Emerging RF: From Micro to Waves
Wednesday 5 May 2010
Room A1 16:00-18:00 Moderators: Nicola F. De Zanche and Lawrence L. Wald

16:00 422.  

Micro-Electromechanical Systems (MEMS) Based RF-Switches in MRI – a Performance Study
Miguel Fuentes1, Ewald Weber1, Stephen Wilson1, Bing Keong Li1, Stuart Crozier1
The School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, Queensland, Australia

This work presents a method of controlling and switching multiple receiver coil-arrays in a manner that will reduce power consumption, relax cabling requirements and increase overall SNR through the use of micro-electromechanical systems (MEMS) RF switches. We have focused on parameters relevant to T/R switching applications in MR coil arrays. The MEMS devices evaluated here show favourable, quantifiable performance on the bench and in MR environment testing, and are found to be acceptable for use in multi-element coil switching roles.

16:12 423.  

Micro-Scale Inductively Coupled Radiofrequency Resonators on Fluidic Platforms for Wireless Nuclear Magnetic Resonance Spectroscopy
Anja Zass1, Kailiang Wang1, Marcel Utz1
Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA, United States

Nuclear magnetic resonance (NMR) spectroscopy is an ideal tool for metabolomics. On microfluidic platforms, small pickup coils are needed for good sensitivity. Usually, this requires electrical connections between chip and spectrometer. Micro-scale inductively coupled rf resonators enable the wireless investigation of small volumes in the NMR. The approach has the advantage of focussing the sensitivity and rf power on the sample, without the need for connections to the spectrometer. Preceding research demonstrated that inductively coupled coils can rival the performance of directly connected ones. We present planar inductively coupled, self-resonant microcoils that showed promising resolution and sensitivity on first tests.

16:24 424

Digitally Controlled μ-Chip Capacitor Array for an Implantable Multiple Frequency Coil
Walker J. Turner1, Zhiming Xiao1, Sien Wu1, Barbara L. Beck2, Rizwan Bashirullah1, Thomas H. Mareci3
Electrical and Computer Engineering, University of Florida, Gainesville, FL, United States; 2McKnight Brain Institute, University of Florida, Gainesville, FL, United States; 3Biochemistry and Molecular Biology, University of Florida, Gainesville, FL, United States

This digitally controlled capacitor array is designed to have a variable capacitance, set through a digital input, to be implemented as a multiple frequency coil for the NMR measurements of multiple nuclei in an implantable artificial pancreas for Type I diabetes.  The test chip of the capacitor array successfully demonstrates the effectiveness of digitally setting the capacitance for resonance while producing reasonable signal sensitivity.  This design can be implemented further for the resonance at additional frequencies.

16:36 425.  

Thin-Film Catheter-Based RF Detector System - not available
Richard R. Syms1, Ian R. Young2, Munir M. Ahmad3, Marc Rea4
EEE Dept., Imperial College London, London, Middlesex, United Kingdom; 2EEE Dept., Imperial College London, London, United Kingdom; 3EEE Dept., Imperial College London, United Kingdom; 4Radiology Dept., Imperial College NHS Trust, London, United Kingdom

Procedures such as biliary endoscopy require imaging modalities such as MRI if soft tissue contrast is to be improved. Local signal detection is then required to achieve adequate signal-to-noise ratio at high resolution. Small RF detector coils have been integrated with catheter probes, but the reliable combination of a coil, tuning and matching capacitors and an output cable is difficult in the limited available space. Here we demonstrate a catheter-based detector entirely formed from thin-film components, fabricated by double-sided patterning of copper-clad polyimide to form a resonant detector with integrated tuning and matching capacitors and a thin-film interconnect.

16:48 426.

Time-Interleaved Radiation Damping Feedback for Increased Steady-State Signal Response
Florian Wiesinger1, Eric W. Fiveland2, Albert J. Byun2, Pekka Sipilae1, Christopher J. Hardy2

1Imaging Technologies, GE Global Research, Munich, Germany; 2MRI Laboratory, GE Global Research, Niskayuna, NY, United States

Radiation damping (RD) describes a second-order effect where the signal-induced current in the receiver coils acts back onto the primary spin system. According to Lenz’s law, the RD acts in a way to oppose its original cause.  In that sense RD can be understood as a self-regulating flip-back pulse. Recently, RD feedback loops have been introduced into the RF signal path to boost the natural RD effect.  While previous RD circuits were limited in terms of feedback gain, here we present a new feedback circuit, which principally circumvents this problem via time separation of RD receive and transmit.

