RF Arrays & Related Technology

Wednesday 14 May 2014

Bastien Guerin¹, Jorge F Villena², Athanasios G Polimeridis², Elfar Adalsteinsson^2,3, Luca Daniel², Jacob White², and Lawrence L Wald^1,3
1A. A. Martinos Center for Biomedical Imaging, Dpt. of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, ²Dpt. of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, ³Division of Health Sciences Technology, Harvard-MIT, Cambridge, MA, United States

The ultimate SNR (uSNR) and SAR (uSAR) have been computed in uniform spheres and cylinders. These ultimate metrics have been useful to rank coils with respect to the ultimate performance and to suggest design modifications. However the performance of parallel imaging and the computation of SAR are strongly affected by the geometry of the head and the non-uniform distributions of the conductivity and permittivity in vivo. We compute for the first time the uSNR and USAR in complex, non-uniform body models and use these metrics to assess the performance of receive (transmit) arrays with up to 128 (16) channels.

Daniel K. Sodickson^1,2, Bei Zhang³, Qi Duan⁴, Ryan Brown¹, Riccardo Lattanzi^1,2, Karthik Lakshmanan¹, Manuskha V. Vaidya^1,2, Alicia Yang^1,2, Robert Rehner⁵, Markus Vester⁵, Stefan Popescu⁵, Stefan Biber⁵, Bernd Stoeckel⁶, Hugo Chang⁶, and Graham C. Wiggins^1
1Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY, United States, ²The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY, United States, ³Translational and Molecular Imaging Institute, Icahn School of Medicine at Mount Sinai, New York, NY, United States, ⁴Advanced MRI Section, Laboratory of Functional and Molecular Imaging, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD, United States,⁵Siemens Healthcare, Erlangen, Germany, ⁶Siemens Medical Solutions, New York, NY, United States

If a many-element RF coil array could be mounted behind the scanner covers while preserving much of the SNR performance of close-fitting arrays, the benefits for patient comfort and simplicity of workflow would be dramatic. In numerical simulations and experimental evaluations of progressively larger encircling prototypes, we explored the feasibility of remote body array designs. Smaller-scale models performed well as compared with close-fitting counterparts. However, with a 50cm-diameter 124-element prototype, we encountered unexpected practical challenges, most notably relating to preamplifier noise coupling, which becomes significant in lightly-loaded many-element arrays. Effective remote array designs will have to contend with these challenges.

Jonas Reber¹, Josip Marjanovic¹, David Otto Brunner¹, Thomas Schmid¹, Urs Moser¹, Benjamin Emanuel Dietrich¹, Christoph Barmet^1,2, and Klaas Paul Pruessmann^1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland, ²Skope Magnetic Resonance Technologies, Zurich, Switzerland

The increasing need for RF acquisition channel counts from receiver coil arrays but also applications such as field monitoring, RF safety surveillance of parallel transmit systems and other applications make cable routing and its implications for patient comfort, system handling and safety a cumbersome task. Further ultra-high field system push the frequency span of which the receiver has to be able to operate with. A novel platform is presented with broadband sampling ADCs communicating via digital high speed optical links to a host. It was found that the performance of rack mounted system can be matched at tolerable power consumptions.

G Shajan¹, Christian Mirkes^1,2, Jens Hoffmann¹, Klaus Scheffler^1,2, and Rolf Pohmann^1
1Max Planck Institute for Biological Cybernetics, Tuebingen, Baden Wuerttemberg, Germany, ²Department of Biomedical Magnetic Resonance, University Hospital, Tuebingen, Baden Wuerttemberg, Germany

Dual tuned coils, often used for X-nuclei imaging, compromise coil performance at both frequencies. Moreover, to acquire proton reference image covering the whole brain at 400 MHz, conventional dual tuned birdcage cannot be used due to wavelength effects. A combination of three coil arrays was developed to maximize the receive sensitivity at the sodium frequency and to provide proton signal covering the whole brain for anatomical localization and B0 off-resonance correction.

Gang Chen¹, Martijn Cloos¹, Daniel Sodickson¹, and Graham Wiggins^1
1The Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY, United States

Electric dipole antennas are seeing increasing use for 7T MR imaging. With 8 elements surrounding a body sized tissue equivalent phantom and array of dipole antennas can achieve higher central SNR than an array of loops, while simultaneously having greater coverage along Z. Most 7T head coils today rely on birdcage, TEM volume coils, or arrays of loops for transmit. Signal dropout in inferior brain regions such as the cerebellum and brain stem is a common problem. We describe here a dipole array for 7T head imaging designed for extended coverage and evaluate its performance in phantom and volunteer imaging.

