Andrew Webb
Leiden University, Radiology, Faculty Member
Direct spectroscopic quantification of small molecules using low cost, low field (< 0.1 T) large bore portable magnets is not possible using conventional techniques due the presence of strong homonuclear coupling which results in... more
Direct spectroscopic quantification of small molecules using low cost, low field (< 0.1 T) large bore portable magnets is not possible using conventional techniques due the presence of strong homonuclear coupling which results in complicated spectral patterns with resonances separated by much less than the achievable spectral linewidth. In contrast, a method using the signals from a Carr–Purcell–Meiboom–Gill (CPMG) train, in which the data are Fourier transformed in this indirect dimension, can produce so-called J-spectra in which several distinct spectral features can be distinguished. In this work, we evaluate this technique to quantify the amount of alcohol (ethanol) in intact bottles of wines or spirits.
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Quantitative MRI and MRS of muscle are increasingly being used to measure individual pathophysiological processes in Becker muscular dystrophy (BMD). In particular, muscle fat fraction was shown to be highly associated with functional... more
Quantitative MRI and MRS of muscle are increasingly being used to measure individual pathophysiological processes in Becker muscular dystrophy (BMD). In particular, muscle fat fraction was shown to be highly associated with functional tests in BMD. However, the muscle strength per unit of contractile cross‐sectional area is lower in patients with BMD compared with healthy controls. This suggests that the quality of the non‐fat‐replaced (NFR) muscle tissue is lower than in healthy controls. Consequently, a measure that reflects changes in muscle tissue itself is needed. Here, we explore the potential of water T2 relaxation times, diffusion parameters and phosphorus metabolic indices as early disease markers in patients with BMD. For this purpose, we examined these measures in fat‐replaced (FR) and NFR lower leg muscles in patients with BMD and compared these values with those in healthy controls. Quantitative proton MRI (three‐point Dixon, multi‐spin‐echo and diffusion‐weighted spin‐...
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To demonstrate a simple head-sized phantom for realistic static and RF field characterization in high field systems. The head-sized phantom was composed of an ellipsoidal compartment and a spherical cavity to mimic the nasal cavity. The... more
To demonstrate a simple head-sized phantom for realistic static and RF field characterization in high field systems. The head-sized phantom was composed of an ellipsoidal compartment and a spherical cavity to mimic the nasal cavity. The phantom was filled with an aqueous solution of polyvinylpyrrolidone (PVP), to mimic the average dielectric properties of brain tissue. The static and RF field distributions were characterized on a 7T MRI system and compared to in vivo measurements and simulations. MR thermometry was performed, and the results were compared to thermal simulations for RF validation purposes. Accurate reproduction of both static and RF fields patterns observed in vivo was confirmed experimentally and was shown to be strongly affected by the inclusion of the spherical cavity. MR thermometry and transmit efficiency ( B1+) measurements were obtained in close agreement with simulations (peak values agreeing within 0.3 °C and 0.02 μT/√W) as well as fiber optic thermal probes...
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Three‐dimensional phase‐encoded magnetic resonance microscopy is the most promising method for obtaining images with isotropic spatial resolutions on the order of a few micrometers. The attainable spatial resolution is limited by the... more
Three‐dimensional phase‐encoded magnetic resonance microscopy is the most promising method for obtaining images with isotropic spatial resolutions on the order of a few micrometers. The attainable spatial resolution is limited by the available gradient strength (Gmax) and the molecular self‐diffusion coefficient (D) of the sample. In this study, numerical simulations in the microscopic‐size regime are presented in order to show that for given values of Gmax and D, there exists an optimum number of phase‐encoding steps that maximize the spatial resolution in terms of minimizing the full‐width at half‐maximum (FWHM) of the image point spread function (PSF). Unlike the case of “macroscopic” imaging, in which diffusion plays an insignificant role in determining spatial resolution, acquiring data beyond this optimal value actually degrades the image PSF. An alternative version of phase encoding, using a variable phase‐encoding time rather than a variable gradient strength, is analyzed in...
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Research Interests:
Cortical hyperexcitability due to enhanced glutamatergic activity has been implicated in migraine pathophysiology but direct evidence is lacking. Here we assessed glutamate levels and intracellular mobility of glutamate in the visual... more
Cortical hyperexcitability due to enhanced glutamatergic activity has been implicated in migraine pathophysiology but direct evidence is lacking. Here we assessed glutamate levels and intracellular mobility of glutamate in the visual cortex of migraineurs in-between attacks. We included 50 migraineurs (23 with aura and 27 without aura) and 24 age- and gender-matched non-headache controls. We used proton magnetic resonance spectroscopy (1H-MRS) and diffusion weighted spectroscopy at 7 T with a single volume of interest (2 × 2 × 3 cm) located in the primary and secondary visual cortex. For 1H-MRS we used a semi-LASER sequence with water referencing for absolute quantification. For diffusion weighted spectroscopy we used an adapted PRESS sequence with gradients applied in three directions and two different gradient amplitudes. Between-group differences were evaluated using analysis of covariance with the grey matter fraction in the volume of interest as covariate and post hoc compariso...
