Niravkumar Darji¹, Kyoung-Nam Kim², Gopesh Patel¹, Hans-Peter Fautz³, Johannes Bernarding², and Oliver Speck^1
1Biomedical Magnetic Resonance, Otto-von-Guericke University, Magdeburg, Saxony Anhalt, Germany, ²Institute for Biometry and Medicine Informatics, Otto-von-Guericke University, Magdeburg, Saxony Anhalt, Germany, ³Siemens Healthcare, Erlangen, Germany

Human MR System equipped with a multi Tx hardware typically have a much smaller number of Tx channels than Receive (Rx) channels. The purpose of this study was to evaluate whether more of Tx channels could improve the Tx field performance. B₁⁺ field inhomogeneity at 7T can be minimized to a certain level by varying the phase and amplitude of individual channels of the 16 channel transmit array. However, even with a larger number of channels, complex B₁⁺ distributions cannot be corrected by means of static RF-shimming only. Transmit-SENSE with spoke trajectories may be more efficient in correcting inhomogeneities even with a lower number of channels.

Martijn Anton Cloos^1,2, Guillaume Ferrand², Nicolas Boulant¹, Michel Luong², Christopher J Wiggins¹, Denis Le Bihan¹, and Alexis Amadon^1
1LRMN, CEA, DSV, I2BM, NeuroSpin, Gif-Sur-Yvette, ile-de-France, France, ²CEA, DSM, IRFU, Gif-Sur-Yvette, ile-de-France, France

Transmit-SENSE gives the opportunity to implement short excitation pulses with good flip-angle homogeneity. Commonly, a transmit-array system used for brain imaging at 7 Tesla consists of 8 independently modulated amplifiers in combination with a dedicated 8-channel RF coil. In this work we explore the possibility to drive an 8-element transmit coil with only 2 independent transmit-channels, significantly reducing the required preparation measurements and overall complexity of the system, while retaining the ability to perform adequate flip-angle homogenization in the human brain at 7 Tesla. Initial results indicate that the FA variation could be reduced to less than 10%.

Christof Thalhammer^1,2, Wolfgang Renz^1,3, Harald Pfeiffer⁴, Jan Rieger¹, Lukas Winter¹, Fabian Hezel¹, Frank Seifert⁴, Werner Hoffmann⁴, Reiner Seemann⁴, and Thoralf Niendorf^1,5
1Berlin Ultrahigh Field Facility, Max-Delbrueck Center for Molecular Medicine, Berlin, Germany, ²University of Regensburg, Regensburg, Germany, ³Siemens Healthcare, Erlangen, Germany, ⁴Physikalisch-Technische Bundesanstalt (PTB), Braunschweig und Berlin, Germany, ⁵Experimental and Clinical Research Center (ECRC), Charité Campus Buch, Berlin, Germany

Ultrahigh field cardiac MR is challenged by non-uniform B1+-distributions. A two-dimensional 16-channel transceive surface coil array based on loop elements is proposed to improve B1+-homogeneity and parallel imaging for cardiac MR at 7 T. The RF characteristics were satisfying without the need for subject-specific tuning and matching. MR images were acquired showing a rather uniform intensity over the whole cardiac region and a high myocardium/blood contrast without subject-specific B1+-shimming.

Wouter Koning¹, Erwin Langenhuizen¹, Alexander J.E. Raaijmakers¹, Cornelis A.T. van den Berg¹, Jaco J.M. Zwanenburg¹, Peter R. Luijten¹, and Dennis W.J. Klomp^1
1University Medical Center, Utrecht, Utrecht, Netherlands

For imaging the carotid arteries in the neck at 7T, a 6 channel radiative transmit array was designed and constructed with low RF power deposition and relative uniform B1+. The array was designed consisting of six separate single-side adapted dipole antennas attached to a neck pillow filled with water. It was able to deliver 20μT at the carotid arteries. Combined with a dedicated 16 channel small element receive coil a TSE sequence could be implemented to show that 7T can be used for clinical high spatial resolution imaging to asses carotid vessel wall integrity.

