ISMRM 21st Annual Meeting & Exhibition 20-26 April 2013 Salt Lake City, Utah, USA

Transmit Arrays & RF Safety
Tuesday 23 April 2013
Room 255 EF  13:30 - 15:30 Moderators: Pierre-Francois A. Van de Moortele, Mark A. Griswold

13:30 0284.   in vivo Radio-Frequency Heating Did Not Change Due to the Power Deposition from Similar 3T and 7T Head Coils
Devashish Shrivastava1, Lynn Utecht1, Jinfeng Tian1, Rachana Visaria2, John Hughes1, and University of Minnesota University of Minnesota Vaughan1
1University of Minnesota, Minneapolis, MN, United States, 2MR Safe Devices, Burnsville, MN, United States

Radio-frequency (RF) heating was measured using fluoroptic probes in the scalp; 5 mm, 10 mm, 15 mm, 20 mm and 25 mm in the brain; and rectum in eight anesthetized swine due to a continuous wave power deposition from similar 3T and 7T TEM head coils for three hours (N=4 for each coil). No significant difference was found between the heating produced due to the 3T and 7T coils in the brain and rectum.

13:42 0285.   
Temperature and Perfusion Monitoring During RF Heating of the Human Calf Muscle
Frank F.J. Simonis1, Esben Thade Petersen1, Alexander J.E. Raaijmakers1, Jan J.W. Lagendijk1, and Cornelis A.T. van den Berg1
1Radiotherapy, UMC Utrecht, Utrecht, Utrecht, Netherlands

Recent simulation studies indicate that in clinical MRI experiments local tissue temperature and RF exposure can exceed safety guidelines. By using PRFS MR thermometry and ASL, one can monitor rises in tissue temperatures and perfusion. By combining these techniques during RF heating, we hope to obtain insight in the local thermoregulatory system of the body. Temperature measurements in the calf muscle during a high SAR scan at clinical settings showed that RF heating can lead to significant heating of the outer region of the calf muscle. An increase in perfusion was also detected, in spite of very low perfusion levels.

13:54 0286.   Rapid RF Safety Evaluation for Transmit-Array Coils
Martijn A. Cloos1, Leoor Anon2, Gang Chen2, Graham C. Wiggins3, and Daniel Sodickson2,3
1Radiology, New York University School of Medicine, New York, NY, United States, 2The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY, United States, 3Radiology, NYU Langone Medical Center, New York, NY, United States

Parallel transmission facilitates greatly enhanced flexibility for RF-pulse design, but can potentially lead to localized RF hot-spots. Commonly used RF safety-concepts rely on coil specific RF-simulations. In this work, we present a novel method for transmit-array evaluation based on a single 15 minute MR experiment. The resulting safety limits, albeit conservative, are sufficiently lenient to facilitate in-vivo evaluation of coil performance. This enables a rapid iterative transmit-array coil design procedure driven by representable in-vivo data without the need for a time-consuming simulation and validation process at every step.

14:06 0287.   
Parallel Transmit Excitation at 1.5 T Based on the Minimization of a Driving Function for Device Heating
Natalia Gudino1,2, Merdim Sonmez2, Anthony Z. Faranesh2, Robert J. Lederman2, Robert S. Balaban2, Michael Schacht Hansen2, and Mark A. Griswold1,3
1Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH, United States, 2Division of Intramural Research, National Heart Lung and Blood Institute, National Institutes of Health, Bethesda, MD, United States, 3Department of Radiology, Case Western Reserve University, Cleveland, OH, United States

A transmit planar coil array driven by near-coil current-source amplifier was implemented in iMRI setup to reduce the RF E-field coupling and consequent heating in a wire/guidewire. An optimum set of phases was obtained through minimization of a driving function (W) for device heating and tested in a MRI phantom and animal experiments. Up to 92 % temperature reduction was measured at the tip of the device when transmitting with the array in an optimum phase configuration instead of using the conventional body coil transmitter. B1 constrained minimization of W at predefined ROIs was proposed and tested on the benchtop.

14:18 0288.   
Local SAR Estimation for Human Brain Imaging Using Multi-Channel Transceiver Coil at 7T
Xiaotong Zhang1, Sebastian Schmitter2, Jiaen Liu1, Pierre-Francois Van de Moortele2, and Bin He1,3
1Department of Biomedical Engineering, University of Minnesota, Minneapolis, MN, United States, 2Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States, 3Institute for Engineering in Medicine, University of Minnesota, Minneapolis, MN, United States

Parallel transmission technique has been recognized as a potential powerful tool for B1 inhomogeneities compensation at UHF; however, elevated SAR associated with increased main magnetic field strength remains as a major safety concern in its application. A B1-based subject-specific local SAR estimation approach for single channel transmission is presented in this study, and has been demonstrated in a human brain imaging experiment at 7T using a 16-channel transceiver head coil. The present approach holds strong promises for enabling subject-specific local SAR prediction, which in turn can be used for explicit constraint in B1 shimming and multi-transmit RF pulse design.

