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The human body’s ultra-high field magnetic resonance imaging suffers from the inhomogeneity of the radio frequency magnetic field $B_{1}^{+}$ and the high-peak levels of SAR created in body tissues during transmission. Both issues are typically addressed in the research domain using a tight-fit local array of antenna elements attached to the body and driven with customized phases by multiple transmit channels. The same approach faces difficulties in fast and reliable SAR evaluation, precise positioning, and patient comfort in the clinical environment. Recently, a novel bore-integrated array of stripline antenna elements with substantial free space was introduced for whole-body imaging at 7 T, showing, however, insufficient transmission efficiency (the level of $B_{1}^{+}$ at the unit input power). This work aims to improve the efficiency of the bore-integrated array configuration, which, unlike the tight-fit configuration, may be beneficial for prospective clinical workflow. After analyzing the proposed solutions, particularly the stripline antenna arrays, we found that one of the main limitations is the high level of losses within the array element. We propose using patch antenna elements separated by compact dual-loop decoupling resonators to overcome this limitation. Comparing the proposed elements with striplines in the same bore-integrated eight-channel configuration, we demonstrate numerically and experimentally on the bench a transmit efficiency improvement at least 3 times depending on the excitation mode. Furthermore, the proposed array is shown to overcome both the stripline array and volumetric TEM coil in terms of SAR efficiency approaching the level of the state-of-the-art tight-fit array of fractionated dipole antennas. Our results support the feasibility of integrating multichannel transmit array coils into the bore liner of a 7 T system while compromising transmit efficiency and providing competitive SAR efficiency.

Purpose: Compared with lower field strengths, DWI at 7 T faces the combined challenges of increased distortion and blurring due to B0 inhomogeneity, and increased signal dropouts due to B1+ inhomogeneity. This study addresses the B1+ limitations using slice‐specific static parallel transmission (pTx) in a multi‐shot, readout‐segmented EPI diffusion imaging sequence. Methods: DWI was performed in 7 healthy subjects using MRI at 7 T and readout‐segmented EPI. Data were acquired with non‐pTx circular‐polarized (CP) pulses (CP‐DWI) and static pTx pulses (pTx‐DWI) using slice‐specific B1+ shim coefficients. Each volunteer underwent two scan sessions on the same day, with two runs of each sequence in the first session and one run in the second. The sequences were evaluated by assessing image quality, flip‐angle homogeneity, and intrasession and intersession repeatability in ADC estimates. Results: pTx‐DWI significantly reduced signal voids compared with CP‐DWI, particularly in inferior brain regions. The use of pTx also improved RF uniformity and symmetry across the brain. These effects translated into improved intrasession and intersession repeatability for pTx‐DWI. Additionally, re‐optimizing the pTx pulse between repeat scans did not have a negative effect on ADC repeatability. Conclusion: The study demonstrates that pTx provides a reproducible image‐quality increase in multishot DWI at 7 T. The benefits of pTx also extend to quantitative ADC estimation with regard to the improvement in intrasession and intersession repeatability. Overall, the combination of multishot imaging and pTx can support the development of reliable, high‐resolution DWI for clinical studies at 7 T. [ABSTRACT FROM AUTHOR]

Purpose: To introduce universal modes by applying the universal pulse concept to time‐interleaved acquisition of modes (TIAMO), thereby achieving calibration‐free B1+$$ {B}_1^{+} $$ inhomogeneity mitigation for body imaging at ultra‐high fields. Methods: Two databases of different RF arrays were used to demonstrate the feasibility of universal modes. The first comprised 31 cardiac in vivo data sets acquired at 7T while the second consisted of 6 simulated 10.5T pelvic data sets. Subject‐specific solutions and universal modes were computed and subsequently evaluated alongside predefined default modes. For the cardiac database, subdivision into subpopulations was investigated. The optimization was performed using least‐squares (LS) TIAMO and acquisition modes optimized for refocused echoes (AMORE). Finally, universal modes based on simulated pelvis data were applied in vivo at 10.5T. Results: In all studied cases, the universal modes yield improvements over the predefined default modes of up to 51% (cardiac) and 30% (pelvic) in terms of median excitation error when using two modes. The subpopulation‐specific cardiac solutions revealed a further improvement of universal modes at the expense of increased errors when applied outside the appropriate subpopulation. Direct application of simulation‐based universal modes in vivo resulted in up to a 14% reduction in excitation error compared to default modes and up to a 34% reduction in peak 10 g local specific absorption rate (SAR) compared to subject‐specific solutions. Conclusions: Universal modes are feasible for calibration‐free B1+$$ {B}_1^{+} $$ inhomogeneity mitigation at ultra‐high fields. In addition, simulation‐based solutions can be applied directly in vivo, eliminating the need for large in vivo databases. [ABSTRACT FROM AUTHOR]

Purpose: This study evaluates the imaging performance of two‐channel RF‐shimming for fetal MRI at 3 T using four different local specific absorption rate (SAR) management strategies. Methods: Due to the ambiguity of safe local SAR levels for fetal MRI, local SAR limits for RF shimming were determined based on either each individual's own SAR levels in standard imaging mode (CP mode) or the maximum SAR level observed across seven pregnant body models in CP mode. Local SAR was constrained either indirectly by further constraining the whole‐body SAR (wbSAR) or directly by using subject‐specific local SAR models. Each strategy was evaluated by the improvement of the transmit field efficiency (average |B1+|) and nonuniformity (|B1+| variation) inside the fetus compared with CP mode for the same wbSAR. Results: Constraining wbSAR when using RF shimming decreases B1+ efficiency inside the fetus compared with CP mode (by 12%–30% on average), making it inefficient for SAR management. Using subject‐specific models with SAR limits based on each individual's own CP mode SAR value, B1+ efficiency and nonuniformity are improved on average by 6% and 13% across seven pregnant models. In contrast, using SAR limits based on maximum CP mode SAR values across seven models, B1+ efficiency and nonuniformity are improved by 13% and 25%, compared with the best achievable improvement without SAR constraints: 15% and 26%. Conclusion: Two‐channel RF‐shimming can safely and significantly improve the transmit field inside the fetus when subject‐specific models are used with local SAR limits based on maximum CP mode SAR levels in the pregnant population. [ABSTRACT FROM AUTHOR]

Purpose: Neurovascular MRI suffers from a rapid drop in B1+ into the neck when using transmit head coils at 7 T. One solution to improving B1+ magnitude in the major feeding arteries in the neck is to use custom RF shims on parallel‐transmit head coils. However, calculating such shims requires robust multichannel B1+ maps in both the head and the neck, which is challenging due to low RF penetration into the neck, limited dynamic range of multichannel B1+ mapping techniques, and B0 sensitivity. We therefore sought a robust, large‐dynamic‐range, parallel‐transmit field mapping protocol and tested whether RF shimming can improve carotid artery B1+ magnitude in practice. Methods: A pipeline is presented that combines B1+ mapping data acquired using circularly polarized (CP) and CP2‐mode RF shims at multiple voltages. The pipeline was evaluated by comparing the predicted and measured B1+ for multiple random transmit shims, and by assessing the ability of RF shimming to increase B1+ in the carotid arteries. Results: The proposed method achieved good agreement between predicted and measured B1+ in both the head and the neck. The B1+ magnitude in the carotid arteries can be increased by 43% using tailored RF shims or by 37% using universal RF shims, while also improving the RF homogeneity compared with CP mode. Conclusion: B1+ in the neck can be increased using RF shims calculated from multichannel B1+ maps in both the head and the neck. This can be achieved using universal phase‐only RF shims, facilitating easy implementation in existing sequences. [ABSTRACT FROM AUTHOR]

