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Objective: To simulate the magnetic and electric fields produced by RF coil geometries commonly used at low field. Based on these simulations, the specific absorption rate (SAR) efficiency can be derived to ensure safe operation even when using short RF pulses and high duty cycles. Methods: Electromagnetic simulations were performed at four different field strengths between 0.05 and 0.1 T, corresponding to the lower and upper limits of current point-of-care (POC) neuroimaging systems. Transmit magnetic and electric fields, as well as transmit efficiency and SAR efficiency were simulated. The effects of a close-fitting shield on the EM fields were also assessed. SAR calculations were performed as a function of RF pulse length in turbo-spin echo (TSE) sequences. Results: Simulations of RF coil characteristics and B1+ transmit efficiencies agreed well with corresponding experimentally determined parameters. Overall, the SAR efficiency was, as expected, higher at the lower frequencies studied, and many orders of magnitude greater than at conventional clinical field strengths. The tight-fitting transmit coil results in the highest SAR in the nose and skull, which are not thermally sensitive tissues. The calculated SAR efficiencies showed that only when 180° refocusing pulses of duration ~ 10 ms are used for TSE sequences does SAR need to be carefully considered. Conclusion: This work presents a comprehensive overview of the transmit and SAR efficiencies for RF coils used for POC MRI neuroimaging. While SAR is not a problem for conventional sequences, the values derived here should be useful for RF intensive sequences such as T1ρ, and also demonstrate that if very short RF pulses are required then SAR calculations should be performed. [ABSTRACT FROM AUTHOR]

Purpose: Development of a novel quadrature inductively driven transceive wireless coil for breast MRI at 1.5 T. Methods: A quadrature wireless coil (HHMM‐coil) design has been developed as a combination of two linearly polarized coils: a pair of 'metasolenoid' coils (MM‐coil) and a pair of Helmholtz‐type coils (HH‐coil). The MM‐coil consisted of an array of split‐loop resonators. The HH‐coil design included two electrically connected flat spirals. All the wireless coils were coupled to a whole‐body birdcage coil. The HHMM‐coil was studied and compared to the linear coils in terms of transmit and SAR efficiencies via numerical simulations. A prototype of HHMM‐coil was built and tested on a 1.5 T scanner in a phantom and healthy volunteer. We also proposed an extended design of the HHMM‐coil and compared its performance to a dedicated breast array. Results: Numerical simulations of the HHMM‐coil with a female voxel model have shown more than a 2.5‐fold increase in transmit efficiency and a 1.7‐fold enhancement of SAR efficiency compared to the linearly polarized coils. Phantom and in vivo imaging showed good agreement with the numerical simulations. Moreover, the HHMM‐coil provided good image quality, visualizing all areas of interest similar to a multichannel breast array with a 32% reduction in signal‐to‐noise ratio. Conclusion: The proposed quadrature HHMM‐coil allows the B1+$$ {\mathrm{B}}_1^{+} $$‐field to be significantly better focused in the region‐of‐interest compared to the linearly polarized coils. Thus, the HHMM‐coil provides high‐quality breast imaging on a 1.5 T scanner using a whole‐body birdcage coil for transmit and receive. [ABSTRACT FROM AUTHOR]

This article reviews recent developments in designing and testing new types of materials which can be: (i) placed around the body for in vivo imaging, (ii) be integrated into a conventional RF coil, or (iii) form the resonator itself. These materials can improve the quality of MRI scans for both in vivo and magnetic resonance microscopy applications. The methodological section covers the basic operation and design of two different types of materials, namely high permittivity materials constructed from ceramics and artificial dielectrics/metasurfaces formed by coupled conductive subunits, either in air or surrounded by dielectric material. Applications of high permittivity materials and metasurfaces placed next to the body to neuroimaging and extremity imaging at 7 T, body and neuroimaging at 3 T, and extremity imaging at 1.5 T are shown. Results using ceramic resonators for both high field in vivo imaging and magnetic resonance microscopy are also shown. The development of new materials to improve MR image quality remains an active area of research, but has not yet found significant use in clinical applications. This is mainly due to practical issues such as specific absorption rate modelling, accurate and reproducible placement, and acceptable size/weight of such materials. The most successful area has been simple "dielectric pads" for neuroimaging at 7 T which were initially developed somewhat as a stop-gap while parallel transmit technology was being developed, but have continued to be used at many sites. Some of these issues can potentially be overcome using much lighter metasurfaces and artificial dielectrics, which are just beginning to be assessed. [ABSTRACT FROM AUTHOR]

