Christoph Michael Schildknecht¹, Markus Weiger¹, Romain Froidevaux¹, and Klaas Paul Pruessmann^1
1Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zürich, Switzerland

For imaging of short-T2 samples often very short excitation pulses are desired, which requires high peak RF power. In this work, a transmit-receive switch is presented that can handle 18kW peak RF power and switches its state in less than 1µs. The novel design uses a timed cascade of anti-parallel PIN diode pairs to achieve this performance.

Ming lu^1,2, Bei Zhang³, John C. Gore^1,2, and Xinqiang Yan^1,2
1Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, TN, United States, ²Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, TN, United States, ³Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, United States

Preamplifier decoupling is a standard way of suppressing inter-element inductive coupling in MRI receive arrays. A well-known method to evaluate and optimize RF coils before practical fabrication is to use circuit-level and electromagnetic (EM) field-level simulations. However, to be best of our knowledge, currently there is not a solid method to evaluate the preamp decoupling effect with circuitry and EM field analysis. In this work, we used RF circuit and EM co-simulation4 to evaluate the preamplifier decoupling effect and tested it on the conventional coil (LIC) and the recently introduced high impedance coil (HIC).

Sigrun Roat¹, Andre Kuehne², Lena Nohava^1,3, and Elmar Laistler^1
1High Field MR Center, Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria, ²MRI.TOOLS GmbH, Berlin, Germany, ³CEA, CNRS, Inserm, BioMaps (Laboratoire d'Imagerie Biomédicale Multimodale Paris Saclay), Université Paris-Saclay, Orsay, France

Flexible RF coils gained large interest in recent years. One possibility to achieve flexibility is using transmission line resonator-type coils made from coaxial cable. These self-resonant structures can be constructed without lumped elements along the loop enhancing flexibility. No consensus has been reached yet whether these coils should be driven at their respective self-resonance. To investigate this question, we have performed a simulation study investigating 11 different setups at 3 frequencies each

Tiago Martins¹, Tales Santini¹, Jacob Berardinelli¹, Anthony DeFranco¹, and Tamer S Ibrahim^1
1University of Pittsburgh, Pittsburgh, PA, United States

In this work, we explore a conformal Tic Tac Toe (TTT) head coil design for ultra-high field MRI. By conforming the antennas to the human head, we can extract better homogeneity allowing for better radiofrequency shimming. The comparison between the traditional TTT design and the conformal TTT design shows expanded coverage, reduction of the coefficient of variation by 0.4% and same intensity for the 16-channel version. The conformal designs are extremely flexible as its advantages can be explored for different configurations and sizes.

Filiz Yetisir¹, Esra Abaci Turk^1,2, P. Ellen Grant^1,2,3, Elfar Adalsteinsson^4,5, and Lawrence L. Wald^3,5,6
1Fetal-Neonatal Neuroimaging and Developmental Science Center, Boston Children's Hospital, Boston, MA, United States, ²Department of Pediatrics, Harvard Medical School, Boston, MA, United States, ³Department of Radiology, Harvard Medical School, Boston, MA, United States, ⁴Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, ⁵Harvard-MIT Division of Health Science and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States, ⁶Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States

3T MRI provides increased SNR but poses technical challenges for fetal imaging such as increased field inhomogeneity and SAR. RF shimming can address some of these challenges but also adds safety considerations. Using a diverse set of 5 numerical pregnant body models generated from patient MRI datasets, we found that 2-channel RF shimming can improve transmit field amplitude and uniformity by up to 19% and 36% respectively without increasing maternal or fetal SAR. The biggest difference in SAR and transmit field patterns was observed between the supine models and the left lateral model.

Giske Opheim¹, Vincent O. Boer², Esben Thade Petersen^2,3, Martin Prener¹, Olaf B. Paulson^1,4, and Jan Ole Pedersen^5
1Neurobiology Research Unit, Dept. of Neurology, Copenhagen University Hospital Rigshospitalet, Copenhagen, Denmark, ²Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Hvidovre, Denmark, ³Section for Magnetic Resonance, DTU Health Tech, Technical University of Denmark, Kgs. Lyngby, Denmark, ⁴Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark, ⁵Philips Healhtcare, Copenhagen, Denmark

7T MRI has several cases of demonstrated clinical yield, but is challenged by spatial B₁ inhomogeneity. Bias fields in structural images may be acceptable for visual and computational analysis, and can to some extent be accounted for by bias-field corrections. However, this is significantly complicated  since the characteristics of the bias field often varies between patients. By analyzing stability of bias fields in 3D MPRAGE images, we compared two dielectric pad setups and the impact of 3D-based B₁⁺ scaling, i.e., optimization of the RF gain. We found increased stability by using large pads and 3D-based B₁⁺ scaling in combination.