Magnetic resonance electrical properties tomography (MREPT) is an emerging imaging modality to noninvasively measure tissue conductivity and permittivity. Implementation of MREPT in the clinic requires repeatable measurements at a short scan time and an appropriate protocol. The aim of this study was to investigate the repeatability of conductivity measurements using phase-based MREPT and the effects of compressed SENSE (CS), and RF shimming on the precision of conductivity measurements. Conductivity measurements using turbo spin echo (TSE) and three-dimensional balanced fast field echo (bFFE) with CS factors were repeatable. Conductivity measurement using bFFE phase showed smaller mean and variance that those measured by TSE. The conductivity measurements using bFFE showed minimal deviation with CS factors up to 8, with deviation increasing at CS factors > 8. Subcortical structures produced less consistent measurements than cortical parcellations at higher CS factors. RF shimming using full slice coverage 2D dual refocusing echo acquisition mode (DREAM) and full coverage 3D dual TR approaches further improved measurement precision. BFFE is a more optimal sequence than TSE for phase-based MREPT in brain. Depending on the area of the brain being measured, the scan can be safely accelerated with compressed SENSE without sacrifice of precision, offering the potential to employ MREPT in clinical research and applications. RF shimming with better field mapping further improves precision of the conductivity measures. [ABSTRACT FROM AUTHOR]

This study shows how inter-subject variation over a dataset of 72 head models results in specific absorption rate (SAR) and B 1 + field homogeneity differences using common shim scenarios. MR-CT datasets were used to segment 71 head models into 10 tissue compartments. These head models were affixed to the shoulders and neck of the virtual family Duke model and placed within an 8 channel transmit surface-loop array to simulate the electromagnetic fields of a 7T imaging experiment. Radio frequency (RF) shimming using the Gerchberg-Saxton algorithm and Circularly Polarized shim weights over the entire brain and select slices of each model was simulated. Various SAR metrics and B 1 + maps were calculated to demonstrate the contribution of head variation to transmit inhomogeneity and SAR variability. With varying head geometries the loading for each transmit loop changes as evidenced by changes in S-parameters. The varying shim conditions and head geometries are shown to affect excitation uniformity, spatial distributions of local SAR, and SAR averaging over different pulse sequences. The Gerchberg-Saxton RF shimming algorithm outperforms circularly polarized shimming for all head models. Peak local SAR within the coil most often occurs nearest the coil on the periphery of the body. Shim conditions vary the spatial distribution of SAR. The work gives further support to the need for fast and more subject specific SAR calculations to maintain safety. Local SAR 10g is shown to vary spatially given shim conditions, subject geometry and composition, and position within the coil. • 7T decoupled coil array transmit metrics simulated for 72 head models. • RF coil traces within 2 cm of the subject may exceed tissue heating limits. • Non-linear RF shimming over a population outperforms circularly polarized shims. • Average tissue heating rates differ for each patient depending on the sequence. [ABSTRACT FROM AUTHOR]

Purpose: To develop and evaluate a novel RF shimming optimization strategy tailored to improve the transmit efficiency in turbo spin echo imaging when performing time‐interleaved acquisition of modes (TIAMO) at ultrahigh fields. Theory and Methods: A nonlocalized efficiency shimming cost function is proposed and extended to perform TIAMO using acquisition modes optimized for refocused echoes (AMORE). The nonlocalized efficiency shimming was demonstrated in brain and knee imaging at 7 Tesla. Phantom and in vivo torso imaging studies were performed to compare the performance between AMORE and previously proposed TIAMO mode optimizations with and without localized constraints in turbo spin echo and gradient echo acquisitions. Results: The proposed nonlocalized efficiency RF shimming produced a circularly polarized‐like field with fewer signal dropouts in the brain and knee. For larger targets, AMORE was used and required a significantly lower transmitter voltage to produce a similar contrast to existing TIAMO mode design approaches for turbo spin echo as well as gradient echo acquisitions. In vivo, AMORE effectively reduced signal dropout in the interior torso while providing more uniform contrast with reduced transmit power. A local constraint further improved performance for a target region while maintaining performance in the larger FOV. Conclusion: AMORE based on the presented nonlocalized efficiency shimming cost function demonstrated improved contrast and SNR uniformity as well as increased transmit efficiency for both gradient echo and turbo spin echo acquisitions. [ABSTRACT FROM AUTHOR]

In a multi-element microstrip transmission line (MTL) transmit array coil, the transmit field ( $B_{1}^{+}$ ) distribution is inhomogeneous due to its standing-wave nature, and the interference effects can severely degrade the $B_{1}^{+}$ and imaging. Therefore, to improve the homogeneity and strength of $B_{1}^{+}$ , this study focuses on the development of a multi-element MTL transmit array coil integrated with a dielectric liner (DL) material. Furthermore, the transmission efficiency ( $T_{x,eff}$ ) is improved in the head region. An eight-element MTL transmits array head coil is investigated using thinner DLs, and the optimized dimensions of the DL are found from its resonant mode at 7 Tesla (T). Simulations and measurements are performed with an MTL transmit array coil at 7 T, and the performance is analyzed for the DL positions and dimensions. Remarkably, the proposed DLs-integrated transmit array coil system offered significant improvements in the $T_{x,eff}$ at different DLs positions. Compared to the case without the DL, the $T_{x,eff}$ is improved by 9% (DL close to the head model), 15% (DL at center), and 39% (DL close to the RF coil). Interestingly, the DL acts as an efficiency tuner element exhibiting a 9% to 39% $T_{x,eff}$ tuning range of improvement. In addition, the RF-shimming technique improves the $B_{1}^{+}$ homogeneity and $T_{x,eff}$ of the coil with DLs by 21% and 42%, respectively, compared to the case without the DL. Moreover, the specific absorption rate (SAR) analysis of the MTL transmit array coil with the DLs is performed. The peak 10g-averaged SAR is reduced from 3.1 W/kg to 2.13 W/kg in the head using DLs and RF-shimming technique. Finally, we used the bench measurement setup to obtain the measured magnetic field. The proposed work exhibits salient features with a smaller DL size and multi-element MTL transmit array coil than other proposed works.

Purpose: Compared with lower field strengths, DWI at 7 T faces the combined challenges of increased distortion and blurring due to B 0 inhomogeneity, and increased signal dropouts due to B 1 + inhomogeneity. This study addresses the B 1 + limitations using slice-specific static parallel transmission (pTx) in a multi-shot, readout-segmented EPI diffusion imaging sequence., Methods: DWI was performed in 7 healthy subjects using MRI at 7 T and readout-segmented EPI. Data were acquired with non-pTx circular-polarized (CP) pulses (CP-DWI) and static pTx pulses (pTx-DWI) using slice-specific B 1 + shim coefficients. Each volunteer underwent two scan sessions on the same day, with two runs of each sequence in the first session and one run in the second. The sequences were evaluated by assessing image quality, flip-angle homogeneity, and intrasession and intersession repeatability in ADC estimates., Results: pTx-DWI significantly reduced signal voids compared with CP-DWI, particularly in inferior brain regions. The use of pTx also improved RF uniformity and symmetry across the brain. These effects translated into improved intrasession and intersession repeatability for pTx-DWI. Additionally, re-optimizing the pTx pulse between repeat scans did not have a negative effect on ADC repeatability., Conclusion: The study demonstrates that pTx provides a reproducible image-quality increase in multishot DWI at 7 T. The benefits of pTx also extend to quantitative ADC estimation with regard to the improvement in intrasession and intersession repeatability. Overall, the combination of multishot imaging and pTx can support the development of reliable, high-resolution DWI for clinical studies at 7 T., (© 2024 International Society for Magnetic Resonance in Medicine.)

