Local multi-channel RF surface coil versus body RF coil transmission for cardiac magnetic resonance at 3 Tesla: which configuration is winning the game?

INTRODUCTION: The purpose of this study was to demonstrate the feasibility and efficiency of cardiac MR at 3 Tesla using local four-channel RF coil transmission and benchmark it against large volume body RF coil excitation. METHODS: Electromagnetic field simulations are conducted to detail RF power deposition, transmission field uniformity and efficiency for local and body RF coil transmission. For both excitation regimes transmission field maps are acquired in a human torso phantom. For each transmission regime flip angle distributions and blood-myocardium contrast are examined in a volunteer study of 12 subjects. The feasibility of the local transceiver RF coil array for cardiac chamber quantification at 3 Tesla is demonstrated. RESULTS: Our simulations and experiments demonstrate that cardiac MR at 3 Tesla using four-channel surface RF coil transmission is competitive versus current clinical CMR practice of large volume body RF coil transmission. The efficiency advantage of the 4TX/4RX setup facilitates shorter repetition times governed by local SAR limits versus body RF coil transmission at whole-body SAR limit. No statistically significant difference was found for cardiac chamber quantification derived with body RF coil versus four-channel surface RF coil transmission. Our simulation also show that the body RF coil exceeds local SAR limits by a factor of ~2 when driven at maximum applicable input power to reach the whole-body SAR limit. CONCLUSION: Pursuing local surface RF coil arrays for transmission in cardiac MR is a conceptually appealing alternative to body RF coil transmission, especially for patients with implants

Development of multi-channel radio frequency technology for sodium and potassium magnetic resonance imaging at 7.0 Tesla: design and clinical application