Sriranga Kashyap¹, Roy A. M. Haast², Thomas F. Kirk³, An T. Vu^4,5, Denizhan Kurban¹, Ron Hellenbrand¹, Christopher J. Wiggins⁶, Alard Roebroeck¹, Ali R. Khan², David A. Feinberg^1,7,8, Benedikt A. Poser¹, and Dimo Ivanov^1
1Department of Cognitive Neuroscience, Maastricht University, Maastricht, Netherlands, ²Centre for Functional and Metabolic Mapping, Western University, London, ON, Canada, ³University of Oxford, Oxford, United Kingdom, ⁴University of California, San Francisco, CA, United States, ⁵San Francisco Veteran Affairs Health Care System, San Francisco, CA, United States, ⁶Scannexus B.V., Maastricht, Netherlands, ⁷Advanced MRI Technologies, Sebastopol, CA, United States, ⁸Helen Wills Neuroscience Institute, University of California, Berkeley, CA, United States

In this 7T study, we examined potential causes of apparent absence of perfusion in the right inferior temporal lobe, by (i) assessing the B₁⁺ distributions on two 1Tx/32Rx head-coils to rule out a hardware source, and (ii) optimising the spatial positioning of dielectric pads to achieve a more symmetric B₁⁺ field. We observed that using an optimised dielectric pad positioning allows for a near whole-brain symmetric B₁⁺, thereby, ensuring the adiabatic condition for the inversion pulse is fulfilled, resulting in the robust measurement of perfusion in the right temporal lobes.

Stephen E. Ogier¹, Shaihan Malik¹, and Joseph Hajnal^1,2
1Biomedical Engineering Department, School of Biomedical Engineering and Imaging Sciences, King's College London, London, United Kingdom, ²Centre for the Developing Brain, School of Biomedical Engineering and Imaging Sciences, King's College London, London, United Kingdom

Parallel transmit using equal numbers of RF power amplifiers and coil elements has emerged as a common way to improve the homogeneity of the transmit field through RF shimming. When performing RF shimming, in many cases the magnitude of the transmit field is limited by the maximum power limit of a single RF amplifier. We propose networks that can be used to share power between transmit channels, increasing the maximum transmit field. These networks are shown to provide substantial flexibility in RF power allocation for certain conditions on the relative phases of the channels.

Ehsan Kazemivalipour^1,2, Giorgio Bonmassar³, Laleh Golestanirad^4,5, and Ergin Atalar^1,2
1Electrical and Electronics Engineering Department, Bilkent University, Ankara, Turkey, ²National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey, ³AA. Martinos Center, Massachusetts General Hospital (MGH), Harvard Medical School, Boston, MA, United States, ⁴Department of Radiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, United States, ⁵Department of Biomedical Engineering, McCormick School of Engineering, Northwestern University, Evanston, IL, United States

The vast majority of RF coil design techniques are based on reducing the coupling level to increase the coils' power efficiency. The assessment of scattering (S) parameters plays a crucial role in these techniques. However, just by checking S-parameters, we cannot be sure that the coil's accepted power generates a field with useful characteristics. Here, we propose to perform the co-simulation on transmit array coils and replace unknown capacitors with power sources. Extraction of the field profiles of the coil's lumped ports/elements could provide the opportunity to involve the field-dependent parameters in the minimization procedure of finding the coil's capacitors.

Sofia Guterres¹, Ana Rodrigues Fouto¹, Nuno André Silva², Pedro Vilela³, and Patrícia Figueiredo^1
1Institute for Systems and Robotics - Lisboa and Department of Bioengineering, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal, ²Hospital da Luz Learning Health, Lisboa, Portugal, ³Hospital da Luz, Lisboa, Portugal

The labelling efficiency of pCASL perfusion imaging sequences can be hampered by B0 field inhomogeneities. We assessed the impact of shimming the labelling region using a commercial RF coil with dedicated shiming channels in the neck, on data acquired from healthy volunteers. Brain perfusion was quantified, as well as the labelling efficiency (based on B0 fieldmaps and numerical simulations). Simulation experiments show that labelling efficiency significantly depends on B0 field inhomogeneity. In vivo experiments, however, showed subtle inhomogeneity reduction within the arteries in the labelling region, yielding no significant improvements in pCASL labelling efficiency or the respective perfusion signal.

