RF Engineering

Tuesday 2 June 2015

Gerd Weidemann¹, Frank Seifert¹, Werner Hoffmann¹, Rainer Seemann¹, Patrick Waxmann¹, and Bernd Ittermann^1
1Physikalisch-Technische Bundesanstalt, Braunschweig und Berlin, Germany

The reflected RF power as a function of steering conditions and the coupling of coil elements can be determined from S-parameters of a MR transmit array. We developed a versatile system for in situ measurements of S-parameter matrices, which can be used to monitor the influence of loading conditions or coupling to the coil environment on the power balance and the coupling of a pTx coil. For an 8 channel coil the complete S-parameter matrix can be measured in less than one minute in situ. Since the system is independent of scanner hardware and does not interfere with a running sequence it can be used for monitoring any pTx system in situ, e.g. for the detection of abnormalities like metallic implants.

Ashraf Abuelhaija¹ and Klaus Solbach^2
1Duisburg-Essen University, Duisburg, Select a state, Germany, ²Duisburg-Essen University, Duisburg, Germany

For Tx arrays, the concept of mismatched termination of coils by power amplifiers (PA) for coil isolation has been proposed. This contribution presents an investigation by using the output impedance of a 1 kW power amplifier which was designed for a 7 Tesla 32 channel near-magnet Tx array. Isolation performance is proven by a measurement using a pair of decoupled pick-up coils and EM simulations of coupled coils terminated by the PA output impedance as well as of the PA large-signal reflection coefficients as function of output power or (coupled) reverse power.

Shubharthi Sengupta¹, Gregor Adriany², Valentin G. Kemper¹, Jan Zimmermann³, Rainer Goebel¹, and Alard Roebroeck^1
1Dept. of Cognitive Neuroscience, Maastricht University, Maastricht, Netherlands, ²Dept. of Radiology, University of Minnesota, Minnesota, United States, ³New York University, New York, United States

We present an RF coil with separate transmit and receive arrays on a conformal holder for imaging the human visual cortex at 7T. We discuss the methods and inspiration behind the construction of these arrays and with the help of high resolution data try to show the advantages of this coil when imaging the peripheral and inferior extents of the human brain.

Xiaoqing Hu¹, Lei Zhang¹, Chao Zou¹, Huabin Zhu², Xiaoliang Zhang³, Yiu-cho Chung¹, Xin Liu¹, Hairong Zheng¹, and Ye Li^1
1Lauterbur Research Center for Biomedical Imaging, Shenzhen Institutes of Advanced Technology of Chinese Academy of Sciences, Shenzhen, Guangdong, China, ²Suzhou Medcoil Healthcare Co.,Ltd, Suzhou, Jjiangsu, China, ³Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, United States

A 32-channel intracranial and extracranial vascular array for three dimension arterial wall MR imaging at 3T

Mark Schuppert¹, Karl-Friedrich Kreitner², Stefan Fischer¹, Simon Wein¹, Boris Keil³, Lawrence L Wald^3,4, and Laura M Schreiber^1,5
1Section of Medical Physics, Department of Radiology, Johannes Gutenberg University Medical Center, Mainz, Germany, ²Department of Radiology, Johannes Gutenberg University Medical Center, Mainz, Germany, ³A.A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, United States,⁴Harvard Medical School, Boston, MA, United States, ⁵Department of Cellular and Molecular Imaging, Comprehensive Heart Failure Center, Wuerzburg, Germany

SNR and g-factor (R = 3, 5, 7, and 8) contribution of coil elements as a function of the coil element distance to the heart was calculated from unaccelerated cine cardiac MRI data obtained with a 64-channel cardiac coil by consecutive addition of coil elements in concentric rings around a center element above the heart. Lateral coil elements do neither contribute substantially to SNR nor improve the g-factor. In 2D imaging at R=5 adequate image quality is achievable with this coil. Omitting lateral coil elements does not seem to compromise the accelerated imaging performance of this coil.

Robin Etzel^1,2, Xueming Cao^1,3, Choukri Mekkaoui¹, David E. Sosnovik¹, Timothy G. Reese¹, Mark Schuppert⁴, Laura M. Schreiber^4,5, Martin Fiebich², Lawrence L. Wald¹, and Boris Keil^1
1A.A. Martinos Center for Biomedical Imaging, Dept. of Radiology, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, United States, ²Mittelhessen University of Applied Sciences, Institute for Medical Physics and Radiation Protection, Giessen, Germany, ³Medical Physics, Department of Radiology, University Medical Center Freiburg, Freiburg, Germany, ⁴Department of Radiology, Johannes Gutenberg University Medical Center, Section of Medical Physics, Mainz, Germany, ⁵Comprehensive Heart Failure Center, Department of Cellular and Molecular Imaging, Wuerzburg, Germany

In cardiac imaging, parallel acquisition has impacted clinical applications to the point where nearly every cardiac examination is performed with an array comprising multiple smaller surface coil elements. In this study we re-design, construct, and evaluate a 64-channel cardiac array coil and compare it to a 34-ch commercially available coil array. We validated the arrays with SNR and g-factor maps and accelerated DWI in vivo imaging.

Jörg Felder¹ and N. Jon Shah^1,2
1Institute of Neuroscience and Medicine, Forschungszentrum Jülich, Jülich, NRW, Germany, ²Faculty of Medicine, Department of Neurology, JARA, RWTH Aachen University, Aachen, NRW, Germany

High field strength coil performance is difficult to predict in all facets based on electromagnetic field simulations alone. Bloch co-simulation can help to improve a priori performance evaluation, e.g., for parallel transmission and SAR predictions. Here we demonstrate co-simulation in order to evaluate SAR and image homogeneity for an eight-element transceiver coil operating at 9.4T.

Bei Zhang¹, Zahi A. Fayad¹, Junqian Xu¹, Bernd Stoeckel², and Priti Balchandani^1
1Translational and Molecular Imaging Institute, Icahn School of Medicine at Mount Sinai, New York, New York, United States, ²Siemens Medical Solution, New York, New York, United States

Both simulations and experiments show that the tuning and matching of RF coils changed when dielectric pads were used. For a dense receive array, such as the Nova Medical single channel transmit 32-channel receive 7T head coil, these changes can affect the coupling between the coil elements and therefore the effective sum-of-squares (SOS) SNR. When calcium titanate dielectric pads (r= 137.4)were placed in the 32-channel head coil, increased coil coupling was apparent in the noise coefficient matrix and the SOS SNR decreased by 30%.However, the optimal SNR slightly increased when using the dielectric pads. Therefore, in order to maximize the advantage of dielectric pads, they should be integrated into coil design and fabrication.

