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B1 Field (b1 + field)

Distribution by Scientific Domains

Medical Sciences	100%

Selected Abstracts

Use of mutually inductive coupling in probe design,

CONCEPTS IN MAGNETIC RESONANCE, Issue 4 2002
D.I. Hoult
Abstract An analysis is presented of mutually inductive coupling in probe design. It is assumed that near field couplings predominate and that lumped constants may therefore be employed. Using three published designs as examples, analytic techniques are presented for assessing B1 field strength, losses, and signal-to-noise ratio in increasingly complex situations. The perturbing effect of the B1 field from a matching coil is examined and it is shown that if the coil is too close to the sample there can be an asymmetry introduced in the rotating frame B1 field. It is then shown that such asymmetries are potentially a general feature of inductively coupled, loaded coils. The importance of suppressing unwanted resonances is highlighted if tuning and matching are to be orthogonal, a potential advantage of mutually inductive matching. Finally, a lumped-constant simulation is briefly described for those situations where an analytic approach becomes too cumbersome. © 2002 Wiley Periodicals, Inc. Concepts in Magnetic Resonance (Magn Reson Engineering) 15: 262,285, 2002 [source]

Parallel transmit and receive technology in high-field magnetic resonance neuroimaging

INTERNATIONAL JOURNAL OF IMAGING SYSTEMS AND TECHNOLOGY, Issue 1 2010
Andrew G. Webb
Abstract The major radiofrequency engineering challenges of high-field MR neuroimaging are as follows: (1) to produce a strong, homogeneous transmit B1 field, while remaining within regulatory guidelines for tissue power deposition and (2) to receive the signal with the maximum signal-to-noise and the greatest flexibility in terms of utilizing the benefits of parallel imaging. Borrowing from developments in electromagnetic hyperthermia, the first challenge has been met by the use of transmit arrays, in which the input power to each element of the array can be varied in terms of magnitude and phase. Optimization of these parameters, as well as the form of the applied RF pulse, leads to very homogeneous B1 fields throughout the brain. The design of large receive arrays, using impedance-mismatched preamplifiers and geometrical overlap for interelement isolation, has resulted in significant sensitivity improvements as well as large acceleration factors in parallel imaging. © 2010 Wiley Periodicals, Inc. Int J Imaging Syst Technol, 20, 2,13, 2010 [source]

Improved homogeneity of the transmit field by simultaneous transmission with phased array and volume coil

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2010
Nikolai 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]

Manipulation of image intensity distribution at 7.0 T: Passive RF shimming and focusing with dielectric materials,

JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2006
Qing X. Yang
Abstract Purpose To investigate the effects of high dielectric material padding on RF field distribution in the human head at 7.0 T, and demonstrate the feasibility and effectiveness of RF passive shimming and focusing with such an approach. Materials and Methods The intensity distribution changes of gradient-recalled-echo (GRE) and spin-echo (SE) images of a human head acquired with water pads (dielectric constant = 78) placed in specified configurations around the head at 7.0 T were evaluated and compared with computer simulation results using the finite difference time domain (FDTD) method. The contributions to the B1 field distribution change from the displacement current and conductive current of a given configuration of dielectric padding were determined with computer simulations. Results MR image intensity distribution in the human head with an RF coil at 7.0 T can be changed drastically by placing water pads around the head. Computer simulations reveal that the high permittivity of water pads results in a strong displacement current that enhances image intensity in the nearby region and alters the intensity distribution of the entire brain. Conclusion The image intensity distribution in the human head at ultra-high field strengths can be effectively manipulated with high permittivity padding. Utilizing this effect, the B1 field inside the human head of a given RF coil can be adjusted to reduce the B1 field inhomogeneity artifact associated with the wave behavior (RF passive shimming) or to locally enhance the signal-to-noise ratio (SNR) in targeted regions of interest (ROIs; RF field focusing). J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc. [source]

Rapid magnetic resonance quantification on the brain: Optimization for clinical usage