Daniel Wenz^1,2 and Rolf Gruetter^1,3
1CIBM Center for Biomedical Imaging, Lausanne, Switzerland, ²Animal Imaging and Technology, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland, ³Laboratory of Functional and Metabolic Imaging (LIFMET), Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland

This work investigates quasi-transverse electric modes which can be excited within rectangular dielectric blocks which are often used to shorten dipole antennas for MRI at 7T. In simulations we studied different parameters which can affect the performance of a dielectrically-shortened dipole antenna. We showed that an optimal geometry for the case when the block and the sample are physically separated differs significantly from the case when perfect direct contact is achieved. Two prototypes were built and used in an in vivo MRI experiments involving one subject showing how different dielectric modes can affect the image quality.

Myung Kyun Woo¹, Lance DelaBarre¹, Matt Waks¹, Russell Lagore¹, Jeromy Thotland¹, Uk-Su Choi², Andrea Grant¹, Steve Jungst¹, Nader Tavaf¹, Yigitcan Eryaman¹, Kamil Ugurbil¹, and Gregor Adriany^1
1Center for Magnetic Resonance Research, Minneapolis, MN, United States, ²Center for Information and Neural Networks, Osaka, Japan

We designed an elliptically arranged novel 32-channel Sleeve antenna receiver array for human whole brain imaging at 10.5 tesla. To demonstrate the signal-to-noise ratio (SNR) performance of this array, we evaluated and compared it with a standard 32-channel loop receiver array.

Thomas Dardano¹, Rolf Gruetter^1,2, and Daniel Wenz^2,3
1Laboratory of Functional and Metabolic Imaging (LIFMET), Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland, ²CIBM Center for Biomedical Imaging, Lausanne, Switzerland, ³Animal Imaging and Technology, Ecole Polytechnique Federale de Lausanne (EPFL), Lausanne, Switzerland

Dipole antennas can be used in multi-channel loop/dipole arrays to boost the signal-to-noise ratio in MRI at 7T (f=300MHz). For this purpose, dipole antennas need to be physically shorter. In this work we conducted electromagnetic field simulations and phantom experiments at 7T to compare the performance of an inductively-shortened dipole antenna with a dielectrically-shortened dipole antenna in a loop/dipole combination. We evaluated the performance of both designs in different loading conditions and we found that the dielectrically-shortened dipole antenna performed in a very robust manner providing apparent receive field gains when compared with its inductively-shortened counterpart.

Ming Lu^1,2, John C. Gore^1,2, and Xinqiang Yan^1,2
1Vanderbilt University Institute of Imaging Science, Nashville, TN, United States, ²Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, TN, United States

At high fields, B1 inhomogeneity is one of the major challenges that limit imaging performance. Parallel transmission with an array of coils is a recognized solution to B1 inhomogeneity. The self-decoupled coil is attractive in Tx array design since it exhibits intrinsic inter-element isolation that may ease the coil fabrication, and allows the dipole mode as well as loop mode. In this work, we evaluated a series of self-decoupled coils in terms of these parameters and aimed to provide guidance for the practical building of a high-performance self-decoupled Tx array.

Mark Gosselink¹, Tijl van der Velden¹, Hans Hoogduin¹, Martijn Froeling¹, and Dennis W. J. Klomp^1
1University Medical Center Utrecht, Utrecht, Netherlands

A 64 channel head coil receiver is presented that operates with low noise coupling inside a continuously tuned 8-channel transceiver coil. When used as a 72-channel receiver the setup outperforms the default 32 channel receiver array in acceleration, demonstrated by high-resolution MPRAGE images of the human brain.