Masoumeh Dehghani M.¹, Arthur Magill W.², Yves Pilloud¹, Nicolas Kunz², and Rolf Gruetter^1,2
1Laboratory for Functional and Metabolic Imaging, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Vaud, Switzerland, ²Centre d’Imagerie Biomédicale, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Vaud, Switzerland

The design and operation of a transmit birdcage coil combined with a surface receive loop for small animal imaging and spectroscopy at 600MHz (1H at 14.1T) is presented. The transmit coil is a 16-rung high pass birdcage driven in quadrature and detuned during reception by PIN diodes placed between every rung on one end ring. The receive loop is tuned using a varactor diode, eliminating the need for variable capacitors and detuning PIN diodes on the loop, saving considerable space in the probe design. This system provides homogeneous excitation and high sensitivity for functional, anatomic, and other MRI/S studies.

Bijaya Thapa¹, Joshua Kaggie¹, Nabraj Sapkota¹, and Eun Kee Jeong^1,2
1Dept. of Physics and Astronomy, Utah Center for Advanced Imaging Research, University of Utah, Salt Lake City, Utah, United States, ²Dept. of Radiology, Korea University, Seoul, Korea

TR switches are used to route high power RF pulses from the RF amplifier to the coil during transmit and the low power MR signal from the coil to the preamplifier during receive. This work developed a custom TR switch with high isolation that protected the preamplifiers from large transmit power. In this study, we designed and developed a TR switch with high isolation and low insertion loss at low cost and reasonable development time, that enabled both linear and quadrature modes for a double frequency coil. The switch's performance was comparable to a commercial TR switch.

Benson Yang¹, Clare McElcheran², Fred Tam¹, and Simon Graham^1,2
1Physical Sciences, Sunnybrook Research Institute, Toronto, ON, Canada, ²Medical Biophysics, The University of Toronto, Toronto, ON, Canada

Parallel transmit systems for MRI applications are not widely available, but offer advantages in B1 shimming, patient safety, imaging acceleration and application-specific radio frequency pulse design. Our prototype system includes field-programmable gate array (FPGA) technology for system control and safety monitoring as well as integrated circuit devices for radio frequency signal modulation. The prototype single channel modulation module is demonstrated during MRI and results are as expected. Future demonstrations will include a software interface for our FPGA module.

Stephen E. Ogier¹ and Steven M. Wright^1,2
1Electrical and Computer Engineering, Texas A&M University, College Station, TX, United States, ²Biomedical Engineering, Texas A&M University, College Station, TX, United States

In vivo NMR spectroscopy has the potential to provide clinically useful data, but it has seen limited use due to poor Signal-to-Noise Ratio. The use of array coils to improve SNR is well established in MRI and is extremely attractive for less-sensitive nuclei such as ¹³C. Array receivers on clinical systems are usually ¹H only, but frequency translation with RF mixers allows us to adapt ¹H receivers for use with other nuclei. By careful selection of the frequency mixed to, we can even perform ¹H decoupled ¹³C spectroscopy with a frequency translating receiver.

John C. Bosshard¹ and Steven M. Wright^1
1Department of Electrical & Computer Engineering, Texas A&M University, College Station, TX, United States

Standalone receivers are becoming more widespread and there is interest in 'wireless' receiver coils for a number of potential applications. In some instances these receivers must run without a direct connection to the spectrometer, using a separate clock, in which case there will generally be a random phase variation between the transmitter and receiver. This abstract examines two methods for proper phasing of the received signal in asynchronous receive: recovering the phase from the residual FID from the 180 pulse, prior to phase encoding, and injecting an RF pilot signal outside of the signal bandwidth through an additional probe.

Filiz Ece Sagcan¹, Bulent Sen¹, and Aylin Bayram^1
1Power Amplifier Technologies, ASELSAN A.S., Ankara, Ankara, Turkey

All digital transmitters are gaining interest in magnetic resonance imaging applications. It provides high power efficiency during the course of imaging operation. We propose a novel, delta sigma modulation based all digital RF modulator. The proposed digital modulation architecture takes advantage of a digital single side band (SSB) modulator to save power; and a CORDIC algorithm to generate multi-band MRI signal with very high frequency resolution. The modulated digital signal can then be amplified with a switch-mode power amplifier to get high efficiency.

Seunghoon Ha¹, Haoqin Zhu¹, and Labros Petropoulos^1
1R&D, IMRIS Inc., Minnetonka, MN, United States

A novel Wi-Fi tuning/detuning RF switch was introduced as part of the wireless inductive coil technology that will enable us to introduce and control multichannel wireless inductive coil structures covering large portion of the human anatomy and will be ideal for multi-channels receiver coil used in multi-positioned intraoperative MRI. MR compatibility tests indicate that the suggested Wi-Fi switch is MR safe and it did not introduce any detrimental effects in terms on the wireless coil’s SNR value, as well as its RF field uniformity and sensitivity characteristics

Sung-Min Sohn¹, Lance DelaBarre¹, and J. Thomas Vaughan^1
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, United States

This study proposes a new Pi-network to tune and match RF coils and shows comparisons of impacts of three networks (L-, Pi-, and T-network) on MR images. The results of this study also present a proper tuning and matching method for a respective application.

Jarek Wosik^1,2, Krzysztof Nesteruk³, I-Chih Tan⁴, Kuang Qin¹, and James A. Bankson^5
1Electrical and Computer Engineering, University of Houston, Houston, Texas, United States, ²Texas Center for Superconductivity, University of Houston, Houston, Texas, United States, ³Institute of Physics Polish Academy of Sciences, Warsaw, Poland, ⁴Center for Molecular Imaging, The University of Texas Health Science Center, Houston, Texas, United States, ⁵Department of Imaging Physics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas, United States

We report on a planar 75-MHz cryogenic (both copper and superconducting) coils for hyperpolarized 13C MRI at 7T. In principle, cryogenic and especially superconducting receive coils can provide very significant SNR gain for 13C detection at 7 T, due to lower than for protons Larmor frequency of 13C nuclei and resulting lower body loss compared to 1H. The double-sided coils were fabricated by patterning Cu and YBCO on laminate and 0.33 mm thick sapphire substrates, respectively. Practical limits of SNR gains of 60K 75 MHz 13C coils and their maximum sizes to-be-used, were analyzed and compared with those of 1H 300 MHz coils.

