1.5T On-Coil Current-Mode Class-D (CMCD) Amplifier with Amplitude Modulation Feedback and Voltage-Mode Class-D (VMCD) Preamplifier
Natalia Gudino¹, Matthew J. Riffe¹, Lisa Bauer², Jeremiah A. Heilman³, Mark A. Griswold^4
1Biomedical Engineering, Case Western Reserve University, Cleveland, OH, United States; ²Physics Department, Case Western Reserve University, Cleveland, OH, United States; ³3Quality Electrodynamics, Mayfield Village, OH, United States; ⁴Radiology Department, Case Western Reserve University, Cleveland, OH, United States

We present a Current-Mode Class-D (CMCD) Feedback amplifier with class-D preamplification that avoids the characteristic DC losses of linear preamplification. We demonstrated a good wave profile of the AM feedback system that modulates the RF pulse and preliminary images that prove successful operation of the system in the scanner.

RF Sensor Considerations for Input Predistortion Correction of Transmit Arrays
Pascal Stang¹, Marta Zanchi¹, William Grissom¹, Adam Kerr¹, John Pauly¹, Greig Scott^1
1Electrical Engineering, Stanford University, Stanford, CA, United States

Transmit arrays promise accelerated excitation, B1 shimming, and the potential for SAR and RF safety management. Yet good results demand high-fidelity RF playback in a challenging multi-channel environment.  Parallel transmit RF systems must overcome a host of issues including mutual coupling, loading variations, RF amplifier non-linearity, ill-defined impedances, and memory effects.  We have proposed Vector Iterative Predistortion and Cartesian Feedback as input predistortion methods to address PTx challenges.  We now present our on-coil and in-line RF feedback sensors critical to these technologies, and discuss their relative capabilities in the context of PTx array control.

Efficient EPI Friendly 3x3 Array with Receive-Only Array Insert
Tamer S. Ibrahim¹, Tiejun Zhao², Fernando E. Boada^3
1Departments of Bioengineering and Radiology, University of Pittsburgh, Pittsburgh, PA, United States; ²Siemens Medical Solutions; ³Department of Radiology, University of Pittsburgh, Pittsburgh, PA, United States

In this work, we will examine the efficiency of Tic Tac Toe RF array designs including the original 2x2 and new 3x3 versions and extend their usefulness to become more application friendly.   This will be achieved by yielding optimal SNR (through a combination with a separate 7-channel receive-only array) and by designing echo-planar imaging (EPI) compatible prototypes.  The results show excellent improvement in eddy current reduction and SNR enhancement with a receive-only array insert.

Constellation Coil
Yudong Zhu¹, Ryan Brown¹, Cem Deniz¹, Leeor Alon¹, Kellyanne Mcgorty¹, Daniel Sodickson^1
1New York University School of Medicine, New York, United States

An RF coil plays a central role in the induction of a B1 field for creating an excitation profile, and meanwhile, a concomitant E field that causes undesirable RF loss and SAR. A coil structure that supports flexible current distribution control is essential for management of both the excitation profile and RF power, and is hence a key factor in parallel Tx performances. We developed a “constellation coil” which prioritizes field optimization-based Tx/Rx improvement with a continuum structure, and accommodates scalability supporting highly parallel Tx/Rx. Preliminary 7T MRI results obtained with prototype parallel Tx and Rx constellation coils are presented.

Reduce Power Deposition Using Microstrip Array with Tilted Elements at 7T
Yong Pang¹, Bing Wu², Xiaoliang Zhang^2,3
1Radiology and Biomedical imaging, University of California San Francisco, San Francisco, CA , United States; ²Radiology and Biomedical imaging, University of California San Francisco, San Francisco, CA, United States; ³UCSF/UC Berkeley Joint Graduate Group in Bioengineering, San Francisco & Berkeley, CA, United States

Power deposition increases with the static magnetic field strength. In this work, a microstrip array with tilted elements is built and the electric field E and magnetic field B1+ are simulated using FDTD method. Their ratio E/B is used to predict the power deposition for two type of different arrays: microstrip array with regular elements and tilted elements. Results show that using the tilted array, coil efficiency and decoupling between elements can be increased. The reduction in E/B ratio indicates possible reduction in power deposition.

High-Field Imaging at Low SAR: Tx/Rx Prostate Coil Array Using Radiative Elements for Efficient Antenna-Patient Power Transfer
Alexander Raaijmakers¹, Ozlem Ipek¹, Dennis Klomp¹, Hugo Kroeze¹, Bart van de Bank¹, Vincent Boer¹, Paul Harvey², Cecilia Possanzini², Jan Lagendijk¹, Nico van den Berg^1
1Radiotherapy, UMC Utrecht, Utrecht, Netherlands; ²Philips Healthcare, Best, Netherlands

Abdominal imaging at 7 T is challenging due to reduced RF penetration at 298 MHz. Conventional high-field surface coil arrays with stripline elements deposit high SAR levels and suffer from inhomogeneous B1-field distribution. We present results of a prototype coil array consisting of so-called radiative antennas. These elements emit power to the region of interest more efficiently. Simulations and volunteer measurements show reduced SAR levels and increased image homogeneity.

