SAR Implications of Different RF Shimming Techniques in the Body at 7
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.
Comprehensive RF Safety Concept for Parallel Transmission Systems
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.
Poseable Male and Female Numerical Body Models for Field Calculations in
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.
Numerical Field Evaluation of Healthcare Workers When Bending Towards
High-Field MRI Magnets
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.
Detailed Experimental and Computational Analyses of the RF Field at
7T: Effects of the Load Content on Achieving 90o Tip Without SAR/Power
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.
Dyadic Green’s Functions for Electrodynamic Calculations of Ideal
Current Patterns for Optimal SNR and SAR
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.
Specific Absorption Rate Mapping Using MR Thermometry in a
Transmit-Receive Head Coil at 3.0T
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.
Direct SAR Measurements Using MRI: A Phantom Study of Proton
Decoupling at 7 Tesla
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.
Measurement of Electric Fields Induced in a Human Subjects Due to
Alternating Magnetic Gradient Fields and Natural Body Movements in
Static Magnetic Fields
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.
Human Nerve Stimulation Threshold Determination in a Planar Gradient
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.