Balthazar Pierre Lechene¹, Anita Flynn¹, Joseph Corea¹, Michael Lustig¹, and Ana Claudia Arias^1
1Electrical Engineering and Computer Science, University of California Berkeley, Berkeley, CA, United States

Despite offering a unique practical advantage, printed flexible MRI surface coils still trail conventional ones when compared head-to-head. Here, we investigate how to improve printed coils by varying the materials used for their fabrication. Two silver inks are compared in terms of resistivity and AC resistance, and those properties are then linked to the imaging quality of resulting coils. The choice of substrate is also studied to enhance the coils’ flexibility while retaining good electrical properties. The results show promise for bringing printed coils to the standards of conventional ones

Cristen LaPierre^1,2, Mathieu Sarracanie^1,2, David E J Waddington^1,3, and Matthew S Rosen^1,2
1MGH/A.A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States, ²Department of Physics, Harvard University, Cambridge, MA, United States, ³ARC Centre of Excellence for Engineered Quantum Systems, School of Physics, University of Sydney, Sydney, NSW, Australia

MRI at low magnetic field (<10 mT) presents unique engineering challenges. Imaging coils must maximize coverage over the volume of interest while minimizing losses in the Johnson noise dominated regime. We designed a spiral volume coil design for combined Tx and Rx and optimized for human head imaging in the transverse B0 field of our low field scanner. A 30-turn single channel volume spiral coil designed for low field imaging significantly outperformed our 8-channel array coil. By minimizing losses in the coil, maximizing filling factor and eliminating coupling issues, we were able to obtain significantly higher SNR.

Helmar Waiczies¹, Alexandra Petiet², Elodie Laffrat^3,4, Darius Lysiak¹, Stephane Hunot^3,4, Thoralf Niendorf¹, and Jan Rieger^1
1MRI.TOOLS GmbH, Berlin, Berlin, Germany, ²Center for Neuroimaging Research, Brain and Spine Institute, ICM, Paris, Paris, France, ³Inserm U 1127, CNRS UMR 7225, Sorbonne Universités, UPMC Univ Paris, Paris, France, ⁴Institut du Cerveau et de la Moelle épinière, ICM, Paris, Paris, France

The squirrel monkey has similar functional and microstructural organization of the central nervous system of humans. Therefore it holds the potential to provide guidance for further explorations into the brain structure and function especially if investigated at ultrahigh magnetic field strengths. However, the dimensions of the preclinical MR systems, the size of the squirrel monkey’s head and the necessary animal handling present considerable constraints for the RF coil desing. Accepting this challenge, this work presents a design and first results of a dedicated RF coil with a large inner diameter and a thin wall for MR imaging at 11.7 T.

Hai Lu¹ and Shumin Wang^1
1Auburn University, Auburn, AL, United States

Conventional RF coils are resonant structures tuned to one or two fixed Larmor frequencies. They are high-Q structures sensitive to different loading conditions. This can be especially problematic for high-field applications where the coil/subject coupling is expected to be much stronger. In order to avoid frequency tuning, a broadband RF coil that utilizes the transverse electromagnetic (TEM) mode of a parallel-plate waveguide was developed. Due to the lack of resonance, it is more stable with respect to different loadings. Furthermore, the same coil can be utilized at distinct frequencies due of the frequency-independent nature of the TEM mode.

Hsuan-Han Chiang¹, Ming-Jye Chen¹, Chien-Cheng Kuo², You-Yin Chen³, Changwei W Wu⁴, and Li-Wei Kuo^1
1Institute of Biomedical Engineering and Nanomedicine, National Health Research Institutes, Miaoli County, Taiwan, ²Graduate Institute of Energy Engineering/Thin Film Technology Center, National Central University, Taoyuan County, Taiwan, ³Department Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan, ⁴Graduate Institute of Biomedical Engineering, National Central University, Taoyuan County, Taiwan

In the past few years, graphene has been widely developed and considered as evolutional nano-materials, which could potentially advance the scientific researches in physics, electronics, optics and biomedical applications due to its remarkable physical characteristics. In this study, we aimed to develop surface RF coils coated with graphene materials for 3T MRI use and perform a quantitative comparison in signal-to-noise ratio (SNR) of graphene and conventional copper RF coils. Our preliminary results show a consistent improvement of SNR of approximately 1.38 folds in average, which could be potentially useful to help gain higher spatial resolution or temporal resolution than conventional copper coil in future studies.

Selaka B Bulumulla¹, Eric Fiveland¹, Keith Park¹, and Joseph Iannotti^1
1GE Global Research, Niskayuna, New York, United States

MR compatible micro electro-mechanical switches (MEMS) have the potential to reconfigure coils ‘on-the-fly’ by enabling/disabling conducting paths and/or circuit components. In this work, we demonstrate the feasibility of reconfiguring (1) coil geometry to switch field of view (FOV) and (2) coil frequency to image multiple nuclei. The geometry reconfiguration (15cm x 11cm vs 8cm x 11cm coil) was realized with two MEMS and frequency reconfiguration (64MHz vs 16MHz) was realized with one MEMS. In each case, two additional MEMS were used in impedance matching circuits to transform coil impedance at each configuration to 50 ohms.

Oliver Heid¹, Jürgen Heller¹, Yong Wu², Xiaoyu Yang², and Hiroyuki Fujita^2
1CT NTF HTC, Siemens AG, Erlangen, Bavaria, Germany, ²Quality Electrodynamics, Mayfield Village, Ohio, United States

A passive, non-wireless RF coil connection method is demonstrated which obviates the need of RF coil identification and selection, by automatically activating any RF coil within the FoV and safely deavtivating any other coil. This is especially important for interactive real time and interventional MRI. Experimental evidence is given.

Junghwan Kim^1,2, Narayan Krishnamurthy¹, Yujuan Zhao¹, Tiejun Zhao³, Kyongtae Ty Bae^1,2, and Tamer Ibrahim^1,2
1Bioengineering, University of Pittsburgh, Pittsburgh, PA, United States, ²Radiology, University of Pittsburgh, Pittsburgh, PA, United States, ³Siemens Medical Solution USA, Inc, Pittsburgh, PA, United States

Numerical calculation of electromagnetic (EM) field using full wave 3D FDTD method was demonstrated in the four different tissue ratio breast models and compared with breast phantom. B1+ homogeneity at sagittal, coronal, and transverse and whole volume was calculated in the models and compared to the phantom. In addition, peak/global SAR was calculated and compared with the temperature change measured at four locations.

Gemma R. Cook¹, Martin J. Graves^1,2, Fraser J. Robb³, and David J. Lomas^1
1Department of Radiology, University of Cambridge, Addenbrooke's Hospital, Cambridge, United Kingdom, ²MRIS, Cambridge University Hospitals NHS Foundation Trust, Addenbrooke's Hospital, Cambridge, United Kingdom, ³GE Healthcare Coils, Aurora, OH, United States

A lack of dedicated paediatric coil designs means adult coils must often be employed in MRI of infants. This study simulated the sensitivity and acceleration potential of such adult coils and an in-house 1.5T prototype with two wrap configurations (providing body conformity different patient sizes). Code was written to calculate the noise correlation matrix and element B1- field profiles for an array, then produce the resultant uniform noise and sum of squares maps. Though all designs had similar accelerated SNR performance, the prototype showed increased B1- uniformity and higher SOS values in phantom and infant models.

