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

RF Circuits & Concepts
Thursday 25 April 2013
Room 150 AG  16:00 - 18:00 Moderators: Andreas Bitz, Arthur W. Magil

16:00 0722.   
A High-Efficiency Linear MRI Transmit Amplifier Using Envelope-Tracking
Pascal P. Stang1, John M. Pauly1, and Greig C. Scott1
1Electrical Engineering, Stanford University, Stanford, CALIFORNIA, United States

Linear Class-AB RF amplifiers are favored in MRI for their excellent linearity and phase stability. Yet the wide peak-to-average power spread common in MRI transmit pulse sequences forces linear amplifiers to run at dismal efficiencies, producing high levels of pulsatile heat dissipation which negatively impacts design, performance, and operating consistency (e.g. memory effects). We apply envelope-tracking to classic linear amplifiers to achieve higher efficiency and reduced thermal stress without sacrificing performance or fidelity. Comparative results are presented on sinc and hard pulses at a range of amplitudes. Broad DC-to-RF efficiency improvements up to 3.5x are observed, translating into reductions up to 40% in dissipated power while delivering the same RF output.

16:12 0723.   Image-Based Validation of Optically Coupled Current Sensor for RF Safety Monitoring
Maryam Etezadi-Amoli1, Pascal P. Stang1, Adam Kerr1, John Pauly1, and Greig C. Scott1
1Electrical Engineering, Stanford University, Stanford, CA, United States

We validated the accuracy of an optically coupled, photonically powered RF current sensor by comparing both submerged and free space current sensor measurements to current computed from B1 maps within the imaging volume. The sensor and image-based measurements differed by less than 10%. Such quantitative methods of real-time current monitoring could greatly enhance the safety of MRI scans performed in the presence of long conductors, such as interventional guidewires and ablation devices.

16:24 0724.   Magnetic Walls for RF Coil Elements Decoupling
Mohamed Ahmed Abou-Khousa1, Joseph S. Gati1, and Ravi Menon1,2
1Robarts Research Institute, Western University, London, Ontario, Canada, 2Dept. of Medical Biophysics, Western University, London, Onatrio, Canada

Novel and essentially lossless RF array coil elements decoupling method is introduced and analyzed. The proposed method is based on utilizing artificially engineered “magnetic walls” to effectively isolate the array elements. The efficacy of the decoupling method is demonstrated with 7T linear Tx/Rx visual cortex array for high-resolution fMRI. The method can be used quite systematically to decouple transmit-, receive- as well as transceive-arrays.

16:36 0725.   
Enhancement Mode GaN (EGaN) FETs for On-Coil MRI Transmit Amplifiers
Michael Twieg1, Matthew J. Riffe2, Natalia Gudino2, and Mark A. Griswold1,3
1Dept. of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, OH, United States, 2Dept. of Biomedical Engineering, Case Western Reserve University, Cleveland, OH, United States, 3Dept. of Radiology, Case Western Reserve University, Cleveland, OH, United States

We present the first use of enhancement-mode Gallium Nitride (eGaN) FETs, a novel emerging transistor technology, in on-coil transmit amplifiers for MRI. This work demonstrates the operation of a complete miniature on-coil amplifier module using the current mode class D (CMCD) topology as well as AM modulation via envelope elimination and restoration (EER). We show that the small size and low cost of eGaN FETs offer significant advantages over traditional Silicon (Si) FET technology for small and high efficiency RF amplifiers.

16:48 0726.   Muti-Channel, In-Bore Power Amplifiers for Multi-Channel Coil at 7T
Lance DelaBarre1, Daniel Myer2, and University of Minnesota University of Minnesota Vaughan3
1Radiology - Center for MR Research, University of Minnesota, Minneapolis, MN, United States, 2CPC Amps, Inc., Hauppauge, NY, United States, 3Radiology - Center for MR Research, U. of Minnesota, Minneapolis, MN, United States

Parallel transmission in MRI has created a need for multiple, independently controlled power amplifier channels. Meeting this need with multiple, intermediate powered amplifiers opens the possibility to move the amplifiers closer to the RF coil to eliminate cable losses. Eight 1 kW A/B linear RF Amplifiers operating inside a 7T magnet bore are demonstrated.