Research Interests: Chemistry, Magnetic Resonance Spectroscopy, Medicine, Glutamate, Brain, and 15 moreHumans, Female, Migraine, Journal Article, Male, Aura, Adult, Glutamic Acid, Cortical Spreading Depression, Case Control Studies, Glutamate Receptor, proton magnetic resonance spectroscopy, Diffusion magnetic resonance imaging, Medical and Health Sciences, and Migraine disorders
Dielectric resonator antenna (DRAs) are compact structures that exhibit low coupling between adjacent elements and therefore can be used as MRI transmit arrays. In this study, we use very high permittivity materials to construct modular... more
Dielectric resonator antenna (DRAs) are compact structures that exhibit low coupling between adjacent elements and therefore can be used as MRI transmit arrays. In this study, we use very high permittivity materials to construct modular flexible transceive arrays of a variable numbers of elements for operation at 7T. DRAs were constructed using rectangular blocks of ceramic (lead zirconate titanate, εr = 1070) with the transverse electric (TE)01 mode tuned to 298 MHz. Finite-difference time-domain simulations were used to determine the B1 and specific absorption rate distributions. B1+ maps were acquired in a phantom to validate the simulations. Performance was compared to an equally sized surface coil. In vivo images were acquired of the wrist (four elements), ankle (seven elements), and calf muscle (16 elements). Coupling between DRAs spaced 5 mm apart on a phantom was -18.2 dB compared to -9.1 dB for equivalently spaced surface coils. DRAs showed a higher B1+ intensity close to ...
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Research Interests: Engineering, Magnetic Resonance Imaging, Nuclear medicine, Medicine, Brain Tumor, and 15 moreHumans, Male, Physical sciences, Perfusion, Aged, Middle Aged, Image Enhancement, Adult, Blood Flow Velocity, Contrast Media, Brain Neoplasms, Magnetic resonance angiography, Cerebral Blood Volume, Arterioles, and Medical and Health Sciences
... The authors thank Dr. Stefanie Glaser at the Biozentrum, Universität Würz-burg, for providing the oocytes, and Dr. Mark Gris-wold for elements of the Biot-Savart programme. REFERENCES ... 10. Wang H, Ciobanu L, Edison AS, Webb AG.... more
... The authors thank Dr. Stefanie Glaser at the Biozentrum, Universität Würz-burg, for providing the oocytes, and Dr. Mark Gris-wold for elements of the Biot-Savart programme. REFERENCES ... 10. Wang H, Ciobanu L, Edison AS, Webb AG. 2004. ...
Research Interests: Materials Science, Biomedical Engineering, Nuclear Magnetic Resonance, Magnetic field, Electromagnetic Shielding, and 11 moreData acquisition, Parallel Imaging, Single Cell, Solenoid Valve, Xenopus laevis, Three Dimensional, Magnet, Spatial resolution, Small samples, Static Magnetic Field, and Electromagnetic coil
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... more
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 ...
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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... more
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.
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Research Interests:
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... more
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. 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. There 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...
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Plant oil has become an important component in the search for a replacement for non-renewable energy sources, as well as being used for a wide range of industrial purposes, all in addition to its vital importance for human diet. Detailed... more
Plant oil has become an important component in the search for a replacement for non-renewable energy sources, as well as being used for a wide range of industrial purposes, all in addition to its vital importance for human diet. Detailed knowledge of the lipid distribution in plants is fundamental for the understanding of local regulatory networks covering storage metabolism, and for the development of new approaches for plant breeding and transgenic research. We here review a measurement protocol or &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;tool&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; based on magnetic resonance imaging (MRI), which allows the non-invasive detection and quantitative visualization of lipid in living plant tissue. The method provides quantitative lipid maps with a resolution close to the cellular level and can be used on a wide range of plants and is applicable at the level of individual tissues, organs, or entire plants during ontogeny. Lipid imaging is designed for both biotechnology and basic science and can be combined with histological, biochemical, and gene expression analysis. Here we present the method as practiced in our group, and discuss unique advantages and limitations of the lipid-imaging tool. Seeds of barley and rapeseed were used as a model for visualization of local oil accumulation at the organ- and tissue-specific scale.
