Home About us Contact
|
Home About us Contact | ||
|
|
||
Field Homogeneity (field + homogeneity)
Selected AbstractsSimple circuit to improve electric field homogeneity in contour-clamped homogeneous electric field chambersELECTROPHORESIS, Issue 7-8 2003José A. Herrera Abstract We redesigned contour-clamped homogeneous electric field (CHEF) circuitry to eliminate crossover distortion, to set identical potentials at electrodes of each equipotential pair and to drive pairs with transistors in emitter follower stages. An equipotential pair comprised the two electrodes set at the same potential to provide electric field homogeneity inside of the hexagonal array. The new circuitry consisted of two identical circuits, each having a resistor ladder, diodes and transistors. Both circuits were interconnected by diodes that controlled the current flow to electrodes when the array was energized in the ,A' or ,B' direction of the electric field. The total number of transistors was two-thirds of the total number of electrodes. Average voltage deviation from potentials expected at electrodes to achieve a homogeneous electric field was 0.06 V, whereas 0.44 V was obtained with another circuit that used transistors in push-pull stages. The new voltage clamp unit is cheap, generated homogeneous electric field, and gave reproducible and undistorted DNA band patterns. [source] Improved homogeneity of the transmit field by simultaneous transmission with phased array and volume coilJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2010Nikolai I. Avdievich PhD Abstract Purpose: To improve the homogeneity of transmit volume coils at high magnetic fields (,4 T). Due to radiofrequency (RF) field/tissue interactions at high fields, 4 T to 8 T, the transmit profile from head-sized volume coils shows a distinctive pattern with relatively strong RF magnetic field B1 in the center of the brain. Materials and Methods: In contrast to conventional volume coils at high field strengths, surface coil phased arrays can provide increased RF field strength peripherally. In theory, simultaneous transmission from these two devices could produce a more homogeneous transmission field. To minimize interactions between the phased array and the volume coil, counter rotating current (CRC) surface coils consisting of two parallel rings carrying opposite currents were used for the phased array. Results: Numerical simulations and experimental data demonstrate that substantial improvements in transmit field homogeneity can be obtained. Conclusion: We have demonstrated the feasibility of using simultaneous transmission with human head-sized volume coils and CRC phased arrays to improve homogeneity of the transmit RF B1 field for high-field MRI systems. J. Magn. Reson. Imaging 2010;32:476,481. © 2010 Wiley-Liss, Inc. [source] Asymmetric quadrature split birdcage coil for hyperpolarized 3He lung MRI at 1.5TMAGNETIC RESONANCE IN MEDICINE, Issue 2 2008Nicola De Zanche Abstract An asymmetric quadrature birdcage body coil for hyperpolarized (HP) 3He lung imaging at 1.5T is presented. The coil is designed to rest on top of the patient support and be used as a temporary insert in a clinical system. A two-part construction facilitates patient access and the asymmetric design makes maximal use of available bore space to ensure comfort. Highly homogeneous, circularly polarized RF magnetic fields are produced at 48.5 MHz using a conformal mapping method for the geometrical design, combined with an algebraic method to calculate the individual capacitance values on the birdcage coil's ladder network. Efficiency and isolation from the system's proton body coil are ensured by an integrated RF screen. The design methodology is readily applicable to other field strengths or nuclei. Improvements over existing 3He coils were found in terms of sensitivity and transmit field homogeneity, an important feature in HP MRI. Magn Reson Med 60:431,438, 2008. © 2008 Wiley-Liss, Inc. [source] 3T MR of the prostate: Reducing susceptibility gradients by inflating the endorectal coil with a barium sulfate suspensionMAGNETIC RESONANCE IN MEDICINE, Issue 5 2007Yael Rosen Abstract Most prostate MRI/MRS examinations are performed with an endorectal coil inflated with air, leading to an air,tissue interface that induces magnetic susceptibility gradients within the gland. Inflation of the coil with a barium sulfate suspension is described and compared to inflation with air or liquid perfluorocarbon (PFC). The B0 field in the prostate gland was mapped for five healthy volunteers when the endorectal coil was inflated with each of the three agents. A marked decrease in the posterior-anterior (P-A) field gradient and a significant improvement in field homogeneity were evident in the presence of a barium suspension and PFC relative to air. MRS data acquired from the prostate gland in the presence of air, PFC, and a barium suspension in the endorectal coil showed similar trends, demonstrating