17:00 0727.   Highly Accelerated Parallel Imaging Using Rotating Radiofrequency Coil Array at 7T
Mingyan Li1, Jin Jin1, Feng Liu1, Adnan Trakic1, Ewald Weber1, and Stuart Crozier1
1School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, Queensland, Australia

In this work, the novel 4-element rotating radiofrequency coil array (RRFCA) was numerically modelled, and its potential to accelerate imaging in different modes was investigated at 7T. Compared with lower field, its acceleration ability was strengthened owing to improved electromagnetic isolation and more distinctive sensitivity profiles. As demonstrated by calculated g-maps and reconstructed images, the RRFCA illustrated clear advantages in imaging acceleration compared with 8-element stationary phased-array coils at 7T.

17:12 0728.   Energy Harvesting Towards Autonomous MRI Detection
Jens Höfflin1, Elmar Fischer2, Jürgen Hennig2, and Jan G. Korvink1,3
1Lab of Simulation - Department of Microsystems Engineering, University of Freiburg - IMTEK, Freiburg, Germany, 2Department of Radiology, University Medical Center Freiburg, Freiburg, Germany, 3Freiburg Institute for Advanced Studies - FRIAS, University of Freiburg, Freiburg, Germany

We present a first prototype of an energy harvesting coil made from insulated copper wire wound on a PMMA holder, designed to fit inside the RF transmission coil of a Bruker AVANCE III MRI spectrometer. The harvesting coil scavenges power from gradient switches and RF pulses and converts these AC signals into a DC voltage suitable for driving a receiving coil’s preamplifier. To limit power consumption, we used a monolithic IC preamplifier manufactured at the Fraunhofer IAF III-V foundry. We were able to show that the preamplifier could be driven solely by our harvester with no external power supply necessary.

17:24 0729.   A Quantum Description of Signal Transduction in Magnetic Resonance: Analytical Results, Cavity Damping, and Relaxation
James Tropp1
1GE Healthcare Technologies, Fremont, CA, United States

We expand our earlier quantum mechanical treatment of NMR transduction by the Jaynes-Cummings formalism for a spin ½ coupled to a quantized cavity, to include the effects of cavity losses, and spin relaxation. Some analytical results for Rabi oscillation are also given.

17:36 0730.   
Signal and Noise Propagation in MR Receive Arrays: Theory and Experimental Validation
Matteo Pavan1, David Otto Brunner1,2, Manuel Schneider1, and Klaas P. Pruessmann1
1ETH and University Zurich, Zurich, Switzerland, 2University and ETH Zurich, Zurich, Switzerland

Receive coil arrays are design to maximize Signal to Noise Ratio of the detected spin magnetization. Recent work reveals an experimental case in which high noise correlation could be tolerated in order to increase the SNR performance of a two channels array; a theory that describes this behavior is here explained and experimentally validated. This work is targeted at MRI technologists dealing with receive coil arrays and aims to address a new theoretical understanding of SNR behavior depending on array’s matching condition with clear distinction between the impact of different components along the receive chain.

17:48 0731.   RF Coil Design with Automatic Tuning and Matching
Sung-Min Sohn1, Lance DelaBarre2, Anand Gopinath1, and University of Minnesota University of Minnesota Vaughan1,2
1Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN, United States, 2Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

RF coils with transmission line elements are commonly used for high-field MRI, as both transmitter and transceiver elements. The RF coil element is typically terminated at the variable trimmer capacitors commonly called matching capacitor (Cm) and tuning capacitor (Ct) at one end, and a fixed value capacitor (Cf) at the other to form a capacitively tuned, matched, and shortened half-wave resonator. These resonant coil elements usually have a high quality factor (Q). High transmit efficiency and receiver signal-to-noise (SNR) are dependent on a well tuned (to Larmor frequency) and matched (to load) resonance for the element. Conversely, variable body loading of these coil elements can adversely impact both tuning and matching, and therefore efficiency and SNR of the coil. This study demonstrates a high-speed, electronically actuated, automatic impedance matching and tuning technique to assure optimal coil efficiency and SNR over a range of patient-coil loading conditions.