Showing papers in "Concepts in Magnetic Resonance Part B-magnetic Resonance Engineering in 2003"
TL;DR: In this paper, a direct relationship between the magnetic field perturbation and the susceptibility distribution inside the MR magnet using a first order perturbations approach to Maxwell's magneto-static equations, combined with the Fourier transformation technique to solve partial derivative equations was obtained.
Abstract: A fast calculation method for the magnetic field distribution due to (dynamic) changes in susceptibility may allow real-time interventional applications. Here it is shown that a direct relationship can be obtained between the magnetic field perturbation and the susceptibility distribution inside the MR magnet using a first order perturbation approach to Maxwell's magneto-static equations, combined with the Fourier transformation technique to solve partial derivative equations. The mathematical formalism does not involve any limitation with respect to shape or homogeneity of the susceptibility field. A first order approximation is sufficient if the susceptibility range does not exceed 10−4 (or 100 ppm). The formalism allows fast numerical calculations using 3D matrices. A few seconds computation time on a PC is sufficient for a 128 × 128 × 128 matrix size. Predicted phase maps fitted both analytical and experimental data within 1% precision. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 19B: 26–34, 2003.
396 citations
TL;DR: The multi-infinity dipole magnet is a new design capable of generating a relatively high and homogeneous permanent magnetic field (0.1-3 Tesla and homogeneity 20 ppm for 3 mm) that is suitable for nuclear magnetic resonance (NMR) or electron spin resonance (ESR) studies of small volume chemical or biological samples as discussed by the authors.
Abstract: The multi-infinity dipole magnet is a new design capable of generating a relatively high and homogeneous permanent magnetic field (0.1–3 Tesla and homogeneity 20 ppm for 3 mm) that is suitable for nuclear magnetic resonance (NMR) or electron spin resonance (ESR) studies of small volume chemical or biological samples. The theoretical design and practical aspects involved in the construction of a prototype tabletop NMR system (0.6 T and homogeneity of 10 ppm in a ellipsoidal region of 3 × 3 × 5 mm3) are described. A simple NMR relaxation time experiment was performed on a water sample doped with 40 mM of CuSO4. This system in its current configuration is suitable for relaxation time studies of materials or for educational use. The introduction of shim coils would increase the homogeneity of the magnet by a factor of 100, allowing the system to be used as a portable NMR spectrometer. In addition the introduction of gradient coils between the poles would transform the spectrometer to a MRI portable system for tissue analysis or simple MR microimaging at fields up to 3 T. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 19B: 35–43, 2003.
132 citations
TL;DR: In this paper, the authors investigated the effect of common mode signals on the shield of a coaxial cable and investigated techniques to reduce their effects at high fields of ≥ 3 Tesla.
Abstract: This experiment was conducted to identify common mode currents that flow on the shield of a coaxial cable and to investigate techniques that reduce their effects at high fields of ≥3 Tesla. It will be shown that there are two different methods of common mode interference for two different coil types. First, is the transceive case, where common mode cable shield currents flow as a result of unbalanced loop voltages that couple to the cable shield. In the second case, unwanted common mode signals are induced from the field of the transmission coil onto the cable shield of the receive-only coil. Two important factors relating to the investigation of common mode signals are danger to the patient from an RF burn and the reduction of the signal-to-noise ratio (SNR) because of extra noise being introduced into the MRI environment. It will be shown that through the use of cable shield traps and balun matching circuits, common mode signals can be reduced significantly, increasing patient safety and SNR. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 19B: 1–8, 2003.
66 citations
TL;DR: In this paper, a family of planar spiral RF microcoils (500 MHz, Archimedean geometry, 1-6 turns, inner radius 0.75 to 4 mm, and conductor widths of 75, 100, and 200 μm) are described.
