Ultra-High Field Engineering: Coils, Hardware & Methods

Tuesday 13 May 2014      13:30 - 14:30

Space 1/Power Poster Theatre & Traditional Poster Hall 
Moderators: Nikolai I. Avdievitch, Ph.D. & Gregor Schaefers, Dipl.-Ing.

Click on this video icon to view the introductory session.

Seung-Kyun Lee¹, Jean-Baptiste Mathieu¹, Joseph E Piel¹, Christopher J Hardy¹, John F Schenck¹, Ek Tsoon Tan¹, Eric Budesheim¹, Eric Fiveland¹, Keith Park¹, Kenneth Rohling¹, Yihe Hua², Jian Lin², Matthew A Bernstein³, John Huston III³, Yunhong Shu³, and Thomas K-F Foo^1
1GE Global Research, Niskayuna, NY, United States, ²GE Global Research, China Technology Center, Shanghai, China, ³Mayo Clinic, Rochester, MN, United States

A dedicated head-only gradient set with asymmetric transverse coils and a hollow-conductor z coil was built and tested. The coil incorporated force-balanced, torque-balanced, and eddy current-controlled design. The coil was tested inside a conventional whole-body 3T magnet to assess image quality and peripheral nerve stimulation (PNS). The gradient was integrated with a custom-built birdcage transmit/receive coil and a 32-channel receiver array for in-vivo imaging. Initial results demonstrated high quality brain images with transverse gradients at 80 mT/m and 500 T/m/s (simultaneously) without PNS.

Xiaoping Wu¹, An T. Vu¹, Sebastian Schmitter¹, Edward Auerbach¹, Steen Moeller¹, Christophe Lenglet¹, Essa Yacoub¹, Pierre-Francois Van de Moortele¹, and Kamil Ugurbil^1
1CMRR, Radiology, University of Minnesota, Minneapolis, Minnesota, United States

Simultaneous Multi-Slice (SMS) imaging using MultiBand (MB) RF pulses has shown great success in functional and diffusion weighted (DW) MRI studies of the brain. Recently, this technique has been combined with parallel transmission (pTx) and it has been demonstrated that pTx MB pulse design can significantly improve transmit B1 homogeneity at 7T and/or reduce RF power consumption relative to a single channel Circular Polarized mode application. Those results were obtained for a few slices in the brain using a gradient echo imaging sequence and did not tackle the problem of whole brain coverage or more challenging spin-echo based acquisitions. In the present study, advances towards volumetric coverage with pTx MB pulses is reported and results of whole brain single-shot, diffusion weighted echo planar imaging (DW-EPI) at 7T using pTx MB RF pulses are presented.

Yolanda Duerst¹, Michael Wyss¹, Bertram J Wilm¹, Benjamin E Dietrich¹, Simon Gross¹, David O Brunner¹, Thomas Schmid¹, and Klaas P Pruessmann^1
1ETH Zurich, Zurich, ZH, Switzerland

Respiratory motion leads to field changes that affect brain MRI. Measurements of breathing induced field changes showed that spectral power in the frequency range associated with breathing is strongest in B0, X (ap), and Z (fh) shims but also non-negligible in some higher order terms. Furthermore, volunteers with higher body mass index generally showed more breathing induced field oscillations which, as expected, decreased with increasing distance from the chest. Such field changes lead to image artifacts due to incorrect spatial encoding and signal loss. Using real-time field feedback to correct for spatiotemporal field changes strongly reduced artifacts in T2*-weighted images. Such field changes lead to image artifacts due to incorrect spatial encoding and signal loss. Using real-time field feedback to correct for spatiotemporal field changes strongly reduced artifacts in T2*-weighted images.

Ian R.O. Connell^1,2, Kyle M. Gilbert², Mohammed A. Abou-Khousa², and Ravi S. Menon^2
1University of Western Ontario, London, Ontario, Canada, ²Centre for Functional and Metabolic Mapping, Robarts Research Institute, London, Ontario, Canada

Mutual coupling is a long-standing and critical issue that affects the performance of parallel transmit arrays. Here, we implement a frequency selective surface method to decouple array elements that helps realize the benefits of RF shimming and pulse shaping algorithms, while also increasing array transmit efficiency.

Alexander J.E. Raaijmakers¹, Dennis W.J. Klomp¹, Fredy Visser^1,2, Hans Hoogduin³, Peter R. Luijten¹, and Cornelis A.T. van den Berg^4
1Radiology, UMC Utrecht, Utrecht, Utrecht, Netherlands, ²Philips Healthcare, Best, Netherlands, ³Brain Division, UMC Utrecht, Utrecht, Netherlands,⁴Radiotherapy, UMC Utrecht, Utrecht, Utrecht, Netherlands

Dipole antennas are emerging rapidly in the UHF community. In antenna theory, Poynting vector and wave impedance play an important role. Following these concepts, we found that dipole antenna performance can be enhanced by dividing the legs in segments and interconnecting these segments by inductors. In this setup, the required inductors are realized by meanders. The resulting elements we call ‘fractionated dipole antennas’. These elements provide good B1 efficiency with low SAR levels. An array of 8 of these elements has been constructed. With this array, impressive image quality has been obtained for prostate, kidneys and coronary arteries.

Karthik Lakshmanan¹, Martijn Cloos¹, Ricardo Lattanzi¹, Daniel Sodickson¹, Dmitry Novivkov¹, and Graham Wiggins^1
1Department of Radiology, New York University School of Medicine, NewYork, NewYork, United States

A dipole antenna element which creates circular polarization and is sensitive to magnetic dipole and electric dipole fields.