MAGNETIC RESONANCE IN MEDICINE, Issue 2 2008
J.B.M. Warntjes
Abstract A method is presented for rapid simultaneous quantification of the longitudinal T1 relaxation, the transverse T2 relaxation, the proton density (PD), and the amplitude of the local radio frequency B1 field. All four parameters are measured in one single scan by means of a multislice, multiecho, and multidelay acquisition. It is based on a previously reported method, which was substantially improved for routine clinical usage. The improvements comprise of the use of a multislice spin-echo technique, a background phase correction, and a spin system simulation to compensate for the slice-selective RF pulse profile effects. The aim of the optimization was to achieve the optimal result for the quantification of magnetic resonance parameters within a clinically acceptable time. One benchmark was high-resolution coverage of the brain within 5 min. In this scan time the measured intersubject standard deviation (SD) in a group of volunteers was 2% to 8%, depending on the tissue (voxel size = 0.8 × 0.8 × 5 mm). As an example, the method was applied to a patient with multiple sclerosis in whom the diseased tissue could clearly be distinguished from healthy reference values. Additionally it was shown that, using the approach of synthetic MRI, both accurate conventional contrast images as well as quantification maps can be generated based on the same scan. Magn Reson Med 60:320,329, 2008. © 2008 Wiley-Liss, Inc. [source]

Saturated double-angle method for rapid B1+ mapping

MAGNETIC RESONANCE IN MEDICINE, Issue 6 2006
Charles H. Cunningham
Abstract For in vivo magnetic resonance imaging at high field (,3 T) it is essential to consider the homogeneity of the active B1 field (B1+), particularly if surface coils are used for RF transmission. A new method is presented for highly rapid B1+ magnitude mapping. It combines the double angle method with a B1 -insensitive magnetization-reset sequence such that the choice of repetition time (TR) is independent of T1 and with a multislice segmented (spiral) acquisition to achieve volumetric coverage with adequate spatial resolution in a few seconds. Phantom experiments confirmed the accuracy of this technique even when TR , T1, with the side effect being lowered SNR. The speed of this method enabled B1+ mapping in the chest and abdomen within a single breath-hold. In human cardiac imaging, the method enabled whole-heart coverage within a single 16-s breath-hold. Results from phantoms and healthy volunteers at 1.5 T and 3 T are presented. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source]

Accurate phosphorus metabolite images of the human heart by 3D acquisition-weighted CSI

MAGNETIC RESONANCE IN MEDICINE, Issue 5 2001
Rolf Pohmann
Abstract Fourier imaging modalities suffer from significant signal contamination between adjacent voxels, especially when the spatial resolution is comparable to the size of the anatomical structures. This contamination can be positive or negative, depending on the spatial response function and the geometry of the object. Such a situation arises in human cardiac 31P chemical shift imaging (CSI). Acquisition-weighted CSI reduces this contamination substantially, which is demonstrated by comparing conventional CSI to Hanning-weighted 3D 31P-CSI experiments in 13 healthy volunteers at 2 T. The nominal spatial resolution and the total number of scans were identical for both experiments. The improved spatial response function of the acquisition-weighted experiment led to a significantly (P < 0.0001) higher myocardial PCr/ATP ratio (2.05 ± 0.31, mean ± SD, N = 33, corrected for saturation and blood contribution) compared to the conventional CSI experiment (1.60 ± 0.46). This is explained by the absence of negative contamination from skeletal muscle, which also resulted in an increase of the observed SNR (from 5.4 ± 1.4 to 7.2 ± 1.4 for ATP). With acquisition-weighted CSI, metabolic images with a nominal resolution of 16 ml could be obtained in a measurement time of 30 min. After correction for the inhomogeneous B1 field of the surface coil, these images show uniform ATP distribution in the entire myocardium, including the posterior wall. Magn Reson Med 45:817,826, 2001. © 2001 Wiley-Liss, Inc. [source]