Abstract: NMR investigation of volume-limited chemical and biological samples requires a radio frequency (RF) microcoil with high signal-to-noise ratio (SNR) over the region of interest. Conventional approaches using solenoidal microcoils provide high sensitivity (RF field strength per unit current) and spectral resolution but require the sample and sample container to be enclosed by the coil. Planar surface microcoils provide an alternative configuration that allows direct access to the sample, but with the sacrifice of RF field uniformity. In this study we evaluate a family of planar RF microcoils (500 MHz, Archimedean geometry, 1–6 turns, inner radius 0.75 to 4 mm, and conductor widths of 75, 100, and 200 μm). The design, fabrication, and performance (electrical and NMR) of the coils are described. This coil configuration exhibits a high local SNR, easy fabrication, good electrical properties, and moderate spectral uniformity (suitable for imaging and low resolution NMR spectroscopy). This design can also be scaled to smaller dimensions. These results suggest that planar spiral RF microcoils will find applications in multimodality microscopy and microfludic devices where sample manipulation and coil integration with the microanalysis systems is necessary. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 17B: 1–10, 2003.
56 citations
TL;DR: In this paper, a two-layer transverse gradient coil was designed and constructed for use in NMR microscopy, and the effect of constructing the coil using wires or cuts in a continuous conducting surface has also been assessed.
Abstract: In small, cylindrical gradient coils consisting of a single layer of wires, the limiting factor in achieving large magnetic field gradients is the rapid increase in coil resistance with efficiency. This behavior results from the decrease in the maximum usable wire diameter as the number of turns is increased. By adopting a multilayer design in which the coil wires are allowed to spread out into multiple layers wound at increasing radii, a more favorable scaling of resistance with efficiency is achieved, thus allowing the design of more powerful gradient coils with acceptable resistance values. By extending the theory used to design standard cylindrical gradient coils, mathematical expressions have been developed that allow the design of multilayer coils. These expressions have previously been applied to the design of a four-layer z-gradient coil. As a further development, the equations have now been modified to allow the design of multilayer transverse gradient coils. The variation in coil performance with the number of layers employed has been investigated for coils of a size suitable for use in NMR microscopy, and the effect of constructing the coil using wires or cuts in a continuous conducting surface has also been assessed. We find that at fixed resistance a small wire-wound two-layer coil offers an increase in efficiency of a factor of aboul 1.5 compared with a single-layer coil. In addition, a two-layer coil of 10-mm inner diameter has been designed and built. This coil had an efficiency of 0.41 Tm-1 A-1, a resistance of 0.96 ± 0.01 Ω, and an inductance of 22.3 ± 0.2 µH. The coil produces a gradient that deviates from linearity by less than 5% over a central cylindrical region of interest of height and length 6.2 mm.
27 citations
TL;DR: In this paper, a 2D full wave model for the birdcage coil is described that includes the effects of shields, lossy layered media, and finite leg widths.
Abstract: A simple 2-D full wave model for the birdcage coil is described that includes the effects of shields, lossy layered media, and finite leg widths. This model has been implemented in Matlab, which provides a simple user interface. Using this model, various coil parameters such as signal-to-noise ratio (SNR), magnetic field homogeneity, magnetic field magnitude, etc. can be computed quickly. The efficiency of this model allows optimization of the coil geometry for particular applications. A method for finding the optimal coil radius in terms of SNR and homogeneity, given shield and sample radii, is presented. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 18B: 15–23, 2003.
18 citations
TL;DR: The use of planar submillimeter diameter radio frequency (RF) microcoils at 500 MHz (11.74 T) to image biologic samples to demonstrate that it is possible to obtain images of living cells with high temporal and spatial resolution under conditions where viability in maintained and functional stimulation is possible.
Abstract: We describe the use of planar submillimeter diameter radio frequency (RF) microcoils at 500 MHz (11.74 T) to image biologic samples, specifically late-stage frog oocytes (typical diameter 750–1000 μm) and isolated rat pancreatic islets (typical diameter 150–200 μm). For the frog oocyte we obtained a 25 × 25 μm in-plane resolution on a 100 μm slice in 34 min, and for the pancreatic islets we obtained a 14 × 14 μm in-plane resolution on a 100-μm slice in 68 min. These results demonstrate that it is possible to obtain images of living cells with high temporal and spatial resolution under conditions where viability in maintained and functional stimulation is possible. The extension of these studies to smaller diameter microcoils or arrays of microcoils may allow magnetic resonance imaging to be performed on individual cells in culture, and could lead to in vivo assessment of cell function within biocapsules or tissue implants. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 18B: 1–8, 2003
17 citations
TL;DR: In this article, the authors show that NMR can be achieved even in pulsed high-field magnets, and discuss why this technique is able to become a promising research tool.