14:30 0289.   
Generating Anatomical Human Head Model for Specific Absorption Rate Estimation in Parallel MR Excitation
Ara K. Yeramian1, Martin Haas1, Maxim Zaitsev1, Chris A. Cocosco1, Jan G. Korvink2,3, and Jürgen Hennig1
1Dep. of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, BW, Germany, 2IMTEK, University of Freiburg, Freiburg, BW, Germany, 3FRIAS, Freiburg, BW, Germany

Specific Absorption Rate (SAR) estimation in parallel MR excitation is achieved through EM simulations of RF parallel excitation coils and 3D virtual models of human body parts in interest. To estimate SAR in the human head, we present a segmentation scheme to efficiently generate patient-specific head models, based on MP RAGE and T1-UTE acquired datasets. The multi-stage segmentation scheme comprises morphological and set operations, 2D and 3D component connectivity check, ITK-based hole filling, and two user-interactive stages. Segmentation output are nine head tissues: brain GM, WM, CSF, cortical bone, skull, skin, air cavities, the eye region, and soft tissue.

14:42 0290.   
Use of "Dark Modes" in a Loop + Dipole Array to Reduce SAR in 7T C-Spine Imaging
Yigitcan Eryaman1,2, Elfar Adalsteinsson3,4, and Lawrence L. Wald2,4
1Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, United States, 2Martinos Center for Biomedical Imaging, Dept. of Radiology, MGH, Charlestown, MA, United States, 3Dept. of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, 4Harvard-MIT Health Sciences and Technology, Cambridge, MA, United States

We demonstrated that dipole elements can be used with loop arrays to reduce the local SAR in spine imaging. We placed dipole elements in the middle of each loop of a previously constructed spine array. Adding excitation by the dipoles did not alter the overall B1+ pattern around the spinal cord significantly, yet the 10 g peak local SAR and the global SAR in a human model was reduced by energizing the dipole elements at 7 T.

14:54 0291.   
RF Shimming Capabilities at 9.4 Tesla Using a 16-Channel Dual-Row Array
Jens Hoffmann1, Gunamony Shajan1, Klaus Scheffler1,2, and Rolf Pohmann1
1High-Field Magnetic Resonance Center, Max Planck Institute for Biological Cybernetics, Tuebingen, Baden-Wuerttemberg, Germany, 2Department for Biomedical Magnetic Resonance, University of Tuebingen, Tuebingen, Baden-Wuerttemberg, Germany

Multi-row arrays offer improved coverage in z-direction along with the capability to control the B1+ field in all spatial dimensions for high-field applications. This can be used to trade B1+ homogeneity against power efficiency in arbitrary volumes, depending on the desired application. However, numerical simulations must be provided to assess local SAR. In this work, we demonstrate the RF phase shimming capability of an inductively decoupled dual-row array for 9.4 T neuroimaging using time-domain simulations together with circuit co-simulation as well as RF shimmed, in vivo TSE and MP2RAGE images.

15:06 0292.   
Parallel Transceiver Array Design Using the Modified Folded Dipole for 7T Body Applications
Wonje Lee1, Martijn A. Cloos2, Daniel Sodickson1, and Graham Wiggins1
1Radiology, NYUMC, New York, NY, United States, 2Radiology, New York University School of Medicine, New York, NY, United States

Despite SNR benefits at high fields, body imaging at 7T is challenging as major difficulties remain in strong B1 inhomogeneity, less penetration depth, B1 twisting, and increased SAR in conductive objects. To allow for effective excitations for body applications with a broad variety of pulse design methods, we adopt a new antenna design, “modified-folded dipole (mf-dipole)”, as a transceiver array. The array performance was found to be beneficial to UHF body imaging in favor of transmit efficiency and better B1 characterization. Initial in-vivo images with a 3 spoke pTx pulse aiming for uniform excitation are presented.

15:18 0293.   
Concurrent Monitoring of RF and Gradient Waveforms of Parallel Transmission Pulses by a Field Camera
David Otto Brunner1, Benjamin E. Dietrich1, Mustafa Cavusoglu1, Christoph Barmet1,2, Bertram J. Wilm1, and Klaas P. Pruessmann1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Zurich, Switzerland, 2Skope Magnetic Resonance Technologies, Zurich, Switzerland

Advanced schemes of parallel excitation build upon the close and accurate interplay between several RF channels and gradient waveforms. We present a method based on gradient magnetic field monitoring to measure the temporal evolution of both, the gradient magnetic field (in principle to higher orders) and the RF excitation pulses played by multiple transmit channels fully concurrently a system which is independent from the scanner and the coil array.