labeled or unlabeled blood with different encoding patterns for each feeding artery, whose inflow into the brain tissue is monitored continuously. This approach leads to unique signal fluctuation within each flow territory, enabling reconstruction of individual flow territories by means of clustering techniques followed by linear regression. VE-DASL was implemented and validated both as single slice and whole-brain method. In vivo results showed reasonable agreement with the "gold-standard" reference maps obtained from VE-pCASL. The Dice similarity coefficient which represents the fractional overlap between VE-DASL and "gold-standard" VE-pCASL territories ranged from 83.4% to 87.7% for the right internal cerebral artery (RICA), 81.7% to 83.1% for the left internal cerebral artery (LICA) and 64.3% to 71.8% for the vertebral arteries. VE-DASL has the potential to map the main flow territories with whole-brain coverage in a short scan duration (∼30 s). Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc. © 2015 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 06/2015; DOI:10.1002/mrm.25806 · 3.57 Impact Factor
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    ABSTRACT: Fluctuations of the background magnetic field (B0 ) due to body and breathing motion can lead to significant artifacts in brain imaging at ultrahigh field. Corrections based on real-time sensing using external field probes show great potential. This study evaluates different aspects of field interpolation from these probes into the brain which is implicit in such methods. Measurements and simulations were performed to quantify how well B0 -fluctuations in the brain due to body and breathing motion are reflected in external field probe measurements. Field probe measurements were compared with scanner acquired B0 -maps from experiments with breathing and shoulder movements. A realistic simulation of B0 -fluctuations caused by breathing was performed, and used for testing different sets of field probe positions. The B0 -fluctuations were well reflected in the field probe measurements in the shoulder experiments, while the breathing experiments showed only moderate correspondence. The simulations showed the importance of the probe positions, and that performing full 3(rd) order corrections based on 16 field probes is not recommended. Methods for quantitative assessment of the field interpolation problem were developed and demonstrated. Field corrections based on external field measurements show great potential, although potential pitfalls were identified. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc. © 2015 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 06/2015; DOI:10.1002/mrm.25802 · 3.57 Impact Factor
  • Wyger M. Brink · Rob F. Remis · Andrew G. Webb ·
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    ABSTRACT: PurposeIn this study, we analyzed dielectric shimming by formulating it as an electromagnetic scattering problem using integral equations.Methods Three-dimensional simulations of the radiofrequency field in two configurations using different materials were analyzed in terms of induced currents and secondary fields. A two-dimensional integral equation method with different backgrounds was used to identify the underlying physical mechanisms. This framework was then used to develop an inversion method for the design of dielectric pads.ResultsThe effects of a dielectric pad can be attributed to the interference of a secondary field that is produced by the currents induced in the dielectric pad, radiating in an inhomogeneous background. The integral equation method with inhomogeneous background reduces the complexity of the forward and inverse problem significantly and can be used to optimize the permittivity distribution for a desired field. Agreement with experimental maps was obtained in a cylindrical phantom, demonstrating the validity of the method.Conclusions The integral equation method with inhomogeneous background yields an efficient numerical framework for the analysis and inverse design of dielectric shimming materials. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 06/2015; DOI:10.1002/mrm.25783 · 3.57 Impact Factor
  • A G Webb · P F Van de Moortele ·
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    ABSTRACT: In this article we present our projections of future hardware developments on 7 T human MRI systems. These include compact cryogen-light magnets, improved gradient performance, integrated RF-receive and direct current shimming coil arrays, new RF technology with adaptive impedance matching, patient-specific specific absorption rate estimation and monitoring, and increased integration of physiological monitoring systems. Copyright © 2015 John Wiley & Sons, Ltd. Copyright © 2015 John Wiley & Sons, Ltd.
    NMR in Biomedicine 05/2015; DOI:10.1002/nbm.3315 · 3.04 Impact Factor
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    ABSTRACT: Diffusion-weighted MRS (DWS) of brain metabolites enables the study of cell-specific alterations in tissue microstructure by probing the diffusion of intracellular metabolites. In particular, the diffusion properties of neuronal N-acetylaspartate (NAA), typically co-measured with N-acetylaspartyl glutamate (NAAG) (NAA + NAAG = tNAA), have been shown to be sensitive to intraneuronal/axonal damage in pathologies such as stroke and multiple sclerosis. Lacking, so far, are empirical assessments of the reproducibility of DWS measures across time and subjects, as well as a systematic investigation of the optimal acquisition parameters for DWS experiments, both of which are sorely needed for clinical applications of the method. In this study, we acquired comprehensive single-volume DWS datasets of the human corpus callosum at 3T and 7T. We investigated the inter- and intra-subject variability of empirical and modeled diffusion properties of tNAA [Davg (tNAA) and Dmodel (tNAA), respectively]. Subsequently, we used a jackknife-like resampling approach to explore the variance of these properties in partial data subsets reflecting different total scan durations. The coefficients of variation (CV ) and repeatability coefficients (CR ) for Davg (tNAA) and Dmodel (tNAA) were calculated for both 3T and 7T, with overall lower variability in the 7T results. Although this work is limited to the estimation of the diffusion properties in the corpus callosum, we show that a careful choice of diffusion-weighting conditions at both field strengths allows the accurate measurement of tNAA diffusion properties in clinically relevant experimental time. Based on the resampling results, we suggest optimized acquisition schemes of 13-min duration at 3T and 10-min duration at 7T, whilst retaining low variability (CV ≈ 8%) for the tNAA diffusion measures. Power calculations for the estimation of Dmodel (tNAA) and Davg (tNAA) based on the suggested schemes show that less than 21 subjects per group are sufficient for the detection of a 10% effect between two groups in case-control studies. Copyright © 2015 John Wiley & Sons, Ltd. Copyright © 2015 John Wiley & Sons, Ltd.
    NMR in Biomedicine 05/2015; 28(8). DOI:10.1002/nbm.3340 · 3.04 Impact Factor

