Dense lattices of photonic crystals can serve as artificial materials, with light propagation in these structures described by effective material parameters that surpass the capabilities of natural materials. In this study, we introduce a metamaterial that supports quadrupole magnetization, a characteristic rarely observed in existing structures. We experimentally demonstrate a magnetic quadrupole metamaterial associated with Mie-resonance-excited stop bands below the Bragg band. Additionally, we develop a theoretical model that addresses both dispersion and boundary conditions within this framework. Using a Fabry-Perot resonator as a case study, we validate our model and reveal that the quadrupole metamaterial can exhibit a markedly different reflection/transmission spectrum, including zero reflection at normal incidence. Our findings underscore the practical potential for both experimental and theoretical investigations of metamaterials that extend beyond the dipole approximation.

Antennas suitable for 6G applications can potentially operate in the W-band (75-110 GHz), which requires fine manufacturing accuracy, particularly for fine surface processing. Such antennas with fine surfaces can be produced using many different techniques, such as chemical etching, milling, and additive manufacturing. However, even fine manufacturing leads to certain imperfections on the antenna surface that lead to the performance degradation of an antenna. As a result, an undesirable difference between the numerical and experimental results can occur. To study this effect, we performed electromagnetic simulations of surface roughness in multiple conventional electromagnetic devices suitable for 6G applications. As antennas under investigation, we have chosen a horn antenna and a patch antenna. In addition, we consider a rectangular WR-10 waveguide and a 50 Ohm microstrip transmission line. Surface roughness has been implemented as a set of hemispheres, which are randomly distributed, added, or subtracted on the surface of the antenna. After performing a set of simulations, we evaluated the S-parameters, antenna efficiency, and directivity patterns. The results are then compared with reference devices with ideal surfaces to find the optimal surface requirement for antenna manufacturing.

Satellite communication systems for mounting on vehicles are commonly based on phased array antennas with electronical beam steering. In this work we develop active metasurface-based receive-transmit antenna arrays for newly deployed satellite constellation system running in Ku-band. Both receive and transmit subarrays are based on patch-antenna elements driven in circular polarization using 90 circle hybrids. In both subarrays we optimized elements sizes and array period to have optimal ellipticity, gain, side lobe level. In electromagnetic simulations we achieved realized gain of 32.2 dBi for the Rx subarray and 33.4 dBi for the Tx subarray obtained in a broad scanning angle range from -15 circle to + 45 circle.

In this paper, we investigate a novel subwavelength scatterer, the layout of which is inspired by the Mo-bius strip. The characteristic size of the proposed particle is about 10 mm. So that we consider the frequency range from 1 to 30 GHz in numerical simulation for field distribution calculation of scattering task using finite element method. The results of spectral analysis of multipolar coefficients of far electric field allows us to conclude that with certain specified geometric parameters of the scatterer, co-directional electric and magnetic dipole moments are simultaneously excited in it. Due to this behav-ior at multiple frequencies, the proposed particle is transparent to one circular polarization, while the other is effectively scattered. Due to the reciprocity principle, such scatterer can also be used as an an-tenna with a predictable radiation pattern in several frequency ranges.

PurposeTo demonstrate the feasibility of using octafluorocyclobutane (OFCB, c‐C4F8) for T1 mapping of lungs in 19F MRI.MethodsThe study was performed at 7 T in three healthy rats and three rats with pulmonary hypertension. To increase the sensitivity of 19F MRI, a bent‐shaped RF coil with periodic metal strips structure was used. The double flip angle method was used to calculate normalized transmitting RF field (B1n+) maps and for correcting T1 maps built with the variable flip angle (VFA) method. The ultrashort TE pulse sequence was applied for acquiring MR images throughout the study.ResultsThe dependencies of OFCB relaxation times on its partial pressure in mixtures with oxygen, air, helium, and argon were obtained. T1 of OFCB linearly depended on its partial pressure with the slope of about 0.35 ms/kPa in the case of free diffusion. RF field inhomogeneity leads to distortion of T1 maps built with the VFA method, and therefore to high standard deviation of T1 in these maps. To improve the accuracy of the T1 maps, the B1n+ maps were applied for VFA correction. This contributed to a 2–3‐fold decrease in the SD of T1 values in the corresponding maps compared with T1 maps calculated without the correction. Three‐dimensional T1 maps were obtained, and the mean T1 in healthy rat lungs was 35 ± 10 ms, and in rat lungs with pulmonary hypertension – 41 ± 9 ms.ConclusionOFCB has a spin‐rotational relaxation mechanism and can be used for 19F T1 mapping of lungs. The calculated OFCB maps captured ventilation defects induced by edema.

