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Bob van den Bergen¹, Cornelis A. T. van den Berg¹, Peter Luijten¹, Jan J. W. Lagendijk¹

¹University Medical Center Utrecht, Utrecht, Netherlands

The development of ultra high field MRI systems has introduced serious challenges for body imaging with respect to SAR and B₁⁺ field non-uniformity, which results in signal voids in the region of interest. RF shimming can remove these signal voids by optimizing the phases and amplitudes of the individual RF excitation elements. We present an overview of the implications of three different RF shimming techniques both for the average and for the local maximum SAR. All three can improve the B₁⁺ field and reduce the average SAR, but only one method results in a reduction of local SAR peaks.

Ingmar Graesslin¹, Dennis Glaesel¹, Sven Biederer², Peter Vernickel¹, Ulrich Katscher¹, Ferdinand Schweser¹, Bjoern Annighoefer¹, Henk Dingemans³, Giel Mens³, Gert v. Yperen³, Paul Harvey³

¹Philips Research Europe, Hamburg, Germany; ²Lübeck University, Lübeck, Germany; ³Philips Medical Systems, Best, Netherlands

RF safety is a prerequisite for in vivo parallel transmission MRI experiments. In this abstract, a comprehensive RF patient safety concept is proposed to ensure scanning within the SAR limits. The concept builds on two different measures. Before the scan, SAR calculations are carried out in (almost) real-time for the desired multi-transmit RF demand waveform, to verify its conformity with existing SAR limits according to IEC. During the scan, the deviation from the desired waveform is monitored to detect unintended violations of the SAR limits or any unsafe conditions.

Zhangwei Wang¹, Christopher W. Penney², Raymond J. Luebbers², Christopher M. Collins¹

¹The Pennsylvania State University, Hershey, Pennsylvania, USA; ²Remcom, inc., State College, Pennsylvania, USA

Different body positions can dramatically affect the SAR distribution, but due to limitations in available numerical body model positions some researchers have resorted to removing portions of the arms to eliminate contacts between the hands and the torso. In this paper, we apply our recently-developed methods for making models poseable to investigate effects of arm position on the SAR distribution for thoracic imaging at 64MHz (1.5T) in a whole-body high-pass birdcage coil.

Hua Wang¹, Adnan Trakic¹, Feng Liu¹, Stuart Crozier¹

¹The University of Queensland, Brisbane, Australia

In MRI, healthcare workers may be exposed to strong static and dynamic magnetic fields outside of the imager. Presented are numerical evaluations of electric fields/currents in anatomically-equivalent male and female human models (healthcare workers) as they lean towards the bores of three superconducting magnet models (1.5T, 4T and 7T) and x, y and z- gradient coils. The combined effect of the 1.5T superconducting magnet and the three gradient coils on the body models is compared with the contributions of the magnet and gradient coils separately. The simulation results indicate that it is possible to induce field quantities above regulatory guidelines, especially when the MRI operator is bending close towards the main magnet and all three gradient coils are switched simultaneously.

YiK-Kiong Hue¹, Tamer S. Ibrahim¹

¹University of Pittsburgh, Pittsburgh, Pennsylvania, USA

In ultra high field human experiment, different subject/loading have significant influence on the performance of the coils. Computational and experimental analyses at 7T are presented to examine the effect of loading on achieving a 90 degree tip angle without local/global SAR violations. The relationship between the flip angles and the local/global SAR value is also studied.

Riccardo Lattanzi¹, ², Daniel K. Sodickson³

¹Harvard-MIT, Boston, Massachusetts, USA; ²Beth Israel Deaconess Medical Center, Boston, Massachusetts, USA; ³New York University Medical Center, New York, New York, USA

In this work we present a semi-analytical method to calculate SNR and SAR based on a mode  expansion using dyadic Green’s functions in a dielectric sphere.  Ultimate intrinsic SNR and SAR can be computed by employing a complete set of surface current modes, and the corresponding ideal surface current patterns can be derived.  This formalism holds also in the case of actual coils and can be useful to investigate the physical behaviors of the RF fields during MR reception and MR excitation.  Ideal current patterns can be used as a reference to improve coil design.

Sukhoon Oh¹, Christopher M. Collins¹

¹The Pennsylvania State University, Hershey, USA

In this study, experimentally-acquired temperature maps are used to calculate the SAR distribution from a head-sized transmit-receive birdcage coil in an agar-gel phantom at 3.0T. A single quadrature birdcage coil was used for both heating and imaging purposes, in contrast to studies in diathermy and ablation using separate coils at different frequencies. Results agree well with expectations from electromagnetic theory. Future experiments will focus on high-power RF heating in phantoms with transmit arrays for the purposes of validating predictions from sophisticated numerical calculations.

Andrew Webb¹, Thomas Neuberger

¹Penn State University, University Park, Pennsylvania, USA

SAR at high field is conventionally estimated using either electromagnetic simulations or fibre optic measurements at a very few physical locations. However, by measuring the slope of temperature rise vs. time, the SAR can actually be measured directly using MRI. One of the areas where SAR limitations are most relevant is the use of proton decoupling in heteronuclear spectroscopy experiments. This study measures local SAR using MRI in phantoms with different conductivities using a range of proton decoupling powers.

Paul Glover¹, Richard Bowtell¹

¹University of Nottingham, Nottingham, UK

A method for measurement of electric fields induced in conductive samples by temporally changing magnetic fields is described. The method was first verified experimentally.  In vivo human measurements of electric fields were made with suitable custom built dipole probes.  Measurements of electric fields were then made using natural movements in the vicinity of a 3T magnet as well as due to gradient switching. Induced electric fields at the surface of the torso were found to be typically 0.15 (m) times the applied dB/dt.

Rebecca Emily Feldman¹, Christopher Judson Hardy², Bulent Aksel², John Fredric Schenck², Blaine Alexander Chronik¹

¹University of Western Ontario, London, Canada; ²GE Global Research, Niskayuna, New York, USA

Gradient induced peripheral nerve stimulation in humans limits MRI operation. The Z-axis of a novel planar gradient system was tested to determine the nerve stimulation thresholds. Rise times between 200 and 1400 μ s were tested. The resulting threshold curve was higher than any previously reported gradient system stimulation thresholds. The planar gradients may be operated at both gradient strengths and slew rates more than twice that of conventional cylindrical gradient designs.