Qi Duan¹, Natalia Gudino¹, Jacco A. de Zwart¹, Peter van Gelderen¹, Joe Murphy-Boesch¹, Jeff H. Duyn¹, and Hellmut Merkle^1
1LFMI, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, United States

The feasibility of using electric dipole antennas for RF transmission for 7T spine MRI is explored. A transmitter consisting of two electric dipole antennas parallel to B₀ is evaluated. The capability of B₁⁺ field steering was demonstrated on a phantom. The transmit efficiency for the optimal setup was compared to a previously published design for a 7T spine array. The proposed transmit structure allows easy integration with a receive-only phased array. Because of design simplicity and favorable B₁⁺ efficiency, dipole antennas provide a promising alternative to conventional design for spine MRI at high field.

Gregor Adriany¹, Scott Schillak², Matt Waks², Brandon Tramm², Tommy Vaughan¹, Kamil Uğurbil¹, Pierre-Francois van de Moortele¹, and Sebastian Schmitter^1
1University of Minnesota, Minneapolis, MN, United States, ²Virtumed LLC, Minneapolis, MN, United States

A combination between a bilateral 2x4 channel T/R Arterial Spin Labeling (ASL) array and a 8 channel T/R head coil at 7 Tesla was developed. The achievable B1+ transmit efficiency of the ASL array is in the order of 0.8 μT/Volt and corresponding post-shim TX efficiency was calculated to be ~80% within the carotid arteries.

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, Minneapolis, Minnesota, United States

An eight-channel RF transceive head coil with automatic tuning and matching was implemented and evaluated at 7T. The electrically controlled feedback system rapidly manipulates the frequency tune and impedance match and is compatible with any existing MRI system. The results show the feasibility of the automated system to tune and match transmit and/or receive elements along with multi-channel RF coils.

Christopher Ellenor¹, Pascal Stang^1,2, Maryam Etezadi-Amoli¹, John Pauly¹, and Greig Scott^1
1Electrical Engineering, Stanford University, Stanford, CA, United States, ²Procyon Engineering, San Jose, CA, United States

A new, completely wireless current sensor is demonstrated for the measurement of RF current on wires. This small and lightweight sensor will find application where wire current must be monitored, such as in interventional MRI procedures, and where current is minimized, such as in device design or in the development of new scanning protocols. The new sensor measures only the magnitude of the current, and an RF pulse is proposed and demonstrated, with which current magnitude is sufficient to identify null modes.

Natalia Gudino¹, Jacco A de Zwart¹, Qi Duan¹, Stephen J Dodd², Peter Van Gelderen¹, and Jeff H Duyn^1
1Advanced MRI section, LFMI, NINDS, National Institutes of Health, Bethesda, MD, United States, ²LFMI, NINDS, National Institutes of Health, Bethesda, MD, United States

On-coil switch mode amplification has been presented as a promising alternative for the implementation of multi-channel transmit systems by reducing cost and complexity. This non-conventional RF amplification is driven by separate optical carrier and envelope signals, which require a special interface, not available with current scanner systems. Here we present a simple low-cost prototype that can generate multiple independently controlled RF carriers and envelope signals from the single low power RF pulse waveform input to the MRI system’s RF amplifier. The interface is designed to operate over a large range of magnetic field strengths without hardware adjustments.

Wyger Brink¹, Johan van den Brink², and Andrew Webb^1
1Radiology, Leiden University Medical Center, Leiden, Zuid-Holland, Netherlands, ²Philips Healthcare, Best, Noord-Brabant, Netherlands

This work explores the implications of using high permittivity dielectric pads on the SAR and B₁⁺ shimming behaviour of a dual-transmit system at 3 T.

Devashish Shrivastava¹, J Thomas Vaughan¹, and Rachana Visaria^2
1University of Minnesota, Minneapolis, MN, United States, ²In Vivo Temperatures, Minnesota, United States

Underestimation error associated with the conventional measurement of local specific absorption rate (SAR) in perfused tissues is quantified as a function of local tissue perfusion and the temperature difference between local tissue and blood. Results show that this error may exceed ‘safe’ local SAR thresholds, as determined by regulatory bodies. Correcting for this SAR estimation error is necessary to build and operate radiofrequency (RF) coils safely to conduct human imaging at high and ultra-high fields.

Giuseppe Carluccio¹, Yu-Shin Ding¹, and Christopher Michael Collins^1
1Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, New York, United States

MR/PET is able to combine the high resolution and soft tissue contrast of MRI with the ability of PET to provide information about metabolic rate. However, metabolic rate is influenced by local temperature. In this work we have investigated how temperature increase by SAR absorption during an MRI scan may affect metabolic rate, important parameter in PET images.

Leeor Alon^1,2, Cem M. Deniz^1,2, Giuseppe Carluccio^1,2, Mary Bruno¹, Daniel K. Sodickson^1,2, and Christopher C. Collins^1,2
1Department of Radiology, Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY, United States, ²Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY, United States

Currently, safety of RF coils is often evaluated in part using computer simulations performed on a limited number of body models. In this work, we propose a method for the creation of body models that have a multitude of tissues and that closely matched the anatomy and orientation of a subjects being scanned. The method is currently automated and takes roughly 15 minutes to complete. Results are shown for the head region of two random subjects.