Purpose: A ratio adjustable power splitter (RAPS) circuit was recently proposed for add‐on RF shimming and array‐compressed parallel transmission. Here we propose a new RAPS circuit design based on off‐the‐shelf components for improved performance and manufacturability. Theory and Methods: The original RAPS used a pair of home‐built Wilkinson splitter and hybrid coupler connected by a pair of connectorized coaxial cables. Here we propose a new hybrid‐pair RAPS (or HP‐RAPS) circuit that replaces the home‐built circuits with two commercially available hybrid couplers and replaces connectorized cables with interchangeable microstrip lines. We derive the relation between the desired splitting ratio and the required phase shifts for HP‐RAPS and investigate how to generate arbitrary splitting ratios using paired meandering and straight lines. Several HP‐RAPSs with different splitting ratios were fabricated and tested on the workbench and MRI experiments. Results: The splitting ratio of an HP‐RAPS circuit has a tan or cot dependence on the meandering line's additional length compared to the straight line. The fabricated HP‐RAPSs exhibit accurate splitting ratios as expected (<4% deviations) and generate transmit fields that well agree with predicted fields. They also demonstrated a low insertion loss of 0.33 dB, high output isolation of −26 dB, and acceptable impedance matching of −16 dB. Conclusion: A novel HP‐RAPS circuit was developed and implemented. It is easy‐to‐fabricate/reproduce with minimal expertise. It also preserves the features of the original RAPS circuit (ratio‐adjustable, small footprint, etc.) with lower insertion loss. [ABSTRACT FROM AUTHOR]

Purpose: Achieving a desired RF transmit field (B1+) in small regions of interest is critical for single‐voxel MRS at ultrahigh field. Radio‐frequency (RF) shimming, using parallel transmission, requires B1+ mapping and optimization, which limits its ease of use. This work aimed to generate calibration‐free RF shims for predefined target regions of interest, which can be applied to any participant, to produce a desired absolute magnitude B1+ (|B1+|). Methods: The RF shims were found offline by joint optimization on a database comprising B1+ maps from 11 subjects, considering regions of interest in occipital cortex, hippocampus and posterior cingulate, as well as whole brain. The |B1+| achieved was compared with a tailored shimming approach, and MR spectra were acquired using tailored and calibration‐free shims in 4 participants. Global and local 10g specific‐absorption‐rate deposition were estimated using Duke and Ella dielectric models. Results: There was no difference in the mean |B1+| produced using calibration‐free versus tailored RF shimming in the occipital cortex (p =.15), hippocampus (p =.5), or posterior cingulate (p =.98), although differences were observed in the RMS error |B1+|. Spectra acquired using calibration‐free shims had similar SNR and low residual water signal. Under identical power settings, specific‐absorption‐rate deposition was lower compared with operating in quadrature mode. For example, the total head specific absorption rate was around 35% less for the occipital cortex. Conclusion: This work demonstrates that static RF shims, optimized offline for small regions, avoid the need for B1+ mapping and optimization for each region of interest and participant. Furthermore, power settings may be increased when using calibration‐free shims, to better take advantage of RF shimming. [ABSTRACT FROM AUTHOR]

Purpose: With increased interest in parallel transmission in ultrahigh‐field MRI, methods are needed to correctly calculate the S‐parameters and complex field maps of the parallel transmission coil. We present S‐parameters paired with spatial field optimization to fully simulate a double‐row 16‐element transceiver array for brain MRI at 7 T. Methods: We implemented a closed‐form equation of the coil S‐parameters and overall spatial B1+ field. We minimized a cost function, consisting of coil S‐parameters and the B1+ homogeneity in brain tissue, by optimizing transceiver components, including matching, decoupling circuits, and lumped capacitors. With this, we are able to compare the in silico results determined with and without B1+ homogeneity weighting. Using the known voltage range from the host console, we reconstructed the B1+ maps of the array and performed RF shimming with four realistic head models. Results: As performed with B1+ homogeneity weighting, the optimized coil circuit components were highly consistent over the four heads, producing well‐tuned, matched, and decoupled coils. The mean peak forward powers and B1+ statistics for the head models are consistent with in vivo human results (N = 8). There are systematic differences in the transceiver components as optimized with or without B1+ homogeneity weighting, resulting in an improvement of 28.4 ± 7.5% in B1+ homogeneity with a small 1.9 ± 1.5% decline in power efficiency. Conclusion: This co‐simulation methodology accurately simulates the transceiver, predicting consistent S‐parameters, component values, and B1+ field. The RF shimming of the calculated field maps match the in vivo performance. [ABSTRACT FROM AUTHOR]

Purpose: To provide transmit whole‐brain coverage at 9.4 T using an array with only eight elements and improve the specific absorption rate (SAR) performance, a novel dipole array was developed, constructed, and tested. Methods: The array consists of eight optimized bent folded‐end dipole antennas circumscribing a head. Due to the asymmetrical shape of the dipoles (bending and folding) and the presence of an RF shield near the folded portion, the array simultaneously excites two modes: a circular polarized mode of the array itself, and the TE mode ("dielectric resonance") of the human head. Mode mixing can be controlled by changing the length of the folded portion. Due to this mixing, the new dipole array improves longitudinal coverage as compared with unfolded dipoles. By optimizing the length of the folded portion, we can also minimize the peak local SAR (pSAR) value and decouple adjacent dipole elements. Results: The new array improves the SEE (< B1+>/√pSAR) value by about 50%, as compared with the unfolded bent dipole array. It also provides better whole‐brain coverage compared with common single‐row eight‐element dipole arrays, or even to a more complex double‐row 16‐element surface loop array. Conclusion: In general, we demonstrate that rather than compensating for the constructive interference effect using additional hardware, we can use the "dielectric resonance" to improve coverage, transmit field homogeneity, and SAR efficiency. Overall, this design approach not only improves the transmit performance in terms of the coverage and SAR, but substantially simplifies the common surface loop array design, making it more robust, and therefore safer. [ABSTRACT FROM AUTHOR]