  • European Journal of Paediatric Neurology; 05/2015
  • Wyger M Brink · Vikas Gulani · Andrew G Webb ·
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    ABSTRACT: This article reviews the principle of dual-channel transmit MRI and highlights current clinical applications which are performed primarily at 3 Tesla. The main benefits of dual-channel transmit compared with single-transmit systems are the increased image contrast homogeneity and the decreased scanning time due to the more accurate local specific absorption ratio estimation, meaning that less conservative safety limits are needed. The dual-transmit approach has been particularly beneficial in body imaging applications, and is also promising in terms of cardiac, spine, and fetal imaging. Future advances in transmit SENSE, the combination of dual-channel transmit with high permittivity pads, as well as the potential increase in the number of transmit channels are also discussed. J. Magn. Reson. Imaging 2015. © 2015 Wiley Periodicals, Inc.
    Journal of Magnetic Resonance Imaging 04/2015; 42(4). DOI:10.1002/jmri.24791 · 3.21 Impact Factor
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    Carson Ingo · Yi Sui · Yufen Chen · Todd B Parrish · Andrew G Webb · Itamar Ronen ·
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    ABSTRACT: In this paper, we provide a context for the modeling approaches that have been developed to describe non-Gaussian diffusion behavior, which is ubiquitous in diffusion weighted magnetic resonance imaging of water in biological tissue. Subsequently, we focus on the formalism of the continuous time random walk theory to extract properties of subdiffusion and superdiffusion through novel simplifications of the Mittag-Leffler function. For the case of time-fractional subdiffusion, we compute the kurtosis for the Mittag-Leffler function, which provides both a connection and physical context to the much-used approach of diffusional kurtosis imaging. We provide Monte Carlo simulations to illustrate the concepts of anomalous diffusion as stochastic processes of the random walk. Finally, we demonstrate the clinical utility of the Mittag-Leffler function as a model to describe tissue microstructure through estimations of subdiffusion and kurtosis with diffusion MRI measurements in the brain of a chronic ischemic stroke patient.
    Frontiers in Physics 04/2015; 3(11). DOI:10.3389/fphy.2015.00011
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    ABSTRACT: The metabolic syndrome (MetS) is characterized by ectopic lipid accumulation. Magnetic resonance (MR) imaging and spectroscopy can quantify ectopic lipid accumulation. Consequences of MetS can be evaluated with MR on a whole-body level. In the liver, several techniques are used to quantify hepatic steatosis and differentiate stages of nonalcoholic fatty liver disease. Cardiac MR can quantify myocardial steatosis and associated complications. In the brain, magnetization transfer imaging and diffusion tensor imaging can detect microstructural brain damage. Various other organs can be assessed with MR. MR is a powerful tool to unravel whole-body MetS pathophysiology, monitor therapeutic efficacy, and establish prognosis. Copyright © 2015 Elsevier Inc. All rights reserved.
    Magnetic Resonance Imaging Clinics of North America 02/2015; 23(1):41-58. DOI:10.1016/j.mric.2014.09.010 · 0.99 Impact Factor
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    ABSTRACT: Purpose Recent studies on ocular shape have raised an increased interest in the peripheral characteristics of the eye, as it potentially triggers changes in the central vision. Current techniques are, however, not capable of accurately measuring the three-dimensional shape of the retina. We describe a new MRI-based method to obtain the retinal shape with high precision and use it to assess if differences in retinal shape could explain previously described trends in peripheral refraction. Methods Twenty-one healthy subjects were examined using high-field ocular MRI. The resulting data were automatically segmented and processed to calculate the retinal topographic map. We validated the method against partial coherence interferometry and assessed the reproducibility for four subjects. Results The retinal topographic maps describe the retinal shape with a sub-pixel reproducibility (-0.01mm; SD=0.15mm). Comparison with partial coherence interferometry showed a mean difference of 0.08mm, 95% confidence interval -0.39-0.55mm, with a standard deviation of 0.23mm. The data give a possible geometric explanation for the previously described trend that myopic eyes have relatively hyperopic refraction in the periphery, with full three-dimensional information. The retinal maps furthermore show small, sub-millimeter, irregularities, that could have an important influence on the subjects' peripheral vision. Conclusions The possibility to quantitatively characterize the full three dimensional retinal shape by MRI offers new ophthalmologic possibilities, such as the quantitative geometric description of staphyloma. It could in addition be used as a validation technique, independent of standard optical methods, to measure the peripheral retinal shape. Copyright © 2015 by Association for Research in Vision and Ophthalmology.
    Investigative Ophthalmology &amp Visual Science 01/2015; 56(2). DOI:10.1167/iovs.14-15161 · 3.40 Impact Factor

