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Parallel imaging is one of the key MRI technologies that allow reduction of image acquisition time. However, the parallel imaging reconstruction commonly leads to a signal‐to‐noise ratio (SNR) drop evaluated using a so‐called geometrical factor (g‐factor). The g‐factor is minimized by increasing the number of array elements and their spatial diversity. At the same time, increasing the element count requires a decrease in their size. This may lead to insufficient coil loading, an increase in the relative noise contribution from the RF coil itself, and hence SNR reduction. Previously, instead of increasing the channel number, we introduced the concept of electronically switchable time‐varying sensitivities, which was shown to improve parallel imaging performance. In this approach, each reconfigurable receive element supports two spatially distinct sensitivity profiles. In this work, we developed and evaluated a novel eight‐element human head receive‐only reconfigurable coaxial dipole array for human head imaging at 9.4 T. In contrast to the previously reported reconfigurable dipole array, the new design does not include direct current (DC) control wires connected directly to the dipoles. The coaxial cable itself is used to deliver DC voltage to the PIN diodes located at the ends of the antennas. Thus, the novel reconfigurable coaxial dipole design opens a way to scale the dynamic parallel imaging up to a realistic number of channels, that is, 32 and above. The novel array was optimized and tested experimentally, including in vivo studies. It was found that dynamic sensitivity switching provided an 8% lower mean and 33% lower maximum g‐factor (for Ry × Rz = 2 × 2 acceleration) compared with conventional static sensitivities. [ABSTRACT FROM AUTHOR]

The purpose of the current study was to implement unequal microstrip power splitters for parallel transmission at 7 T that are optimized for size and loss and that can be configured for a wide range of power ratios. The splitters will enable the use of more transmit coils without a corresponding increase in the number of transmit channels or amplifiers to control specific absorption rate, shorten RF pulses, and shim inhomogeneous RF fields. Wilkinson unequal power splitters based on a novel microstrip network design were optimized to minimize their size under 8 cm in length and 9 cm in width, enabling them to be included in coil housing or cascaded in multiple stages. Splitters were designed and constructed for a wide range of output power ratios at 298 MHz. Simulations and bench tests were performed for each ratio, and a methodology was established to adapt the designs to other ratios and frequencies. The designs and code are open source and can be reproduced as is or reconfigured. The single‐stage designs achieved good matches and isolations between output ports (worst isolation −15.9 dB, worst match −15.1 dB). A two‐stage cascaded (one input to four outputs) power splitter with 1:2.5, 1:10, 1:3, and 1:6 ratio outputs was constructed. The worst isolation between output ports was −19.7 dB in simulation and the worst match of the three ports was −17.8 dB. The measured ratios for one‐ and two‐stage boards were within 10% of the theoretical ratios. The power‐handling capability of the smallest trace was approximately 70 W. Power loss for the one‐ and two‐stage boards ranged from 1% to 3% in simulation compared with 5.1% to 7.2% on the bench. It was concluded that Wilkinson unequal microstrip power splitters can be implemented with a small board size (low height) and low loss, and across a wide range of output power ratios. The splitters can be cascaded in multiple stages while maintaining the expected ratios and low loss. This will enable the construction of large fixed transmit array‐compression matrices with low loss. [ABSTRACT FROM AUTHOR]

MRI at ultra‐high field (UHF, ≥7 T) provides a natural strategy for improving the quality of X‐nucleus magnetic resonance spectroscopy and imaging due to the intrinsic benefit of increased signal‐to‐noise ratio. Considering that RF coils require both local transmission and reception at UHF, the designs of double‐tuned coils, which often consist of several layers of transmit and receive resonant elements, become quite complex. A few years ago, a new type of RF coil, ie a dipole antenna, was developed and used for human body and head imaging at UHF. Due to the mechanical and electrical simplicity of dipole antennas, combining an X‐nucleus surface loop array with 1H dipoles can substantially simplify the design of a double‐tuned UHF human head array coil. Recently, we developed a novel bent folded‐end dipole transceiver array for human head imaging at 9.4 T. The new eight‐element dipole array demonstrated full brain coverage, and transmit efficiency comparable to that of the substantially more complex 16‐element surface loop array. In this work, we developed, constructed and evaluated a double‐tuned 13C/1H human head 9.4 T array consisting of eight 13C transceiver surface loops and eight 1H transceiver bent folded‐end dipole antennas all placed in a single layer. We showed that interaction between loops and dipoles can be minimized by placing four 1H traps into each 13C loop. The presented double‐tuned RF array coil substantially simplifies the design as compared with the common double‐tuned surface loop arrays. At the same time, the coil demonstrated an improved 1H longitudinal coverage and good transmit efficiency. [ABSTRACT FROM AUTHOR]

