ISMRM 24th Annual Meeting & Exhibition • 07-13 May 2016 • Singapore

Scientific Session: MR Safety

Tuesday, May 10, 2016
Summit 2
10:00 - 12:00
Moderators: Andreas Bitz, Christopher Collins

Investigating the Effects of 10.5T Static Field Exposure on Blood Pressure and Heart Rate in Anesthetized Pigs
Yigitcan Eryaman1, Patrick Zhang1, Lynn Utecht1, Russell L Lagore1, Jeramy Kulesa1, Lance DelaBarre1, Kivanc Kose2, Lynn E. Eberly3, Gregor Adriany1, Kamil Ugurbil1, and J. Thomas Vaughan1
1CMRR,Radiology, University of Minnesota, Minneapolis, MN, United States, 2Dermatology Service, Memorial Sloan Kettering Cancer Center, New York, NY, United States, 3Division of Biostatistics,School of Public Health, University of Minnesota, Minneapolis, MN, United States
Preliminary studies are conducted to investigate the effects of 10.5 T whole body exposure on anesthetized pigs. Blood pressure was measured invasively, recorded and analyzed to calculate the systolic/diastolic blood pressure levels as well as the heart rate. 

Does trans-membrane stimulation occur in peripheral nerve stimulation: why the SENN does not fit the data? - Permission Withheld
Donald McRobbie1,2
1South Australian Medical Imaging, Flinders Medical Centre, Adelaide, Australia, 2Surgery, Imperial College, London, United Kingdom
The Spatial Extended Non-linear Node (SENN) model currently used in MR safety guidelines does not adequately predict the behaviour of magnetic stimulation in terms of its time constant and waveform dependence. This has implications for the setting of gradient limits in MRI. Stimulation of the nerve by induced electric fields perpendicular to the nerve axis may remove the inconsistencies. A better model of magnetic stimulation is required.

Thermo-Acoustic Ultrasound Detection of RF Tip Heating in MRI
Neerav Dixit1, Pascal Stang2, John Pauly1, and Greig Scott1
1Electrical Engineering, Stanford University, Stanford, CA, United States, 2Procyon Engineering, San Jose, CA, United States
Thermo-acoustic ultrasound uses pressure waves generated by thermoelastic expansion to measure heating. This technique can be used to detect excessive local SAR and RF tip heating in implanted or interventional devices. We compare the signal quality and inherent properties of several modulation schemes for thermo-acoustic ultrasound. We then interface a system for thermo-acoustic detection of heating with an MRI scanner and demonstrate the first use of thermo-acoustic ultrasound to detect RF tip heating from an MRI scanner.

Butler matrix transmit channel compression in pTx: a SAR-aware study.
Mihir Rajendra Pendse1, Riccardo Stara1,2,3, Gianluigi Tiberi4, Alessandra Retico2, Michela Tosetti5, and Brian Rutt1
1Stanford University, Stanford, CA, United States, 2Istituto Nazionale di Fisica Nucleare (Pisa), Pisa, Italy, 3Universita' di Pisa, Pisa, Italy, 4IMAGO7, Pisa, Italy, 5ISRCC Stella maris, Calambrone (Pisa), Italy
The use of a Butler matrix in pTx is thought to allow transmit channel compression by a factor of 2 or more compared to direct drive, while maintaining similar flip angle control. However, the SAR-related consequences of this compression strategy are relatively unexplored. Using a SAR-aware pTx design method (IMPULSE), we demonstrate that excellent flip angle uniformity is indeed possible using only 2 or 4 Butler modes compared to 8 direct drive channels; however, this comes at the expense of increased SAR. We also present a generalized strategy for selecting the optimal subset of Butler modes, i.e. the subset that provides adequate flip angle control at minimum SAR. 

Ultimate hyperthermia: Computation of the best achievable radio-frequency hyperthermia treatments in non-uniform body models
Bastien Guerin1,2, Jorge F. Villena3, Athanasios G. Polimeridis4, Elfar Adalsteinsson5,6,7, Luca Daniel5, Jacob K. White5, Bruce R. Rosen1,2,6, and Lawrence L. Wald1,2,6
1A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, 2Harvard Medical School, Boston, MA, United States, 3Cadence Design Systems, Feldkirchen, Germany, 4Skolkovo Institute of Science and Technology, Moscow, Russian Federation, 5Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, 6Harvard-MIT Division of Health Sciences Technology, Cambridge, MA, United States, 7Institute for Medical Engineering and Science, Massachusetts Institute of Technology, Cambridge, MA, United States
We propose a framework for the computation of the ultimate hyperthermia, which is the best possible hyperthermia treatment for a given frequency and non-uniform body model achievable by any multi-channel hyperthermia coil. We compute the ultimate hyperthermia treatment of two shallow (close to skull) and deep (close to ventricle) brain tumors in the realistic “Duke” body model and for treatment frequencies ranging from 64 MHz to 600 MHz. We characterize the convergence to the ultimate SAR pattern as well as temperature increase associated with the ultimate SAR distribution in the presence of non-uniform perfusion effects.