Zhiyong Zhai¹ and Michael Morich^1
1Philips Healthcare, Cleveland, Ohio, United States

For a birdcage T/R head coil at 7T, B1+ field distribution in the head is very non-uniform due to wavelength effects in tissue medium. Using two independent transmit channels to drive the two birdcage modes one can slightly improve B1+ uniformity, but further improvement is needed. Here we propose a modified 7T T/R head coil that maintains the simplicity of two T/R channels but with improved RF shimming capability. Simulations show that this head coil can improve B1+ uniformity in the head considerably more than a conventional birdcage head coil alone with two independent T/R channels.

Yizhe Zhang¹, Yan Liu¹, Bingyao Sun¹, Xiaoliang Zhang², and Xiaohua Jiang^1
1Department of Electrical Engineering, Tsinghua University, Beijing, China, ²Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, United States

In this work, we developed a multi-channel transmission system with compact MOSFET amplifiers for parallel excitation in 7T animal MRI scanner. The system is composed of a multi-channel RF controller and 4 independent RF power amplifiers. Each amplifier contains two stages (using transistor MRF321 and MOSFET MRF177 for the first and second stage respectively). The design was validated by simulation and bench test. The power gain for the amplifier is 18.7dB at 300MHz, demonstrating the sufficient amplification capability of the transmission system for small animal parallel transmission applications at 7T.

Roland Müller¹, Mikhail Kozlov¹, and Harald E. Möller^1
1Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Saxony, Germany

Low-cost USB DVB-T sticks can operate as frontend for multipurpose Software Defined Radio (SDR) receivers. Bypassing any decoder, real/inphase and imaginary/quadrature byte streams are provided. Many applications are conceivable in MRI, including amplitude demodulation to restore the RF pulse envelope and spectral analysis. Recording transmitted RF pulses can help to find mistakes during sequence development or to identify malfunctioning scanner hardware. More and more electronic devices are operated within the examination room or even inside the scanner bore. Unwanted emissions near the Larmor frequency and insufficient shielding can already be identified in the RF lab.

Hedok Lee^1,2 and Andrew Ravin^1
1Anesthesiology, State University of New York at Stony Brook, Stony Brook, New York, United States, ²Radiology, State University of New York at Stony Brook, Stony Brook, New York, United States

Utility of 3D printers have been recognized in MRI research but the impact of building materials onto static magnetic field (B0) and RF field (B1) has not been investigated at 9.4T field strength. In this study three commonly used building materials (ABS, PLA and photo-polymer resin) were tested for their impact on B0 and B1 fields, and we report that B0 field is affected in the presence of the materials while B1 field remains unaffected.

J. Andrew Derbyshire¹ and Peter A. Bandettini^1
1fMRI Core, National Institute of Mental Health, NIH, Bethesda, MD, United States

This work describes an approach for extending the capabilities and features of MRI systems in directions that are outside the scope of the standard pulse sequence development environments. We demonstrate the ability to control the MR system with a pulse-sequence independent implementation of a real-time scan plane control system.

Jinfeng Tian¹, Lance Delabarre¹, Greg Metzger¹, and J. Thomas Vaughan^1
1U. of Minnesota, Minneapolis, Minnesota, United States

The matching network is significant, yet sometimes overlooked, component of MRI RF Coils. The matching network can have an impact on the local SAR values, especially when 1): the simulated coils are designed for use inside the subject/patient body 2): the tissues near the matching network are already exposed to high SAR values without the inclusion of the matching network. A case study of an endo-rectal coil for 7T prostate imaging with numerical tool showed that the matching network has large impact on the local SAR and is not negligible for MRI RF coils specified above.

Lukas Winter¹ and Thoralf Niendorf^1,2
1Berlin Ultrahigh Field Facility (B.U.F.F.), Max-Delbrück Center for Molecular Medicine, Berlin, Germany, ²Experimental and Clinical Research Center (ECRC), a joint cooperation between the Charité and the Max-Delbrueck Center for Molecular Medicine, Berlin, Germany

UHF MR (B0≥7T) and EPR share the suffering from electrodynamic constraints dictated by an increase in spin excitation frequency rendering human in vivo applications challenging if not elusive. Recognizing the opportunities of antenna array design to offset some of these constraints, this work utilizes electromagnetic field simulations in phantoms and human voxel models to analyze transmission field (B1+) and specific absorption rate (SAR) distributions of multi-channel phased array antennas for a frequency range of 300–3000MHz corresponding to an effective field strength of 7–70T for proton MRI or ~10-100mT for EPR.

Xinqiang Yan^1,2, Xiaoliang Zhang³, Long Wei², Yuqian Liu², and Rong Xue^1
1State Key Laboratory of Brain and Cognitive Science, Beijing MRI Center for Brain Research, Institute of Biophysics, Chinese Academy of Sciences, Beijing, Beijing, China, ²Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, Beijing, China, ³Department of Radiology and Biomedical Imaging, University of California San Francisco and UCSF/UC Berkeley Joint Graduate Group in Bioengineering, San Francisco, California, United States

In this study, a hybrid monopole/loop technique which could combine the advantages of loop arrays and monopole arrays was proposed. To investigate this technique, a hybrid transceiver coil array containing 4 monopole channels and 4 loop channels was developed for human head imaging at 7T. Compared with the monopole-only and loop-only arrays, the proposed hybrid array has higher SNR and better parallel imaging performance. With increased independent transmit channels, the hybrid array promises additional degrees of freedom for the RF shimming and parallel transmission (pTx), which might be advantageous to provide more homogeneous transmit fields.

Mikhail Kozlov¹, Roland Müller¹, and Harald Möller^1
1MPI Leipzig, Leipzig, Saxony, Germany

We investigated how trade-off between B1+ averaged over the volume of interest, BV_sar= B1+V /„©SAR10g, the safety excitation efficiency, and IB1+V (B1+ root-mean-square inhomogeneity) depends on overlapped dual-row transmit array geometry, position of a head inside the array and array tuning condition. Different tuning conditions resulted in larger peak-to-peak variation for CP excitation than for excitations that minimized IB1+V. For these excitations IB1+V decreased from 15% to 12% when brain was located in middle of the coil. This position is clearly preferable for keeping the safety excitation efficiency unaffected. For an overlapping dual row array with four elements in each row designed for full-brain coverage, a small overlap is preferable.

Stefan H. G. Rietsch^1,2, Harald H. Quick^1,2, and Stephan Orzada^1
1Erwin L. Hahn Institute for MR Imaging, Essen, Germany, ²High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany

Micro strip line elements and dipole antennas have successfully been incorporated as radiofrequency transmit elements in 7T high field MRI. Some inherent features of both types are combined in meander elements (ME). In this work finite difference time domain field simulations are used to investigate the impact of changing meander size on RF transmit performance and on decoupling between neighboring ME. Results show that medium meander sizes allow for maximum central field strength, while providing the best mutual decoupling to adjacent elements. In conclusion, RF transmit-arrays consisting of ME should be based on elements with medium meander size.