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Nutrients are absorbed in the small intestines at a mucosal epithelium that covers multitudes of villi, fingerlike protrusions ~200-400 mum in scale. The villi line the mucosal surface and have been observed to move in response to local... more
Nutrients are absorbed in the small intestines at a mucosal epithelium that covers multitudes of villi, fingerlike protrusions ~200-400 mum in scale. The villi line the mucosal surface and have been observed to move in response to local stimulation. Luminal contractions (motility) create macro-scale fluid motions that transport nutrient molecules to the epithelium surrounding these moving micro-scale villi. We combine
Research Interests: Computer Science, Physics, Microfluidics, Principal Component Analysis, Magnetic Resonance Imaging, and 15 moreImage segmentation, Motility, Deformation, Non Functional Requirement, Space Time, Lattice Boltzmann, Principle component analysis, Lattice Boltzmann Method, Second Order, Three Dimensional, Moving Boundary, Lattice Boltzmann Methods, Small Intestine, Magnetic resonance image, and Lattice Boltzmann model
... Belinda Akpa University of Illinois at Chicago Department of Chemical Engineering 810 S. Clinton St., Chicago, IL 60607 akpa@uic.edu Xiaohong Joe Zhou University of Illinois at Chicago Center for Magnetic Resonance Imaging 1801 W.... more
... Belinda Akpa University of Illinois at Chicago Department of Chemical Engineering 810 S. Clinton St., Chicago, IL 60607 akpa@uic.edu Xiaohong Joe Zhou University of Illinois at Chicago Center for Magnetic Resonance Imaging 1801 W. Taylor St. ...
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Non‐invasive determination of mitochondrial content is an important objective in clinical and sports medicine. 31P MRS approaches to obtain information on this parameter at low field strength typically require in‐magnet exercise. Direct... more
Non‐invasive determination of mitochondrial content is an important objective in clinical and sports medicine. 31P MRS approaches to obtain information on this parameter at low field strength typically require in‐magnet exercise. Direct observation of the intra‐mitochondrial inorganic phosphate (Pi) pool in resting muscle would constitute an alternative, simpler method. In this study, we exploited the higher spectral resolution and signal‐to‐noise at 7T to investigate the MR visibility of this metabolite pool. 31P in vivo MR spectra of the resting soleus (SOL) muscle were obtained with 1H MR image‐guided surface coil localization (six volunteers) and of the SOL and tibialis anterior (TA) muscle using 2D CSI (five volunteers). A resonance at a frequency 0.38 ppm downfield from the cytosolic Pi resonance (Pi1; pH 7.0 ± 0.04) was reproducibly detected in the SOL muscle in all subjects and conditionally attributed to the intra‐mitochondrial Pi pool (Pi2; pH 7.3 ± 0.07). In the SOL muscl...
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Diffusion weighted spectroscopy can provide microstructural information that is specific to compartmental geometry. So far, in human brain, apparent diffusion coefficients (ADCs) of only the metabolites N‐acetyl aspartate, creatine (tCr)... more
Diffusion weighted spectroscopy can provide microstructural information that is specific to compartmental geometry. So far, in human brain, apparent diffusion coefficients (ADCs) of only the metabolites N‐acetyl aspartate, creatine (tCr) and choline (tCho) have been assessed. High field MR at 7 T allows the collection and analysis of diffusion weighted spectroscopy data of additional metabolites of interest such as glutamate (Glu), N‐acetyl aspartyl glutamate, and glutamine (Gln), which are of interest due to their different compartmentalization and role in brain physiology. In this study, we performed 1H diffusion weighted spectroscopy at 7 T using a diffusion‐weighted PRESS sequence in parietal white matter (n = 6) and occipital grey matter (n = 7). Data were analyzed using the LCmodel. ADCs could reliably be obtained of N‐acetyl aspartate, tCr, tCho, Glu, Gln in grey and white matter, and N‐acetyl aspartyl glutamate in white matter. Significant differences in ADC values were obse...