Objective: High-permittivity pads have shown promising results in enhancing SNR and transmit efficiency when used for MRI of the brain, but fewer studies have been conducted to examine the performance of high-permittivity pads in other parts of the patient. In this work, we evaluate the impact on SNR and transmit efficiency distributions when high-permittivity pads with different thickness are positioned near the chest of the patient in combination with a transmit/receive array coil. Methods: The performance of the pads is evaluated through numerical simulations, and both the SNR distribution and the transmit efficiency maps are compared with those obtained when the pads are not present and the distance between the coils and the patient is minimal. The average improvement of SNR and transmit efficiency in the heart is also evaluated for different values of the permittivity of the pads. Results: In the scenario examined, high-permittivity pads can increase SNR and transmit efficiency in the heart volume by as much as 16% and 65%, respectively. [ABSTRACT FROM AUTHOR]

Purpose: To present a 3T brain imaging study using a conformal prototype helmet constructed with an ultra‐high dielectric constant (uHDC; εr ~ 1000) materials that can be inserted into standard receive head‐coils. Methods: A helmet conformal to a standard human head constructed with uHDC materials was characterized through electromagnetic simulations and experimental work. The signal‐to‐noise ratio (SNR), transmit efficiency, and power deposition with the uHDC helmet inserted within a 20‐channel head coil were measured in vivo and compared with a 64‐channel head coil and the 20‐channel coil without the helmet. Seven healthy volunteers were analyzed. Results: Simulation and in vivo experimental results showed that transmit efficiency was improved by nearly 3 times within localized regions for a quadrature excitation, with a measured global increase of 58.21 ± 6.54% over 7 volunteers. The use of a parallel transmit spokes pulse compensated for severe degradation of B1+ homogeneity, at the expense of higher global and local specific absorption rate levels. A SNR histogram analysis with statistical testing demonstrated that the uHDC helmet enhanced a 20‐channel head coil to the level of the 64‐channel head coil, with the improvements mainly within the cortical brain regions. Conclusion: A prototype uHDC helmet enhanced the SNR of a standard head coil to the level of a high density 64‐channel coil, although transmit homogeneity was compromised. Further improvements in SNR may be achievable with optimization of this technology, and could be a low‐cost approach for future radiofrequency engineering work in the brain at 3T. [ABSTRACT FROM AUTHOR]

