Andrew G Webb

Leiden University Medical Centre, Leyden, South Holland, Netherlands

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Publications (322)1064.92 Total impact

  • [Show abstract] [Hide abstract]
    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 · 3.40 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 · 3.40 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 · 3.40 Impact Factor
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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 · 3.56 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; DOI:10.1002/nbm.3340 · 3.56 Impact Factor
  • European Journal of Paediatric Neurology 05/2015; 19:S65-S66. DOI:10.1016/S1090-3798(15)30216-6 · 1.93 Impact Factor
  • 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; DOI:10.1002/jmri.24791 · 2.79 Impact Factor
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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 · 0.80 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 · 3.66 Impact Factor
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    ABSTRACT: Studies have suggested that, in subjects with subjective cognitive impairment (SCI), Alzheimer's disease (AD)-like changes may occur in the brain. Recently, an in vivo study has indicated the potential of ultra-high-field MRI to visualize amyloid-beta (Aβ)-associated changes in the cortex in patients with AD, manifested by a phase shift on T2 *-weighted MRI scans. The main aim of this study was to investigate whether cortical phase shifts on T2 *-weighted images at 7 T in subjects with SCI can be detected, possibly implicating the deposition of Aβ plaques and associated iron. Cognitive tests and T2 *-weighted scans using a 7-T MRI system were performed in 28 patients with AD, 18 subjects with SCI and 27 healthy controls (HCs). Cortical phase shifts were measured. Univariate general linear modeling and linear regression analysis were used to assess the association between diagnosis and cortical phase shift, and between cortical phase shift and the different neuropsychological tests, adjusted for age and gender. The phase shift (mean, 1.19; range, 1.00-1.35) of the entire cortex in AD was higher than in both SCI (mean, 0.85; range, 0.73-0.99; p < 0.001) and HC (mean, 0.94; range, 0.79-1.10; p < 0.001). No AD-like changes, e.g. increased cortical phase shifts, were found in subjects with SCI compared with HCs. In SCI, a significant association was found between memory function (Wechsler Memory Scale, WMS) and cortical phase shift (β = -0.544, p = 0.007). The major finding of this study is that, in subjects with SCI, an increased cortical phase shift measured at high field is associated with a poorer memory performance, although, as a group, subjects with SCI do not show an increased phase shift compared with HCs. This increased cortical phase shift related to memory performance may contribute to the understanding of SCI as it is still unclear whether SCI is a sign of pre-clinical AD. Copyright © 2014 John Wiley & Sons, Ltd. Copyright © 2014 John Wiley & Sons, Ltd.
    NMR in Biomedicine 12/2014; DOI:10.1002/nbm.3248 · 3.56 Impact Factor
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    ABSTRACT: OBJECT Accurate edge tracing segmentation remains an incompletely solved problem in brain image analysis. The authors propose a novel algorithm using a particle filter to follow the boundary of the brain in the style often used in autonomous air and ground vehicle navigation. Their goals were to create a versatile tool to segment brain and fluid in MRI and CT images of the developing brain, lay the foundation for an intelligent automated edge tracker that is modality independent, and segment normative data from MRI that can be applied to both MRI and CT. METHODS Simulated MRI data sets were used to train and evaluate the particle filter segmentation algorithm. The method was then applied to produce normative growth curves for children and adolescents from 0 to 18 years of age for brain and fluid from MR images from the National Institutes of Health pediatric database and these data were compared to historical results. The authors further adapted this method for use with CT images of pediatric hydrocephalus and compared the results with hand-segmented data. RESULTS Segmentation of simulated MRI data with varied levels of noise (0%-9%) and spatial inhomogeneity (0%-40%) resulted in percent errors ranging from 0.06% to 5.38% for brain volume and 2.45% to 22.3% for fluid volume. The authors used this tool to create normal brain and CSF growth curves from MR images. The calculated growth curves showed excellent consistency with