Transmit Arrays & SAR Monitoring
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Tuesday 8 May 2012
Room 201  16:00 - 18:00 Moderators: Stuart R. Crozier, David I. Hoult

16:00 0305.   
Design, Evaluation and Application of a Modular 32 Channel Transmit/Receive Surface Coil Array for Cardiac MRI at 7T
Andreas Grń▀l1, Wolfgang Renz2, Fabian Hezel1, Tobias Frauenrath1, Harald Pfeiffer3, Werner Hoffmann3, Peter Kellman4, Conrad Martin1, and Thoralf Niendorf1,5
1Berlin Ultrahigh Field Facility, Max-Delbrueck-Center for Molecular Medicine, Berlin, Germany, 2Siemens Healthcare, Erlangen, Germany, 3Physikalisch-Technische Bundesanstalt (PTB), Berlin, Germany, 4Laboratory of Cardiac Energetics, National Institutes of Health/NHLBI, Bethesda, MD, United States,5Experimental and Clinical Research Center (ECRC), CharitÚ Campus Buch, Humboldt-University, Berlin, Germany

Ultrahigh field cardiac MR is challenged by non-uniform B1+-distributions. A modular two-dimensional 32-channel transceiver surface coil array based on loop elements is proposed to improve parallel imaging and B1+ homogeneity for cardiac MR at 7 T. The RF characteristics were satisfying without the need for subject-specific tuning and matching. MR images were acquired showing a rather uniform intensity over the whole cardiac region and a high myocardium/blood contrast without subject-specific B1+-shimming.

16:12 0306.   
7 Tesla MRI of the shoulder and upper extremities using an 8-channel Tx/Rx Coil
Oliver Kraff1,2, Stephan Orzada1,2, Mohamed Choukri1, Anja Fischer1,2, Susanne C Ladd1,2, Mark E Ladd1,2, and Andreas K Bitz1,2
1Erwin L. Hahn Institute for Magnetic Resonance Imaging, University Duisburg-Essen, Essen, Germany, 2Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Essen, Germany

A transmit/receive RF array was built for imaging the shoulder and upper extremities at 7 Tesla. Eight meander stripline elements were aligned on a c-shaped holder which can be split into two parts for easy patient positioning. The coil allows imaging of either the right or left upper extremity. In vivo images of the shoulder and elbow revealed uniform excitation over the whole field-of-view in both gradient and spin echo images, rendering fine anatomical details of cartilage, bone, nerves, vasculature, and tendons.

16:24 0307.   
7 T cardiac imaging with array of radiative antennas compared to loop coil array
Alexander J.E. Raaijmakers1, Maarten J. Versluis2, Sebastian A. Aussenhofer2, Hamza El Aidi1, Peter Luijten1, Tim Leiner1, Cornelis A.T. van den Berg1, and Andrew Webb2
1Imaging Division, UMC Utrecht, Utrecht, Netherlands, 2J.C. Gorter Institute, LUMC, Leiden, Netherlands

Recently, a novel coil array of radiative antennas was introduced for prostate imaging. In this study, the same array is used for cardiac imaging at 7 Tesla. An additional coil array consisting of large loop coils has been constructed for comparison. Both arrays are characterized by FDTD simulations. These show that the array of radiative antennas has much higher B1+ and B1- while the array of large loop coils has much lower SAR levels. Cine MR cardiac images have been obtained, showing higher SNR for the array of radiative antennas. With this array the right coronary artery was successfully depicted.

16:36 0308.   A 16-Element Dual-row Transmit Coil Array for 3D RF Shimming at 9.4 T
G Shajan1, Jens Hoffmann1, Klaus Scheffler1,2, and Rolf Pohmann1
1Max Planck Institute for Biological Cybernetics, Tuebingen, Baden Wuerttemberg, Germany, 2Department for Neuroimaging, University Hospital, Tuebingen, Germany

The well known transmit field inhomogeneity at ultra high field strengths, caused by the shorter RF wavelength in tissue, is most severe in the lower half of the brain. A dual-row 16 element transmit coil that gives the additional flexibility to shim for the lower brain was designed. Numerical investigation revealed excellent static B1 shimming performance. In vivo shimming on a coronal slice demonstrates the ability to improve the transmit field in brain regions that are challenging to image using single-row transmit arrays. The coil can be combined with receive-only arrays for highly sensitive parallel signal reception.

16:48 0309.   A Highly Decoupled 8 Channel Transmit-Receive Loop Array for 7T with Diverse B1 Profiles
Graham Charles Wiggins1, Bei Zhang1, Gang Chen2, and Daniel Sodickson1
1The Bernard and Irene Schwartz Center for Biomedical Imaging, NYU Medical Center, New York, NY, United States, 2The Sackler Institute of Graduate Biomedical Science, NYU School of Medicine, New York, NY, United States

While neighboring coil elements can be decoupled by various means, it is more difficult to mitigate the often substantial coupling to next nearest neighbors. This is particularly problematic with transmit arrays, since preamp decoupling cannot be used to control next nearest neighbor coupling. We present a novel array design with triangular elements which allows capacitive decoupling between neighboring coils and inductive decoupling between next nearest neighbor coils. The triangular elements also present diverse B1 profiles, allowing for acceleration along the Z direction.

