Transmit Array Technology
Wednesday 22 April 2009
Room 312 10:30-12:30


Peter Ullmann and Yudong Zhu

10:30  390. 8-Channel Eigenmode Tx-Array at 3T for Tx-SENSE
    Scott B. King1, Jarod Matwiy1, Mike J. Smith1, Ulrich Fontius2, Franz Schmitt2, Boguslaw Tomanek3
National Research Council of Canada, Institute for Biodiagnostics, Winnipeg, Manitoba, Canada; 2Siemens AG, Erlangen, Germany; 3National Research Council of Canada, Institute for Biodiagnostics (West), Calgary, Alberta, Canada
    Tx-array elements that have B1 field distributions that are completely orthogonal may allow the complimentary effect of Rx-array data compression but on the transmit side for Tx-SENSE applications, minimizing the number of transmit channels required to achieve a certain Tx-SENSE reduction factor. Here we report on an eigenmode Tx-array solution that generates 8-channel orthogonal B1 fields used in conjunction with an 8-channel transmitter MRI system for Tx-SENSE MRI.
10:42 391. Clinical Imaging at 7T with a 16 Channel Whole Body Coil and 32 Receive Channels
    J. Thomas Vaughan1, Carl Snyder1, Lance Delabarre1, Jinfeng Tian1, Gregor Adriany1, Peter Andersen1, John Strupp1, Kamil Ugurbil1
Radiology, University of Minnesota, Minneapolis, MN, USA
    Reported is the first demonstration of “clinical mode” whole body imaging at 7T, using a body coil for best uniformity together with local receivers for improved sensitivity. Body imaging at 7T however requires new technology and methods to mitigate the severe RF artifacts encountered by conventional approaches. To achieve safe and successful body images, an actively detuned, 16-channel TEM body coil was used together with a pair of 8 channel receive arrays. B1 shimming was employed to optimize the RF field over the ROI. 32 combined channels from the body coil and the receive arrays were used to maximize signal.
10:54 392. In-Vivo RF Power-Controlled B1 Shimming with Tx/Rx Array and with Tx Array Combined Rx Only Coil Without B1 Measurements
    Tamer S. Ibrahim1, Yik-Kiong Hue, Lin Tang2, Tiejun Zhao3, Fernando E. Boada, Howard J. Aizenstein
1Bioengineering and Radiology, University of Pittsburgh, Pittsburgh, PA, USA; 2University of Oklahoma; 3Siemens Medical Solutions
    Several major obstacles have dampened wide implementation of multi-transmission methods including requirements to have the knowledge of how the RF fields produced by current MRI coils/arrays behave in every imaged subject using B1 field measurement/mapping prior to performing the multi-transmission experiment. This work aims overcoming this subject-dependence issue while maintaining a high-SNR intact through the development of subject-insensitive multi-transmit arrays with receive-only inserts.