Previously, we have shown in simulations that electronically modulated time-varying receive sensitivities can improve parallel imaging reconstruction when fast modulations are applied during acquisition of k-space lines. Here, we demonstrate this concept experimentally with a prototype 8-channel reconfigurable receive coil, for which B1- modulation is achieved by fast switching PIN diodes in the receive loops. With this setup, MR measurements were performed in both phantom and human subject. Lower reconstruction errors and g-factors (~25% improvement for R=4) were observed for the case of rapidly switched sensitivities compared to conventional reconstruction with static sensitivities.

Homogeneity and longitudinal coverage of transmit (Tx) human head RF coils at ultrahigh field (UHF, ≥7 T) can be improved by 3D RF shimming, which requires using multi‐row Tx arrays. Examples of 3D RF shimming using double‐row UHF loop transceiver (TxRx) and Tx arrays have been described previously. Dipole antennas provide unique simplicity and robustness while offering comparable Tx efficiency and signal‐to‐noise ratio to conventional loop designs. Single‐row Tx and TxRx human head UHF dipole arrays have been previously described by multiple groups. Recently, we developed a novel type of dipole antenna, a folded‐end dipole, and presented single‐row eight‐element array prototypes for human head imaging at 7 and 9.4 T. These studies have shown that the novel antenna design can improve the longitudinal coverage and minimize peak local specific absorption rate (SAR) as compared with common unfolded dipoles. In this work, we developed, constructed, and evaluated a 16‐element double‐row TxRx folded‐end dipole array for human head imaging at 9.4 T. To minimize cross‐talk between neighboring dipoles located in different rows, we used transformer decoupling, which decreased coupling to a level below −20 dB. The developed array design was demonstrated to be capable of 3D static RF shimming and can be potentially used for dynamic shimming using parallel transmission. For optimal phase shifts between the rows, the array provides 11% higher SAR efficiency and 18% higher homogeneity than a folded‐end dipole single‐row array of the same length. The design also offers a substantially simpler and more robust alternative to the common double‐row loop array with about 10% higher SAR efficiency and better longitudinal coverage.

At 9.4T, transmit-only RF array coils for human brain imaging provide mediocre transmit efficiency due to a weak loading factor, which implies low power dissipation in the tissues. In this work we aim to optimize using the full-wave simulations a dual row 16-channel transmit-only array to improve the transmit performance. For that purpose, we simulated eight different array designs containing the loops with overlapping and gaps between the rows and two configurations of loopoles. We have shown that the optimized design with the overlapped loops provides improvement ~15% in Tx-efficiency and 16.5% improvement in SAR-efficiency compared to the reference coil.

Additive technologies show promising results in 6G antennas operating in W-band. However, quality of additive manufacturing strongly depends on many crucial factors related to the manufacturing process. In this work we perform electromagnetic simulations to study an impact of surface roughness to the performance of some conventional antennas suitable for 6G applications. Finally, we could define the surface roughness requirements for the additive antennas.

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 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.

The advancement of clinical applications of 7T MRI depends heavily on the development of new RF coil designs. Recent works based on Ultimate Intrinsic SNR theory demonstrated that an optimal central SNR at 7T requires combining surface loops with dipole antennas. In this work, we developed and evaluated two novel 32-element 7T human head loop/dipole array designs. Both coils demonstrated superior Tx-efficiency, longitudinal coverage, and SNR in comparison to widely used commercial array coil. While the transceiver (TxRx)-dipole/receive(Rx)-loop array demonstrated best SNR, the TxRx-loop/Rx-dipole array showed the best Tx-efficiency. In addition, double-row TxRx-loop/Rx-dipole array provides 3D RF shimming capability.

Purpose: To extend the concept of 3D dynamic parallel imaging, we developed a prototype of an electronically reconfigurable dipole array that provides sensitivity alteration along the dipole length. Methods: We developed a radiofrequency array coil consisting of eight reconfigurable elevated-end dipole antennas. The receive sensitivity profile of each dipole can be electronically shifted toward one or the other end by electrical shortening or lengthening the dipole arms using positive-intrinsic-negative-diode lump-element switching units. Based on the results of electromagnetic simulations, we built the prototype and tested it at 9.4 T on phantom and healthy volunteer. A modified 3D SENSE reconstruction was used, and geometry factor (g-factor) calculations were performed to assess the new array coil. Results: Electromagnetic simulations showed that the new array coil was capable of alteration of its receive sensitivity profile along the dipole length. Electromagnetic and g-factor simulations showed closely agreeing predictions when compared to the measurements. The new dynamically reconfigurable dipole array provided significant improvement in geometry factor compared to static dipoles. We obtained up to 220% improvement for 3 x2 (Ry x Rz) acceleration compared to the static configuration case in terms of maximum g-factor and up to 54% in terms of mean g-factor for the same acceleration. Conclusion: We presented an 8-element prototype of a novel electronically reconfigurable dipole receive array that permits rapid sensitivity modulations along the dipole axes. Applying dynamic sensitivity modulation during image acquisition emulates two virtual rows of receive elements along the z-direction, and therefore improves parallel imaging performance for 3D acquisitions.

