Room 150 AG

16:00 - 18:00

Moderators:

Andreas Bitz, Arthur W. Magil

Pascal P. Stang¹, John M. Pauly¹, and Greig C. Scott^1
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.

Maryam Etezadi-Amoli¹, Pascal P. Stang¹, Adam Kerr¹, John Pauly¹, and Greig C. Scott^1
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.

Mohamed Ahmed Abou-Khousa¹, Joseph S. Gati¹, and Ravi Menon^1,2
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.

Michael Twieg¹, Matthew J. Riffe², Natalia Gudino², and Mark A. Griswold^1,3
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.

Lance DelaBarre¹, Daniel Myer², and University of Minnesota University of Minnesota Vaughan^3
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.

Mingyan Li¹, Jin Jin¹, Feng Liu¹, Adnan Trakic¹, Ewald Weber¹, and Stuart Crozier^1
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.

Jens Höfflin¹, Elmar Fischer², Jürgen Hennig², and Jan G. Korvink^1,3
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.

James Tropp^1
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.

Matteo Pavan¹, David Otto Brunner^1,2, Manuel Schneider¹, and Klaas P. Pruessmann^1
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.

Sung-Min Sohn¹, Lance DelaBarre², Anand Gopinath¹, and University of Minnesota University of Minnesota Vaughan^1,2
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.