Sodium (Na+) and potassium (K+) ions play key roles in the physiology and metabolism of living cells. Primary active transport, which is carried out by sodium/potassium pumps (Na+/K+-ATPase), maintains the ion concentration gradient between intra- and extracellular space. Changes in Na+ and K+ concentration (and distribution) might reflect ongoing pathological processes within a tissue what might be relevant for various types of cardiovascular and ocular disorders. Ultrahigh magnetic resonance imaging (UHF-MRI) provides new opportunities to non-invasively investigate changes in Na+ and K+ concentrations with spatial resolution and within total scan times that are reaching ranges acceptable for clinical applications. Despite an intrinsic to UHF-MRI gain in signal-to-noise ratio (SNR), nuclear magnetic resonance (NMR) signals of sodium (23Na) and potassium (39K) being detected remain very weak. The NMR sensitivity of 23Na is about 9%, while 39K is 0.05% the one of proton (1H). Therefore, radio frequency (RF) coils, which are used to capture these signals, should be optimized for a given anatomical structure in order to improve the SNR. The goal of this work is to develop two separate RF coils which would enable high-resolution in vivo 23Na MRI of the human eye and in vivo 39K MRI of the human heart at 7.0 Tesla. To achieve these goals, the six-channel transmit receive 23Na coil array and a four/two-channel 1H/39K coil array have been designed, built and tested. The performance of the developed RF coils has been evaluated using RF circuit, electromagnetic field (EMF) and specific absorption rate (SAR) simulations. Phantom as well as in vivo experiments involving several healthy volunteers have been conducted. The experiments have revealed that the developed six-channel transmit/receive coil array supports in vivo 23Na MRI of the human eye with nominal spatial resolution of (1.0 x 1.0 x 1.0) mm3 and within scan time of 10 minutes. This work also demonstrates that the proposed four/two-channel 1H/39K coil array enabled obtaining the world’s first in vivo 39K image of the human heart with nominal spatial resolution of (14.5 x 14.5 x 14.5) mm3 and within total scan time of 30 minutes. The results demonstrate that sodium content in the lens is distinguishable from sodium content in the aqueous and vitreous humor. There is strong evidence that sodium concentration in the compartments of the eye should change in diseases like cataract, glaucoma and ocular melanoma. The broad roles of this element in processes related to eye physiology suggest a range of questions for ophthalmological investigations. This work also shows that in vivo potassium MRI of the human heart is feasible. Previous reports, suggesting that potassium concentration is expected to alter in arrhythmia, ischemia or irreversible injury to miocytes, provides encouragement for future in vivo studies involving patients who suffer from various cardiovascular disorders.Natrium- (Na+) und Kaliumionen (K+) spielen kritische Rollen in der Physiologie und dem Metabolismus lebender Zellen. Der primär-aktive Transport, der den Ionenkonzentrationsgradienten zwischen intra- und extrazellulärer Maxtrix aufrecht hält, wird von Natrium/Kaliumpumpen durchgeführt. Änderungen der Na+ - und K+-Konzentration und -Verteilung könne auf pathologische Prozesse in einem Gewebe zurückzuführen sein. Dies ist sehr relevant für eine Vielzahl von Krankheiten, einschließlich Herz-Kreislauf- und Augenerkrankungen. Ultrahohe Magnetresonanztomographie (UHF-MRT) bietet neue Möglichkeiten zur nicht-invasiven Untersuchung von Änderungen in Na+- und K+-Konzentrationen mit hoher räumlicher Auflösung, die innerhalb für klinische Anwendungen akzeptabler Gesamtabtastzeiten durchgeführt werden können. Trotz eines UHF-MRT-spezifischen Anstiegs des Signal-Rausch-Verhältnisses (SNR) bleiben die nachgewiesenen kernmagnetischen Resonanzsignale (NMR) von Natrium (23Na) und Kalium (39K) sehr schwach. Die NMR-Empfindlichkeit von 23Na beträgt etwa 9%, während 39K - 0.05% des Protons (1H) beträgt. Daher sollten Hochfrequenzspulen (HF), die zur Erfassung dieser Signale verwendet werden, für eine bestimmte anatomische Struktur optimiert werden, um das SNR zu verbessern. Ziel dieser Arbeit ist es, zwei separate HF-Spulen zu entwickeln, die eine hochauflösende in vivo 23Na-MRT des menschlichen Auges und eine in vivo 39K-MRT des menschlichen Herzens bei 7.0 Tesla ermöglichen. Um diese Ziele zu erreichen, wurden das/ein 23Na-Spulenarray mit sechs Sende- und Empfangskanälen und ein 1H/39K-Spulenarray mit vier/zwei Kanälen entworfen, gebaut und getestet. Es wurden Experimente an Messphantomen sowie In-vivo-Testmessungen von mehreren gesunden Freiwilligen durchgeführt. Die Messungen haben gezeigt, dass das in dieser Arbeit entwickelte 6-Kanal-Sende- / Empfangsspulenarray in vivo 23Na MRT des menschlichen Auges mit einer nominalen räumlichen Auflösung von (1.0x1.0x1.0) mm3 innerhalb der Scanzeit von 10 Minuten ermöglicht. Diese Arbeit zeigt auch, dass es dank des 1H/39K -Spulenarray mit vier/zwei Kanälen gelang, das weltweit erste 39K -Bild des menschlichen Herzens in vivo mit einer nominalen räumlichen Auflösung von (14.5x14.5x14.5) mm3 und einer Gesamtabtastzeit von zu 30 Minuten aufzunehmen. Die Ergebnisse zeigen, dass der Natriumgehalt in der Linse vom Natriumgehalt im wässrigen und im Glaskörper unterscheidbar ist. Es gibt starke Hinweise darauf, dass sich die Natriumkonzentration in den Kompartimenten des Auges bei Erkrankungen wie Katarakt, Glaukom und okularem Melanom ändern sollte. Diese Arbeit zeigt auch, dass eine in vivo Kalium-MRT des menschlichen Herzens möglich ist. Frühere Berichte, aus denen hervorgeht, dass sich die Kaliumkonzentration voraussichtlich bei Arrhythmie, Ischämie oder irreversiblen Verletzungen der Miozyten ändert, ermutigen zukünftige In-vivo-Studien mit Patienten, die an verschiedenen Herz-Kreislauf-Erkrankungen leiden

In vivo sodium MR imaging of the abdomen at 3T

URPOSE: Transmembrane sodium ((23)Na) gradient is critical for cell survival and viability and a target for the development of anti-cancer drugs and treatment as it serves as a signal transducer. The ability to integrate abdominal (23)Na MRI in clinical settings would be useful to non-invasively detect and diagnose a number of diseases in various organ systems. Our goal in this work was to enhance the quality of (23)Na MRI of the abdomen using a 3-Tesla MR scanner and a novel 8-channel phased-array dual-tuned (23)Na and (1)H transmit (Tx)/receive (Rx) coil specially designed to image a large abdomen region with relatively high SNR. METHODS: A modified GRE imaging sequence was optimized for (23)Na MRI to obtain the best possible combination of SNR, spatial resolution, and scan time in phantoms as well as volunteers. Tissue sodium concentration (TSC) of the whole abdomen was calculated from the inhomogeneity-corrected (23)Na MRI for absolute quantification. In addition, in vivo reproducibility and reliability of TSC measurements from (23)Na MRI was evaluated in normal volunteers. RESULTS: (23)Na axial images of the entire abdomen with a high spatial resolution (0.3 cm) and SNR (~20) in 15 min using the novel 8-channel dual-tuned (23)Na and (1)H transmit/receive coil were obtained. Quantitative analysis of the sodium images estimated a mean TSC of the liver to be 20.13 mM in healthy volunteers. CONCLUSION: Our results have shown that it is feasible to obtain high-resolution (23)Na images using a multi-channel surface coil with good SNR in clinically acceptable scan times in clinical practice for various body applications