17:00 427

A Double Maxwell Sine Field RF Coil for a TRASE RF Phase Gradient Coil Set
Qunli Deng1, Scott B. King2, Vyacheslav Volotovskyy2, Boguslaw Tomanek1, Jonathan C. Sharp1

1Institute for Biodiagnostics (West), National Research Council of Canada, Calgary, AB, Canada; 2Institute for Biodiagnostics, National Research Council of Canada, Winnipeg, MB, Canada

TRASE is a new k-space imaging method which uses transmit RF phase gradients for spatial encoding instead of B0-gradients. RF coil design is particularly important for TRASE as the image quality largely depends upon the RF phase gradient fields. Here we report an improved design for a sine profile field, which is a necessary component of an RF phase gradient set. By considering the concomitant z-directed RF field, and by 2D and 3D simulations, a double Maxwell design was arrived at and constructed. The double Maxwell coil shows a 91% larger imaging volume than the previous single Maxwell design.

17:12 428

Targeted Traveling Wave MRI
Marco Mueller1, Stefan Alt, Reiner Umathum, Wolfhard Semmler, Michael Bock
1DKFZ, Heidelberg, Baden-Württemberg, Germany

The travelling wave concepts can be used for whole body MRI at high fields but suffers from high energy deposition (SAR). We introduce a coaxial targeted travelling wave RF coil, which guides the wave to any desired region in the body. To limit whole body SAR, the wave-propagation range is confined to the imaging region. Imaging results with a coil prototype show that the B1 field is focused to the targeted imaging region, and a homogeneous B1 field distribution is achieved outside the magnet’s symmetry axis.

17:24 429

Mid-Bore Excitation of Traveling Waves with an Annular Ladder Resonator for 7T Body Imaging with Reduced SAR
Graham Charles Wiggins1, Bei Zhang1, Riccardo Lattanzi1, Daniel Sodickson1

1Radiology, NYU Medical Center, New York, NY, United States

Traveling wave imaging has previously been demonstrated using a patch antenna placed at one end of the scanner bore. For body imaging, reflections and attenuation result in very low B1+ in the torso. Attempting torso imaging by boosting the transmit power can create too much heating of tissue between the antenna and the region of interest, particularly in the head. We propose a novel coil design which can be placed at or near isocenter to create a traveling wave excitation which is strongest in the torso, with significantly reduced SAR in distant tissues.

17:36 430.

An Advantageous Combination of Travelling Wave and Local Receive for Spine MR Imaging at 7T: Local SAR Reduction and SENSE Reconstruction
Anna Andreychenko1, Ingmar Voogt2, Hugo Kroeze2, Dennis W. Klomp2, Jan J. Lagendijk1, Peter Luijten2, Cornelis A.T. van den Berg1
1Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands; 2Radiology, University Medical Center Utrecht, Utrecht, Netherlands

Spine structure contains a lot of fine details and, thus, high field spine MR imaging would benefit from the increased image resolution due to SNR gain. In case of a local transmit coil its performance is limited by SAR restrictions. In this work we explore a possible combination of the novel travelling wave RF excitation combined with local receive array to image the lumbar spine at 7T. We have demonstrated that transmitting with the travelling wave significantly reduces local SAR values, using local receive coils improves B1- sensitivity and available reference scan allows optimal SENSE image reconstruction.

17:48 431

A Comparison of a Patch Antenna to an End-Fire Helix Antenna for Use in Travelling Wave MRI
Daniel James Lee1, Paul M. Glover1

1Physics and Astronomy, SPMMRC, University of Nottingham, Nottingham, Notitnghamshire, United Kingdom

So far, most travelling wave studies have used a patch antenna to create the travelling wave, as they are simple in design and can be constructed rapidly at little cost. In this study, both a patch antenna and an end-fire helix antenna are simulated and constructed to allow their relative merits to be assessed.  Simulations are used to asses specific absorption rates (SAR) and experimental data are used to assess the signal to noise ratio (SNR) and B1 homogeneity of both antennas.



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