Purpose: To improve the labeling efficiency of pseudo‐continuous arterial spin labeling (PCASL) at 7T using parallel transmission (pTx). Methods: Five healthy subjects were scanned on an 8‐channel‐transmit 7T human MRI scanner. Time‐of‐flight (TOF) angiography was acquired to identify regions of interest (ROIs) around the 4 major feeding arteries to the brain, and B1+ and B0 maps were acquired in the labeling plane for tagging pulse design. Complex weights of the labeling pulses for each of the 8 transmit channels were calculated to produce a homogenous radiofrequency (RF) ‐shimmed labeling across the ROIs. Variable‐Rate Selective Excitation (VERSE) pulses were also implemented as a part of the labeling pulse train. Whole‐brain perfusion‐weighted images were acquired under conditions of RF shimming, VERSE with RF shimming, and standard circularly polarized (CP) mode. The same subjects were scanned on a 3T scanner for comparison. Results: In simulation, VERSE with RF shimming improved the flip‐angles across the ROIs in the labeling plane by 90% compared with CP mode. VERSE with RF shimming improved the temporal signal‐to‐noise ratio by 375% compared with CP mode, but did not outperform a matched 3T sequence with a matched flip‐angle. Conclusion: We have demonstrated improved PCASL tagging at 7T using VERSE with RF shimming on a commercial head coil under conservative SAR limits at 7T. However, improvements of 7T over 3T may require strategies with less conservative SAR restrictions. [ABSTRACT FROM AUTHOR]

To investigate velocity encoded and velocity compensated variants of multi-spoke RF pulses that can be used for flip-angle homogenization at ultra-high fields (UHF). Attention is paid to the velocity encoding for each individual spoke pulse and to displacement artifacts that arise in Fourier transform imaging in the presence of flow. A gradient waveform design for multi-spoke excitation providing an algorithm for minimal TE was proposed that allows two different encodings. Such schemes were compared to an encoding approach that applies an established scheme to multi-spoke excitations. The impact on image quality and quantitative velocity maps was evaluated in phantoms using single- and two-spoke excitations. Additional validation measurements were obtained in-vivo at 7 T. Phantom experiments showed that keeping the first gradient moment constant for all k-space lines eliminates any displacements in phase-encoding and slice-selection direction for all spoke pulses but leads to artifacts for non-zero velocity components along readout direction. Introducing variable but well-defined first gradient moments in the phase-encoding direction creates displacements along the velocity vector and thus minimizes velocity-induced geometrical distortions. Phase-resolved mean volume flow in the ascending and descending aorta obtained from two-spoke excitation showed excellent agreement with single-spoke excitation over the cardiac cycle (mean difference 0.8 ± 16.2 ml/s). The use of single- and multi-spoke RF pulses for flow quantification at 7 T with controlled displacement artifacts has been successfully demonstrated. The presented techniques form the basis for correct velocity quantification and compensation not only for conventional but also for multi-spoke RF pulses allowing in-plane B 1 + homogenization using parallel transmission at UHF. [ABSTRACT FROM AUTHOR]

A dedicated, novel, multichannel transmit/receive (Tx/Rx) coil array was designed, simulated, and tested for cardiac magnetic resonance imaging (cMRI) of pigs at 7 T. The cardiac array was composed of 16 elements with physically independent anterior and posterior parts. The array was connected to the MRI scanner in the parallel transmit (pTx) mode forming an 8-channel transmit and 16-channel receive (8Tx/16Rx) coil configuration. The anterior array was composed of four central octagonal elements, which were decoupled using a common central ring (CCR) and shared decoupling capacitors. The surrounding four triangular elements were decoupled from the central four elements using gaps and capacitive decoupling. In total, the eight elements of the anterior array resembled a coil with a symmetrical circular shape. Due to the advantageous arrangement of the elements in conjunction with the novel decoupling method of a CCR, the coupling between the adjacent loop elements was minimized. The posterior array was built using a $4 \times 2$ standard rectangular, symmetric element configuration. The dedicated cardiac array was tested in phantom and in ex vivo MR measurements with a female pig of 46 kg. The dedicated coil array enhanced the signal-to-noise ratio (40 ± 27) in ex vivo measurements of the pig heart by about three times compared to a coil prototype for cardiac human imaging (11.8 ± 7). High-resolution, ex vivo cardiac images were acquired with an in-plane resolution of up to 0.3 mm $\times0.3$ mm using the dedicated pig coil array. The novel cardiac array supports parallel imaging with an acceleration factor (R) of 4 without increasing the mean $g$ -factor within the heart region beyond 1.15. [ABSTRACT FROM AUTHOR]

• The antisymmetric array enhanced the SNR by about two-fold compared to the rectilinear array design. • The novel design demonstrated good decoupling between all neighboring elements of the entire array. • Improved control of the B 1 + field distributions using RF shimming was demonstrated in EM-simulations. • The antisymmetric array enabled the application of GRAPPA acceleration factor R = 4 with mean g-factor ± SD of only 1.13 ± 0.07. • High resolution ex-vivo images were acquired using standard clinical protocols (in-plane resolution = 0.3mm × 0.3mm). The design, simulation, assembly and testing of a novel dedicated antisymmetric transmit/receive (Tx/Rx) coil array to demonstrate the feasibility of cardiac magnetic resonance imaging (cMRI) in pigs at 7 T was described. The novel antisymmetric array is composed of eight elements based on mirrored and reversed loop orientations to generate varying B 1 + field harmonics for RF shimming. The central four loop elements formed together a pair of antisymmetric L-shaped channels to allow good decoupling between all neighboring elements of the entire array. The antisymmetric array was compared to a standard symmetric rectilinear loop array with an identical housing dimension. Both arrays were driven in the parallel transmit (pTx) mode forming an 8-channel transmit and 16-channel receive (8Tx/16Rx) coil array, where the same posterior array was combined with both anterior arrays. The hardware and imaging performance of the dedicated cardiac arrays were validated and compared by means of electromagnetic (EM) simulations, bench-top measurements, phantom, and ex-vivo MRI experiments with 46 kg female pig. Combined signal-to-noise ratio (SNR), geometry factor (g-factor), noise correlation maps, and high resolution ex-vivo cardiac images were acquired with an in-plane resolution of 0.3 mm × 0.3 mm using both arrays. The novel antisymmetric array enhanced the SNR within the heart by about two times and demonstrated good decoupling and improved control of the B 1 + field distributions for RF shimming compared to the standard coil array. Parallel imaging with acceleration factor (R) up to 4 was possible using the novel antisymmetric coil array while maintaining the mean g-factor within the heart region of 1.13. [ABSTRACT FROM AUTHOR]

The Human Connectome Project (HCP) has a 7T component that aims to study the human brain's organization and function with high spatial and temporal resolution fMRI and diffusion-weighted acquisitions. For whole brain applications at 7T, a major weakness however remains the heterogeneity of the radiofrequency transmission field ( B 1 + ), which prevents from achieving an optimal signal and contrast homogeneously throughout the brain. In this work, we use parallel transmission (pTX) Universal Pulses (UP) to improve the flip angle homogeneity and demonstrate their application to highly accelerated multi-band EPI (MB5 and GRAPPA2, as prescribed in the 7T HCP protocol) sequence, but also to acquire at 7T B 1 + -artefact-free T 1 - and T 2 -weighted anatomical scans used in the pre-processing pipeline of the HCP protocol. As compared to typical implementations of pTX, the proposed solution is fully operator-independent and allows "plug and play" exploitation of the benefits offered by multi-channel transmission. Validation in five healthy adults shows that the proposed technique achieves a flip angle homogeneity comparable to that of a clinical 3 T system. Compared to standard single-channel transmission, the use of UPs at 7T yielded up to a two-fold increase of the temporal signal-to-noise ratio in the temporal lobes as well as improved detection of functional connectivity in the brain regions most strongly affected by B 1 + inhomogeneity. [ABSTRACT FROM AUTHOR]