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The role of diet and fat consumption in the pathogenesis of breast cancer is an important subject. We report a method for noninvasive determination of lipid composition in human breast by proton magnetic resonance spectroscopy (MRS) at 7... more
The role of diet and fat consumption in the pathogenesis of breast cancer is an important subject. We report a method for noninvasive determination of lipid composition in human breast by proton magnetic resonance spectroscopy (MRS) at 7 T. Two respiratory‐triggered TE‐averaged stimulated echo acquisition mode (STEAM) acquisitions were performed on the adipose tissue of 10 healthy volunteers where the second acquisition had all gradients inverted. This acquisition protocol allows the suppression of modulation sidebands that complicate spectral analysis at the short TEavg = 24.5 ms. The entire acquisition takes ∼10 min. Ten lipid peaks were typically resolved. T1 and T2 were also measured and used to correct the peak intensities. The calculated average lipid composition for saturated was 28.7 ± 8.4%, monounsaturated, 48.5 ± 7.9%, and polyunsaturated, 22.7 ± 3.1%, in close agreement with reported values from subcutaneous adipose measurements. Intrasubject variability was 2.0, 1.6, and...
Research Interests: Algorithms, Chemistry, Biomedical Engineering, Nuclear Magnetic Resonance, Magnetic Resonance Spectroscopy, and 15 moreMedicine, Adipose tissue, Humans, Fatty acids, Female, Nmr, PUFA, Breast, Middle Aged, Adult, Magnetic, Adipose, Reproducibility of Results, Sensitivity and Specificity, and Protons
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The effects of chemical fixation are known to alter MR parameters, such as relaxation times and the apparent diffusion coefficient (ADC) of water. It is often assumed that such changes are reversible after samples have been reimmersed in... more
The effects of chemical fixation are known to alter MR parameters, such as relaxation times and the apparent diffusion coefficient (ADC) of water. It is often assumed that such changes are reversible after samples have been reimmersed in a buffer solution for a sufficient period of time. In this study we characterize the changes associated with fixation of single Xenopus laevis oocytes and their subsequent reimmersion in buffer. Substantial reductions in both T1 and T2 values were measured for all compartments of the cell after fixation, with the cytoplasm showing larger changes than the nucleus. After reimmersion in buffer, there were small but statistically significant differences in MR parameters between fresh and reimmersed cells. Experiments with a gadolinium (Gd) contrast agent showed evidence of irreversible changes in the permeability of cellular membranes to small molecules. Magn Reson Med, 2006. © 2006 Wiley‐Liss, Inc.
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The purpose of this communication is to investigate (1) the feasibility of carrying out longitudinal magnetic resonance imaging (MRI) studies in animals with implanted microwire electrodes adapted for MRI compatibility, (2) the effect of... more
The purpose of this communication is to investigate (1) the feasibility of carrying out longitudinal magnetic resonance imaging (MRI) studies in animals with implanted microwire electrodes adapted for MRI compatibility, (2) the effect of MRI studies on the quality of neurophysiological recordings, (3) the use of MRI to study the extent and recovery of tissue damage due to electrode insertion and (4) histological tissue damage due to MRI. There was no evidence of chronic neural damage caused by repeated MRI by any of the measures used nor any statistical difference in the quality of the electrophysiological recordings between animals that had undergone MRI scans and those that had not.
Research Interests: Materials Science, Biomedical Engineering, Magnetic Resonance Imaging, Neurophysiology, Medicine, and 11 moreNeural Engineering, Humans, Cerebral Cortex, Feasibility Studies, Behavioral Animal Models, Clinical Sciences, Radiation Injuries, Neurosciences, Magnetic resonance image, Brain injuries, and Equipment safety
Research Interests: Engineering, Materials Science, Magnetic Resonance Imaging, Medicine, Magnetic field, and 15 moreHigh Frequency, Dielectric Resonator Antenna, DIELECTRIC, Physical sciences, Magnetic Resonance, Ceramic, Magnetics, Image Enhancement, Oxides, Calcium Compounds, Electric Field, Equipment Design, Equipment Failure Analysis, Permittivity, and High magnetic field
Research Interests: Engineering, Physics, Medicine, Humans, Computer Simulation, and 15 moreFemale, Data acquisition, Male, Physical sciences, Magnetic Resonance, Electromagnetic Fields, Phased Array, Image Enhancement, Adult, Radiofrequency, Radio Waves, Field of View, Osteochondrosis, Electromagnetic coil, and spinal column
In the past decade, MRI perfusion imaging has become increasingly important in the radiological clinic. Herein, arterial spin labeling (ASL) represents a truly noninvasive method for assessment of hemodynamic changes. Recently, whole body... more
In the past decade, MRI perfusion imaging has become increasingly important in the radiological clinic. Herein, arterial spin labeling (ASL) represents a truly noninvasive method for assessment of hemodynamic changes. Recently, whole body scanners that operate at a field strength of 7 Tesla or above have become available, carrying the promise of higher signal to noise ratio. In this article, we discuss the basic ASL methods that are available today and the problems that may be encountered when implementing ASL on a high field scanner. Particularly, B0 and B1 field inhomogeneities and fluctuations in precession frequency due to respiration were measured at the level of the brain feeding arteries. The effect of these disturbances on ASL is being discussed as are other issues such as specific absorption rate and relaxation rates. It can be concluded that implementation of ASL on MRI systems with high field strength is not trivial, although the feasibility of ASL measurements has been d...