This study comprehensively assesses radiofrequency (RF) volumetric wireless coils utilizing artificial materials for clinical breast MRI. In particular, we evaluated the transmit efficiency, RF safety, and homogeneity of magnetic field amplitude distribution for four structures electromagnetically coupled with a whole-body birdcage coil: extremely high permittivity ceramic coil, solenoid coil, Helmholtz coil, and metamaterial-inspired coil based on periodically coupled split-loop resonators. These coils exhibit favorable attributes, including lightweight construction, compactness, cost-effectiveness, and ease of manufacturing. The results of this study demonstrated that the metamaterial-inspired coil outperforms other wireless coils considered for addressing a specific problem in terms of the set of characteristics. In particular, the metamaterial-inspired coil achieved 85% and 88% homogeneity in magnetic field amplitude distribution at 3 T and 1.5 T MRI, respectively. Also, the 1.5 T metamaterial-inspired coil demonstrated the best performance, increasing the efficiency gain of the birdcage coil by 4.93 times and improving RF safety by 2.96 times. This research explains the limitations and peculiarity of utilizing the volumetric wireless coils in 1.5 and 3 T MRI systems. [Display omitted] • Volumetric wireless coils were compared for a set of electromagnetic characteristics. • Wireless coils enhance the body coil's Tx efficiency and RF safety. • Metasolenoid outperforms other coils in Tx efficiency and RF safety gain. • The Helmholtz coil is the best in terms of Q-factor and ease of manufacture. • Shielding a coil from the body boosts its Tx efficiency and limits the field of view. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The local transmit/ receive efficiency can be increased at 1.5 T using high permittivity materials. • These materials also result in a reduction in the maximum specific absorption rate. • These materials can potentially be used to scan patients with metallic implants. Abstract Objective The purpose of this work is to investigate the use of ceramic materials (based on BaTiO 3 with ZrO 2 and CeO 2 -additives) with very high relative permittivity (ε r ∼ 4500) to increase the local transmit field and signal-to-noise ratio (SNR) for commercial extremity coils on a clinical 1.5 T MRI system. Methods Electromagnetic simulations of transmit efficiency and specific absorption rate (SAR) were performed using four ferroelectric ceramic blocks placed around a cylindrical phantom, as well as placing these ceramics around the wrist of a human body model. Results were compared with experimental scans using the transmit body coil of the 1.5 T MRI system and an eight-element extremity receive array designed for the wrist. SNR measurements were also performed for both phantom and in vivo scans. Results Electromagnetic simulations and phantom/in vivo experiments showed an increased in the local transmit efficiency from the body coil of ∼20–30%, resulting in an ∼50% lower transmit power level and a significant reduction in local and global SAR throughout the body. For in vivo wrist experiments, the SNR of a commercial eight-channel receive array, integrated over the entire volume, was improved by ∼45% with the ceramic. Conclusion The local transmit efficiency as well as the SNR can be increased for 1.5 T extremity MRI with commercial array coils by using materials with very high permittivity. [ABSTRACT FROM AUTHOR]

Purpose: Design and characterization of a new inductively driven wireless coil (WLC) for wrist imaging at 1.5 T with high homogeneity operating due to focusing the B1 field of a birdcage body coil. Methods: The WLC design has been proposed based on a volumetric self‐resonant periodic structure of inductively coupled split‐loop resonators with structural capacitance. The WLC was optimized and studied regarding radiofrequency fields and interaction to the birdcage coil (BC) by electromagnetic simulations. The manufactured WLC was characterized by on‐bench measurements and in vivo and phantom study in comparison to a standard cable‐connected receive‐only coil. Results: The WLC placed into BC gave the measured B 1 + increase of the latter by 8.6 times for the same accepted power. The phantom and in vivo wrist imaging showed that the BC in receiving with the WLC inside reached equal or higher signal‐to‐noise ratio than the conventional clinical setup comprising the transmit‐only BC and a commercial receive‐only flex‐coil and created no artifacts. Simulations and on‐bench measurements proved safety in terms of specific absorption rate and reflected transmit power. Conclusions: The results showed that the proposed WLC could be an alternative to standard cable‐connected receive coils in clinical magnetic resonance imaging. As an example, with no cable connection, the WLC allowed wrist imaging on a 1.5 T clinical machine using a full‐body BC for transmitting and receive with the desired signal‐to‐noise ratio, image quality, and safety. [ABSTRACT FROM AUTHOR]