Purpose: Parallel transmission techniques in MRI have the potential to improve the image quality near metal implants at 3 T. However, current testing of implants only evaluates the risk of radiofrequency (RF) heating in phantoms in circularly polarized mode. We investigate the influence of changing the transmission settings in a 2‐channel body coil on the peak temperature near 2 CoCrMo hip prostheses, using adaptive specific absorption rate (SAR) as an estimate of RF heating. Methods: Adaptive SAR is a SAR averaging method that is optimized to correlate with thermal simulations and limit the temperature to 39°C near hip implants. The simulated peak temperature was compared when using whole‐body SAR, SAR10g, and adaptive SAR as a constraint for the maximum allowed input power. Adaptive SAR was used as a fast estimate of temperature to evaluate the trade‐off between good image quality and low heating near the hip implants. Electromagnetic simulations were validated by simulating and measuring B1 maps and electric fields in a phantom at 3 T. Results: Simulations and measurements showed excellent agreement. Limiting whole‐body SAR to 2 W/kg and SAR10g to 10 W/kg resulted in temperatures up to 49.3°C and 40.7°C near the hip implants after 30 minutes of RF exposure, respectively. Predictions based on adaptive SAR limited the temperature to 39°C, and allowed to improve the B1 field distribution while preventing peak temperatures near the hip implants. Conclusion: Significant RF heating can occur at 3 T near hip implants when parallel transmission is used. Adaptive SAR can be integrated in RF shimming algorithms to improve the uniformity and reduce heating. [ABSTRACT FROM AUTHOR]

Purpose: Neurovascular MRI suffers from a rapid drop in B 1 + into the neck when using transmit head coils at 7 T. One solution to improving B 1 + magnitude in the major feeding arteries in the neck is to use custom RF shims on parallel-transmit head coils. However, calculating such shims requires robust multichannel B 1 sensitivity. We therefore sought a robust, large-dynamic-range, parallel-transmit field mapping protocol and tested whether RF shimming can improve carotid artery B + maps in both the head and the neck, which is challenging due to low RF penetration into the neck, limited dynamic range of multichannel B 1 + mapping techniques, and B 0 sensitivity. We therefore sought a robust, large-dynamic-range, parallel-transmit field mapping protocol and tested whether RF shimming can improve carotid artery B 1 + magnitude in practice., Methods: A pipeline is presented that combines B 1 + for multiple random transmit shims, and by assessing the ability of RF shimming to increase B 1 + for multiple random transmit shims, and by assessing the ability of RF shimming to increase B 1 + in the carotid arteries., Results: The proposed method achieved good agreement between predicted and measured B 1 + in both the head and the neck. The B 1 + magnitude in the carotid arteries can be increased by 43% using tailored RF shims or by 37% using universal RF shims, while also improving the RF homogeneity compared with CP mode., Conclusion: B 1 + in the neck can be increased using RF shims calculated from multichannel B 1 + maps in both the head and the neck. This can be achieved using universal phase-only RF shims, facilitating easy implementation in existing sequences., (© 2023 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.)

Purpose: To obviate online slice‐by‐slice RF shim optimization and reduce B 1 + mapping requirements for patient‐specific RF shimming in high‐field magnetic resonance imaging. Theory and Methods: RF Shim Prediction by Iteratively Projected Ridge Regression (PIPRR) predicts patient‐specific, SAR‐efficient RF shims with a machine learning approach that merges learning with training shim design. To evaluate it, a set of B 1 + maps was simulated for 100 human heads for a 24‐element coil at 7T. Features were derived from tissue masks and the DC Fourier coefficients of the coils’ B 1 + maps in each slice, which were used for kernelized ridge regression prediction of SAR‐efficient RF shim weights. Predicted shims were compared to directly designed shims, circularly polarized mode, and nearest‐neighbor shims predicted using the same features. Results: PIPRR predictions had 87% and 13% lower B 1 + coefficients of variation compared to circularly polarized mode and nearest‐neighbor shims, respectively, and achieved homogeneity and SAR similar to that of directly designed shims. Predictions were calculated in 4.92 ms on average. Conclusion: PIPRR predicted uniform, SAR‐efficient RF shims, and could save a large amount of B 1 + mapping and computation time in RF‐shimmed ultra‐high field magnetic resonance imaging. [ABSTRACT FROM AUTHOR]

Purpose: Image quality obtained for brain imaging at 7T can be hampered by inhomogeneities in the static magnetic field, B0, and the RF electromagnetic field, B1. In imaging sequences such as fluid‐attenuated inversion recovery (FLAIR), which is used to assess neurological disorders, these inhomogeneities cause spatial variations in signal that can reduce clinical efficacy. In this work, we aim to correct for signal inhomogeneities to ensure whole‐brain coverage with 3D FLAIR at 7T. Methods: The direct signal control (DSC) framework was used to optimize channel weightings applied to the 8 transmit channels used in this work on a pulse‐by‐pulse basis through the echo train in the FLAIR sequences. 3D FLAIR brain images were acquired on 5 different subjects and compared with imaging using a quadrature‐like mode of the transmit array. Precomputed “universal” DSC solutions calculated from a separate set of 5 subjects were also explored. Results: DSC consistently enabled improved imaging across all subjects, with no dropouts in signal seen over the entire brain volume, which contrasted with imaging in quadrature mode. Further, the universal DSC solutions also consistently improved imaging despite not being optimized specifically for the subject being imaged. Conclusion: 3D FLAIR brain imaging at 7T is substantially improved using DSC and is able to recover regions of low signal without increasing imaging time or interecho spacing. [ABSTRACT FROM AUTHOR]

Purpose: To design and evaluate an 8/15‐channel transmit/receive (Tx/Rx) head‐neck RF coil combination with region‐specific B 1 + shimming for whole‐brain MRI with focus on improved functional MRI of the cerebellum at 7 T. Methods: An 8‐channel transceiver RF head coil was combined with a 7‐channel receive‐only array. The noise parameters and acceleration capabilities of this 8Tx/15Rx coil setup were compared with a commercially available 1Tx/32Rx RF head coil. Region‐specific 8‐channel B 1 + shimming was applied when using the 8Tx/15Rx RF coil. To evaluate the capability for functional MRI of the cerebellum, temporal SNR and statistical nonparametric maps for finger‐tapping experiments with 14 healthy subjects were derived by applying a variable slice thickness gradient‐echo echo‐planar functional MRI sequence. Results: The 8Tx/15Rx setup had a lower maximum noise correlation between channels, but higher average correlations compared with the 1Tx/32Rx coil. Both RF coils exhibited identical g‐factors in the cerebellum with R = 3 acceleration. The enlarged FOV of the 8Tx/15Rx coil in combination with region‐specific B 1 + shimming increased homogeneity of the transmission field and temporal SNR in caudal cerebellar regions. Temporal SNR losses in cranial parts were reduced, resulting in more highly significant voxels in the caudally activated areas and identical patterns in the cranial cerebellar parts during a finger‐tapping task. Conclusion: Compared with the 1Tx/32Rx RF coil, the presented 8Tx/15Rx RF coil combination successfully improves functional MRI of the human cerebellum at 7 T while maintaining whole‐brain coverage. A clear temporal SNR gain in caudal cerebellar regions is shown. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this study was to evaluate the effect of integrated high‐permittivity materials (HPMs) on excitation homogeneity and global specific absorption rate (SAR) for transmit arrays at 7T. Methods: A rapid electrodynamic simulation framework was used to calculate L‐curves associated with excitation of a uniform 2D profile in a dielectric sphere. We used ultimate intrinsic SAR as an absolute performance reference to compare different transmit arrays in the presence and absence of a layer of HPM. We investigated the optimal permittivity for the HPM as a function of its thickness, the sample size, and the number of array elements. Results: Adding a layer of HPM can improve the performance of a 24‐element array to match that of a 48‐element array without HPM, whereas a 48‐element array with HPM can perform as well as a 64‐element array without HPM. Optimal relative permittivity values changed based on sample and coil geometry, but were always within a range obtainable with readily available materials (εr = 100–200). Conclusion: Integration of HPMs could be a practical method to improve RF shimming performance, alternative to increasing the number of coils. The proposed simulation framework could be used to explore the design of novel transmit arrays for head imaging at ultra‐high field strength. Magn Reson Med 80:391–399, 2018. © 2017 International Society for Magnetic Resonance in Medicine. [ABSTRACT FROM AUTHOR]