PURPOSE:To investigate how electronically modulated time-varying receive sensitivities can improve parallel imaging reconstruction at ultra-high field. METHODS:Receive sensitivity modulation was achieved by introducing PIN diodes in the receive loops, which allow rapid switching of capacitances in both arms of each loop coil and by that alter B1- profiles, resulting in two distinct receive sensitivity configurations. A prototype 8-channel reconfigurable receive coil for human head imaging at 9.4T was built, and MR measurements were performed in both phantom and human subject. A modified SENSE reconstruction for time-varying sensitivities was formulated, and g-factor calculations were performed to investigate how modulation of receive sensitivity profiles during image encoding can improve parallel imaging reconstruction. The optimized modulation pattern was realized experimentally, and reconstructions with the time-varying sensitivities were compared with conventional static SENSE reconstructions. RESULTS:The g-factor calculations showed that fast modulation of receive sensitivities in the order of the ADC dwell time during k-space acquisition can improve parallel imaging performance, as this effectively makes spatial information of both configurations simultaneously available for image encoding. This was confirmed by in vivo measurements, for which lower reconstruction errors (SSIM = 0.81 for acceleration R = 4) and g-factors (max g = 2.4; R = 4) were observed for the case of rapidly switched sensitivities compared to conventional reconstruction with static sensitivities (SSIM = 0.74 and max g = 3.2; R = 4). As the method relies on the short RF wavelength at ultra-high field, it does not yield significant benefits at 3T and below. CONCLUSIONS:Time-varying receive sensitivities can be achieved by inserting PIN diodes in the receive loop coils, which allow modulation of B1- patterns. This offers an additional degree of freedom for image encoding, with the potential for improved parallel imaging performance at ultra-high field.

PurposeTo explore the potential of deuterium metabolic imaging (DMI) in the human brain in vivo at 7 T, using a multi‐element deuterium (2H) RF coil for 3D volume coverage.Methods1H‐MR images and localized 2H MR spectra were acquired in vivo in the human brain of 3 healthy subjects to generate DMI maps of 2H‐labeled water, glucose, and glutamate/glutamine (Glx). In addition, non‐localized 2H‐MR spectra were acquired both in vivo and in vitro to determine T1 and T2 relaxation times of deuterated metabolites at 7 T. The performance of the 2H coil was assessed through numeric simulations and experimentally acquired B1+ maps.Results3D DMI maps covering the entire human brain in vivo were obtained from well‐resolved deuterated (2H) metabolite resonances of water, glucose, and Glx. The T1 and T2 relaxation times were consistent with those reported at adjacent field strengths. Experimental B1+ maps were in good agreement with simulations, indicating efficient and homogeneous B1+ transmission and low RF power deposition for 2H, consistent with a similar array coil design reported at 9.4 T.ConclusionHere, we have demonstrated the successful implementation of 3D DMI in the human brain in vivo at 7 T. The spatial and temporal nominal resolutions achieved at 7 T (i.e., 2.7 mL in 28 min, respectively) were close to those achieved at 9.4 T and greatly outperformed DMI at lower magnetic fields. DMI at 7 T and beyond has clear potential in applications dealing with small brain lesions.

Purpose To improve whole‐brain SNR at 7 Tesla, a novel 32‐element hybrid human head array coil was developed, constructed, and tested. Methods Our general design strategy is based on 2 major ideas: Firstly, following suggestions of previous works based on the ultimate intrinsic SNR theory, we combined loops and dipoles for improvement of SNR near the head center. Secondly, we minimized the total number of array elements by using a hybrid combination of transceive (TxRx) and receive (Rx) elements. The new hybrid array consisted of 8 folded‐end TxRx‐dipole antennas and 3 rows of 24 Rx‐loops all placed in a single layer on the surface of a tight‐fit helmet. Results The developed array significantly improved SNR in vivo both near the center (∼20%) and at the periphery (∼20% to 80%) in comparison to a common commercial array coil with 8 transmit (Tx) and 32 Rx‐elements. Whereas 24 loops alone delivered central SNR very similar to that of the commercial coil, the addition of complementary dipole structures provided further improvement. The new array also provided ∼15% higher Tx efficiency and better longitudinal coverage than that of the commercial array. Conclusion The developed array coil demonstrated advantages in combining complementary TxRx and Rx resonant structures, that is, TxRx‐dipoles and Rx‐loops all placed in a single layer at the same distance to the head. This strategy improved both SNR and Tx‐performance, as well as simplified the total head coil design, making it more robust.