On the subjective acceptance during cardiovascular magnetic resonance imaging at 7.0 Tesla

PURPOSE: This study examines the subjective acceptance during UHF-CMR in a cohort of healthy volunteers who underwent a cardiac MR examination at 7.0T. METHODS: Within a period of two-and-a-half years (January 2012 to June 2014) a total of 165 healthy volunteers (41 female, 124 male) without any known history of cardiac disease underwent UHF-CMR. For the assessment of the subjective acceptance a questionnaire was used to examine the participants experience prior, during and after the UHF-CMR examination. For this purpose, subjects were asked to respond to the questionnaire in an exit interview held immediately after the completion of the UHF-CMR examination under supervision of a study nurse to ensure accurate understanding of the questions. All questions were answered with "yes" or "no" including space for additional comments. RESULTS: Transient muscular contraction was documented in 12.7% of the questionnaires. Muscular contraction was reported to occur only during periods of scanning with the magnetic field gradients being rapidly switched. Dizziness during the study was reported by 12.7% of the subjects. Taste of metal was reported by 10.1% of the study population. Light flashes were reported by 3.6% of the entire cohort. 13% of the subjects reported side effects/observations which were not explicitly listed in the questionnaire but covered by the question about other side effects. No severe side effects as vomiting or syncope after scanning occurred. No increase in heart rate was observed during the UHF-CMR exam versus the baseline clinical examination. CONCLUSIONS: This study adds to the literature by detailing the subjective acceptance of cardiovascular magnetic resonance imaging examinations at a magnetic field strength of 7.0T. Cardiac MR examinations at 7.0T are well tolerated by healthy subjects. Broader observational and multi-center studies including patient cohorts with cardiac diseases are required to gain further insights into the subjective acceptance of UHF-CMR examinations

Patient-specific RF safety assessment in MRI: Progress in creating surface-based human head and shoulder models

The interaction of electromagnetic (EM) fields with the human body during magnetic resonance imaging (MRI) is complex and subject specific. MRI radiofrequency (RF) coil performance and safety assessment typically includes numerical EM simulations with a set of human body models. The dimensions of mesh elements used for discretization of the EM simulation domain must be adequate for correct representation of the MRI coil elements, different types of human tissue, and wires and electrodes of additional devices. Examples of such devices include those used during electroencephalography, transcranial magnetic stimulation, and transcranial direct current stimulation, which record complementary information or manipulate brain states during MRI measurement. The electrical contact within and between tissues, as well as between an electrode and the skin, must also be preserved. These requirements can be fulfilled with anatomically correct surface-based human models and EM solvers based on unstructured meshes. Here, we report (i) our workflow used to generate the surface meshes of a head and torso model from the segmented AustinMan dataset, (ii) head and torso model mesh optimization for three-dimensional EM simulation in ANSYS HFSS, and (iii) several case studies of MRI RF coil performance and safety assessment

Sodium MRI of the human heart at 7.0 T: preliminary results

The objective of this work was to examine the feasibility of three-dimensional (3D) and whole heart coverage 23Na cardiac MRI at 7.0 T including single-cardiac-phase and cinematic (cine) regimes. A four-channel transceiver RF coil array tailored for 23Na MRI of the heart at 7.0 T (f = 78.5 MHz) is proposed. An integrated bow-tie antenna building block is used for 1H MR to support shimming, localization and planning in a clinical workflow. Signal absorption rate simulations and assessment of RF power deposition were performed to meet the RF safety requirements. 23Na cardiac MR was conducted in an in vivo feasibility study. 3D gradient echo (GRE) imaging in conjunction with Cartesian phase encoding (total acquisition time TAQ = 6 min 16 s) and whole heart coverage imaging employing a density-adapted 3D radial acquisition technique (TAQ = 18 min 20 s) were used. For 3D GRE-based 23Na MRI, acquisition of standard views of the heart using a nominal in-plane resolution of (5.0 x 5.0) mm2 and a slice thickness of 15 mm were feasible. For whole heart coverage 3D density-adapted radial 23Na acquisitions a nominal isotropic spatial resolution of 6 mm was accomplished. This improvement versus 3D conventional GRE acquisitions reduced partial volume effects along the slice direction and enabled retrospective image reconstruction of standard or arbitrary views of the heart. Sodium cine imaging capabilities were achieved with the proposed RF coil configuration in conjunction with 3D radial acquisitions and cardiac gating. Cardiac-gated reconstruction provided an enhancement in blood-myocardium contrast of 20% versus the same data reconstructed without cardiac gating. The proposed transceiver array enables 23Na MR of the human heart at 7.0 T within clinical acceptable scan times. This capability is in positive alignment with the needs of explorations that are designed to examine the potential of 23Na MRI for the assessment of cardiovascular and metabolic diseases

W(h)ither human cardiac and body magnetic resonance at ultrahigh fields? Technical advances, practical considerations, applications, and clinical opportunities.