17:00 0310.   
16-channel degenerate birdcage T/R loop array head coil for parallel transmit MRI at 7 T
Wei Zhao1, Borjan Gagoski2, Khaldoun Makhoul1, Boris Keil1, Azma Mareyam1, Philipp Hoecht3, and Lawrence L Wald1
1Athinoula A. Martinos Center for Biomedical Imaging, Dept. of Radiology Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts, United States, 2Electrical engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3Siemens Medical Solutions USA Inc., Charlestown, Massachusetts, United States

Parallel transmit method has provided the potential to shape excitations with practical pulse lengths and mitigate B1+ inhomogeneity and B0 inhomogeneity at high field. Increasing the number of transmit channels offers the flexibility for adjusting the phase and amplitude of each channel and the degree of the freedom for improved pulse design and SAR. In this work, we developed a 16-channel T/R degenerate birdcage head coil in a single cylindrical row, with good decoupling (>15dB) between the next neighbors and high B1+ efficiency as compared with that of an 8-channel concentrically shielded T/R array coil.

17:12 0311.   RF safety of the combination of a 31P Tx/Rx endorectal coil and a 1H Tx/Rx body array for 31P MRSI of the prostate at 7T
Andreas K. Bitz1,2, Thiele Kobus3, Tom W. J. Scheenen1,3, and Mark E. Ladd1,2
1Erwin L. Hahn Institute for Magnetic Resonance Imaging, University Duisburg-Essen, Essen, Germany, 2Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Essen, Germany, 3Department of Radiology, Radboud University Nijmegen Medical Centre, Nijmegen, Netherlands

To relate information of cancer-related metabolites to anatomical location within the human prostate, a 31P endorectal coil (ERC) was combined with an 8-channel 1H body array. For compliance testing, RF coupling between both coils was investigated. The compliance test for the coil combination was performed by simulation and in vivo measurements. It could be shown that both coils are well decoupled at both operating frequencies. Whereas the maximum permissible input power of 33 W for the body array was derived from SAR simulations, the maximum permissible input power of 1.9 W for the ERC was derived from in vivo temperature measurements.

17:24 0312.   
An In Vivo Study on Fast PRF Temperature Imaging based on Compressed Sensing: An Alternative Approach to Monitor RF Safety?
Zhipeng Cao1, Sukhoon Oh2, Philipp Ehses3, Giuseppe Carluccio4, Christopher M. Collins2, and Mark A. Griswold5
1Bioengineering, Penn State University, Hershey, PA, United States, 2Radiology, Penn State University, Hershey, PA, United States, 3Neuroimaging, University Hospital Tubingen, Tubingen, Germany, 4Electrical Engineering, The University of Illinois at Chicago, Chicago, IL, United States, 5Radiology, Case Western Reserve University, Cleveland, OH, United States

A proposed image reconstruction method based on compressed sensing to accelerate MR PRF temperature imaging procedure is validated on an in vivo dataset with a simple Cartesian undersampling trajectory. Results imply a novel alternative approach to ensure RF safety for high field MR systems.

17:36 0313. Safe MR scan times based on CEM43 tissue damage thresholds, using electromagnetic and thermal simulations with anatomically correct human models and considering local thermoregulation
Manuel Murbach1,2, Esra Neufeld1, Klaas P Pruessmann2, and Niels Kuster1,3
1IT'IS Foundation, Zurich, Switzerland, 2Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland, 3Swiss Federal Institute of Technology (ETH), Zurich, Switzerland

This study investigates the MR induced thermal load in local temperature hotspots for various anatomical human models and positions. The simulation results are compared to temperature measurements in humans. Based on CEM43 (cumulative equivalent minutes at 43░C) tissue damage thresholds, thermally safe scan times are derived. Local thermoregulation can have a strong impact on SAR induced heating. Safety concerns arise especially for patients with disabled or partially dysfunctional perfusion abilities (e.g. the elderly, diabetics). The high estimated and measured temperature rise indicates the necessity of considering thermal dose models such as CEM43, which take into account exposure time and temperature.

17:48 0314.   
An anatomically realistic temperature phantom of the head for validation of SAR calculations
Nadine N Graedel1, Jonathan R Polimeni1,2, Bastien Guerin1, Borjan Gagoski3, and Lawrence L Wald1,4
1A. A. Martinos Center for Biomedical Imaging, Dept. of Radiology, MGH, Charlestown, MA, United States, 2Harvard Medical School, Boston, MA, United States, 3Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, 4Harvard-MIT Division of Health Sciences and Technology, MA, Cambridge, United States

We constructed a human head phantom which allows 3D temperature mapping using the temperature sensitive contrast agent TmDOTMA. The phantom contains multiple tissue compartments that match those of the human head both in terms of their geometry and their electrical properties.