Abstract: NMR has benefited tremendously from superconducting magnet technology providing stable, ever increasing magnetic fields over the last few decades. However, since superconductors expel magnetic field and with no suitable material in sight this trend has come to a halt at ∼20 T, and resistive magnet designs are being used again for high-field applications. Outside NMR, much higher magnetic fields (∼85 T) can be achieved with pulsed, resistive magnets. A low duty cycle and a small magnet volume cut down on the average losses and make these systems affordable. Here, we show for the first time that NMR can be accomplished even in pulsed high-field magnets, and we discuss why this technique is able to become a promising research tool. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 19B: 9–13, 2003
14 citations
TL;DR: The system exploits the advantages of parallel computing, coupled with an efficient general linear model (GLM) coefficient estimation algorithm, to overcome several issues constraining the analysis of the whole-brain fMRI data in real time.
Abstract: A system for the real-time analysis of functional magnetic resonance imaging (fMRI) time series is evaluated. The system exploits the advantages of parallel computing, coupled with an efficient general linear model (GLM) coefficient estimation algorithm, to overcome several issues constraining the analysis of the whole-brain fMRI data in real time. The highly parallel, voxel-wise processing of fMRI data motivated the use of a cluster of personal computers for parallel computation. Aside from gaining a significant increase in computational speed, the PC cluster provides a versatile way to handle the computational requirements of the system. The use of GLM in the supporting software allows substantial parametric analysis to be performed. Results of the real-time analysis of the whole-brain fMRI data of a normal subject performing a simple finger-tapping task demonstrated the capabilities of the system. For a real-time statistical analysis including real-time image reconstruction, realignment for motion correction, smoothing, GLM coefficient estimation, statistical analysis, and update of the displayed activation map, the time required to process the data for each image volume is about 1.034 s for a 64 × 64 × 30 image volume and 2.561 s for a 128 × 128 × 20 image volume, less than the TR set to 3 s. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 19B: 14–25, 2003.
13 citations
TL;DR: In this article, a simple description of what it means to fold in specific spatial frequency information is given in terms of the structure of the point spread function, which is useful for designers of k -space sampling schemes that deliberately incorporate undersampled regions.
Abstract: It is well known that to avoid aliasing in the reconstruction of functions on n that have support on a compact set B (having n -volume B ), it is necessary to sample k -spacewith a density equal to or higher than one sample per 1/ B n -volume of k -space. It is also wellknown that undersampling any particular region of k -space will lead to an aliasing, or “folding in”of image information containing the spatial frequency information of the undersampled region.Here a simple description of what it means to fold in specific spatial frequency information isgiven in terms of the structure of the point spread function. The description should be usefulfor designers of k -space sampling schemes that deliberately incorporate undersampled regions. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 17B: 17–24, 2003 KEY WORDS: k -space; reconstruction; Nyquist criterion In order to avoid aliasing artifact in a reconstructedmagnetic resonance image, it is known that theNyquist sampling criterion in
10 citations
TL;DR: In this article, a surface coil that reduces contaminating signals from the surface-lying tissues and improves uniformity of the B1 field within the same preselected volume for both 31P and 1H is described.
Abstract: To perform multinuclear magnetic resonance spectroscopy and imaging, a coil with a high signal-to-noise ratio and spatially uniform B1 field that covers the same volume of interest at several frequencies is preferred. The design of a surface coil that reduces contaminating signals from the surface-lying tissues and improves uniformity of the B1 field within the same preselected volume for both31P and 1H is described. Testing of the coil demonstrated acquisition of proton images and 31P spectra from a selected voxel in rat liver at 7 T. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 17B: 11–16, 2003.
TL;DR: In this article, the authors presented a new method of simulating high-temperature superconducting (HTS) RF coils using an electromagnetically trained artificial neural network (EM-ANN).
Abstract: This article presents a new method of simulating high-temperature superconducting (HTS) RF coils using an electromagnetically trained artificial neural network (EM-ANN). This design is based on a spiral planar coil with distributed capacitance fabricated with Y1Ba2Cu3O7 (YBCO) films. Simulation time with this new method can be reduced to only one millionth of the time required by the commercial electromagnetic software programme HP Momentum. The new method can also exploit the properties of an artificial neural network by providing an inverse algorithm based on a resonant frequency input to derive other properties of an RF coil. This inverse algorithm using EM-ANN is easier, faster, and more interactive than the traditional “moment method.” The simulation results also show excellent agreement with experimental measurements, with a margin of error of less than 3%. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 18B: 9–14, 2003
TL;DR: A toroid cavity autoclave (TCA) is a metal pressure vessel that simultaneously functions as an NMR resonator as discussed by the authors, and has been used for catalysis in supercritical fluids, rotating-frame imaging of materials and evaluation of transport phenomena.