Nikolai I. Avdievich¹, Jullie W. Pan¹, and Hoby P. Hetherington^1
1Neurosurgery, Yale University, New Haven, CT, United States

At 7T, interactions between the human brain and RF result in poor homogeneity and efficiency when volume coils are used for transmission. Although short (9cm) transceiver arrays can dramatically improve both axial homogeneity and efficiency (17uT/kW) using RF shimming, coverage in the longitudinal direction is limited. Increasing the length of the array to 15cm provides only modest improvements in longitudinal coverage at the cost of significant decreases in sensitivity and transmit efficiency. To overcome this limitation we have developed an inductively decoupled 16 coil transceiver array, with the individual coils arranged in two rows along the z-axis of 8 coils each (2x8 array). The array provides dramatically improved longitudinal coverage and SNR while retaining high efficiency.

Kyle Michael Gilbert¹, Andrew T Curtis¹, Joseph S Gati¹, L Martyn Klassen¹, and Ravi S Menon^1
1Robarts Research Institute, The University of Western Ontario, London, Ontario, Canada

A 16-channel transmit/receive array was developed that conforms to the human head. Elements of the array were decoupled using circumferential shields that extended orthogonally from the coil former. The conformal geometry increased transmit efficiency and receive sensitivity. The decoupling scheme permitted high isolation between coil elements, which improved RF shimming and parallel imaging performance. High cortical SNR and low geometry factors make this coil well suited to functional MRI.

Tamer S. Ibrahim¹, Tiejun Zhao², and Fernando E Boada^3
1University of Pittsburgh, Pittsburgh, PA, United States, ²Siemens Medical Solutions, ³University of Pittsburgh

Several major obstacles have prevented the widespread implementation of main-stream parallel transmission methods, specifically Tx-SENSE for 7T head imaging. These include a) the need for accurate B1+ field maps, b) coil and subject dependent increases in local SARs as a result of RF excitation, and 3) concerns regarding the unclear RF safety assurance of the PTX experiment. This work demonstrates consistent (12 subjects were tested) high acceleration factor in-vivo Tx-SENSE with a generic measured or simulated set of B1+ Maps on load independent whole-head 7T Tx Arrays. The Load Independent 16-channel Tx array covers the whole head volume.

Pedram Yazdanbakhsh¹, and Klaus Solbach^1
1HFT, Duisburg-Essen University, Duisburg, NRW, Germany

In this abstract, a new method for the measured scattering transmission coefficients of the 8×8 and 16×16 Butler Matrices, used to drive the 8 and 16 coil array in a 7-Tesla MRI system, has been introduced. This method allows for interpretation of amplitude and phase errors in terms of CP-modes, which would be excited in the MRI coil system. Using this method any given distribution of signals at the output ports can be represented as the superposition of an infinite number of CP-modes in the same way as a periodic signal can be represented as a combination of harmonic signals.

Pascal P Stang¹, Marta Gaia Zanchi¹, Adam Kerr¹, John Mark Pauly¹, and Greig C Scott^1
1Electrical Engineering, Stanford University, Stanford, California, United States

Transmit coil arrays are subject to mutual coupling and loading variations, while RF power amplifiers exhibit non-linearity, ill-defined output impedances, and memory effects. We examine and successfully demonstrate Frequency-Offset Cartesian Feedback (FOCF) as a method to address these challenges, showing improved RF amplifier linearity, output impedance manipulation, and active coil decoupling in MR images. Corrections are performed using only an in-line vector coupler at the RF amplifier, with no on-coil sensor needed. Tuning for active decoupling and stability sensitivities are discussed.

Ke Feng¹, Mary P McDougall², 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

Single Echo Acquisition uses only one echo for an image and is thus capable of high speed imaging. Because the voxel size is the same as the coil size, a phase compensation gradient is necessary. However, this compensation gradient must be the same as coil direction. Because of this requirement, current configuration is limited to planar array. When a single coil is used for both transmitting and receiving (TR mode), phase imparted during transmitting is “refocused” during receiving, thus eliminating the need for phase compensation gradient. Preliminary results shows single shot curved slice excitation and receiving are shown.