Andrew G Webb

Leiden University, Leyden, South Holland, Netherlands

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Publications (350)1124.37 Total impact

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    ABSTRACT: Purpose: To explore the effects of high permittivity dielectric pads on the transmit and receive characteristics of a 3 Tesla body coil centered at the thighs, and their implications on image uniformity in receive array applications. Theory and methods: Transmit and receive profiles of the body coil with and without dielectric pads were simulated and measured in healthy volunteers. Parallel imaging was performed using sensitivity encoding (SENSE) with and without pads. An intensity correction filter was constructed from the measured receive profile of the body coil. Results: Measured and simulated data show that the dielectric pads improve the transmit homogeneity of the body coil in the thighs, but decrease its receive homogeneity, which propagates into reconstruction algorithms in which the body coil is used as a reference. However, by correcting for the body coil reception profile this effect can be mitigated. Conclusion: Combining high permittivity dielectric pads with an appropriate body coil receive sensitivity filter improves the image uniformity substantially compared with the situation without pads. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
    No preview · Article · Dec 2015 · Magnetic Resonance in Medicine
  • Rita Schmidt · Andrew Webb
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    ABSTRACT: Electrical Properties Tomography (EPT) using MRI is a technique that has been developed to provide a new contrast mechanism for in vivo imaging. Currently the most common method relies on the solution of the homogeneous Helmholtz equation, which has limitations in accurate estimation at tissue interfaces. A new method proposed in this work combines a Maxwell's integral equation representation of the problem, and the use of high permittivity materials (HPM) to control the RF field, in order to reconstruct the electrical properties image. The magnetic field is represented by an integral equation considering each point as a contrast source. This equation can be solved in an inverse method. In this study we use a reference simulation or scout scan of a uniform phantom to provide an initial estimate for the inverse solution, which allows the estimation of the complex permittivity within a single iteration. Incorporating two setups with and without the HPM improves the reconstructed result, especially with respect to the very low electric field in the center of the sample. Electromagnetic simulations of the brain were performed at 3 T to generate the B1+ field maps and reconstruct the electric properties images. The standard deviations of the relative permittivity and conductivity were within 14% and 18%, respectively for a volume consisting of white matter, gray matter and cerebellum.
    No preview · Article · Dec 2015 · Journal of Magnetic Resonance
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    ABSTRACT: The purpose of this article was to compare the sensitivity of proton observed phosphorus editing (POPE) with direct (31) P MRS with Ernst angle excitation for (1) H-(31) P coupled metabolites at 7 T. POPE sequences were developed for detecting phosphocholine (PC), phosphoethanolamine (PE), glycerophosphocholine (GPC), and glycerophosphoethanolamine (GPE) on the (1) H channel, thereby using the enhanced sensitivity of the (1) H nuclei over (31) P detection. Five healthy volunteers were examined with POPE and (31) P-MRS. POPE editing showed a more than doubled sensitivity in an ideal phantom experiment as compared with direct (31) P MRS with Ernst angle excitation. In vivo, despite increased relaxation losses, significant gains in signal-to-noise ratio (SNR) of 30-40% were shown for PE and GPE + PC levels in the human brain. The SNR of GPC was lower in the POPE measurement compared with the (31) P-MRS measurement. Furthermore, selective narrowband editing on the (31) P channel showed the ability to separate the overlapping GPE and PE signals in the (1) H spectrum. POPE can be used for enhanced detection of (1) H-(31) P coupled metabolites in vivo. Copyright © 2015 John Wiley & Sons, Ltd.
    No preview · Article · Nov 2015 · NMR in Biomedicine
  • 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.
    No preview · Article · Oct 2015 · Journal of Magnetic Resonance
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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.
    No preview · Article · Oct 2015 · NMR in Biomedicine
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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.
    No preview · Article · Oct 2015 · Investigative radiology

  • No preview · Article · Oct 2015 · Neuromuscular Disorders
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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.
    Full-text · Article · Sep 2015 · Journal of Cardiovascular Magnetic Resonance
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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.
    No preview · Article · Aug 2015 · NMR in Biomedicine
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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.
    No preview · Article · Aug 2015 · Journal of Magnetic Resonance Imaging

  • No preview · Article · Jul 2015 · Otology & Neurotology
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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.
    No preview · Article · Jun 2015 · Magnetic Resonance in Medicine
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    Full-text · Dataset · Jun 2015
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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.
    No preview · Article · Jun 2015 · Magnetic Resonance in Medicine
  • 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.
    No preview · Article · Jun 2015 · Magnetic Resonance in Medicine
  • 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.
    No preview · Article · May 2015 · NMR in Biomedicine
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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.
    Full-text · Article · May 2015 · NMR in Biomedicine

  • No preview · Conference Paper · May 2015

  • No preview · Conference Paper · May 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.
    No preview · Article · Apr 2015 · Journal of Magnetic Resonance Imaging

Publication Stats

9k Citations
1,124.37 Total Impact Points


  • 2009-2015
    • Leiden University
      Leyden, South Holland, Netherlands
    • Leiden University Medical Centre
      • Department of Radiology
      Leyden, South Holland, Netherlands
    • Curium-LUMC
      Leyden, South Holland, Netherlands
  • 2013
    • Technische Universiteit Eindhoven
      • Department of Biomedical Engineering
      Eindhoven, North Brabant, 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
  • 1992
    • University of Groningen
      • Department of Cell Biology
      Groningen, Groningen, Netherlands