Abstract: A toroid cavity autoclave (TCA) is a metal pressure vessel that simultaneously functions as an NMR resonator Since the introduction of the TCA concept, many adaptations and optimizations were put together to suit particular applications such as catalysis in supercritical fluids, rotating-frame imaging of materials, and evaluation of transport phenomena In this article, we present recent achievements in TCA engineering that were carried out for both major fields of research with TCAs, ie, high-pressure and imaging studies A new modular TCA design is introduced that makes it possible to easily access all individual parts of the autoclave for maintenance and replacement The TCA is double tuned to a high frequency for 1H and 19F resonances and a low frequency for hetero nuclei It withstands pressures up to 400 bar at room temperature and 300 bar at temperatures up to 100°C The autoclave sample volume is electrically separated into two compartments with only the lower compartment NMR sensitive With the two-compartment TCA, it is possible to monitor multiple-phase systems or gas/liquid reactions in situ without sacrificing high-resolution qualities of the probe In addition, special features are presented, such as the utilization of a second pressure line for injecting key reagents that initiate a chemical reaction or for vigorously mixing gases with a reactive solution A newly designed resistive heater that does not introduce stray magnetic fields is also described
TL;DR: In this paper, the authors describe a method of lines for analyzing radio frequency (RF) coils for high-field magnetic resonance imaging, where only two of the three geometric dimensions are discretized, while seeking an analytic solution in the remaining space dimension, an efficient solution can be obtained at reasonable geometric restrictions.
Abstract: The method of lines is described as a novel and powerful way to analyze radio frequency (RF) coils for high-field magnetic resonance imaging. Originally developed for optical and microwave circuits, this method enables a full-wave solution to Maxwell's field equations without the computational burden of pure finite element or finite difference methods. Because only two of the three geometric dimensions are discretized, while seeking an analytic solution in the remaining space dimension, an efficient solution can be obtained at reasonable geometric restrictions. After outlining the theoretical formulation and the computer implementation of this technique, we will apply it to the modeling of microstrip RF transverse electromagnetic resonator coils at 200 and 400 MHz as well as a high-pass shielded birdcage resonator and a single-loop surface coil. With single-frequency solutions obtainable within 5-10 s on a personal computer, the approach proves highly suitable as a design tool and for rapid prototyping.
TL;DR: In this article, the relationship between cavity geometry and coil performance was studied and five variations of the ReCav were built where either the diameter ratio between the inner and outer cylinders or the extent of reentrant wall were varied.
Abstract: The continual march to higher magnetic fields for MRI/S has brought about a change in RF coil methodology that incorporates the use of cavity designs. One such design, the ReCav coil, has shown improvement in signal-to-noise (SNR) over standard low-pass birdcages. This work studies the relationship between cavity geometry and coil performance. Five variations of the ReCav were built where either the diameter ratio between the inner and outer cylinders or the extent of reentrant wall were varied. The Q-damping and SNR were measured for each design. It is shown that the reentrant wall in the ReCav plays an important role in improving the performance of the coil. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 18B: 24–27, 2003
TL;DR: MRS_Controller as mentioned in this paper is a free software written for Windows that can be used to control magnetic resonance spectrometers or postprocess data, and is composed of four modules that may carry out field stabilization, graphical pulse sequence edition, experiment planning, and spectral data postprocessing.
Abstract: Freeware called MRS_Controller and written for Windows is described. It may be used to control magnetic resonance spectrometers or postprocess data, and is composed of four modules that may carry out field stabilization, graphical pulse sequence edition, experiment planning, and spectral data postprocessing. The control of a commercial Tesla BS587A spectrometer was performed with this software through an independent dynamic link library. It may easily be rewritten to allow the control of other types of spectrometers. This software can also be used for spectral data postprocessing and for planning or preparing experiments offline because hardware access is disabled when an instrument is not detected at startup.