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Magnetic resonance imaging (MRI)-based temperature mapping very close to the surface of an ultrasound transducer is not possible due to the large magnetic susceptibility- induced image artifacts that arise from the materials used in... more
Magnetic resonance imaging (MRI)-based temperature mapping very close to the surface of an ultrasound transducer is not possible due to the large magnetic susceptibility- induced image artifacts that arise from the materials used in transducer construction. Here, it is shown in phantoms that &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;susceptibility-compensated&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; MRI sequences can be used to measure thermal increases approximately 1 mm from the surface of a 4-element cymbal array transducer, which has been used widely for noninvasive transdermal drug delivery. The estimated temperatures agree well with those measured using thermocouples.
Research Interests: Engineering, Computer Science, Acoustics, Materials Science, Magnetic Resonance Imaging, and 15 moreThermography, Medicine, Physical sciences, Temperature, Magnetic Susceptibility, Artifacts, Temperature measurement, Reproducibility of Results, Sensitivity and Specificity, Thermocouple, Nuclear Magnetic Resonance Imaging, Equipment Failure Analysis, Transducer, Transdermal Drug Delivery, and Magnetic resonance image
Research Interests: Magnetic Resonance Imaging, Biology, Drug delivery, Medicine, Biomimicry, and 15 moreBiological Sciences, Dogs, Anatomy, Deer, Female, Animals, Male, Canis Familiaris, Morphometric Analysis, High Resolution, Airway, Frontal Sinus, Fractal Dimension, Magnetic resonance image, and Medical and Health Sciences
Given the several orders of magnitude fewer spins per voxel for MR microscopy than for conventional MRI, efficient coil design is important to obtain sufficient signal‐to‐noise within reasonable data acquisition times. As MR microscopy is... more
Given the several orders of magnitude fewer spins per voxel for MR microscopy than for conventional MRI, efficient coil design is important to obtain sufficient signal‐to‐noise within reasonable data acquisition times. As MR microscopy is typically performed using very high magnetic fields, coil design must also incorporate the effects of increased component losses and skin‐depth‐dependent resistance, as well as radiation losses and phase effects for coils when conductor dimensions constitute a substantial fraction of the electromagnetic wavelength. For samples much less than 1 mm in size, wire solenoids or microfabricated planar coils are used. For samples with diameters of several millimeters, saddle, birdcage, Alderman–Grant or millipede coils become the preferred choice. Recent advances in multiple‐coil probes and phased arrays have been used to reduce data acquisition time and/or increase sample throughput, and small superconducting coils have shown significant improvements in ...
Research Interests: Materials Science, Biomedical Engineering, Microscopy, Nuclear Magnetic Resonance, Fluorescence Microscopy, and 12 moreMagnetic Resonance Spectroscopy, Medicine, Computer Simulation, Diffusion, Mice, Animals, Clinical Sciences, Rats, Time Factors, Radiofrequency, Radio Waves, and Electromagnetic coil
Microstructural and metabolic changes directly related to neuronal activation have been investigated using functional diffusion-weighted spectroscopy. The volume of interest was positioned in the primary visual cortex. A time series of... more
Microstructural and metabolic changes directly related to neuronal activation have been investigated using functional diffusion-weighted spectroscopy. The volume of interest was positioned in the primary visual cortex. A time series of alternating diffusion- and non-diffusion-weighted (1)H spectra was acquired at 7 T employing stimulated echo acquisition mode sequence and a long diffusion time (Δ = 245 ms). Time-resolved series of metabolite apparent diffusion coefficient values were derived. Significant increases in apparent diffusion coefficient of 3.3 ± 1.4% (p = 0.05) and 3.9 ± 0.9% (p = 0.002) for total n-acetyl aspartate and total creatine, respectively, and 8.1 ± 2.5% (p = 0.03) for total choline were observed in response to visual stimulation. The increase in apparent diffusion coefficient for these metabolites is a potential indication for microstructural changes in neurons during neural activation and/or for an increase in the energy-dependent cytoplasmic streaming associated with enhanced metabolism during visual stimulation.