historical data. Additionally, compared with manual segmentation the particle filter accurately segmented brain and fluid volumes from CT scans of 5 pediatric patients with hydrocephalus (p < 0.001). CONCLUSIONS The authors have produced the first normative brain and CSF growth curves for children and adolescents 0-18 years of age. In addition, this study includes the first use of a particle filter as an edge tracker in image segmentation and offers a semiautomatic method to segment both pediatric and adult brain data from MR and CT images. The particle filter has the potential to be further automated toward a clinical rather than research tool with both of these modalities. Because of its modality independence, it has the capability to allow CT to be a more effective diagnostic tool for neurological disorders, a task of substantial importance in emergency settings and in developing countries where CT is often the only available method of brain imaging.
    Journal of Neurosurgery Pediatrics 11/2014; 15(2):1-12. DOI:10.3171/2014.9.PEDS12426 · 1.37 Impact Factor
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    ABSTRACT: OBJECT The incidence of temporal lobe epilepsy (TLE) due to mesial temporal sclerosis (MTS) can be high in developing countries. Current diagnosis of MTS relies on structural MRI, which is generally unavailable in developing world settings. Given widespread effects on temporal lobe structure beyond hippocampal atrophy in TLE, the authors propose that CT volumetric analysis can be used in patient selection to help predict outcomes following resection. METHODS Ten pediatric patients received preoperative CT scans and temporal resections at the CURE Children's Hospital of Uganda. Engel classification of seizure control was determined 12 months postoperatively. Temporal lobe volumes were measured from CT and from normative MR images using the Cavalieri method. Whole brain and fluid volumes were measured using particle filter segmentation. Linear discrimination analysis (LDA) was used to classify seizure outcome by temporal lobe volumes and normalized brain volume. RESULTS Epilepsy patients showed normal to small brain volumes and small temporal lobes bilaterally. A multivariate measure of the volume of each temporal lobe separated patients who were seizure free (Engel Class IA) from those with incomplete seizure control (Engel Class IB/IIB) with LDA (p < 0.01). Temporal lobe volumes also separate normal subjects, patients with Engel Class IA outcomes, and patients with Class IB/IIB outcomes (p < 0.01). Additionally, the authors demonstrated that age-normalized whole brain volume, in combination with temporal lobe volumes, may further improve outcome prediction (p < 0.01). CONCLUSIONS This study shows strong evidence that temporal lobe and brain volume can be predictive of seizure outcome following temporal lobe resection, and that volumetric CT analysis of the temporal lobe may be feasible in lieu of structural MRI when the latter is unavailable. Furthermore, since the authors' methods are modality independent, these findings suggest that temporal lobe and normative brain volumes may further be useful in the selection of patients for temporal lobe resection when structural MRI is available.
    Journal of Neurosurgery Pediatrics 11/2014; 15(2):1-11. DOI:10.3171/2014.9.PEDS12428 · 1.37 Impact Factor
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    ABSTRACT: Becker muscular dystrophy (BMD) is characterized by progressive muscle weakness. Muscles show structural changes (fatty infiltration, fibrosis) and metabolic changes, both of which can be assessed using MRI and MRS. It is unknown at what stage of the disease process metabolic changes arise and how this might vary for different metabolites. In this study we assessed metabolic changes in skeletal muscles of Becker patients, both with and without fatty infiltration, quantified via Dixon MRI and 31P MRS. MRI and 31P MRS scans were obtained from 25 Becker patients and 14 healthy controls using a 7 T MR scanner. Five lower-leg muscles were individually assessed for fat and muscle metabolite levels. In the peroneus, soleus and anterior tibialis muscles with non-increased fat levels, PDE/ATP ratios were higher (P < 0.02) compared with controls, whereas in all muscles with increased fat levels PDE/ATP ratios were higher compared with healthy controls (P ≤ 0.05). The Pi/ATP ratio in the peroneus muscles was higher in muscles with increased fat fractions (P = 0.005), and the PCr/ATP ratio was lower in the anterior tibialis muscles with increased fat fractions (P = 0.005). There were no other significant changes in metabolites, but an increase in tissue pH was found in all muscles of the total group of BMD patients in comparison with healthy controls (P < 0.05). These findings suggest that 31P MRS can be used to detect early changes in individual muscles of BMD patients, which are present before the onset of fatty infiltration. Copyright © 2014 John Wiley & Sons, Ltd.