Purpose: Current methods for mitigation of transmit field B 1 + inhomogeneities at ultrahigh field (UHF) MRI by multi‐channel radiofrequency (RF) shimming rely on accurate B 1 + mapping. This can be time consuming when many RF channels have to be mapped for in vivo body MRI, where the B1 maps should ideally be acquired within a single breath‐hold. Therefore, a new B 1 + mapping technique (B1TIAMO) is proposed. Methods: The performance of this technique is validated against an established method (DREAM) in phantom measurements for a cylindrical head phantom with an 8‐channel transmit/receive (Tx/Rx) array. Furthermore, measurements for a 32‐channel Tx/Rx remote array are conducted in a large body phantom and the | B 1 + | map reliability is validated against simulations of the transmit RF field distribution. Finally, in vivo results of this new mapping technique for human abdomen are presented. Results: For the head phantom (8‐channel Tx/Rx coil), the single | B 1 + | comparison between B1TIAMO, the direct DREAM measurements, and simulation data showed good agreement with 10–19% difference. For the large body phantom (32‐channel Tx/Rx coil), B1TIAMO matched the RF field simulations well. Conclusion: The results demonstrate the potential to acquire 32 accurate single‐channel B 1 + maps for large field‐of‐view body imaging within only a single breath‐hold of 16 s at 7T UHF MRI. Magn Reson Med 79:2652–2664, 2018. © 2017 International Society for Magnetic Resonance in Medicine. [ABSTRACT FROM AUTHOR]

B 1 + inhomogeneity in the human body increases as the nuclear magnetic resonance (NMR) frequency increases. Various methods have thus been developed to reduce B 1 + inhomogeneity, such as a dielectric pad, a coupling coil, parallel transmit, and radio-frequency (RF) shimming. However, B 1 + inhomogeneity still remains in some cases of abdominal imaging. In this study, we developed a B 1 -control receive array coil (B-RAC). Unlike the conventional receive array coil, B-RAC reduces B 1 + inhomogeneity by using additional PIN diodes to generate the inductive loop during the RF transmit period. The inductive loop can generate dense and sparse regions of the magnetic flux, which can be used to compensate for B 1 + inhomogeneity. First, B-RAC is modeled in the numerical simulation, and the spatial distributions of B 1 + in a phantom and a human model were analyzed. Next, we fabricated a 12-channel B-RAC and measured receive sensitivity and B 1 + maps in a 3T-MRI experiment. It was demonstrated that B-RAC can reduce B 1 + inhomogeneity in the phantom and human model without increasing the maximum local specific absorption rate (SAR) in the body. B-RAC was also found to have almost the same the receive sensitivity as the conventional receive coil. Using RF shimming combined with B-RAC was revealed to more effectively reduce B 1 + inhomogeneity than using only RF shimming. Therefore, B-RAC can reduce B 1 + inhomogeneity while maintaining the receive sensitivity. [ABSTRACT FROM AUTHOR]

A remote transmission and detection for magnetic resonance imaging (MRI) at 7 T was validated by using a patch antenna that excites the traveling-wave (TW) modes guided by the radio frequency (RF) shield of the MRI system. In this paper, RF simulations were performed at 298 MHz for an eight-element stepped impedance resonators (SIRs) head volume coil combined with a patch antenna. The combined structure is loaded with both a cylindrical phantom and the Duke full-body human voxel model. An optimization routine was developed in MATLAB to provide homogenous B1^{+} field distributions with minimal 10 g averaged specific absorption rate (SAR) values within the phantom and the Duke human biological model. Before optimization, the TW approach achieved a more homogenous B1^{+} field distribution providing a large field of view along the propagation direction compared to the SIRs head volume coil. However, the corresponding peak local SAR values for the TW approach is about 2.4 times higher than those of the SIRs head volume coil. The B1^{+} transmission efficiency for the TW approach is higher than that of the SIRs head volume coil by 10–30% due to the coupling of the TW modes into the volume coil. The RF-shimming technique improves the B1^{+} field homogeneity for the combined approach by 42%, 48%, and 47% in the central axial, sagittal, and coronal slices, respectively, and satisfies the constraints of maximum local SAR in the human head for MRI at 7 T. [ABSTRACT FROM PUBLISHER]

Purpose Elongated implanted conductors can interact with the radiofrequency (RF) transmission field during MRI, posing safety concerns of excessive heating in patients with deep brain stimulators. A technique using parallel RF transmission (pTx) is evaluated on an anthropomorphic heterogeneous model with bilateral and unilateral curved wires. Methods Amplitude and phase were optimized by simulation to minimize heating surrounding the implanted wires and to minimize [ABSTRACT FROM AUTHOR]

Purpose The inhomogeneity of flip angle distribution is a major challenge impeding the application of high-field MRI. We report a method combining spatially selective excitation using generalized spatial encoding magnetic fields (SAGS) with radiofrequency (RF) shimming to achieve homogeneous excitation. This method can be an alternative approach to address the challenge of [ABSTRACT FROM AUTHOR]

To prospectively compare image quality and myocardial T1 relaxation times of modified Look-Locker inversion recovery (MOLLI) imaging at 3.0 T (T) acquired with patient-adaptive dual-source (DS) and conventional single-source (SS) radiofrequency (RF) transmission. Pre- and post-contrast MOLLI T1 mapping using SS and DS was acquired in 27 patients. Patient wise and segment wise analysis of T1 times was performed. The correlation of DS MOLLI measurements with a reference spin echo sequence was analysed in phantom experiments. DS MOLLI imaging reduced T1 standard deviation in 14 out of 16 myocardial segments (87.5%). Significant reduction of T1 variance could be obtained in 7 segments (43.8%). DS significantly reduced myocardial T1 variance in 16 out of 25 patients (64.0%). With conventional RF transmission, dielectric shading artefacts occurred in six patients causing diagnostic uncertainty. No according artefacts were found on DS images. DS image findings were in accordance with conventional T1 mapping and late gadolinium enhancement (LGE) imaging. Phantom experiments demonstrated good correlation of myocardial T1 time between DS MOLLI and spin echo imaging. Dual-source RF transmission enhances myocardial T1 homogeneity in MOLLI imaging at 3.0 T. The reduction of signal inhomogeneities and artefacts due to dielectric shading is likely to enhance diagnostic confidence. [ABSTRACT FROM AUTHOR]