Important issues in designing radiofrequency (RF) coils for human head imaging at ultra‐high field (UHF; ≥7 T) are the inhomogeneity and longitudinal coverage (along the magnet axis) of the transmit (Tx) RF field. Both the homogeneity and coverage produced by Tx volume coils can be improved by means of three‐dimensional (3D) RF shimming, which requires the use of multirow Tx‐arrays. In addition, according to recent findings of the ultimate intrinsic signal‐to‐noise ratio (UISNR) theory, the loop‐only receive (Rx) arrays do not provide optimal SNR near the brain center at UHF. The latter can be obtained by combining complementary conductive structures carrying different current patterns (e.g., loops and dipole antennas). In this work, we developed, constructed, and evaluated a novel 32‐element hybrid array design for human head imaging at 7 T. The array consists of 16 transceiver loops placed in two rows circumscribing the head and 16 folded‐end Rx‐only dipoles positioned in the centers of loops. By placing all elements in a single layer, we increased RF power deposition into the tissue and, thus, preserved the Tx‐efficiency. Using this hybrid design also simplifies the coil structure by minimizing the total number of array elements. The array demonstrated whole brain coverage, 3D RF shimming capability, and high SNR. It provided ~15% higher SNR near the brain center and, depending on the RF shim mode, from 20% to 40% higher Tx‐efficiency than a common commercial head array coil.

The advancement of clinical applications of ultrahigh field (UHF) MRI depends heavily on advances in technology, including the development of new radiofrequency (RF) coil designs. Currently, the number of commercially available 7 T head RF coils is rather limited, implying a need to develop novel RF head coil designs that offer superior transmit and receive performance. RF coils to be used for clinical applications must be robust and reliable. In particular, for transmit arrays, if a transmit channel fails the local specific absorption rate may increase, significantly increasing local tissue heating. Recently, dipole antennas have been proposed and used to design UHF head transmit and receive arrays. The dipole provides a unique simplicity while offering comparable transmit efficiency and signal‐to‐noise ratio with the conventional loop design. Recently, we developed a novel array design in our laboratory using a folded‐end dipole antenna. In this work, we developed, constructed and evaluated an eight‐element transceiver bent folded‐end dipole array for human head imaging at 7 T. Driven in the quadrature circularly polarized mode, the array demonstrated more than 20% higher transmit efficiency and significantly better whole‐brain coverage than that provided by a widely used commercial array. In addition, we evaluated passive dipole antennas for decoupling the proposed array. We demonstrated that in contrast to the common unfolded dipole array, the passive dipoles moved away from the sample not only minimize coupling between the adjacent folded‐end active dipoles but also produce practically no destructive interference with the quadrature mode of the array.

Purpose To theoretically describe, design, and test the new geometry of the birdcage coil for 7 Tesla anatomical brain imaging, which includes a large window on top, without deliberately jeopardizing its homogeneity and efficiency. This opencage will not only improve patient comfort but also enable the volunteer to follow functional MRI stimuli. This design could also facilitate the tracking of patient compliance and enable better correction of the movement. Methods Via the transfer matrix approach, a birdcage‐like coil with a nonperiodic distribution of rungs is constructed with optimized currents in the coil rungs. Subsequently, the coil is adjusted in full‐wave simulations. Then, the coil is assembled, fine‐tuned, and matched on the bench. Finally, these results are confirmed experimentally on a phantom and in vivo. Results Indeed, the computed isolation of −14.9 dB between the feeding ports of the coil and the symmetry of the circular polarized mode pattern transmit RF magnetic field ( ) showed that the coil was properly optimized. An experimental assessment of the developed coil showed competitive transmit efficiency and coverage compared with the conventional birdcage coil of similar size. Conclusion The proposed opencage coil can be designed and work without a dramatic drop of performance in terms of the field homogeneity, transmit efficiency ( / ), peak local specific absorption rate ( ) and SAR efficiency ( / ).

In this work, we compare three types of transceive wireless coils: Helmholtz-type coil, metamaterial-inspired coil and their combination. Each transceive coil is electromagnetically coupled to a body coil “birdcage” type of a 1.5 T MR scanner and improves bilateral breast imaging performance. While Helmholtz-type coil and metamaterial-inspired coil based on coupled split loop resonators are linearly polarized, their combination allows to couple with both linear components of the radiofrequency magnetic field, providing a more significant effect of a local boosting of the body coil’s transmit efficiency and radiofrequency safety in comparison with birdcage coil only.