improvement in line-widths and spectral resolution when the barium suspension or the PFC were inflating the endorectal coil. On this basis we conclude that a barium suspension provides an available, cheap, and safe alternative to PFC, and we suggest that inflating the endorectal coil with a barium suspension should be considered for prostate MR studies, especially at high field strengths (such as 3T). Magn Reson Med 57:898,904, 2007. © 2007 Wiley-Liss, Inc. [source] Robust automated shimming technique using arbitrary mapping acquisition parameters (RASTAMAP)MAGNETIC RESONANCE IN MEDICINE, Issue 5 2004L. Martyn Klassen Abstract Quantitative MRI techniques as well as methods such as blood oxygen level-dependent (BOLD) imaging and in vivo spectroscopy require stringent optimization of magnetic field homogeneity, particularly when using high main magnetic fields. Automated shimming approaches require a method of measuring the main magnetic field, B0, followed by adjusting the currents in resistive shim coils to maximize homogeneity. A robust automated shimming technique using arbitrary mapping acquisition parameters (RASTAMAP) using a 3D multiecho gradient echo sequence that measures B0 with high precision was developed. Inherent compensation and postprocessing methods enable removal of artifacts due to hardware timing errors, gradient propagation delays, gradient amplifier asymmetry, and eddy currents. This allows field maps to be generated for any field of view, bandwidth, resolution, or acquisition orientation without custom tuning of sequence parameters. Field maps of an aqueous phantom show ± 1 Hz variation with altered acquisition orientations and bandwidths. Subsequent fitting of measured shim coil field maps allows calculation of shim currents to produce optimum field homogeneity. Magn Reson Med 51:881,887, 2004. © 2004 Wiley-Liss, Inc. [source] On the application of a non-CPMG single-shot fast spin-echo sequence to diffusion tensor MRI of the human brainMAGNETIC RESONANCE IN MEDICINE, Issue 1 2002Mark E. Bastin Abstract The strong sensitivity of Carr-Purcell-Meiboom-Gill (CPMG) fast spin-echo (FSE) sequences, such as rapid acquisition with relaxation enhancement (RARE), to the phase of the prepared transverse magnetization means that artifact-free single-shot diffusion-weighted images can currently only be obtained with a 30,50% reduction in the signal-to-noise ratio (SNR). However, this phase sensitivity and signal loss can be addressed in FSE sequences that use quadratic phase modulation of the radiofrequency (RF) refocusing pulses to generate a sustained train of stable echoes. Here the first application of such a non-CPMG single-shot FSE (ssFSE) sequence to diffusion tensor MR imaging (DT-MRI) of the human brain is described. This approach provides high SNR diffusion-weighted images that have little or no susceptibility to poor B0 magnetic field homogeneity and the strong eddy currents typically present in DT-MRI experiments. Magn Reson Med 48:6,14, 2002. © 2002 Wiley-Liss, Inc. [source] Clinical applications of 1H-MR spectroscopy in the evaluation of epilepsies , What do pathological spectra stand for with regard to current results and what answers do they give to common clinical questions concerning the treatment of epilepsies?ACTA NEUROLOGICA SCANDINAVICA, Issue 4 2003T. Hammen Nuclear magnetic resonance spectroscopy (1H-MRS) is a non-invasive method in detecting abnormal spectra of various brain metabolites containing N -acetylaspartate (NAA), Choline (Cho), Creatine (Cr), , -Aminobutyric acid (GABA) and Glutamate. Technical processing of the MR-systems, improved automated shimming methods and further development of special shim coils increase the magnetic field homogeneity and lead to a better spectral quality and spectral resolution. The handling of the systems becomes more user-friendly and is more likely to be used in routine diagnostics. The 1H-MRS has become a diagnostic tool for assessing a number of diseases of the central nervous system mainly including epilepsies and brain tumours. The role of 1H-MRS in the assessment of epilepsies will probably increase in future. In the following article, the principles of 1H-MRS and an overview of it in the evaluation and treatment of epilepsies with special regard to temporal lobe epilepsies (TLE) has been illustrated. [source] Multimode Microwave Reactor for Heterogeneous Gas-Phase CatalysisCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 11 2003H. Will In order to investigate heterogeneously catalyzed gas-phase reactions, e.g. the oxidation of propane, on heterogeneous catalysts in the multimode microwave field, a microwave device was modified to achieve an adequate field homogeneity and a continuous power control. To date, no similar apparatus has been described in the literature. The presented microwave apparatus is suitable for carrying out reproducible catalytic investigations in the multimode microwave field. [source] | ||||||||||