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

RF Engineering: Far Fields & High Dielectrics
Wednesday 24 April 2013
Room 151 AG  10:00 - 12:00 Moderators: David O. Brunner, Cornelis A. T. van den Berg

10:00 0389.   Miniaturized Patch Antenna for Traveling-Wave Excitation: Pilot Study at 7 T MRI
Sukhoon Oh1, Elena Semouchkina2, Thomas Neuberger3, Michael T. Lanagan4, Bei Zhang1, Cem Murat Deniz5, and Christopher Michael Collins1
1Center for Biomedical Imaging, School of Medicine, New York University, New York, New York, United States, 2Department of Electrical and Computer Engineering, Michigan Technological University, Houghton, MI, United States, 3Department of Bioengineering, The Pennsylvania State University, University Park, Pennsylvania, United States, 4Engineering Science and Mechanics, The Pennsylvania State University, University Park, Pennsylvania, United States, 5New York University, New York, New York, United States

Traveling-wave excitation in ultra-high field MRI has been introduced in an effort to increase field of view and to enhance RF transmit field homogeneity. Here we present initial results at 300 MHz using a recently-developed patch antenna for excitation. The antenna is 80% smaller in size than conventional patch antenna and has an asymmetric design to produce a circularly-polarized field with only a single feed.

10:12 0390.   
Designing a Practical Dielectric Lining for a Whole Body Traveling Wave Setup at 7T: Tradeoff Between RF Performance and Ease of Handling
Anna Andreychenko1, Hugo Kroeze1, Wouter Koning1, Alexander J.E. Raaijmakers1, Froukje E. Euwe2, Jan J.W. Lagendijk1, Peter R. Luijten1, Dennis W.J. Klomp1, and Cornelis A.T. van den Berg1
1Imaging Division, UMC Utrecht, Utrecht, Utrecht, Netherlands, 2Medical Technology, UMC Utrecht, Utrecht, Utrecht, Netherlands

A multi-mode coaxial waveguide was proposed as an alternative to volume body coil at 7T. However, use of the initial design in clinical practice was not feasible. Here, the design was adapted to use together with the patient table in the bore of a clinical MR scanner. With the first in-vivo MR experiments a sufficient RF shimming performance and B1+ efficiency of the modified design was demonstrated. The modified multi-mode coaxial waveguide is significantly easier to handle and, thus, can be effectively applied in clinical practice as a high field “body coil” at 7T.

10:24 0391.   
Mode Matching for the Modeling and Safety Assessment of Multiple-Channel Waveguide Transmission
Jan Paška1, David Otto Brunner2, Juerg Froehlich1, and Klaas P. Pruessmann2
1Laboratory for Electromagnetic Fields and Microwave Electronics, ETH Zurich, Zurich, Switzerland, 2Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland

Travelling wave with multiple channels (TWM) is a new RF system. First experimental results in phantoms exist. For in-vivo imaging a careful safety assessment is necessary. EM-simulations of the TWM are difficult due to the large setup. In a previous work, the EM-domain was divided into subdomains and combined in a post-processing step, the FDTD method was used for EM-simulations, which has difficulties in modeling evanescent modes and the waveguide discontinuity. In this work this approach was extended. FEM was used for the simulation of all subdomains. The improved method yields more precise EM-simulations and a less conservative power limit.

10:36 0392.   A Travelling Wave Antenna with Matched Waveguide for Head Imaging at 7 T
Daniel James Lee1 and Paul M. Glover1
1SPMMRC, University of Nottingham, Nottingham, Nottinghamshire, United Kingdom

The travelling wave approach to MRI uses an antenna to propagate a TW through the bore of a 7T+ scanner. A novel design where the antenna is incorporated into a dielectric waveguide which then matches the incident wave into the head is presented. Such a set up has been constructed and tested in vivo.

10:48 0393.   
Combined Loop + Dipole Arrays for 7 T Brain Imaging
Yigitcan Eryaman1,2, Bastien Guérin3, Robert Kosior1,2, Elfar Adalsteinsson4,5, and Lawrence L. Wald2,5
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, 3Martinos Center for Biomedical Imaging, Dept. of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, 4Dept. of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, 5Harvard-MIT Health Sciences and Technology, Cambridge, MA, United States

We compared the trade-off between local SAR and excitation fidelity for three 7T head transmit array configurations to determine if the approximate orthogonality between the loop coil and dipole element field patterns could be exploited for pTx. Using simulated E and B fields for 8 and 16 element loop arrays and an 8 loop + 8 dipole array, we calculated SAR constrained pTx and RF shimming pulses. Our results show that an array composed of both loop and dipole elements outperforms an array that is composed of loops only in SAR and excitation fidelity.