Charlotte Sappo^1,2, Gary R Drake^1,3, Xinqiang Yan^1,3, and William A Grissom^1,2,3,4
1Vanderbilt University Institute of Imaging Science, Nashville, TN, United States, ²Biomedical Engineering, Vanderbilt University, Nashville, TN, United States, ³Radiology, Vanderbilt University, Nashville, TN, United States, ⁴Electrical and Computer Engineering, Vanderbilt University, Nashville, TN, United States

A dense array is desirable for ultra-high field in parallel transmission to better mitigate B₁⁺ field inhomogeneities and control specific absorption rate. Most ultra-high field scanners are limited by the existing number of transmit channels. This limitation can be overcome using array-compressed parallel transmission (acpTx) which enables a small number of channels to drive a large number of coils. This work describes the construction of a 30-loop head transmit array using a mixed overlapped and self-decoupled design with a widely-used commercial 32-channel receive insert for human imaging applications. 

Jeremiah Hess¹, Marianne Black², Feliks Kogan², and Brian Hargreaves^1,2,3
1Department of Bioengineering, Stanford University, Stanford, CA, United States, ²Department of Radiology, Stanford University, Stanford, CA, United States, ³Department of Electrical Engineering, Stanford University, Stanford, CA, United States

Flexible extremity coil-arrays for knee imaging in MRI can improve coil positioning and comfort to patients in the scanner and wrap tightly around the knee, placing coil elements close to anatomy of interest. Despite this, flex coils have not been widely adapted due to perceived superiority of the SNR of transmit-receive coils. In this study, we analyzed the SNR of flex coil-arrays versus a rigid coil-array in phantom and in vivo to determine which coil-arrays had improved SNR. Preliminary results suggest that flexible coil-arrays show comparable or increased SNR and generally more uniform SNR over rigid coil-arrays.

Navid PourramzanGandji¹, Christopher T. Sica², Gary W. Yang³, Hannes Wiesner⁴, Soo Han Soon⁴, Xiao-Hong Zhu⁴, Michael Lanagan⁵, Wei Chen⁴, and Qing X. Yang^1
1Neurosurgery, Pennsylvania State University, Hershey, PA, United States, ²Radiology, Pennsylvania State University, Hershey, PA, United States, ³Pennsylvania State University, Hershey, PA, United States, ⁴Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota, Minneapolis, MN, United States, ⁵Materials Science and Engineering, Pennsylvania State University, State College, PA, United States

Prior work has examined the effects of uHDC material upon phased array coils. Several beneficial effects have been observed: reduction of per-channel noise and inter-channel noise correlations, and enhancement of SNR. Due to the high dielectric constant the electric field will be confined to the dielectric material, which leads to a reduction of the primary noise source, the conservative electric field. As a result the non-conservative field becomes more dominant within the sample, with a resulting increase of B1 field intensity and more substantial SNR enhancement.

Aimé Labbé¹, Isabelle Saniour¹, Rose-Marie Dubuisson¹, Jean-Christophe Ginefri¹, Luc Darrasse¹, and Marie Poirier-Quinot^1
1Université Paris-Saclay, CEA, CNRS, Inserm, BioMaps, Orsay, France

Decoupling is a major technological challenge of High-Temperature Superconducting (HTS) surface coils. A promising strategy is to exploit nonlinearities in the HTS coil electric response to achieve decoupling. In this work, we present a new characterization method allowing the evaluation of these nonlinearities using MRI. We first present the theoretical model behind this approach; we then proceed to use it on 3D images of a water phantom in the vicinity of a copper coil and then a HTS coil to validate the model through the extraction of the quality factor Q, compared to the one measured by alternative methods.

Matthäus Poniatowski¹, Ilan Elias², Mirsad Mahmutovic¹, Gurinder Multani¹, Sam-Luca J.D. Hansen¹, Markus W. May¹, Alexander M. König³, Jens H. Figiel³, Andreas H. Mahnken³, and Boris Keil^1
1Institute of Medical Physics and Radiation Protection, TH Mittelhessen University of Applied Sciences, Gießen, Germany, ²Motionrad GmbH, Berlin, Germany, ³Department of Diagnostic and Interventional Radiology, Philipps-University Marburg, Marburg, Germany

Limitations of conventional MRI include its lack of ability to observe controlled joint motion and biomechanics in real-time. Although clinically evident, many injuries, instabilities, and dysfunctions of the musculoskeletal system are frequently not depicted on conventional static MRI. To enable dynamic MRI for joints, an in-bore motion-assisted device and a wearable coil array was designed, constructed, and validated. The combination of robotic assisted joint motion, a tight-fitting coil array that does not restrict the joint's range of motion, and accelerated imaging enabled dynamic MRI of the ankle.