    NMR in Biomedicine 11/2014; 27(11). DOI:10.1002/nbm.3199 · 3.56 Impact Factor
  • Andrew Webb
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    ABSTRACT: Cavity resonators are widely used in electron paramagnetic resonance, very high field magnetic resonance microimaging and also in high field human imaging. The basic principles and designs of different forms of cavity resonators including rectangular, cylindrical, re-entrant, cavity magnetrons, toroidal cavities and dielectric resonators are reviewed. Applications in EPR and MRI are summarized, and finally the topic of traveling wave MRI using the magnet bore as a waveguide is discussed.
    Progress in Nuclear Magnetic Resonance Spectroscopy 10/2014; DOI:10.1016/j.pnmrs.2014.09.003 · 8.71 Impact Factor
  • 19th International Congress of the World-Muscle-Society; 10/2014
  • 19th International Congress of the World-Muscle-Society; 10/2014
  • Joep Wezel, Bert Jan Kooij, Andrew G Webb
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    ABSTRACT: To determine the MR compatibility of common dental retainer wires at 7 Tesla in terms of potential RF heating and magnetic susceptibility effects. Electromagnetic simulations and experimental results were compared for dental retainer wires placed in tissue-mimicking phantoms. Simulations were then performed for a human model with wire in place. Finally, image quality was assessed for different scanning protocols and wires. Simulations and experimental data in phantoms agreed well, with the length of the wire correlating to maximum heating in phantoms being approximately 47 mm. Even in this case, no substantial heating occurs when scanning within the specific absorption rate (SAR) guidelines for the head. Image distortions from the most ferromagnetic dental wire were not significant for any brain region. Dental retainer wires appear to be MR compatible at 7 Tesla. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 10/2014; 72(4). DOI:10.1002/mrm.25019 · 3.40 Impact Factor
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    ABSTRACT: Background Anesthetics are commonly applied in animal studies of gastrointestinal (GI) function. Different anesthetics alter smooth-muscle motility in different ways. The aim of this study is to quantify and compare non-invasively with magnetic resonance imaging (MRI) the motility patterns of the rat gut when anesthetized with inactin vs isoflurane anesthetics in the fed state.Methods Rats were given an oral gavage of MRI contrast agent for improved visualization of the GI tract. Two-dimensional images through the jejunum of the pre- and postanesthetized rat in the fed state were acquired every 168 ms. Image registration, segmentation, and postprocessing algorithms were applied to produce spatio-temporal maps that were used to quantify peristaltic and segmental motions in the jejunum region interspersed between periods of inactivity.Key ResultsThere were significantly longer periods of inactivity in the rats treated with isoflurane than in those treated with inactin (179.9 ± 22.4 s vs 17.7 ± 10.3 s). The speed of propagation and wavelength of peristalsis, and the frequency and speed of pattern switching of segmental motility, were higher (p < 0.05) in rats treated with inactin.Conclusions & InferencesIsoflurane and inactin anesthetics produce significantly different motility behavior with the rat's GI tract in the fed state. Isoflurane anesthetic, results in a reduced frequency of occurrence of motility periods and an overall reduced level of motility in comparison with inactin.
    Neurogastroenterology and Motility 10/2014; 26(10). DOI:10.1111/nmo.12410 · 3.42 Impact Factor

Publication Stats

7k Citations
1,064.92 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
    • University of Groningen
      • Department of Internal Medicine
      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 Civil and Environmental Engineering
      • • Department of Psychology
      Urbana, Illinois, United States
    • University of Florida
      • Department of Radiology
      Gainesville, Florida, United States
  • 2005–2007
    • University of Wuerzburg
      • Division of Experimental Physics VII
      Würzburg, Bavaria, Germany
  • 2000
    • University of Illinois at Chicago
      • Department of Bioengineering
      Chicago, IL, United States