The objective of this study was to document and review advances and groundbreaking progress in cardiac and body MR at ultrahigh fields (UHF, B0 ≥ 7.0 T) with the goal to attract talent, clinical adopters, collaborations and resources to the biomedical and diagnostic imaging communities. This review surveys traits, advantages and challenges of cardiac and body MR at 7.0 T. The considerations run the gamut from technical advances to clinical opportunities. Key concepts, emerging technologies, practical considerations, frontier applications and future directions of UHF body and cardiac MR are provided. Examples of UHF cardiac and body imaging strategies are demonstrated. Their added value over the kindred counterparts at lower fields is explored along with an outline of research promises. The achievements of cardiac and body UHF-MR are powerful motivators and enablers, since extra speed, signal and imaging capabilities may be invested to overcome the fundamental constraints that continue to hamper traditional cardiac and body MR applications. If practical obstacles, concomitant physics effects and technical impediments can be overcome in equal measure, sophisticated cardiac and body UHF-MR will help to open the door to new MRI and MRS approaches for basic research and clinical science, with the lessons learned at 7.0 T being transferred into broad clinical use including diagnostics and therapy guiding at lower field

A Specialized Multi-Transmit Head Coil for High Resolution fMRI of the Human Visual Cortex at 7T

PURPOSE: To design, construct and validate radiofrequency (RF) transmit and receive phased array coils for high-resolution visual cortex imaging at 7 Tesla. METHODS: A 4 channel transmit and 16 channel receive array was constructed on a conformal polycarbonate former. Transmit field efficiency and homogeneity were simulated and validated, along with the Specific Absorption Rate, using [Image: see text] mapping techniques and electromagnetic simulations. Receiver signal-to-noise ratio (SNR), temporal SNR (tSNR) across EPI time series, g-factors for accelerated imaging and noise correlations were evaluated and compared with a commercial 32 channel whole head coil. The performance of the coil was further evaluated with human subjects through functional MRI (fMRI) studies at standard and submillimeter resolutions of upto 0.8mm isotropic. RESULTS: The transmit and receive sections were characterized using bench tests and showed good interelement decoupling, preamplifier decoupling and sample loading. SNR for the 16 channel coil was ∼ 1.5 times that of the commercial coil in the human occipital lobe, and showed better g-factor values for accelerated imaging. fMRI tests conducted showed better response to Blood Oxygen Level Dependent (BOLD) activation, at resolutions of 1.2mm and 0.8mm isotropic. CONCLUSION: The 4 channel phased array transmit coil provides homogeneous excitation across the visual cortex, which, in combination with the dual row 16 channel receive array, makes for a valuable research tool for high resolution anatomical and functional imaging of the visual cortex at 7T

Radiofrequency antenna concepts for human cardiac MR at 14.0 T

OBJECTIVE: To examine the feasibility of human cardiac MR (CMR) at 14.0 T using high-density radiofrequency (RF) dipole transceiver arrays in conjunction with static and dynamic parallel transmission (pTx). MATERIALS AND METHODS: RF arrays comprised of self-grounded bow-tie (SGBT) antennas, bow-tie (BT) antennas, or fractionated dipole (FD) antennas were used in this simulation study. Static and dynamic pTx were applied to enhance transmission field (B(1)(+)) uniformity and efficiency in the heart of the human voxel model. B(1)(+) distribution and maximum specific absorption rate averaged over 10 g tissue (SAR(10g)) were examined at 7.0 T and 14.0 T. RESULTS: At 14.0 T static pTx revealed a minimum B(1)(+)(ROI) efficiency of 0.91 μT/√kW (SGBT), 0.73 μT/√kW (BT), and 0.56 μT/√kW (FD) and maximum SAR(10g) of 4.24 W/kg, 1.45 W/kg, and 2.04 W/kg. Dynamic pTx with 8 kT points indicate a balance between B(1)(+)(ROI) homogeneity (coefficient of variation  1.11 µT/√kW) at 14.0 T with a maximum SAR(10g) < 5.25 W/kg. DISCUSSION: MRI of the human heart at 14.0 T is feasible from an electrodynamic and theoretical standpoint, provided that multi-channel high-density antennas are arranged accordingly. These findings provide a technical foundation for further explorations into CMR at 14.0 T