Andrew G Webb

Leiden University Medical Centre, Leyden, South Holland, Netherlands

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Publications (304)990.14 Total impact

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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.
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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
  • 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: Many MR scans need to be synchronised with external events such as the cardiac or respiratory cycles. For common physiological functions commercial trigger equipment exists, but for more experimental inputs these are not available. This paper describes the design of a multi-purpose open-source trigger platform for MR systems. The heart of the system is an open-source Arduino Due microcontroller. This microcontroller samples an analogue input and digitally processes these data to determine the trigger. The output of the microcontroller is programmed to mimic a physiological signal which is fed into the electrocardiogram (ECG) or pulse oximeter port of MR scanner. The microcontroller is connected to a Bluetooth dongle that allows wireless monitoring and control outside the scanner room. This device can be programmed to generate a trigger based on various types of input. As one example, this paper describes how it can be used as an acoustic cardiac triggering unit. For this, a plastic stethoscope is connected to a microphone which is used as an input for the system. This test setup was used to acquire retrospectively-triggered cardiac scans in ten volunteers. Analysis showed that this platform produces a reliable trigger (>99% triggers are correct) with a small average 8ms variation between the exact trigger points.
    Journal of Magnetic Resonance 10/2014; 247. DOI:10.1016/j.jmr.2014.08.009 · 2.32 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 · 2.94 Impact Factor
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    ABSTRACT: Objective Duchenne muscular dystrophy (DMD) is characterized by progressive muscle weakness caused by DMD gene mutations leading to absence of the full-length dystrophin protein in muscle. Multiple dystrophin isoforms are expressed in brain, but little is known about their function. DMD is associated with specific learning and behavioural disabilities which are more prominent in patients with mutations in the distal part of the DMD gene, predicted to affect expression of shorter protein isoforms. We used quantitative MRI to study brain microstructure in DMD.MethodsT1-weighted and diffusion tensor images (DTI) were obtained on a 3 tesla MR scanner from 30 patients and 22 age-matched controls (ages 8-18 years). All subjects underwent neuropsychological examination. Group comparisons on tissue volume and DTI parameters were made between DMD and controls, and between two DMD subgroups that were classified according to predicted Dp140 isoform expression (DMD_Dp140+ and DMD_Dp140-).ResultsDMD patients had smaller total brain volume, smaller grey matter volume, lower white matter fractional anisotropy, and higher white matter mean and radial diffusivity than healthy controls. DMD patients also performed worse on neuropsychological examination. Subgroup analyses showed that DMD_Dp140- contributed most to the grey matter volume differences and performed worse on information processing.InterpretationBoth grey and white matter is affected in boys with DMD at a whole-brain level. Differences between subgroup DMD_Dp140- and controls indicate an important role for the Dp140 dystrophin isoform in cerebral development. ANN NEUROL 2014. © 2014 American Neurological Association
    Annals of Neurology 09/2014; 76(3). DOI:10.1002/ana.24222 · 11.91 Impact Factor
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    ABSTRACT: In many centers, MR imaging of the inner ear and auditory pathway performed on 1.5T or 3T systems is part of the preoperative work-up of cochlear implants. We investigated the applicability of clinical inner ear MR imaging at 7T and compared the visibility of inner ear structures and nerves within the internal auditory canal with images acquired at 3T.
    American Journal of Neuroradiology 08/2014; 36(2). DOI:10.3174/ajnr.A4084 · 3.68 Impact Factor
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    ABSTRACT: PurposeHigh field T2*-weighted MR images of the cerebral cortex are increasingly used to study tissue susceptibility changes related to aging or pathologies. This paper presents a novel automated method for the computation of quantitative cortical measures and group-wise comparison using 7 Tesla T2*-weighted magnitude and phase images.Methods The cerebral cortex was segmented using a combination of T2*-weighted magnitude and phase information and subsequently was parcellated based on an anatomical atlas. Local gray matter (GM)/white matter (WM) contrast and cortical profiles, which depict the magnitude or phase variation across the cortex, were computed from the magnitude and phase images in each parcellated region and further used for group-wise comparison. Differences in local GM/WM contrast were assessed using linear regression analysis. Regional cortical profiles were compared both globally and locally using permutation testing. The method was applied to compare a group of 10 young volunteers with a group of 15 older subjects.ResultsUsing local GM/WM contrast, significant differences were revealed in at least 13 of 17 studied regions. Highly significant differences between cortical profiles were shown in all regions.Conclusion The proposed method can be a useful tool for studying cortical changes in normal aging and potentially in neurodegenerative diseases. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 08/2014; DOI:10.1002/mrm.25397 · 3.40 Impact Factor