Abstract: A novel radio frequency (RF) coil for ultra‐high‐field MRI in the form of a slotted waveguide array (SWGA) filled with a low‐loss high‐permittivity dielectric is proposed, evaluated, and demonstrated. A comprehensive computational electromagnetics study, along with basic RF measurements, to characterize the SWGA RF coil at 7T is presented. Slotted waveguides are robust structures capable of handling high powers. They are inherently narrow‐band and have well defined linear polarization. When arranged in an array, they effectively generate high‐quality B 1 field with strong B 1 + and extremely low B 1 − and B z components. With added dielectric lenses, the observed transmit efficiencies exceed 2.3 μ T W in the human head model phantom, which is much higher than all results reported in literature. Moreover, we show that the proposed exciter, as an array with well‐decoupled elements (measured isolation between elements is 33 dB or higher), can effectively be used for RF shimming. The novel coil generates RF magnetic field with excellent circular polarization, good uniformity, and negligible axial z‐component, and it provides arbitrarily large or small field of view and excellent transmit efficiency, with and without dielectric lenses. It features well‐defined narrowband operation, excellent isolation between ports/channels, and inherent possibilities for field optimizations via RF shimming and parallel imaging. [ABSTRACT FROM AUTHOR]

Purpose To explore the effect of using extremely high permittivity (εr∼1,000) materials on image quality and power requirements of spine imaging at 3 T. Theory and Methods A linear array of high permittivity dielectric blocks made of lead zirconate titanate (PZT) was designed and characterized by electromagnetic simulations and experiments. Their effect on the transmit efficiency, receive sensitivity, power deposition, and diagnostic image quality was analyzed in vivo in 10 healthy volunteers. Results Simulation results showed that for quadrature mode excitation, the PZT blocks improve the transmit efficiency by 75% while reducing the maximum 10g average specific absorption rate (SAR10) by 20%. In vivo experiments in 10 healthy volunteers showed statistically significant improvements for the transmit efficiency, and image quality. Compared to active radiofrequency shimming, image quality was similar, but the required system input power was significantly lower for quadrature excitation using the PZT blocks. Conclusion For single-channel transmit systems, using high permittivity PZT blocks offer a way to improve transmit efficiency and image quality in the spine. Results show that the effect, and therefore optimal design, is body mass index and sex specific. Magn Reson Med 79:1192-1199, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. [ABSTRACT FROM AUTHOR]

In this work, we experimentally demonstrate an increase in the local transmit efficiency of a 1.5 T MRI scanner by using a metasurface formed by an array of brass wires embedded in a high permittivity low loss medium. Placement of such a structure inside the scanner results in strong coupling of the radiofrequency field produced by the body coil with the lowest frequency electromagnetic eigenmode of the metasurface. This leads to spatial redistribution of the near fields with enhancement of the local magnetic field and an increase in the transmit efficiency per square root maximum specific absorption rate in the region-of-interest. We have investigated this structure in vivo and achieved a factor of 3.3 enhancement in the local radiofrequency transmit efficiency. [ABSTRACT FROM AUTHOR]

Traveling-wave MRI, which uses relatively small and simple RF antennae, has robust matching performance and capability for large field-of-view (FOV) imaging. However, the power efficiency of traveling-wave MRI is much lower than conventional methods, which limits its application. One simple approach to improve the power efficiency is to place passive resonators around the subject being imaged. The feasibility of this approach has been demonstrated in previous works using a single small resonant loop. In this work, we aim to explore how much the improvements can be maintained in human imaging using an array design, and whether electric dipoles can be used as local elements. First, a series of electromagnetic (EM) simulations were performed on a human model. Then RF coils were constructed and the simulation results using the best setup for head imaging were validated in MR experiments. By using the passive local loop and transverse dipole arrays, respectively, the transmit efficiency (B 1 + ) of traveling-wave MRI can be improved by 3-fold in the brain and 2-fold in the knee. The types of passive elements (loops or dipoles) should be carefully chosen for brain or knee imaging to maximize the improvement, and the enhancement depends on the local body configuration. [ABSTRACT FROM AUTHOR]