Nikolai I. Avdievich¹, Andreas Pfrommer¹, Jens Hoffmann¹, Grzegorz L. Chadzynski^1,2, Klaus Scheffler¹, and Anke Henning^1,3
1Max Planck Institute for Biological Cybernetics, Tübingen, Germany, ²Department of Biomedical Magnetic Resonance, University Hospital Tübingen, Tübingen, Germany, ³Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland

Surface loop phased arrays have been shown to improve transmit performance and B1 homogeneity for head imaging up to 9.4T. However, transmit arrays enlarged to fit receive arrays often cannot satisfy the requirements in B1 and bandwidth for ultra-high field spectroscopic imaging. We have developed and constructed a tight fit 400MHz 8-channel transceiver array. The array improved transmit efficiency and homogeneity in the axial slab through the phantom’s center when used in CP mode. B1+ averaged over the central axial slice measured 55.6 nT/V, which corresponds to 12.4 uT per 1 kW of RF power delivered directly to the array.

Jorge Fernandez Villena¹, Athanasios G. Polimeridis¹, Bastien Guerin², Yigitcan Eryaman¹, Lawrence L. Wald^2,3, Elfar Adalsteinsson^1,3, Jacob K. White¹, and Luca Daniel^1
1Research Laboratory of Electronics, Dept. of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, ²A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, ³Harvard-MIT Division of Health Sciences Technology, Cambridge, MA, United States

We propose a methodology for the comprehensive full-wave electromagnetic analysis of arbitrary MRI transmit coils within few minutes for a given realistic body model at a given frequency. It allows us to compute the un-tuned coil port parameters, to obtain the current distribution for the tuned coils, and the corresponding electromagnetic field distribution in the inhomogeneous body for each transmit channel. Such an approach is fast enough (2-3 minutes for 8-coil arrays) to be applied in automatic procedures for the optimization of high field coil designs based on the electromagnetic field distribution in realistic inhomogeneous human body models.

Ryan Brown¹, Martijn A Cloos¹, Christian Geppert², Linda Moy¹, Daniel K Sodickson¹, and Graham C Wiggins^1
1The Bernard and Irene Schwartz Center for Biomedical Imaging, Dept. of Radiology, New York University School of Medicine, New York, NY, United States, ²Siemens Medical Solutions USA Inc., New York, NY, United States

High-field bilateral breast imaging is challenging because of the conflicting requirements of a uniform transmit field and a large field-of-view. This is addressed in the proposed split-symmetric transmission scheme, where symmetries in the body anatomy and coil array are exploited to allow a given RF shim or set of tailored pulses to be replicated across lateral and contralateral coils. This allowed a substantial advantage in the degrees of freedom associated with the transmit optimization problem, and resulted in a uniform excitation over a bilateral field-of-view. Important benefits of this transmit system are that tailored parallel pulses and dedicated lymph node coils provide improved coverage in the posterior breast and in the lymph nodes, both of which have been difficult to visualize at 7T with standard transmit coils.

Madhwesha Rama Rao¹, Fraser Robb², and Jim M Wild^1
1Academic Unit of Radiology, University of Sheffield, Sheffield, South Yorkshire, United Kingdom, ²GEHC Coils, General Electric Company, Aurora, Ohio, United States

Synchronous acquisition of 1H and HP gases in the lungs provide complementary structure function information from the lung with inherent spatial temporal registration. To date 1H images have been acquired using the systems body coil, which has low SNR compared to dedicate receive coils. This study demonstrates the design and application of a dedicated 1H receiver array to improve the proton lung SNR in synchronous acquisition with HP gas 3He and 129Xe at 1.5T. Synchronous acquisition with two birdcage transmitter receiver arrays has also been demonstrated. Both represent novel developments in the field of multi nuclear MRI RF engineering

Gregory J. Metzger¹, Iris Elliott², Jinfeng Tian¹, Devashish Shrivastava¹, Pierre-Francois van de Moortele¹, and Gregor Adriany^1
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States, ²Hologic, Toronto, Ontario, Canada

A reusable, multi-channel ERC with improved receive performance has been constructed and validated for use in both anatomic and demanding functional studies of the prostate at 7T. The availability of a sterilizable coil will make the use of an ERC a viable option for future clinical trials at UHF.