Research Interests: Chemistry, Biomedical Engineering, Nuclear Magnetic Resonance, Creatine, Magnetic Resonance Spectroscopy, and 15 moreMedicine, Visual Cortex, Humans, Diffusion, Female, Male, Apparent Diffusion Coefficient, Clinical Sciences, Adult, Reproducibility of Results, Sensitivity and Specificity, Choline, Metabolite, Effective Diffusion Coefficient, and Diffusion magnetic resonance imaging
Research Interests: Chemistry, Biomedical Engineering, Water, Nuclear Magnetic Resonance, Creatine, and 15 moreMagnetic Resonance Spectroscopy, Medicine, Brain, Humans, Diffusion, Female, Male, Clinical Sciences, White matter, Adult, Time Factors, Choline, Metabolite, Metabolome, and Diffusion magnetic resonance imaging
Cells in the central nervous system, neurons and glia, display a wide range of structural features. Molecular diffusion properties in the intracellular space of these cells reflect this structural diversity, deviating from standard... more
Cells in the central nervous system, neurons and glia, display a wide range of structural features. Molecular diffusion properties in the intracellular space of these cells reflect this structural diversity, deviating from standard Gaussian dynamics and resulting in anomalous subdiffusion. By tracking the displacement of intracellular metabolites, diffusion-weighted magnetic resonance spectroscopy allows for in vivo compartment-specific and cell-preferential morphological analysis of neurons and glia in the human brain. Suggestive of different intracellular environments between tissue type, the neuronal and glial intracellular space in gray matter is significantly more subdiffusive than in white matter. An important difference is found between the subdiffusion of choline, a predominantly glial metabolite, in gray and white matter, potentially reflecting differences in structural complexity between fibrous and protoplasmic astrocytes. The exclusively intracellular metabolite subdiffu...
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Research Interests: Chemistry, Aging, Diffusion Tensor Imaging, DTI, Magnetic Resonance Spectroscopy, and 15 moreMedicine, Anisotropy, Diffusion MRI, Humans, Corpus Callosum, Healthy Aging, Female, Male, Clinical Sciences, Aged, Adult, Fractional Anisotropy, Axons, Diffusion magnetic resonance imaging, and Demyelinating diseases
Systemic lupus erythematosus is an inflammatory autoimmune disease with multi-organ involvement. Central nervous system involvement in systemic lupus erythematosus is common and results in several neurological and psychiatric symptoms... more
Systemic lupus erythematosus is an inflammatory autoimmune disease with multi-organ involvement. Central nervous system involvement in systemic lupus erythematosus is common and results in several neurological and psychiatric symptoms that are poorly linked to standard magnetic resonance imaging outcome. Magnetic resonance imaging methods sensitive to tissue microstructural changes, such as diffusion tensor imaging and magnetization transfer imaging, show some correlation with neuropsychiatric systemic lupus erythematosus (NPSLE) symptoms. Histological examination of NPSLE brains reveals presence of cerebral oedema, loss of neurons and myelinated axons, microglial proliferation and reactive astrocytosis, microinfacrts and diffuse ischaemic changes, all of which can affect both diffusion tensor imaging and magnetization transfer imaging in a non-specific manner. Here we investigated the underlying cell-type specific microstructural alterations in the brain of patients with systemic l...
Research Interests: Pathology, Magnetic Resonance Imaging, Diffusion Tensor Imaging, Creatine, Magnetic Resonance Spectroscopy, and 15 moreMedicine, Diffusion MRI, Brain, Humans, Corpus Callosum, Female, Male, Middle Aged, Adult, Choline, Axons, Case Control Studies, Phosphocreatine, Psychology and Cognitive Sciences, and Medical and Health Sciences
The physiology and mechanics of the small intestine originates with lumen-scale fluid motions generated by enterically controlled muscle wall contractions. Although complex in appearance, we have shown with principle component... more
The physiology and mechanics of the small intestine originates with lumen-scale fluid motions generated by enterically controlled muscle wall contractions. Although complex in appearance, we have shown with principle component decomposition of gut motion from a rat model that simpler component structure may integrate to produce basic peristaltic and segmental motions. To couple these measured modes with fluid mixing and nutrient absorption we have developed 2-D and axisymmetric models of the gut using the lattice-Boltzmann framework with scalar and second order moving boundary conditions. Previous models indicated that peristalsis is detrimental to absorption and therefore that gut motility is likely bimodal, transitioning between peristalsis and segmental modes to optimize the transport of chyme vs. nutrient absorption. However we have since discovered that more complex control is possible due to potential transitions between ``trapped'' vs. ``nontrapped'' peristalt...
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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... more
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...