In this work a new concept for designing transmit coils for magnetic resonance using a plasma is introduced. Unlike conventional coils, a plasma can be turned on and off, eliminating electrical interactions between coils, and enabling radiofrequency-invisibility when desired. A surfatron has been designed to produce a surface-mode wave which propagates along the inner surface of a commercial fluorescent lighting tube. NMR spectra and images have been produced using the plasma as the transmit coil and a copper-based monopole to receive the signal. The transmit efficiency of the plasma tube was estimated, and is currently much lower than that of an equivalently-sized metal-based structure: however, there are many potential methods for increasing the efficiency using a custom-built plasma tube. [ABSTRACT FROM AUTHOR]

[Display omitted] • Wireless coil inductively coupled to the body coil can replace the local Tx/Rx coil. • Wireless coil outperforms the Tx/Rx extremity coil's SNR with up to 1.40-fold gain. • Wireless coil provides 2.5-fold higher global SAR efficiency versus local Tx/Rx coil. This work performs a detailed assessment of radiofrequency (RF) safety and imaging performance of a volumetric wireless coil based on periodically coupled split-loop resonators (SLRs) for 1.5 T wrist MRI versus a commercially available transceive extremity coil. In particular, we evaluated the transmit efficiency and RF safety for three setups: a whole-body birdcage coil, a transceive extremity birdcage coil, and a volumetric wireless coil inductively coupled to the whole-body birdcage coil. The imaging performance of the two latter setups was studied experimentally for nine subjects. The signal-to-noise ratio (SNR) of the images acquired with several standard pulse sequences for osteoarthritis wrist imaging was assessed. Application of the wireless coil significantly improved the specific absorption rate (SAR) efficiency of the whole-body birdcage coil, with at least 4.3-fold and 7.6-fold improvement of local and global SAR efficiencies, respectively. This setup also outperformed the transceive extremity coil in terms of SNR (up to 1.40-fold gain) with a moderate (11%) reduction of the local SAR efficiency. [ABSTRACT FROM AUTHOR]

Purpose A multi-turn transmit surface coil design was presented to improve B1 efficiency when used with current source amplification. Methods Three different coil designs driven by an on-coil current-mode class-D amplifier with current envelope feedback were tested on the benchtop and through imaging in a 1.5 T scanner. Case temperature of the power field-effect transistor at the amplifier output stage was measured to evaluate heat dissipation for the different current levels and coil configurations. In addition, a lower power rated device was tested to exploit the potential gain in B1 obtained with the multi-turn coil. Results As shown both on the benchtop and in a 1.5 T scanner, B1 was increased by almost 3-fold without increasing heat dissipation on the power device at the amplifier's output using a multi-turn surface coil. Similar gain was obtained when connecting a lower power rated field-effect transistor to the multi-turn coil. Conclusion In addition to reduce heat dissipation per B1 in the device, higher B1 per current efficiency allows the use of field-effect transistors with lower current ratings and lower port capacitances, which could improve the overall performance of the on-coil current source transmit system. Magn Reson Med 69:1180-1185, 2013. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Magnetic resonance prostate imaging at 300 MHz is complicated by the effects arising from the short RF wavelength in biological tissue. A radiative antenna leads to a better RF signal penetration for sites located at one or more wavelengths depth, i.e. beyond the near zone. It consists of a dipole antenna placed on a high-dielectric substrate. The effects of changes in substrate dimensions and relative permittivity upon the electromagnetic fields, the B [ABSTRACT FROM AUTHOR]

• A metamaterial inspired wireless coil for targeted breast MRI at 1.5T is presented. • Wireless coil is based on electromagnetically coupled split-loop resonators. • Improvements of local transmit and SAR efficiencies of the body coil were achieved. • The phantom test showed a tenfold increase of SNR when compared to the body coil. In this work, we propose an application of a metamaterial inspired volumetric wireless coil (WLC) based on coupled split-loop resonators for targeted breast MRI at 1.5 T. Due to strong electromagnetic coupling with the body coil, the metamaterial inspired WLC locally focuses radiofrequency (RF) magnetic flux in the target region, thus improving both transmit and receive performance of the external body coil. This leads to substantial enhancement in local transmit efficiency and improvement of RF safety. Phantom images showed a tenfold increase of signal-to-noise ratio (SNR) in the region-of-interest (ROI) and, at the same time, an almost 50-fold reduction in transmit power relative to the same body coil used alone. [ABSTRACT FROM AUTHOR]