Cardiac magnetic resonance imaging (MRI) at high field presents challenges because of the high specific absorption rate and significant transmit field ( B1+) inhomogeneities. Parallel transmission MRI offers the ability to correct for both issues at the level of individual radiofrequency (RF) pulses, but must operate within strict hardware and safety constraints. The constraints are themselves affected by sequence parameters, such as the RF pulse duration and TR, meaning that an overall optimal operating point exists for a given sequence. This work seeks to obtain optimal performance by performing a 'sequence-level' optimization in which pulse sequence parameters are included as part of an RF shimming calculation. The method is applied to balanced steady-state free precession cardiac MRI with the objective of minimizing TR, hence reducing the imaging duration. Results are demonstrated using an eight-channel parallel transmit system operating at 3 T, with an in vivo study carried out on seven male subjects of varying body mass index (BMI). Compared with single-channel operation, a mean-squared-error shimming approach leads to reduced imaging durations of 32 ± 3% with simultaneous improvement in flip angle homogeneity of 32 ± 8% within the myocardium. [ABSTRACT FROM AUTHOR]

A new target-field approach to generating uniform radio frequency (RF) fields within the human body for high field MRI is described. The method involves producing a set of external fields which, after interaction with a dielectric object, superimpose to produce a traveling plane wave, exposing all spins to the same RF amplitude (B1) over a cycle of the harmonic field. Conceptually this is similar to conventional RF shimming, but uses a different RF source design, input data, and objective function. The method requires a detailed knowledge of the coupling between exterior field modes, produced by an array of RF sources, and field modes within the body. Given an estimate of the coupling matrix, the linear superposition of external modes that produces a desired internal target field can be determined. The new method is termed Traveling Internal Plane-wave Synthesis (TIPS). A simple design of a coil array is described that can, in principle, generate the required field modes. Simulations demonstrate that radio frequency magnetic fields of nearly uniform (< 1% variation) magnitude can be produced within dielectric objects larger than a wavelength in size. If the dielectric medium has non-zero conductivity, traveling waves are attenuated as they traverse the object, but field uniformity within planar slices is preserved. For general 3D imaging, a superposition of plane waves can provide field focusing to balance conductive losses, thereby achieving nearly uniform-magnitude B1+ magnetic fields over a volume of interest. [ABSTRACT FROM AUTHOR]

Background Magnetic resonance imaging (MRI) using a 3-T MRI scanner is now widely used for clinical examinations. However, B1 inhomogeneity becomes larger with MRI scanners using 3-T and higher. It especially becomes a problem in the breast. To improve B1 homogeneity, a RF shimming technique has been developed. Purpose To evaluate the B1 homogeneity of breast MRI using RF shimming with individual specific values for subjects after mastectomy. Material and Methods The subjects are healthy female volunteers who underwent normal breast imaging, followed by imaging of one breast while the other breast was bound tightly to the chest by bleached cotton cloths (simulating volunteers after mastectomy). B1 mappings were performed with RF shimming using two techniques: (i) optimized fixed value; and (ii) individual specific values using a 3-T MRI scanner. The means and standard deviations of the B1 maps for all slices in the breast were measured and compared between the fixed value and the individual specific value cases. Results For normal volunteers, the breast B1 variation was not statistically significantly different between the RF shimming techniques. For volunteers after simulated surgery, the breast B1 variation was (1.02 ± 0.29) with the fixed value and (0.98 ± 0.22) with the individual specific value ( P < 0.01). With the individual specific optimization, B1 variation for all slices in the breast was improved for volunteers after simulated surgery. Conclusion RF shimming with individual specific values has the potential to improve the B1 homogeneity of breast MRI in patients after mastectomy. [ABSTRACT FROM AUTHOR]

Purpose To design array-compressed parallel transmit radiofrequency (RF) pulses and compare them to pulses designed with existing transmit array compression strategies. Theory and Methods Array-compressed parallel RF pulse design is proposed as the joint optimization of a matrix of complex-valued compression weights that relate a full-channel physical array to a reduced-channel virtual array, along with a set of RF pulses for the virtual array. In this way, the physics of the RF pulse application determine the coil combination weights. Array-compressed pulse design algorithms are described for four parallel transmit applications: accelerated two-dimensional spiral excitation, multislice RF shimming, small-tip-angle kT-points excitation, and slice-selective spokes refocusing. Array-compressed designs are compared in simulations and an experiment to pulses designed using four existing array compression strategies. Results In all cases, array-compressed pulses achieved the lowest root-mean-square excitation error among the array compression approaches. Low errors were generally achieved without increasing root-mean-square RF amplitudes or maximum local 10-gram specific absorption rate. Leave-one-out multisubject shimming simulations demonstrated that array-compressed RF shimming can identify useful fixed coil combination weights that perform well across a population. Conclusion Array-compressed pulse design jointly identifies the transmit coil array compression weights and RF pulses that perform best for a specific parallel excitation application. Magn Reson Med 76:1158-1169, 2016. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The development of MRI systems operating at or above 7 T has provided researchers with a new window into the human body, yielding improved imaging speed, resolution and signal-to-noise ratio. In order to fully realise the potential of ultrahigh-field MRI, a range of technical hurdles must be overcome. The non-uniformity of the transmit field is one of such issues, as it leads to non-uniform images with spatially varying contrast. Parallel transmission (i.e. the use of multiple independent transmission channels) provides previously unavailable degrees of freedom that allow full spatial and temporal control of the radiofrequency (RF) fields. This review discusses the many ways in which these degrees of freedom can be used, ranging from making more uniform transmit fields to the design of subject-tailored RF pulses for both uniform excitation and spatial selection, and also the control of the specific absorption rate. © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

Ultra-high field MRI is an area of great interest for clinical research and basic science due to the increased signal-to-noise, spatial resolution and magnetic-susceptibility-based contrast. However, the fact that the electromagnetic wavelength in tissue is comparable to the relevant body dimensions means that the uniformity of the excitation field is much poorer than at lower field strengths. In addition to techniques such as transmit arrays, one simple but effective method to counteract this effect is to use high permittivity “pads”. Very high permittivities enable thinner, flexible pads to be used, but the limiting factor is wavelength effects within the pads themselves, which can lead to image artifacts. So far, all studies have used simple continuous rectangular/circular pad geometries. In this work we investigate how the wavelength effects can be partially mitigated utilizing shaped pad with holes. Several arrangements have been simulated, including low order pre-fractal geometries, which maintain the overall coverage of the pad, but can provide better image homogeneity in the region of interest or higher sensitivity depending on the setup. Experimental data in the form of in vivo human images at 7T were acquired to validate the simulation results. [ABSTRACT FROM AUTHOR]

The progression to 7 Tesla (7 T) magnetic resonance imaging (MRI) yields promises of substantial increase in signal-to-noise (SNR) ratio. This increase can be traded off to increase image spatial resolution or to decrease acquisition time. However, renal 7 T MRI remains challenging due to inhomogeneity of the radiofrequency field and due to specific absorption rate (SAR) constraints. A number of studies has been published in the field of renal 7 T imaging. While the focus initially was on anatomic imaging and renal MR angiography, later studies have explored renal functional imaging. Although anatomic imaging remains somewhat limited by inhomogeneous excitation and SAR constraints, functional imaging results are promising. The increased SNR at 7 T has been particularly advantageous for blood oxygen level-dependent and arterial spin labelling MRI, as well as sodium MR imaging, thanks to changes in field-strength-dependent magnetic properties. Here, we provide an overview of the currently available literature on renal 7 T MRI. In addition, we provide a brief overview of challenges and opportunities in renal 7 T MR imaging. [ABSTRACT FROM AUTHOR]