Sung-Min Sohn¹, Lance DelaBarre¹, Anand Gopinath², and J. Thomas Vaughan^1,2
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, United States, ²Department of Electrical and Computer Science Engineering, University of Minnesota, Minnesota, United States

The 2nd prototype automatic RF coil has been built with some major upgrades after we presented the feasibility of applying automatic tuning and matching in a multi-channel transceive RF coil at 7T in the last meeting. The scientific and clinical utility of multi-channel systems that enable parallel MR imaging will not be fully realized without the proposed automatic frequency tuning and impedance matching technology at ultra-high fields.

Madhwesha Rao¹, Juan Parra-Robles¹, Helen Marshall¹, Neil Stewart¹, Guilhem Collier¹, and Jim Wild^1
1University of Sheffield, Sheffield, South Yorkshire, United Kingdom

The hyperpolarized gases, ³He and ¹²⁹Xe have distinct properties, and provide unique and complementary functional information from the lungs. A double/triple-nuclear same-breath imaging exam of the lungs with ¹H, ³He and ¹²⁹Xe can therefore provide exclusive functional information from each of the gas images and complementary co-registered structural information from ¹H images in the same physiological time frame. This study describes the development of a RF instrumentation for same-breath triple-nuclear (³He-¹²⁹Xe-¹H) MR imaging. Application to same-breath ventilation (³He-¹²⁹Xe-¹H) imaging and same-breath ³He-¹²⁹Xe diffusion imaging are demonstrated.

Christoph Leussler¹, Christian Findeklee¹, Peter Vernickel¹, Kay Nehrke¹, and Peter Börnert^1
1Philips GmbH Innovative Technologies, Research Laboratories, Hamburg, Germany

Transmit arrays and their applications must be robust in positioning, transmit homogeneity, coil matching, and required RF power. In this study, we investigate the influence of coil bending of flexible transmit arrays on homogeneity, required RF power and SAR. We present B1+-shimming using flexible 8-channel local TX/RX arrays for the human torso at 3T using fast B1+ mapping by means of DREAM.

Nikolai I Avdievich¹, Ioannis A Giapitzakis¹, and Anke Henning^1,2
1Max Planck Institute for Biological Cybernetics, Tübingen, Germany, ²Institute for Biomedical Engineering, UZH and ETH Zurich, Zurich, Switzerland

Surface loop transceiver head phased arrays improve transmit efficiency in comparison to larger Tx-only arrays due to tighter fit. However, the number of elements is limited by the number of available Tx-channels and decoupling issues, which compromises SNR and parallel receive performance. As a proof of concept we developed and constructed a novel splittable 9.4T transceiver array. Splitting of each Tx-loop doubles the number of Rx-elements without necessity of moving the array away from the subject, which allows optimization of both Tx and Rx performance at the same time. Splitting in a larger number of Rx-elements, (e.g.4, 8), is possible.

Shajan G¹, 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

For head sized volume transmit arrays at ultra-high field, 16 coil elements arranged in 2x8 is beneficial than eight coils arranged in 2x4 configuration. The aim of this project is to develop and qualify an interface to connect a 16-channel transmit array to an 8-channel parallel transmit system. The interface module consists of eight capacitive loaded miniaturized Wilkinson power splitters. Each pTx channel drives two coil elements through the interface module. Transmit (B1+) field maps were acquired in both CP mode and CP2+ mode and compared with the numerical model. The transmit array performance closely followed the predicted results.

Natalia Gudino¹, Qi Duan¹, Jacco A de Zwart¹, Joe Murphy-Boesch¹, Peter van Gelderen¹, and Jeff H Duyn^1
1Advanced MRI section, LFMI, NINDS, National Institutes of Health, Bethesda, Maryland, United States

Parallel transmission (pTX) is a critical technology to improve B1 uniformity and control RF power deposition at high field. Typically, optimization of the pTX excitation is based on prior information from electromagnetic field simulations and MRI. In this work we explored a way to support pTX by obtaining real-time B1 information from the RF current sensor integrated in an on-coil current-source amplifier with envelope feedback. This information was digitally encoded through low power analog-digital converters to be sent optically to a controller located outside the MRI room. Real-time analysis of these signals can be used for RF-safety monitoring and quality control.

Sung-Min Sohn¹, Anand Gopinath², and J. Thomas Vaughan^1,2
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, United States, ²Department of Electrical and Computer Science Engineering, University of Minnesota, Minneapolis, Minnesota, United States

To achieve accurate and stable RF power measurements in MR systems, a high directivity coupler is proposed and compared to a conventional coupler, and a compact MR-compatible on-coil power monitor with the high directivity coupler was built and tested both on the test bench and 7T MRI (Siemens Magnetom, Erlangen, DE).

Redi Poni^1,2, Taner Demir^1,2, and Ergin Atalar^2,3
1Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey, ²UMRAM, Ankara, Turkey, ³Bilkent University, Ankara, Turkey

A digital RF power amplifier is designed for 1.5T. The amplifier achieves higher efficiency, embodies amplifier with the coil and can be digitally controlled with an FPGA. An output power of 65.7 W was obtained.

Mustafa Cavusoglu¹, Benjamin E. Dietrich¹, David O. Brunner¹, and Klaas P. Pruessmann^1
1Biomedical Engineering, ETH Zurich, Zurich, Zurich, Switzerland

Advanced schemes of parallel excitation are based on the exact knowledge of the magnetic field dynamics inside the MR scanner and require highly accurate interplay between several RF channels and gradient waveforms. Further the transmit channels need to be highly synchronized and timed among themselves as well as with the gradient system. We present an optimization framework for spatially selective and sparse-spokes pulse design in parallel RF transmission at 7T based on magnetic field monitoring to measure the temporal evolution of gradient magnetic fields and the multi-channel RF excitation pulses on equal time basis, fully concurrently and at full power.

Cameron M. Hough¹, Russell L. Lagore², Cecilia Possanzini³, and Nicola De Zanche^1
1Department of Oncology, University of Alberta, Edmonton, Alberta, Canada, ²Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, United States, ³Philips Healthcare, Best, Netherlands

In this work we investigate the noise temperature variations of RF preamplifiers when exposed to an external magnetic field. The noise parameters are expressed in terms of the transconductance which varies with the magnetic field and is readily derived from S parameters measured using a network analyzer. Models for field-effect transistors and bipolar-junction transistors were applied and fitted to measured noise temperature data. Noise parameter changes as a function of B₀ are calculated.