• A parametric quantitative study of dipole 7 Tesla array coil was performed. • The transmit, SAR efficiency and mutual coupling due to coil structure were studied. • The study was performed on a homogeneous phantom as well as on a human body model. • SAR efficiency depends on whether the study being done on a phantom or a human model. In this contribution we present numerical and experimental results of a parametric quantitative study of radiative dipole antennas in a phased array configuration for efficient body magnetic resonance imaging at 7 T via parallel transmission. For magnetic resonance imaging (MRI) at ultrahigh fields (7 T and higher) dipole antennas are commonly used in phased arrays, particularly for body imaging targets. This study reveals the effects of dipole positioning in the array (elevation of dipoles above the subject and inter-dipole spacing) on their mutual coupling, B 1 + per P acc and B 1 + per maximum local SAR efficiencies as well as the RF-shimming capability. The numerical and experimental results are obtained and compared for a homogeneous phantom as well as for a real human models confirmed by in vivo experiments. [ABSTRACT FROM AUTHOR]

Objective: To evaluate the transmit efficiency and specific absorption rate (SAR) efficiency of a new eight‐element passively fed meander‐dipole antenna array designed for body MRI at 7 T, and to compare these values with a conventional directly fed meander‐dipole array. Methods: The main radiating element of the passively fed dipole is printed on one side of a dielectric substrate and is capacitively coupled to a shorter feeding element (connected to the coaxial cable) printed on the opposite side of the substrate. The transmit (B1+) field and SAR were simulated on a phantom and on a human voxel model for both a passively fed and a directly fed single element. Two eight‐channel arrays containing, respectively, directly and passively fed meander dipoles were then simulated, and experimental B1+ maps and T2‐weighted spin echo images of the prostate were obtained in vivo for four healthy volunteers. Results: In simulations, the mean transmit efficiency (B1+ per square root input power) value in the prostate was ~ 12.5% lower, and the maximum 10 g average SAR was 44% lower for the array containing passively fed dipoles, resulting in ~ 15% higher SAR efficiency for the passively fed array. In vivo RF‐shimmed turbo spin echo images were acquired from both arrays, and showed image SNRs within 5% of one another. Conclusion: A passive‐feeding network for meander‐dipole antennas has been shown to be a simple method to increase the SAR efficiency of a multi‐element array used for body imaging at high fields. We hypothesize that the main reason for the increase in SAR efficiency is the storage of the strong conservative electric field in the dielectric between the feeding element and the radiating element of the dipole. The passive‐feeding approach can be generalized to other dipole geometries and configurations. [ABSTRACT FROM AUTHOR]

At ultrahigh fields (B0 ≥ 7T), it is challenging to cover a large field of view using single‐row conventional RF coils (standing wave resonators) due to the limited physical dimensions. In contrast, traveling wave approaches can excite large fields of view even using a relatively simple hardware setup, but suffer from poor efficiency and high local specific absorption rate in non‐imaged regions. In this study, we propose and numerically analyze a new coil which combines the concept of traveling wave and standing wave. The new coil consists of a pair of transverse dipole rings (PTDR) whose separation is adjusted according to the desired imaging coverage. The PTDR coil was validated using electromagnetic simulations in phantoms and human leg models, which showed that coverage can be as long as 60 cm. When the coverage of the PTDR coil was shortened to 20 cm to cover the knees only, it's transmit and specific absorption rate efficiencies were 84% and 37% higher than those of the 50 cm coverage, respectively. [ABSTRACT FROM AUTHOR]