Introduction: Multi‐component T2 mapping using a gradient‐ and spin‐echo (GraSE) acquisition has become standard for myelin water imaging at 3 T. Higher magnetic field strengths promise signal‐to‐noise ratio benefits but face specific absorption rate limits and shortened T2 times. This study investigates compartmental T2 times in vivo and addresses advantages and challenges of multi‐component T2 mapping at 7 T. Methods: We acquired 3D multi‐echo GraSE data in seven healthy adults at 7 T, with three subjects also scanned at 3 T. Stimulated echoes arising from B1+ inhomogeneities were accounted for by the extended phase graph (EPG) algorithm. We used the computed T2 distributions to determine T2 times that identify different water pools and assessed signal‐to‐noise and fit‐to‐noise characteristics of the signal estimation. We compared short T2 fractions and T2 properties of the intermediate water pool at 3 T and 7 T. Results: Flip angle mapping confirmed that EPG accurately determined the larger B1+ inhomogeneity at 7 T. Multi‐component T2 analysis demonstrated shortened T2 times at 7 T compared with 3 T. Fit‐to‐noise and signal‐to‐noise ratios were improved at 7 T but depended on B1+ homogeneity. Adjusting the shortest T2 to 8 ms and the T2 threshold that separates different water compartments to 20 ms yielded short T2 fractions at 7 T that conformed to 3 T data. Short T2 fractions in myelin‐rich white matter regions were lower at 7 T than at 3 T, and higher in iron‐rich structures. Discussion: Adjusting the T2 compartment boundaries was required due to the shorter T2 relaxation times at 7 T. Shorter echo spacing would better sample the fast decaying signal but would increase peripheral nerve stimulation. Multi‐channel transmission will improve T2 measurements at 7 T. Conclusion: We used a multi‐echo 3D GraSE sequence to characterize the multi‐exponential T2 decay at 7 T. We adapted T2 parameters for evaluation of the short T2 fraction. Obtained 7 T multi‐component T2 maps were in good agreement with 3 T data. [ABSTRACT FROM AUTHOR]

Purpose Moving to ultra‐high fields (≥7 T), the inhomogeneity of both RF (B1) and static (B0) magnetic fields increases, which further motivates us to design a realistic head‐shaped phantom, especially for spectroscopic imaging. Such phantoms provide images similar to the human brain and serve as a reliable tool for developing and examining methods in MRI. This study aims to develop and characterize a realistic head‐shaped phantom filled with brain‐mimicking metabolites for MRS and magnetic resonance spectroscopic imaging in a 7 T MRI scanner. Methods: A 3D head‐shaped container with three sections—mimicking brain, muscle and precranial lipid—was constructed. The phantom was designed to provide robustness to heating, mechanical damage and leakage, with easy refilling. The head's shape and the agarose mixture were optimized to provide B0 and B1 distributions and T1/T2 relaxation values similar to those of human brain. Eight brain‐tissue‐mimicking metabolites were included for spectroscopy. The phantom was evaluated for localized spectroscopy, fast spectroscopic imaging and fat suppression. Results: The B0 and B1 maps showed distribution similar to that of human brain, with increased B0 inhomogeneity near the nasal and ear areas and reduced B1 in the temporal lobe and brain stem regions, as expected in vivo. The metabolites' concentrations were verified by single‐voxel spectroscopy, showing an average deviation of 11%. Fast spectroscopic imaging and imaging with fat suppression were demonstrated. Conclusion: A 3D head‐shaped phantom for human brain imaging and spectroscopic imaging in 7 T MRI was demonstrated, making it a realistic phantom for methodology development at 7 T. [ABSTRACT FROM AUTHOR]