Research Interests: Mathematics, Biomedical Engineering, Magnetic Resonance Spectroscopy, Medicine, Molecular Imaging, and 15 moreHumans, Corpus Callosum, Diffusion, Female, Feasibility Studies, Male, Clinical Sciences, White matter, Image Enhancement, Adult, Reproducibility, Reproducibility of Results, Sensitivity and Specificity, Tissue distribution, and Diffusion magnetic resonance imaging
The purposes of this study were to determine 1) whether magnetic resonance imaging (MRI)-based T2 mapping and measurements of limb volume can differentiate injured and uninjured tissue after blunt trauma to rat hindlimbs and 2) whether... more
The purposes of this study were to determine 1) whether magnetic resonance imaging (MRI)-based T2 mapping and measurements of limb volume can differentiate injured and uninjured tissue after blunt trauma to rat hindlimbs and 2) whether administration of buprenorphine influences these assessments. Male Wistar rats (age, 3 to 4 mo) underwent blunt contusion injury to the posterior aspect of the hindlimb; MRI was conducted at 6, 12, 24, 48, 72, and 96 h after injury. The imaging results showed that administration of buprenorphine had no effect on the T2 value {area under the receiver operating characteristic [ROC] curve: with drug, 0.869 [95% confidence interval (CI), 0.78 to 0.96]; without drug, 0.809 [95% CI, 0.72 to 0.90]} but did influence limb volume [area under the ROC curve; without drug, 0.954 (95% CI, 0.92 to 0.99); with drug, 0.713 (95% CI, 0.61 to 0.82)]. When using MRI to determine the extent of injury or to track injury over time, calculated limb volumes may lose sensitivi...
Research Interests: Magnetic Resonance Imaging, Inflammation, Medicine, Biological Sciences, Pubmed, and 14 moreAnimals, Male, Buprenorphine, The, ROC Curve, Morphine, Rats, Edema, Wistar Rats, Hemorrhage, Receiver Operating Characteristic, Predictive value of tests, Confidence Interval, and Medical and Health Sciences
Hydrocephalus has traditionally been quantified by linear measures of ventricular size, with adjunct use of cortical mantle thickness. However, clinical outcome depends on cognitive function, which is more directly related to brain volume... more
Hydrocephalus has traditionally been quantified by linear measures of ventricular size, with adjunct use of cortical mantle thickness. However, clinical outcome depends on cognitive function, which is more directly related to brain volume than these previous measures. The authors sought to quantify the dynamics of brain and ventricular volume growth in normal compared with hydrocephalic mice. Hydrocephalus was induced in 14-day-old C57BL/6 mice by percutaneous injection of kaolin into the cisterna magna. Nine hydrocephalic and 6 normal mice were serially imaged from age 2-12 weeks with a 14.1-T MR imaging unit. Total brain and ventricle volumes were calculated, and linear discriminant analysis was applied. Two very different patterns of response were seen in hydrocephalic mice compared with mice with normative growth. In one pattern (3 mice) brain growth was normal despite accumulation of CSF, and in the second pattern (6 mice) abnormal brain enlargement was accompanied by increased...
Research Interests: Algorithms, Neurosurgery, Magnetic Resonance Imaging, Comparative Study, Medicine, and 15 moreCephalometry, Fourier Analysis, Cerebrospinal Fluid, Hydrocephalus, Brain, Mice, Animals, Mathematical Computing, Age Factors, Nuclear Magnetic Resonance Imaging, Frontal Lobe, Intracranial Pressure, Occipital Lobe, brain size, and cisterna magna
Diffusion-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... more
Diffusion-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. A 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. Highly 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 stab...
Research Interests: Chemistry, Biomedical Engineering, Medicine, Diffusion MRI, Brain Mapping, and 14 moreBrain, Humans, Diffusion, Female, Male, Three Dimensional Imaging, Artifacts, Adult, Reproducibility of Results, Brain Chemistry, Chemical Shift Imaging, Metabolite, Effective Diffusion Coefficient, and Diffusion magnetic resonance imaging
Conductor based RF coils used in NMRI devices produce an ac magnetic field (known as the B1 field) that is perpendicular to the main dc magnetic field (B0). Circulating currents within these coils produce the transverse B1 magnetic... more
Conductor based RF coils used in NMRI devices produce an ac magnetic field (known as the B1 field) that is perpendicular to the main dc magnetic field (B0). Circulating currents within these coils produce the transverse B1 magnetic fields. Various types of these conductor based ...