Chunqi Qian¹, Qi Duan¹, Stephen Dodd¹, Alan Koretsky¹, and Joseph Murphy-Boesch^1
1NIH, Bethesda, MD, United States

A localized detection coil with an integrated negative resistance amplifier has been constructed to enhance the local sensitivity of inductively coupled receive coils. When the transistor is grounded on its drain, the feedback effect transforms capacitive impedance on the source into negative resistance on the gate, thus increasing the signal current in the LC resonator. Although the integrated amplifier consumes only 86 μW of power, 14 dB gain is achievable with excellent noise performance. Such a detection scheme enables high resolution animal imaging at 3T with small active coils inductively coupled to larger clinical coils using the standard signal interface.

Roland Müller¹, Torsten Schlumm¹, André Pampel¹, and Harald E. Möller^1
1Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Saxony, Germany

Low-cost USB DVB-T sticks can operate as frontend for multipurpose Software Defined Radio (SDR) receivers. Bypassing any decoder, real/inphase and imaginary/quadrature byte streams are provided. Many applications are conceivable in MRI, including amplitude demodulation to restore the RF pulse envelope and spectral analysis. Recording transmitted RF pulses can help to find mistakes during sequence development or to identify malfunctioning scanner hardware. More and more electronic devices are operated within the examination room or even inside the scanner bore. Unwanted emissions near the Larmor frequency and insufficient shielding can already be identified in the RF lab.

Mark Gosselink¹, Andrea Anzellotti², Giel Mens², Marco Boutelje², Bart Voermans², Hans Hoogduin¹, Peter R. Luijten¹, Cecilia Possanzini², and Dennis W.J. Klomp^1
1University Medical Center Utrecht, Utrecht, Utrecht, Netherlands, ²Philips Healthcare, Best, Netherlands

MRI systems can be equipped with digital on coil receivers, providing increased SNR when requiring high dynamic range, and unlimited number of receiver channels. Higher field systems like 7T may benefit from both features as SNR increases (and thus dynamic range), and higher acceleration factors can be obtained at contained geometry factors (thus benefit from more receiver channels). In this work we demonstrate that using under-sampling combined with Surface Acoustic Wave filters can convert 1.5T- into 7T- digital receivers with low noise figure.

David E J Waddington^1,2, Mathieu Sarracanie^1,3, Najat Salameh^1,3, and Matthew S Rosen^1,3
1MGH/A.A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States, ²ARC Centre of Excellence for Engineered Quantum Systems, School of Physics, University of Sydney, Sydney, NSW, Australia, ³Department of Physics, Harvard University, Cambridge, MA, United States

Overhauser-enhanced MRI (OMRI) is an electron-proton double resonance imaging technique of much interest due to its ability to detect the concentration and distribution of free radicals. Tracking of exogenous free radicals with OMRI in vivo has enabled the development of oxymetry probes and the imaging of redox reactions. The large gyromagnetic ratio of electrons (28 GHz/T) demands that in vivo OMRI is performed at very low magnetic fields (~10 mT) in order to minimize RF heating and penetration depth issues. We report the development of a high performance OMRI probe consisting of a litz wire NMR solenoid (276 kHz) inside a modified Alderman-Grant Resonator (141 MHz) for operation at 6.5 mT.

Branislav M. Notaros¹, Milan M. Ilic¹, Alexey A. Tonyushkin², Nada J. Sekeljic¹, and Pranav Athalye^1
1Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, Colorado, United States, ²Radiology Dept., Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts, United States

We propose a novel method for traveling wave (TW) MRI at high- and ultra-high-fields using a bore-extended, subject-loaded helical antenna as a whole-body TW RF coil. The helical antenna, being essentially a TW antenna, with TW current, radiates a TW field into the subject inside it. The novel RF coil has advantages over patch antennas and other existing TW excitation methods and is simultaneously a solution for a whole-body RF coil at high fields. Our simulations of 3T and 7T MRI systems with quadrifilar helical antenna RF coils show TW RF magnetic field with high field-uniformity and excellent circular polarization.

Jürgen Braun¹, Sebastian Hirsch², Tassilo Heinze³, and Ingolf Sack^2
1Department of Medical Informatics, Charité - Universitätsmedizin Berlin, Berlin, Berlin, Germany, ²Department of Radiology, Charité - Universitätsmedizin Berlin, Berlin, Germany,³SPL Spindel und Präzisionslager GmbH, Sachsen, Germany

A new actuator for MRE is introduced based on transient air pressure waves produced by a fast switching valve and an air-filled cylindrical bellow inside the magnet. This concept provides an economic setup of interference-free, powerful and easy-to-use drivers for potential applications of MRE in obese patients, at low gradient systems and for remote actuation. The feasibility of the new driver is demonstrated by MRE of the brain and remote excitation from the upper thoracic spine. The combination of low drive frequencies and high resolution elasticity reconstruction provided viscoelastic parameter maps in agreement to previous work on cerebral MRE.

MR-PET, Gradients & Other Hardware

Tuesday 2 June 2015

Edwin G.W. ter Voert¹, Helen Davison^1,2, Felipe de Galiza Barbosa^1,3, Martin Huellner^1,4, Patrick Veit-Haibach^1,3, and Gaspar Delso^5
1Department of Medical Imaging, Division of Nuclear Medicine, University Hospital Zurich, Zurich, Zurich, Switzerland, ²Department of Medical Physics, Royal United Hospitals Bath NHS Foundation Trust, Bath, Somerset, United Kingdom, ³Department of Diagnostic and Interventional Radiology, University Hospital Zurich, Zurich, Zurich, Switzerland,⁴Department of Medical Imaging, Clinic of Neuroradiology, University Hospital Zurich, Zurich, Zurich, Switzerland, ⁵GE Healthcare, Waukesha, WI, United States

MR based attenuation correction (MR-AC) is still challenging in simultaneous PET/MR scanning; especially near metal implants. A possible solution could be the inclusion of time-of-flight (TOF) information into the PET image reconstruction algorithm. To evaluate the influence of TOF on artifact reduction and improvement in PET image quality, 35 patients were included and scheduled for a comparative scan in a new simultaneous TOF PET/MR scanner. A total 46 clinical PET image artifacts and 21 simulated artifacts were being evaluated. In conclusion, time-of-flight information in simultaneous TOF PET/MR scanning significantly (p<0.01) reduces PET artifacts and improves clinical reader confidence.

Mark Oehmigen¹, Maike Lindemann¹, Titus Lanz², Sonja Kinner³, and Harald H. Quick^1,4
1High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany, ²Rapid Biomedical GmbH, Rimpar, Germany, ³Institute for Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Essen, Germany, ⁴Erwin L. Hahn Institute for MR Imaging, University Duisburg-Essen, Essen, Germany

To implement a 16-channel RF breast coil for PET/MR hybrid imaging, systematic performance tests were realized. The MRI performance was tested using modular PET/MR breast phantoms featuring high resolution MR-visible inserts and PET-visible spheres with various diameters. Signal-to-noise maps and spatial resolution were evaluated. Whereas the coil was designed to minimize the attenuation of PET photons, CT-based attenuation correction of the coil was performed and PET quantification was evaluated in phantom experiments. The RF coil was then tested in PET/MR breast imaging on four breast cancer patients. Attenuation correction enabled correct PET activity quantification in PET/MR phantom and patient studies.