Ultra-high field (UHF, ≥7 T) tight fit transceiver phased arrays improve transmit (Tx) efficiency ( B1+/√ P) in comparison with Tx-only arrays, which are usually larger to fit receive (Rx)-only arrays inside. One of the major problems limiting applications of tight fit arrays at UHFs is the anticipated increase of local tissue heating, which is commonly evaluated by the local specific absorption rate (SAR). To investigate the tradeoff between Tx efficiency and SAR when a tight fit UHF human head transceiver phased array is used instead of a Tx-only/Rx-only RF system, a single-row eight-element prototype of a 400 MHz transceiver head phased array was constructed. The Tx efficiency and SAR of the array were evaluated and compared with that of a larger Tx-only array, which could also be used in combination with an 18-channel Rx-only array. Data were acquired on the Siemens Magnetom whole body 9.4 T human MRI system. Depending on the head size, positioning and the RF shim strategy, the smaller array provides from 11 to 23% higher Tx efficiency. In general, the Tx performance, evaluated as B1+/√SAR, i.e. the safety excitation efficiency (SEE), is also not compromised. The two arrays provide very similar SEEs evaluated over 1000 random RF shim sets. We demonstrated that, in general, the tight fit transceiver array improves Tx performance without compromising SEE. However, in specific cases, the SEE value may vary, favoring one of the arrays, and therefore must be carefully evaluated. [ABSTRACT FROM AUTHOR]

MRI at ultra-high field (UHF, ≥7 T) provides a natural strategy for improving the quality of X-nucleus magnetic resonance spectroscopy and imaging due to the intrinsic benefit of increased signal-to-noise ratio. Considering that RF coils require both local transmission and reception at UHF, the designs of double-tuned coils, which often consist of several layers of transmit and receive resonant elements, become quite complex. A few years ago, a new type of RF coil, ie a dipole antenna, was developed and used for human body and head imaging at UHF. Due to the mechanical and electrical simplicity of dipole antennas, combining an X-nucleus surface loop array with 1 H dipoles can substantially simplify the design of a double-tuned UHF human head array coil. Recently, we developed a novel bent folded-end dipole transceiver array for human head imaging at 9.4 T. The new eight-element dipole array demonstrated full brain coverage, and transmit efficiency comparable to that of the substantially more complex 16-element surface loop array. In this work, we developed, constructed and evaluated a double-tuned 13 C/ 1 H human head 9.4 T array consisting of eight 13 C transceiver surface loops and eight 1 H transceiver bent folded-end dipole antennas all placed in a single layer. We showed that interaction between loops and dipoles can be minimized by placing four 1 H traps into each 13 C loop. The presented double-tuned RF array coil substantially simplifies the design as compared with the common double-tuned surface loop arrays. At the same time, the coil demonstrated an improved 1 H longitudinal coverage and good transmit efficiency., (© 2021 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.)

Introduction: Multi-component T 2 mapping using a gradient- and spin-echo (GraSE) acquisition has become standard for myelin water imaging at 3 T. Higher magnetic field strengths promise signal-to-noise ratio benefits but face specific absorption rate limits and shortened T 2 times. This study investigates compartmental T 2 times in vivo and addresses advantages and challenges of multi-component T 2 mapping at 7 T., Methods: We acquired 3D multi-echo GraSE data in seven healthy adults at 7 T, with three subjects also scanned at 3 T. Stimulated echoes arising from B 1 + inhomogeneities were accounted for by the extended phase graph (EPG) algorithm. We used the computed T 2 distributions to determine T 2 times that identify different water pools and assessed signal-to-noise and fit-to-noise characteristics of the signal estimation. We compared short T 2 fractions and T 2 properties of the intermediate water pool at 3 T and 7 T., Results: Flip angle mapping confirmed that EPG accurately determined the larger B 1 + inhomogeneity at 7 T. Multi-component T 2 analysis demonstrated shortened T 2 times at 7 T compared with 3 T. Fit-to-noise and signal-to-noise ratios were improved at 7 T but depended on B 1 + homogeneity. Adjusting the shortest T 2 to 8 ms and the T 2 threshold that separates different water compartments to 20 ms yielded short T 2 fractions at 7 T that conformed to 3 T data. Short T 2 fractions in myelin-rich white matter regions were lower at 7 T than at 3 T, and higher in iron-rich structures., Discussion: Adjusting the T 2 compartment boundaries was required due to the shorter T 2 relaxation times at 7 T. Shorter echo spacing would better sample the fast decaying signal but would increase peripheral nerve stimulation. Multi-channel transmission will improve T 2 measurements at 7 T., Conclusion: We used a multi-echo 3D GraSE sequence to characterize the multi-exponential T 2 decay at 7 T. We adapted T 2 parameters for evaluation of the short T 2 fraction. Obtained 7 T multi-component T 2 maps were in good agreement with 3 T data., (© 2020 John Wiley & Sons, Ltd.)