  • Matthias J. P. van Osch, Andrew G. Webb
    08/2014; 2(8). DOI:10.1007/s40134-014-0061-0
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    ABSTRACT: The aim of this study is to explore regional iron-related differences in the cerebral cortex, indicative of Alzheimer's disease pathology, between early- and late-onset Alzheimer's disease (EOAD, LOAD, respectively) patients using 7T magnetic resonance phase images. High-resolution T2(∗)-weighted scans were acquired in 12 EOAD patients and 17 LOAD patients with mild to moderate disease and 27 healthy elderly control subjects. Lobar peak-to-peak phase shifts and regional mean phase contrasts were computed. An increased peak-to-peak phase shift was found for all lobar regions in EOAD patients compared with LOAD patients (p < 0.05). Regional mean phase contrast in EOAD patients was higher than in LOAD patients in the superior medial and middle frontal gyrus, anterior and middle cingulate gyrus, postcentral gyrus, superior and inferior parietal gyrus, and precuneus (p ≤ 0.042). These data suggest that EOAD patients have an increased iron accumulation, possibly related to an increased amyloid deposition, in specific cortical regions as compared with LOAD patients.
  • Alzheimer's and Dementia 07/2014; 10(4):P132-P133. DOI:10.1016/j.jalz.2014.04.065 · 17.47 Impact Factor
  • Alzheimer's and Dementia 07/2014; 10(4):P402. DOI:10.1016/j.jalz.2014.05.498 · 17.47 Impact Factor
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    ABSTRACT: PurposeDiffusion-weighted chemical shift imaging (DW-CSI) of brain metabolites poses significant challenges associated with the acquisition of spectroscopic data in the presence of strong diffusion weighting gradients. We present a reproducible DW-CSI acquisition and processing scheme that addresses most of the potential sources of instability and provides reproducible and anatomically meaningful diffusion-weighted and apparent diffusion coefficient (ADC) metabolite maps.MethodsA real-time navigator-based acquisition scheme was used, allowing instantaneous reacquisition of corrupted k-space data and postprocessing correction of gradient-induced phase fluctuations. Eddy current correction based on residual water resonance was implemented and improved the quality of the data significantly.ResultsHighly reproducible diffusion-weighted metabolite maps of three highest concentration brain metabolites are shown. The navigator-based accept/reject strategy and the postacquisition corrections improved the stability of the DW-CSI signal and the reproducibility of the resulting DW-CSI maps significantly. The metabolite ADC values could be related to the underlying tissue cellular composition.Conclusion Robust investigation of DW-CSI of brain metabolites is feasible and may provide information complementary to that obtained from more sensitive but less specific methods such as diffusion tensor imaging. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 07/2014; DOI:10.1002/mrm.25346 · 3.40 Impact Factor
  • A. G. Webb
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    ABSTRACT: The conventional set-up for MR-monitored focused ultrasound surgery includes a piezoelectric transducer and an acoustic-coupling water bath integrated into the MR patient table; a large surface RF coil is placed close to the patient or, alternatively, the body coil is used as the MR receiver. Potential disadvantages of this approach are that the body coil has low sensitivity because of its low filling factor and the local RF coil can interfere with and cause reflections of the ultrasound irradiation. In this article, a completely new approach is presented, in which an MR transmit/receive coil is not needed at all. Instead, the dimensions of the water bath are adjusted so that a high-order dielectric mode is excited, resulting in efficient MR excitation and reception at the transducer focal point. An example of monitoring ultrasound-mediated heating in a phantom is shown on a 7-T human system, although the new method can also be applied at lower fields. Copyright © 2014 John Wiley & Sons, Ltd.
    NMR in Biomedicine 06/2014; 27(6). DOI:10.1002/nbm.3120 · 3.56 Impact Factor

Publication Stats

7k Citations
990.14 Total Impact Points


  • 2009–2015
    • Leiden University Medical Centre
      • Department of Radiology
      Leyden, South Holland, Netherlands
  • 2011–2014
    • Leiden University
      Leyden, South Holland, Netherlands
  • 2013
    • Technische Universiteit Eindhoven
      • Department of Biomedical Engineering
      Eindhoven, North Brabant, Netherlands
    • Universitair Medisch Centrum Groningen
      • Department of Internal Medicine
      Groningen, Groningen, Netherlands
  • 2012
    • Johns Hopkins University
      • Department of Biological Chemistry
      Baltimore, Maryland, United States
  • 2005–2010
    • Pennsylvania State University
      • Department of Bioengineering
      University Park, MD, United States
  • 2003–2010
    • Urbana University
      Urbana, Illinois, United States
  • 1992–2009
    • University of Illinois, Urbana-Champaign
      • • Department of Electrical and Computer Engineering
      • • Department of Civil and Environmental Engineering
      • • Department of Psychology
      Urbana, Illinois, United States
  • 2006–2007
    • William Penn University
      University Park, 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
  • 1991
    • University of Florida
      Gainesville, Florida, United States