Farouk Nouizi¹, Jaedu Cho¹, Alex Luk², Edward anashkin³, Pavel Stepanov³, Val zavarzin³, Irving weinberg³, Lydia Min-Ying Su¹, Gultekin Gulsen², and Orhan Nalcioglu^2
1Radiology, University of California Irvine, Irvine, CA, United States, ²University of California Irvine, Irvine, CA, United States, ³Weinberg Medical Physics, LLC, Bethesda, MD, United States

Positron Emission Mammography (PEM) is a dedicated breast imaging systems performing breast imaging under gentle compression. PEM exhibited a higher spatial resolution than PET due to the good immobilization of the breast and its small interdetector distance. However, its data acquisition scheme results in a poor axial spatial resolution. In this study, we introduce a new combined PEM/MRI system taking advantage of the high resolution soft-tissue contrast provided by MRI and the best signal to noise ratio (SNR) due to efficient count collection configuration of PEM. The performance of the system is validated on a breast phantom.

Saikat Saha¹ and Kyle Reiser^2
1GE Healthcare, Waukesha, WI, United States, ²GE Healthcare, WI, United States

In conventional whole body MR RF transmit coils, various RF components such as diodes, capacitors, inductors, cables etc. are placed throughout the coil for optimal RF performance. If such components in a simultaneous PET/MR system are placed in the FOV of the PET detector, they will scatter some of the PET signals (511keV annihilation photons), affecting image quality. To address this problem we have created a “zero PET attenuation” whole body Tx/Rx coil with minimal use of high density RF components. We present design and performance for the resulting antenna as implemented in our 3.0T simultaneous whole body PET/MR system.

Yaohui Wang¹, Feng Liu¹, Ewald Weber¹, and Stuart Crozier^1
1School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, Queensland, Australia

A novel acoustic noise reduction method is proposed for the hybrid MRI-LINAC (Linear Accelerator) system. This method is based on the beam-deflection theory. If using some supports or bolts on the split gradient assembly, by adjusting the positions of the supports or bolts, the noise level in the central gap can be significantly attenuated. Theoretical investigation on a 3-dimensional (3D) full acoustic model has verified that this method is available. The overall noise level reduction in the central gap is 8.5 dB. Future work will be done on the position optimization of the supports of bolts.

Md. Shahadat Hossain Akram¹, Koki Matsuzawa¹, Yasuhiko Terada¹, and Katsumi Kose^1
1Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki, Japan

Solid angle coupled circuit method is proposed for Gz and Gx cylindrical type gradient coils. 3D solid angle was calculated for both coils. Actuance calculation was performed to consider the induction effect to a particular subdomain due to eddy currents in the surrounding subdomains. The degradation of magnetic field inside the materials due to induced antiparallel magnetic moments was also considered in the coupling calculation considering subdomains taken inside the materials. Gradient echo shift experiment was conducted for verification. We found good agreements between simulation and experimental results.

Christopher M Ireland^1,2, Randy O Giaquinto², Jean A Tkach², Ronald G Pratt², and Charles L Dumoulin^2
1Department of Biomedical Engineering, University of Cincinnati, Cincinnati, OH, United States, ²Imaging Research Center, Cincinnati Children's Hospital Medical Center, Cincinnati, OH, United States

MRI acoustic exposure has the potential to elicit physiological distress and impact development in preterm and term infants. To mitigate the risk, a novel acoustic quiet body coil for a neonatal MR system has been developed to reduce the sound pressure level experienced by neonates during MR procedures. Acoustic characterization demonstrated the quiet coil was consistently quieter, on average of 9 dBA, compared to a conventional body coil. The quiet coil provides a robust way of attenuating noise that does not require imaging pulse sequence modifications, is patient-independent, and synergistic with other attenuating practices.

Yasuhiko Terada¹, Kazunori Ishizawa¹, and Katsumi Kose^1
1Institue of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki, Japan

Winding patterns of gradient coils are often designed using target-field (TF) methods. However, the TF methods need time-consuming parameter tuning and trial-and-error approach to obtain a desired performance. Moreover, the TF methods ignore the dimensions of winding wires and sometimes lead to unrealizable coils for construction. Here we proposed a simple and straightforward method for designing planar gradient coils with discrete windings, which does not require parameter tuning. In the proposed scheme, the coil geometric parameters were optimized by an artificial bee colony (ABC) algorithm. The use of ABC greatly accelerates the design speed, compared with other discrete-wire based methods, and can also provide coil performances comparable with the TF method. The proposed method was used to design gradient coils for a vertical wide-bore superconducting MRI system (4.74 T), and the validity of the method was experimentally confirmed.

Paul Chang^1,2, Martin Eschelbach¹, Roland Syha³, Klaus Scheffler¹, and Anke Henning^1,4
1Max Planck Institute for Biological Cybernetics, Tuebingen, Baden-Wuerttemberg, Germany, ²Graduate School of Neural & Behavioural Sciences, University of Tuebingen, Tuebingen, Baden-Wuerttemberg, Germany, ³Diagnostic and Interventional Radiology, University Hospital Tuebingen, Tuebingen, Baden-Wuerttemberg, Germany, ⁴Institute for Biomedical Engineering, UZH and ETH Zurich, Zurich, Switzerland

Field monitoring using NMR field cameras typically use the gradients for calculating the positions of the field probes. However this is under the assumption that the gradients are linear which is certainly not the case. The effect of evaluated gradient imperfections on the estimation of the probes was investigated. Furthermore, the effect of the position error on monitoring higher order spherical harmonics was also investigated. A CT scan of the field camera setup was used to correct for errors in the calculations of the probe position.

Stefan Kroboth¹, Kelvin Layton¹, Feng Jia¹, Sebastian Littin¹, Huijun Yu¹, and Maxim Zaitsev^1
1Medical Physics, University Medical Center Freiburg, Freiburg, BW, Germany

Matrix coils have recently been introduced for shimming as well as encoding. To reduce costs, a switching circuit can be used to drive clusters of coil elements simultaneously with a reduced number of amplifiers. This work focuses on algorithmic approaches for finding suitable coil element clusters that are able to create an approximation to a desired target field. We introduce three algorithms and show that fewer amplifiers than coil elements do not necessarily reduce the flexibility of a matrix gradient coil, as long as the switching circuit suitably assigns the coil elements to the available amplifiers.