A method to approximate maximum local SAR in multi-channel transmit MR systems without transmit phase information
Stephan Orzada1, Mark E. Ladd1,2, and Andreas K. Bitz2
1Erwin L. Hahn Institute, Essen, Germany, 2Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany
The capability of multi-channel transmit systems to drive different waveforms in the individual transmit channels results in an increased complexity for the SAR supervision In this work we propose a method based on virtual observation points (VOPs) to derive a conservative upper bound for the local SAR with a reasonable safety margin without knowledge of the transmit phases of the channels. In six different scenarios we demonstrate that the proposed method can be superior to the simple worst case method often used when only amplitude and no phase information is available.

Heat Equation Inversion (HEI) Algorithm Sensitivity Assessment for Computation of SAR from MR Thermometry Acquisitions
Leeor Alon1,2,3,4, Daniel Sodickson1,2,3, and Cem M. Deniz1,2,3,4
1Center for Advanced Imaging Innovation and Research (CAI2R), New York University School of Medicine, New York, NY, United States, 2Center for Biomedical Imaging, New York University School of Medicine, New York, NY, United States, 3NYU Wireless, NYU-Poly, New York, NY, United States, 4RF Test Labs, New York, NY, United States
MR thermometry methods are often used to assess safety of RF antennas. Typically thermal mapping is conducted in phantoms, which measures the temperature change as result of exposure to RF waves. From these temperature difference maps, SAR distribution can be reconstructed using the inverse heat equation (HEI) framework . With a goal of of testing the robustness of this method, in this work, we assessed the fidelity of the algorithm with respect to different regularization parameter, different SAR distributions, excitation frequencies and heating durations.

A Patient-adjustable MRI coil for implant-friendly imaging of deep brain stimulation: Design, construction, and patient-specific numerical simulations
Laleh Golestanirad1, Boris Keil1, Maria Ida Iacono2, Giorgio Bonmassar1, Leonardo M Angelone2, Cristen LaPierre1, and Lawrence L Wald1
1Radiology, Massachusetts General Hospital, Charlestown, MA, United States, 2Division of Biomedical Physics, Office of Science and Engineering Laboratories, Center for Devices and Radiological Health, US Food and Drug Administration, Silver Spring, MD, United States
Recently we presented the feasibility study of using a reconfigurable DBS-friendly head coil, composed of a patient-adjustable rotating birdcage transmitter, and an integrated 32-channel receiver array to reduce SAR during imaging of patients with deep brain stimulation implants. Here we introduce the first prototype of such coil system, and present results of finite element simulations on patient-derived numerical models of realistic DBS lead trajectories, which characterize its SAR reduction performance.

Fast Full-Wave Calculation of Electromagnetic Fields for MRI Applications Based on Weak-Form Volume Integral Equation (VIE)
Wan Luo1,2, Shao Ying Huang1, Jiasheng Su1, Zu-Hui Ma1, and J. Thomas Vaughan3
1Singapore University of Technology and Design, Singapore, Singapore, 2University of Electronic Science and Technology of China, Chengdu, China, People's Republic of, 3University of Minnesota, Minneapolis, MN, United States
When B0 in a MRI system increases, peaks and nulls are formed in the energy/field distribution inside the subject under scan, which causes safety issues and deteriorates imaging accuracy, respectively. Therefore, a quick and accurate electromagnetic simulation of the human body is crucial for predicting the temperature and specific absorption rate distribution before a scan. Here, we develop a solver based on the weak-form volume integral equation (VIE) and accelerated by the fast Fourier transform method. It requires much less CPU time and memory compared with the traditional strong-form VIE and the popular FDTD-based commercial software SEMCAD.

Inter-laboratory study of a computational radiofrequency coil model at 64 MHz
Elena Lucano1,2, Mikhail Kozlov3, Eugenia Cabot4, Sara Louie5, Marc Horner5, Wolfgang Kainz1, Gonzalo G Mendoza1, Aiping Yao4,6, Earl Zastrow4,6, Niels Kuster4,6, and Leonardo M Angelone1
1Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, U.S. Food and Drug Administration, Silver Spring, MD, United States, 2Department of Information Engineering, Electronics and Telecommunications, University of Rome "Sapienza", Rome, Italy, 3MR:comp GmbH, Gelsenkirchen, Germany, 4IT'IS foundation, Zurich, Switzerland, 5ANSYS, Inc., Canonsburg, PA, United States,6Department of Information Technology and Electrical Engineering, ETH, Zurich, Switzerland
Preliminary results of an ongoing inter-laboratory study are presented. The eventual purpose of the effort is to develop a methodology that harmonizes RF-modeling and RF-testing protocol for use in RF exposure assessment. In this phase of the study, numerical and experimental data were collected from four laboratories for unloaded and loaded coil conditions. Only information about the geometry and resonance frequency of the physical coil was provided. Qualitatively good agreement across all teams was found. Subsequent phases of the study shall include a methodology on uncertainty analysis associated with the numerical and experimental methods that can be used in practice

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