11:06 393. 7 Tesla Transmit-Receive Array for Carotid Imaging: Simulation and Experiment
    Graham Wiggins1, Bei Zhang1, Qi Duan1, Riccardo Lattanzi1, Stephan Biber2, Bernd Stoeckel3, Kellyanne McGorty1, Daniel K. Sodickson1
Radiology, Center for Biomedical Imaging, NYU School of Medicine, New York, NY, USA; 2Siemens Healthcare, Erlangen, Germany; 3Siemens Medical Solutions USA Inc., New York, NY, USA
    Full wave electromagnetic simulations were used to explore design options for a 4-element transmit and 8 element receive array for imaging the carotid arteries in humans at 7 Tesla. By phasing the excitation of the transmit elements, B1+ excitation efficiency at the depth of the carotids was improved. The simulations suggested shifting the transmit and receive elements relative to each other to account for the twisting B1+ and B1- fields at 7T. A carotid array was constructed based on these observations, and was compared to a similar receive array at 3 Tesla. Substantial SNR gains were observed.
11:18 394. An Eight-Channel Phased Array RF Coil for Spine MR Imaging at 7 Tesla
    Oliver Kraff1,2, Stefan Kruszona1,2, Andreas K. Bitz1,2, Stephan Orzada1,2, Stefan Maderwald1,2, Lena C. Schaefer1,2, Irina Brote1,2, Mark E. Ladd1,2, Harald H. Quick1,2
Erwin L. Hahn Institute for MRI, Essen, Germany; 2Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Essen, Germany
    A transmit/receive RF array, build of eight overlapping loop coils, has been developed for imaging the human spine at 7T. We characterize this prototype in simulations and bench measurements as well as in phantom and in vivo measurements. Numerical simulations were performed for design optimization as well as safety validation. In vivo images show a good excitation along the spine over a 40 cm FOV. Anatomic details such as the vertebral bodies, the dens, or the longitudinal ligaments are well visualized. Our results indicate that this phased array coil could open a promising new application field in 7T clinical research.
11:30 395. Vector Iterative Pre-Distortion: An Auto-Calibration Method for Transmit Arrays
    Pascal Pawel Stang1, Adam Kerr1, William Grissom1, John Mark Pauly1, Greig Cameron Scott1
Electrical Engineering, Stanford University, Stanford, CA, USA
    Transmit arrays hold growing promise for MRI by enabling improved safety, RF homogeneity, selectivity, and pulse acceleration. Yet, achieving good performance with complex Transmit-SENSE pulses requires that array channels be characterized, closely calibrated, and decoupled. We propose Vector Iterative Pre-distortion (VIP), a multi-channel iterative correction method which pre-distorts RF amplifier input to achieve desired output at the coil. VIP uses current sensor feedback to detect and correct errors in the transmit path yielding substantially improved RF fidelity. We successfully demonstrate VIP as general method for correcting non-ideal transmit path performance, and show improved Transmit-SENSE results when using the method.
11:42 396. Variable Power Combiner for a 7T Butler Matrix Coil Array
    Pedram Yazdanbakhsh1, Markus Fester2, Ralph Oppelt3, Andreas Bitz4, Oliver Kraff4, Stephan Orzada4, Mark E. Ladd4, Klaus Solbach1
High Frequency Technique, University Duisburg-Essen, Duisburg, NRW, Germany; 2Siemens Medical Solutions, Erlangen, Germany; 3Siemens Corporate Technology, Erlangen, Germany; 4Erwin L. Hahn Institute for Magnetic Resonance Imaging, Essen, Germany
    Butler matrix networks are used for the excitation of phase modes of MRI coil arrays. The standard excitation method is to connect each power amplifier output to one of the (input) mode ports of a Butler matrix. This allows all phase modes of the coil array to be excited at equal power level. However, not all modes are equally useful, since the lowest-order CP mode is the dominant mode, while higher modes are less important to excite and CP- modes may be completely unnecessary to excite. In this work an eight-channel variable power combiner has been designed and fabricated using an 8×8 Butler Matrix network to excite the coil array in a 7 T MRI system.
11:54 397. 7T Current-Mode Class-D (CMCD) RF Power Amplifier
    Natalia Gudino1, Jeremiah A. Heilman2, Matthew J. Riffe3, Chris A. Flask4, Mark A. Griswold4
Biomedical Engineering , Case Center for Imaging Research, Cleveland, OH, USA; 2Physics , Case Center for Imaging Research, Cleveland, OH, USA; 3Biomedical Engineering, Case Center for Imaging Research, Cleveland, OH, USA; 4Radiology, Case Center for Imaging Research, Cleveland, OH, USA
    We present preliminary results of the first, to our knowledge, an on-coil class D, current mode (CDCM) amplifier for 7T applications. High efficiency, intrinsic decoupling and the elimination of high power RF cables make this configuration promising for building a 7T array parallel RF transmission We successfully obtained an axial image of a saline phantom using a multislice FLASH sequence in a 7T Bruker Biospec scanner. We think this represents a promising advance for high field multichannel transmit arrays.
12:06 398. Frequency Offset Cartesian Feedback Control System for MRI Power Amplifier
    Marta Gaia Zanchi1, John Mark Pauly1, Greig Cameron Scott1
Electrical Engineering, Stanford University, Stanford, CA, USA
    We present a Cartesian feedback system based on complex baseband loop driver amplifiers to control the radiofrequency signal generated by MRI power amplifiers. By shifting the control baseband frequencies far away from DC, our system eliminates the quadrature excitation ghosts and unmodulated LO leakage—caused by baseband mismatches and DC offsets—that can affect power amplifier control systems based on the classic Cartesian feedback method.
12:18 399. Transmit B1 Field Pattern Control Using RF Current Source Technique
    Wonje Lee1, Eddy B. Boskamp1, Thomas Grist2, Krishna Kurpad2
GE Healthcare, Waukesha, WI, USA; 2Radiology, University of Wisconsin - Madison

In this work a two-channel parallel excitation system incorporating an RF current source technology is described and its practical use is demonstrated at 3T in a head size volume transmit coil. The B1 field pattern was successfully controlled by the current source integration without using any other decoupling strategies, which has the potential to improve parallel transmit systems by forcing the drive currents into current elements in load dependent transmit environments at high fields.