Transmit Technology
Monday 3 May 2010
Room A6 11:00-13:00 Moderators: Kenneth M. Bradshaw and Tamer Ibrahim

11:00 43.

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. Griswold4

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.

11:12   44.

RF Sensor Considerations for Input Predistortion Correction of Transmit Arrays
Pascal Stang1, Marta Zanchi1, William Grissom1, Adam Kerr1, John Pauly1, Greig Scott1
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.

11:24 45. 

Efficient EPI Friendly 3x3 Array with Receive-Only Array Insert
Tamer S. Ibrahim1, Tiejun Zhao2, Fernando E. Boada3
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.

11:36 46. 

Constellation Coil
Yudong Zhu1, Ryan Brown1, Cem Deniz1, Leeor Alon1, Kellyanne Mcgorty1, Daniel Sodickson1
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.

11:48 47.  

Reduce Power Deposition Using Microstrip Array with Tilted Elements at 7T
Yong Pang1, Bing Wu2, Xiaoliang Zhang2,3
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.

12:00 48. 

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 Berg1
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.

12:12  49. 

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 Ugurbil1
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.

12:24   50. 

16-Channel Tx/Rx Body Coil for RF Shimming with Selected Cp Modes at 7T
Stephan Orzada1,2, Stefan Maderwald1,2, Oliver Kraff1,2, Irina Brote1,2, Mark E. Ladd1,2, Klaus Solbach3, Pedram Yazdanbakhsh3, Achim Bahr4, Hans-Peter Fautz5, Andreas K. Bitz1,2
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.

12:36 51.

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. Wiggins1
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.

12:48 52.

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 Crozier1
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.



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