  • Physica Medica 12/2014; 30:e55-e56. DOI:10.1016/j.ejmp.2014.07.169 · 2.40 Impact Factor

Publication Stats

8k Citations
1,089.24 Total Impact Points


  • 2009-2015
    • Leiden University Medical Centre
      • Department of Radiology
      Leyden, South Holland, Netherlands
  • 2010-2014
    • Leiden University
      Leyden, South Holland, Netherlands
  • 2013
    • Technische Universiteit Eindhoven
      • Department of Biomedical Engineering
      Eindhoven, North Brabant, Netherlands
  • 1992-2013
    • University of Groningen
      • • Department of Internal Medicine
      • • Department of Cell Biology
      Groningen, Groningen, Netherlands
  • 2005-2010
    • Pennsylvania State University
      • Department of Bioengineering
      University Park, MD, United States
  • 2003-2010
    • Urbana University
      Urbana, Illinois, United States
  • 2006-2009
    • William Penn University
      Worcester, Massachusetts, United States
  • 1991-2009
    • University of Illinois, Urbana-Champaign
      • • Department of Electrical and Computer Engineering
      • • Department of Psychology
      Urbana, Illinois, United States
    • University of Florida
      Gainesville, Florida, United States
  • 2003-2007
    • University of Wuerzburg
      • • Division of Experimental Physics VII
      • • Institute of Physics
      Würzburg, Bavaria, Germany
  • 2000
    • University of Illinois at Chicago
      • Department of Bioengineering
      Chicago, IL, United States