11:00 0394.   Towards Ultimate SNR: Combination of a Multi-Element TX/RX Dipole Antenna Array with Nested and Meander Shaped RX Dipole Antenna at 7.0 Tesla
Celal Özerdem1, Lukas Winter1, Katharina Fuchs1, and Thoralf Niendorf1,2
1Berlin Ultrahigh Field Faciliy (BUFF), Max Delbrück Center for Molecular Medicine (MDC), Berlin-Buch, Germany, 2Experimental and Clinical Research Center, a joint cooperation between the Charité Medical Faculty, Berlin, Germany

This work demonstrates use of meander formed RX dipoles to improve the receive capability of an array consisting of TX/RX bow tie dipole antennas.For this purpose numerical field simulations and phantom experiments are performed to characterize the transmit behavior of the array and to measure the SNR of the proposed array.

11:12 0395.   
A Two-Dimensional 16 Channel Dipole Transceiver Array for Cardiac MR at 7.0 T: Design, Evaluation of RF Shimming Behavior and Application in CINE Imaging
Andreas Graessl1, Lukas Winter1, Celal Özerdem1, Fabian Hezel1, Katharina Fuchs1, Harald Pfeiffer2, Werner Hoffmann2, and Thoralf Niendorf1,3
1Berlin Ultrahigh Field Facility, Max-Delbrueck-Center for Molecular Medicine, Berlin, Germany, 2Physikalisch-Technische Bundesanstalt (PTB), Berlin, Germany, 3Experimental and Clinical Research Center (ECRC), Charité Campus Buch, Humboldt-University, Berlin, Germany

A two-dimensional 16-channel transceiver array based on dipole elements is designed and applied to Cardiac MR (CMR) at 7.0 Tesla. Dipole Antennas are a promising approach to provide a homogeneous B1+ field distribution and good transmit efficiency in the short wavelength regime of Ultrahigh field MRI. RF efficiency optimization is applied and assessed for slice-by-slice shimming based on simulation data. The transition to in-vivo studies succeeded and revealed excellent B1+-homogeneity and a high myocardium/blood contrast over the whole cardiac region with a single RF shim setting and without subject-specific B1+-shimming or element retuning.

11:24 0396.   Drastic Enhancement and Manipulation of RF Field with Ultra High Dielectric Constant (UHDC) Material at 3T
Sebastian Rupprecht1, Christopher T. Sica1, Raffi Sahul2, Seongtae Kwon2, Michael T. Lanagan3, and Qing X. Yang1,4
1Department of Radiology, The Pennsylvania State University College of Medicine, Hershey, PA, United States, 2TRS Technologies Inc., State College, PA, United States, 3Department of Engineering Science and Mechanics, The Pennsylvania State University, State College, PA, United States, 4Department of Neurosurgery, The Pennsylvania State University College of Medicine, Hershey, PA, United States

Utilization of monolithic materials with ultra high dielectric constant (lower case Greek epsilonr = 800 -1200) to generate a drastically focused and enhanced transmission field in the sample of interest (up to 500% increased) and reception field (up to 4 fold increased) with standard hardware at 3T. Currently this translates to at least 40 % higher signal to noise ratio with strong potential higher values. Additionally the transmission power was reduced to 1/3 (lower case Greek epsilonr =800) and respectively 1/27 (lower case Greek epsilonr =1200) of the initial power with no uHDC material.

11:36 0397.   Practical Methods for Improved B1+-Homogeneity in 3T Breast Imaging
Simone Angela Winkler1 and Brian K. Rutt1
1Department of Radiology, Stanford University, Stanford, CA, United States

3T MRI is increasingly used because of its intrinsic SNR benefits compared to 1.5T MRI. Increased B1+ inhomogeneities in 3T MRI can lead to left-right B1+-asymmetry in the breast. This abstract presents simulations of several different methods for compensating left-right B1+-asymmetries in the breast region by means of 1) 2-channel RF shimming (I/Q-phase/amplitude adjustments); 2) dielectric-absorptive shimming; or 3) a combination. Both approaches are adaptable to a wide range of MR systems to yield a simple, practical, and inexpensive procedure without SAR penalty for uniform contrast and quantitative parameter estimates, and ultimately, more accurate detection and monitoring of breast cancer.

11:48 0398.   
Design of a Resonant Ceramic Array for Cardiac Imaging at High Field Strengths
Sebastian A. Aussenhofer1, Maarten J. Versluis1, and Andrew Webb1
1Department of Radiology Leiden University Medical Center, CJ Gorter Center for High Field MRI, Leiden, South Holland, Netherlands

Most high field imaging coils are based on lumped elements and copper strips or transmission line elements. Recently an alternative design was presented using dipole antennas mounted on high permittivity non-resonant ceramic blocks in order to force the penetrating wave into the far field region. In this current work a new concept is presented which uses unilaterally shielded high permittivity ceramic discs which, when coupled to the body, are designed to resonate in the TE01 mode.