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SummaryIn this article, we present a tool which allows the rapid and non‐invasive detection and quantitative visualization of lipid in living seeds at a variety of stages using frequency‐selected magnetic resonance imaging. The method... more
SummaryIn this article, we present a tool which allows the rapid and non‐invasive detection and quantitative visualization of lipid in living seeds at a variety of stages using frequency‐selected magnetic resonance imaging. The method provides quantitative lipid maps with a resolution close to the cellular level (in‐plane 31 µm × 31 µm). The reliability of the method was demonstrated using two contrasting subjects: the barley grain (monocot, 2% oil, highly compartmentalized) and the soybean grain (dicot, 20% oil, economically important oilseed). Steep gradients in local oil storage were defined at the organ‐ and tissue‐specific scales. These gradients were closely coordinated with tissue differentiation and seed maturation, as revealed by electron microscopy and biochemical and gene expression analysis. The method can be used to elucidate similar oil accumulation processes in different tissues/organs, as well as to follow the fate of storage lipids during deposition and subsequent m...
Research Interests: Technology, Magnetic Resonance Imaging, Biology, Plant biotechnology, Magnetic Resonance Spectroscopy, and 13 moreMedicine, Quantitative analysis, Biological Sciences, Storage, Crop, Soybeans, Seeds, Gramineae, Quantitative Analysis, Nuclear Magnetic Resonance Imaging, Soybean Oil, Plant Oils, and Medical and Health Sciences
Nutrient absorption in the small intestine cannot occur until molecules are presented to the epithelial cells that line intestinal villi, finger-like protrusions under enteric control. Using a two-dimensional multiscale lattice Boltzmann... more
Nutrient absorption in the small intestine cannot occur until molecules are presented to the epithelial cells that line intestinal villi, finger-like protrusions under enteric control. Using a two-dimensional multiscale lattice Boltzmann model of a lid-driven cavity flow with ‘villi’ at the lower surface, we analyse the hypothesis that muscle-induced oscillatory motions of the villi generate a controlled ‘micro-mixing layer’ (MML) that couples with the macro-scale flow to enhance absorption. Nutrient molecules are modelled as passive scalar concentrations at high Schmidt number. Molecular concentration supplied at the cavity lid is advected to the lower surface by a lid-driven macro-scale eddy. We find that micro-scale eddying motions enhance the macro-scale advective flux by creating an MML that couples with the macro-scale flow to increase absorption rate. We show that the MML is modulated by its interactions with the outer flow through a diffusion-dominated layer that separates a...
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Research Interests: Chemistry, Biomedical Engineering, Water, Nuclear Magnetic Resonance, Creatine, and 15 moreMagnetic Resonance Spectroscopy, Medicine, Brain, Humans, Diffusion, Female, Male, Clinical Sciences, White matter, Adult, Time Factors, Choline, Metabolite, Metabolome, and Diffusion magnetic resonance imaging
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Research Interests: Biophysics, Photosynthesis, Biology, Medicine, Biological Sciences, and 8 moreLipids, Chlorophyll, New, Soybean, Soybeans, Seeds, Lipid Droplet, and Chloroplast
Research Interests: Computer Science, Chemistry, Magnetic Resonance Imaging, Cell Migration, Cell Signaling, and 15 moreConfocal Microscopy, Cell Differentiation, Brain, Animals, Embryonic Stem Cell, Animal Model, Iron Oxide, Biodistribution, In Vivo, High Resolution, Magnetics, Cell Survival, Ferric Compounds, Magnetic resonance image, and Cell count
Research Interests: Materials Science, Microscopy, Magnetic Resonance Imaging, Medicine, Multidisciplinary, and 15 moreMice, Animals, Animal Model, In Vivo, Image Enhancement, Myocardium, Reproducibility of Results, Right Ventricle, Heart Ventricles, Chemical Shift Imaging, Cardiac Disease, Contrast Media, Magnetic resonance image, Cardiac function, and High magnetic field
It is important to accurately characterize the heating of tissues due to the radiofrequency energy applied during MRI. This has led to an increase in the use of numerical methods to predict specific energy absorption rate distributions... more
It is important to accurately characterize the heating of tissues due to the radiofrequency energy applied during MRI. This has led to an increase in the use of numerical methods to predict specific energy absorption rate distributions for safety assurance in MRI. To ensure these methods are accurate for actual MRI coils, however, it is necessary to compare to experimental results. Here, we report results of some recent efforts to experimentally map temperature change and specific energy absorption rate in a phantom and in vivo where the only source of heat is the radiofrequency fields produced by the imaging coil. Results in a phantom match numerical simulation well, and preliminary results in vivo show measurable temperature increase. With further development, similar methods may be useful for verifying numerical methods for predicting specific energy absorption rate distributions and in some cases for directly measuring temperature changes and specific energy absorption rate indu...