Journal ArticleDOI
SAR and B1 field distributions in a heterogeneous human head model within a birdcage coil
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TLDR
Calculations of radiofrequency magnetic (B1) field and specific energy absorption rate (SAR) distributions in a sphere of tissue and a multi‐tissue human head model in a 12‐element birdcage coil are presented.Abstract:
Calculations of radiofrequency magnetic (B1) field and specific energy absorption rate (SAR) distributions in a sphere of tissue and a multi-tissue human head model in a 12-element birdcage coil are presented. The coil model is driven in linear and quadrature modes at 63, 175, 200, and 300 MHz. Plots of B1 field magnitude and SAR distributions, average SAR, maximum local SAR, and measures of B1 field homogeneity and signal-to-noise ratio are given. SAR levels for arbitrary pulse sequences can be estimated from the calculated data. Maximum local SAR levels are lower at lower frequencies, in quadrature rather than in linear coils, and in linear fields oriented posterior-to-anterior rather than left-to-right in the head. It should be possible to perform many experiments in the head at frequencies up to 300 MHz without exceeding standard limits for local or average SAR levels.read more
Citations
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Journal ArticleDOI
7T vs. 4T: RF power, homogeneity, and signal-to-noise comparison in head images.
J. T Vaughan,Michael Garwood,Christopher M. Collins,Wanzhan Liu,Lance DelaBarre,Gregor Adriany,Peter Andersen,Hellmut Merkle,Rainer Goebel,Michael B. Smith,Kamil Ugurbil +10 more
TL;DR: Signal‐to‐noise ratio (SNR), RF field (B1), and RF power requirement for human head imaging were examined at 7T and 4T magnetic field strengths and were consistent with calculations performed using a human head model and Maxwell's equations.
Journal ArticleDOI
Challenges for Molecular Magnetic Resonance Imaging
TL;DR: Challenges for CEST Agents and Hyperpolarized Probes: Use of Gd Contrast Agents with HyperPolarized Substances 3038.
Journal ArticleDOI
Parallel excitation with an array of transmit coils
TL;DR: Theoretical and experimental results are presented that establish the value of parallel excitation with a transmit coil array in accelerating excitation and managing RF power deposition and suggest that by exploiting the localization characteristics of the coils, an orchestrated play of shorter RF pulses can achieve desired excitation profiles faster without adding strains to gradients.
Journal ArticleDOI
Sensitivity and power deposition in a high-field imaging experiment.
TL;DR: Image signal‐to‐noise ratio and power dissipation are investigated theoretically up to 400 MHz; the free induction decay is sometimes found to be annulled and the traditional, low‐frequency formulation is compared.
Journal ArticleDOI
B(1) destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil.
Pierre-Francois Van de Moortele,Can Akgun,Gregor Adriany,Steen Moeller,Johannes Ritter,Christopher M. Collins,Michael B. Smith,J. Thomas Vaughan,K. Ugurbil +8 more
TL;DR: This work demonstrates with an appropriately conductive sphere phantom that destructive interferences are responsible for a weak B1 in the periphery, without a significant standing wave pattern, and suggests that measuring spatial B1 phase could help, within an MR session, to perform RF shimming in order to obtain more homogeneous B 1 in user‐defined areas of the brain.
References
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RF magnetic field penetration, phase shift and power dissipation in biological tissue: Implications for NMR imaging
Paul A. Bottomley,E. R. Andrew +1 more
TL;DR: The results indicate that at NMR operating frequencies much greater than about 30 MHz, magnetic field amplitude and phase variations experienced by the nuclei may cause serious distortions in an image of a human torso.
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Comparison of linear and circular polarization for magnetic resonance imaging
Gary H. Glover,Cecil E. Hayes,Norbert J. Pelc,William A. Edelstein,Otward M. Mueller,H. R. Hart,Christopher J. Hardy,Matthew O'Donnell,William Daniel Barber +8 more
TL;DR: In this article, a comparison of experimental imaging results obtained with linearly polarized and circularly polarized radiofrequency excitation and reception is presented, and the expected benefits of circular polarization over linear polarization in reduction of excitation power (up to 50% reduction) and signal-to-noise advantage (√ 2) have been realized in practice with cylindrical objects and human subjects.
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Estimating radiofrequency power deposition in body NMR imaging.
TL;DR: The absence of highly localized intensity artifacts in 63‐MHz body images does not suggest anomalously intense power deposition at localized internal sites, although peak power is difficult to measure.
Journal ArticleDOI
Electromagnetic fields of surface coil in vivo NMR at high frequencies.
TL;DR: A high frequency solution of the electromagnetic field produced by a circular surface coil adjacend to a homogenous conducting, dielectric sphere is used to predict the attainable signal to noise ratio (S/N) and specific absorption rate (SAR) for in vivo 1H NMR spec troscopy experiments from 200 to 430 MHz.