Purpose To show the effect, efficiency, and image quality improvements achievable by Dual Refocusing Echo Acquisition Mode (DREAM)-based [ABSTRACT FROM AUTHOR]

Objective: In microstrip transmission line (MTL) transmit/receive (transceive) arrays used for ultrahigh field MRI, the array length is often constrained by the required resonant frequency, limiting the image coverage. The purpose of this study is to increase the imaging coverage and also improve its parallel imaging capability by utilizing a double-row design. Methods: A 16-channel double-row MTL transceive array was designed, constructed, and tested for human head imaging at 7 T. Array elements between two rows were decoupled by using the induced current elimination or magnetic wall decoupling technique. In vivo human head images were acquired, and g-factor results were calculated to evaluate the performance of this double-row array. Results: Testing results showed that all coil elements were well decoupled with a better than –18 dB transmission coefficient between any two elements. The double-row array improves the imaging quality of the lower portion of the human head, and has low g-factors even at high acceleration rates. Conclusion: Compared with a regular single-row MTL array, the double-row array demonstrated a larger imaging coverage along the z-direction with improved parallel imaging capability. Significance: The proposed technique is particularly suitable for the design of large-sized transceive arrays with large channel counts, which ultimately benefits the imaging performance in human MRI. [ABSTRACT FROM AUTHOR]

Purpose: To improve image contrast and B1+ field homogeneity in 3 Tesla (T) breast MR.Materials and Methods: Two practical B1+ shimming methods for 3T breast MR are presented; low-cost passive shimming using local pads of high dielectric permittivity (εr from 0 to 100), and two-channel radiofrequency (RF) shimming (adjusting Q-I amplitude ratios and phase differences of 0 to -4 dB and 90 to 45 degrees), as well as a combination of both methods. The technique has been studied both in simulation using a numerical body model with added mammary tissue and in vivo in six subjects.Results: Large improvements are observed with both methods, leading to a decrease in left-right B1+ asymmetry ratio of 1.24 to 1.00 (simulation) and from 1.26 to 1.01 (in vivo). RF safety was not adversely affected.Conclusion: Both RF shimming and dielectric shimming were shown to improve inhomogeneity in the B1+ field in 3T breast MR. [ABSTRACT FROM AUTHOR]

Object: In the present study the performance of the dual refocusing echo acquisition mode (DREAM) B mapping sequence is evaluated for RF shimming in the abdomen at 3 T and validated against existing RF shim technology. Materials and methods: In vivo experiments were performed on 19 normal volunteers using a clinical 3 T dual channel MRI system. For each volunteer three different B mapping techniques [DREAM, actual flip angle imaging (AFI) and saturated double angle method (SDAM)] were employed for RF shimming of the liver and to subsequently assess the quality of the obtained RF shim settings in terms of the achieved B homogeneity and accuracy of the mean B. Results: DREAM-based B calibration led to an average homogeneity improvement of 39.1 % (AFI = 38.7 %, SDAM = 38.1 %) and a mean B of 90.9 % of the prescribed B (AFI = 88.9 %, SDAM = 92.0 %). The duration of the B calibration scan was reduced from 30 s (AFI) and 15 s (SDAM) to 2.5 s (DREAM). Conclusion: DREAM accelerates RF shimming of the liver by an order of magnitude without compromising RF shimming performance. [ABSTRACT FROM AUTHOR]

Purpose To evaluate the use of the recently proposed ultrafast B1+ mapping approach DREAM (Dual Refocusing Echo Acquisition Mode) for a refinement of patient adaptive radiofrequency (RF) shimming. Materials and Methods Volumetric DREAM B1+ calibration scans centered in the upper abdomen were acquired in 20 patients and three volunteers with written informed consent at a clinical dual source 3 Tesla (T) MR system. Based on these data, RF transmit settings were optimized by central-slice based RF-shimming (CS-RF shim) and by a refined, multi-slice adaptive approach (MS-RF shim). Simulations were performed to compare flip angle accuracy and B1+ homogeneity (cv = stddev/mean) achieved by CS-RF shim versus MS-RF shim for transversal and coronal slices, and for volume shimming on the spine. Results By MS-RF shim, mean deviation from nominal flip angle was reduced to less than 11% in all slices, all targets, and all subjects. Relative improvements in B1+ cv (MS-RF shim versus CS-RF) were up to 14%/39%/47% in transversal slices/coronal slices/ spine area. Conclusion Volumetric information about B1+ can be used to further improve the accuracy and homogeneity of the B1+ field yielding higher diagnostic confidence, and will also be of value for various quantitative methods which are sensitive to flip angle imperfections. J. Magn. Reson. Imaging 2014;40:857-863. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Objective: To provide a numerical and experimental investigation of the static RF shimming capabilities in the human brain at 9.4 T using a dual-row transmit array. Materials and methods: A detailed numerical model of an existing 16-channel, inductively decoupled dual-row array was constructed using time-domain software together with circuit co-simulation. Experiments were conducted on a 9.4 T scanner. Investigation of RF shimming focused on B homogeneity, efficiency and local specific absorption rate (SAR) when applied to large brain volumes and on a slice-by-slice basis. Results: Numerical results were consistent with experiments regarding component values, S-parameters and B pattern, though the B field was about 25 % weaker in measurements than simulations. Global shim settings were able to prevent B field voids across the entire brain but the capability to simultaneously reduce inhomogeneities was limited. On a slice-by-slice basis, B standard deviations of below 10 % without field dropouts could be achieved in axial, sagittal and coronal orientations across the brain, even with phase-only shimming, but decreased B efficiency and SAR limitations must be considered. Conclusion: Dual-row transmit arrays facilitate flexible 3D RF management across the entire brain at 9.4 T in order to trade off B homogeneity against power-efficiency and local SAR. [ABSTRACT FROM AUTHOR]

Purpose Magnetic resonance imaging of humans at high magnetic field strengths is strongly influenced by the interference of the radiofrequency (RF) electromagnetic field and the body. To minimize this effect, multiple RF sources could be used. A novel setup (called multimode, coaxial waveguide) is proposed that facilitates RF shimming based on the traveling waves. Methods The multimode, coaxial waveguide combines the coaxial waveguide, cylindrical waveguide, high dielectric permittivity lining, and eight radial stub antennas. Each antenna excites multiple waveguide modes. Based on modes orthogonality, a method was devised to decompose an excitation pattern of single stub antenna into waveguide modes. Results The number of modes present in the excitation pattern of a single stub antenna increased with the higher effective permittivity of the dielectric lining. Thus, RF shimming performance of the setup was improved. An average homogeneity of 10% was demonstrated for a single slice of each principle plane in the human head at 7 T. Conclusion Traveling wave RF shimming is feasible both in axial and longitudinal directions and is improved with an increased amount of orthogonal waveguide modes. Nevertheless, with the currently available RF amplifiers at 7 T, the performance of the setup is limited to low flip angles. Magn Reson Med 71:1641-1649, 2014. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]