RF Coil Designs for 7T Cardiac Imaging
John Thomas Vaughan¹, Carl j. Snyder¹, Lance Delabarre¹, Jinfeng Tian¹, Can Akgun¹, Gregor Adriany¹, John Strupp¹, Peter Andersen¹, Eddie Auerbach¹, Pierre-Francois Van de Moortele¹, Kamil Ugurbil^1
1University of Minnesota-Center for Magnetic Resonance Research, Minneapolis, MN, United States

Our objective was to investigate three RF coil approaches to human cardiac imaging at 7T.  The first approach used a 16-channel, whole body coil together with 16 channel local receivers.  The second approach used a 16-channel transceiver array.  And the third approach made use of a close fitting torso coil with local 16 channel receivers.  The three approaches were evaluated by image and efficiency data, as well as practical constraints such as lead placement, receiver coil accommodation, and human comfort.  All three coils were used successfully, and found to offer options and respective trade-offs for successful cardiac imaging at 7T.

16-Channel Tx/Rx Body Coil for RF Shimming with Selected Cp Modes at 7T
Stephan Orzada^1,2, Stefan Maderwald^1,2, Oliver Kraff^1,2, Irina Brote^1,2, Mark E. Ladd^1,2, Klaus Solbach³, Pedram Yazdanbakhsh³, Achim Bahr⁴, Hans-Peter Fautz⁵, Andreas K. Bitz^1,2
1Erwin L. Hahn Institute for Magnetic Resonance Imaging, Essen, NRW, Germany; ²Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Essen, NRW, Germany; ³High Frequency Engineering, University Duisburg-Essen, Duisburg, NRW, Germany; ⁴IMST GmbH, Kamp-Lintfort, Germany; ⁵Siemens Healthcare Sector, Erlangen, Germany

To increase the capability of a 7 Tesla 8-channel RF shimming system, a 16-channel Tx/Rx body coil was built to be used with a 16-channel Butler matrix for mode compression and an 8-channel variable power combiner. The array has a large field of view and shows good homogeneity in gradient echo images. RF shimming with mode compression and variable power combining was successfully performed in human volunteers.

Optimizing 7T Spine Array Design Through Offsetting of Transmit and Receive Elements and Quadrature Excitation
Qi Duan¹, Daniel K. Sodickson¹, Riccardo Lattanzi¹, Bei Zhang¹, Graham C. Wiggins^1
1Center for Biomedical Imaging, Department of Radiology, NYU School of Medicine, New York, NY, United States

This abstract presents a novel 7T spine array design for optimizing SNR at the regions of interest. This design takes into account opportunities for quadrature excitation and the twisting of B1+ and B1- fields to optimize SNR within the ROIs. The design parameters were quantitatively optimized via full wave simulation. The benefits of the proposed design were validated via actual MR scan with higher SNR within ROI, more efficient excitation, and less peak local heating than alternative designs. This design can be easily extended for larger longitudinal coverage, providing a more efficient excitation and MR images without obvious signal nulls.

On the Reduction of the Transmit B1 Non-Uniformity and SAR Using a Single-Element Rotating RF Coil
Feng Liu¹, Ewald Weber¹, Adnan Trakic¹, Hua Wang¹, Stuart Crozier^1
1The School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, St.Lucia, Queensland, Australia

In this work, we presented a complete technological solution for tailoring uniform RF fields and minimizing tissue heating for high field MRI. The success of the new B1 shimming technique is largely facilitated by a mechanically rotating RF coil (RRFC) configuration. The proposed method is explained with a biologically loaded, one-element rotating coil operating at 400 MHz. The coil is modelled using the method of moment (MoM) and tissue-equivalent sphere phantom is loaded and modelled using the Green’s function method. A sensitivity matrix is constructed based on the pre-characterized B1 and electric field profiles of a large number of angular positions around the imaged phantom, an optimization procedure is then employed for the determination of optimal driving configuration by solving the ill-posed linear system equation. Test simulations show that, compared with conventional bird-cage mode driving scheme, the proposed excitation scheme is capable of significant improvement of the B1 -field homogeneity and reduction of the local and global SAR values. This primary study indicates that the proposed RF excitation technology can effectively perform field-tailoring and might hold the potential of solving the high frequency RF problem.