Shogo Horinouchi¹, Etsuko Kumamoto², and Kagayaki Kuroda^3,4
1Graduate School of System Informatics, Kobe University, Kobe, Hyogo, Japan, ²Information Science and Technology Center, Kobe University, Kobe, Japan, ³Graduate School of Engineering, Tokai University, Hiratsuka, Japan, ⁴Center for Frontier Medical Engineering, Chiba University, Chiba, Japan

Temperature distribution of whole body with a cardiac pacemaker by the low frequency band of the gradient magnetic field generated by the Helmholtz coils were simulated by low frequency solver and thermal simulator with steady state solver. Simulations were performed by 40mT/s of slew late and from 100Hz to 2500Hz of frequency of gradient magnet fields. Temperature near the center of the element of Helmholtz coil were higher and maximum temperature was 6.83e-03℃ at 1000Hz. The temperature around a cardiac pacemaker was less than 1.74e-04℃ at 1000Hz. The results leaded there was little substantial risk for heating in this case.

Simon Gross¹, 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 temperature dependent frequency drift of 1H-NMR field probes is reduced by two orders of magnitude using an intrinsic magnetostatic compensation method. This allows extremely precise and stable measurements of magnetic field evolutions e.g. in MRI field monitoring applications independent of the respective thermal dynamics.

Eric Michel¹, Daniel Hernandez¹, Min Hyoung Cho¹, and Soo Yeol Lee^1
1Kyung Hee University, Suwon, Gyeonggi-Do, Korea

Miniaturization of H¹ NMR probes is need it for high resolution Magnetic Field Monitoring (MFM) but it compromises the SNR and the manufacturing process becomes more challenging. We introduce a new design of field monitoring probes consisting in a solenoid placed directly into the sample material, here called, sample-immersed microcoil (SIM) probes having a high-impedance RF shield to avoid eddy currents. We validate this design experimentally in terms of sensitivity, SNR, internal homogeneity and RF shielding efficiency. We observed that this design give us long duration signals without significant SNR losses that could allow us to monitor imaging sequences having resolutions below 200 m.

Elad Bergman¹, Yifat Sarda¹, Noa Ritz², Edmond Sabo², Reuven Bergman², and Uri Nevo^1
1Biomedical Engineering, Tel Aviv University, Tel Aviv, Israel, Israel, ²Departments of Dermatology and Pathology, Rambam Health Care Campus and The Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel, Israel

We present a method for evaluation of skin aging and of sun damage with a unilateral stray-field NMR scanner operating in low field. In-vivo profiles with relaxation times of the sun-protected and sun-exposed forearm skins were measured in 9 female subjects. Biopsies were used as references. A significant increase in the fraction of the slow T_2 component, and in the value of the T_(2 )component itself correlated with chronological aging. A further increase was superimposed in the sun-exposed forearm. The results suggest that this affordable hardware, so far not used clinically, can serve as a tool for skin aging assessment.

Wolfgang Loew¹, Nathan Lamba², Randy Giaquinto¹, Matthew Lanier¹, Lacey Sickinger¹, Brynne Williams¹, Christopher Ireland¹, Yu Li¹, and Charles Dumoulin^1
1Imaging Research Center, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio, United States, ²Ohio State University, Ohio, United States

Traditionally, resonant coils are used for MR signal detection. In this work we present a novel approach for detecting MR signals by combining a non-resonant grid structure with resonant coil elements that are inductively coupled to each segment of the grid. S-Parameters were measured and imaging experiments were performed to evaluate coupling and imaging performance in phantoms.

Saikat Saha^1
1GE Healthcare, Waukesha, WI, United States

Today’s powerful gradient coils produce large eddy currents in the RF shield, resulting in significant heating. The magnitude and location of the eddy currents depend on scan duration and gradients used. Stressful EPI and/or Spiral scans can deposit as much as 1000-1500W of power in the shield and do so non-uniformly. This non-uniform heating is deleterious for the performance of thermally sensitive PET detectors in a simultaneous whole body PET/MR system. To address this problem we present here a novel integrated RF coil and shield which not only provides excellent MR IQ but also provide a cooler, more stable thermal environment for the PET detectors.

Makoto Tsuda¹, Daiki Tamada¹, Yasuhiko Terada¹, and Katsumi Kose^1
1University of Tsukuba, Tsukuba, Ibaraki, Japan

A digital MRI console for a 1.0 T MRI system was developed using a digital oscilloscope, an arbitrary waveform generator, and board computers. Pulse sequences were generated by the board computers in the 20 us resolution, which triggered other instruments. The waveform generator directly generated Larmor frequency (43.85 MHz) RF pulse but the oscilloscope sampled the detected 43.85 MHz NMR signal using the under sampling technique at 6.9 MHz. Unreproducible phase scrambling cased by independent time-base used in each instruments was successfully recovered using the RF pulse simultaneously sampled, which proved a promise of our system.

Michael Twieg¹ and Mark A Griswold^1,2
1Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland, OH, United States, ²Department of Radiology, Case Western Reserve University, Cleveland, OH, United States

Decoupling of elements in a parallel transmit array has proven to be a significant obstacle to implementing parallel transmit (pTX) in MRI applications. Typically, a coil array is designed to reduce mutual impedance between elements, but this approach alone has limitations. Additional methods of decoupling include passive impedance transformations and closed loop control. Such methods have been explored with linear amplifier topologies, but little has been reported about approaches to decoupling high-efficiency switchmode amplifier topologies. Here we present improvements in the decoupling between elements driven by CMCD amplifiers using passive impedance transformation and closed loop current magnitude feedback.

Charles Randall Poole¹, Tanvir Baig¹, Robert Deissler¹, Robert W. Brown¹, and Michael A Martens^1
1Department of Physics, Case Western Reserve University, Cleveland, Ohio, United States

Magnesium diboride (MgB2), a high temperature superconductor, has been considered for use in conduction cooled MRI magnets to reduce the usage of liquid helium. Compared to NbTi wires the thermal normal zone propagation (NZP) in MgB2 is much slower leading to active quench protection designs. The NZP and the temperature rise in the coil were modeled using the Douglas-Gunn method to solve the 3D heat equation for wire with copper percentages varying from 15% to 50%. It was determined that wire with higher percentage of copper is more advantageous for quench protection due to the slower rise in maximum temperature.

Artem Mikheev¹ and Henry Rusinek^1
1Radiology, NYU Langone Medical Center, New York, NY, United States

New algorithm for Bias field correction at 7T and the corresponding software implementation are presented. Algorithm achieves a high correction quality at 7T. New Bias correction evaluation methodology is proposed.

T. Stan Gregory¹, Kevin J. Wu¹, Jasper Yu¹, James Brent Box¹, Rui Cheng¹, Leidong Mao¹, Guoyi Tang², and Zion Tsz Ho Tse^1
1College of Engineering, The University of Georgia, Athens, Georgia, United States, ²Advanced Materials Institute, Tsinghua University, Shenzhen, Guangdong, China

The development of an MRI-compatible actuator based on Magnetohydrodynamic propulsion, potentially allowing for simultaneous endocapsule control during MR imaging sequences.