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Purpose: This work proposes a "hybrid" RF pulse design method for parallel transmit (pTx) systems to simultaneously control flip angle and root‐mean‐squared B1+$$ {\mathrm{B}}_1^{+} $$ (B1rms$$ {B}_1^{\mathrm{rms}} $$). These pulses are generally only designed for flip angle, however, this can lead to uncontrolled B1rms$$ {B}_1^{\mathrm{rms}} $$, which then leads to variable magnetization transfer (MT) effects. We demonstrate the hybrid design approach for quantitative imaging where both flip angle and B1rms$$ {B}_1^{\mathrm{rms}} $$ are important. Theory and Methods: A dual cost function optimization is performed containing the normalized mean squared errors of the flip angle and B1rms$$ {B}_1^{\mathrm{rms}} $$ distributions weighted by a parameter λ$$ \lambda $$. Simulations were conducted to study the behavior of both properties when simultaneously optimizing them. In vivo experiments on a 7T MRI system with an 8‐channel pTx head coil were carried out to study the effect of the hybrid design approach on variable flip angle R1$$ {\mathrm{R}}_1 $$(= 1/T1) mapping. Results: Simulations showed that both flip angle and B1rms$$ {B}_1^{\mathrm{rms}} $$ can be homogenized simultaneously without detriment to either when compared to an individual optimization. By homogenizing flip angle and B1rms$$ {B}_1^{\mathrm{rms}} $$, R1$$ {\mathrm{R}}_1 $$ maps were more uniform (coefficient of variation 6.6% vs. 13.0%) compared to those acquired with pulses that only homogenized flip angle. Conclusion: The proposed hybrid design homogenizes on‐resonance MT effects while homogenizing the flip angle distribution, with only a small detriment in the latter compared to a pulse that just homogenizes flip angle. This improved R1$$ {\mathrm{R}}_1 $$ mapping by controlling incidental MT effects, yielding more uniform R1$$ {\mathrm{R}}_1 $$ maps. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the effects of using different parallel‐transmit (pTx) head coils and specific absorption rate (SAR) supervision strategies on pTx pulse design for ultrahigh‐field MRI using a 3D‐MPRAGE sequence. Methods: The PTx universal pulses (UPs) and fast online‐customized (FOCUS) pulses were designed with pre‐acquired data sets (B0, B1+ maps, specific absorption rate [SAR] supervision data) from two different 8 transmit/32 receive head coils on two 7T whole‐body MR systems. For one coil, the SAR supervision model consisted of per‐channel RF power limits. In the other coil, SAR estimations were done with both per‐channel RF power limits as well as virtual observation points (VOPs) derived from electromagnetic field (EMF) simulations using three virtual human body models at three different positions. All pulses were made for nonselective excitation and inversion and evaluated on 132 B0, B1+, and SAR supervision datasets obtained with one coil and 12 from the other. At both sites, 3 subjects were examined using MPRAGE sequences that used UP/FOCUS pulses generated for both coils. Results: For some subjects, the UPs underperformed when simulated on a different coil from which they were derived, whereas FOCUS pulses still showed acceptable performance in that case. FOCUS inversion pulses outperformed adiabatic pulses when scaled to the same local SAR level. For the self‐built coil, the use of VOPs showed reliable overestimation compared with the ground‐truth EMF simulations, predicting about 52% lower local SAR for inversion pulses compared with per‐channel power limits. Conclusion: FOCUS inversion pulses offer a low‐SAR alternative to adiabatic pulses and benefit from using EMF‐based VOPs for SAR estimation. [ABSTRACT FROM AUTHOR]

Purpose: To extend the coverage of brain coil arrays to the neck and cervical–spine region to enable combined head and neck imaging at 7 Tesla (T) ultra‐high field MRI. Methods: The coil array structures of a 64‐channel receive coil and a 16‐channel transmit coil were merged into one anatomically shaped close‐fitting housing. Transmit characteristics were evaluated in a B1+‐field mapping study and an electromagnetic model. Receive SNR and the encoding capability for accelerated imaging were evaluated and compared with a commercially available 7 T brain array coil. The performance of the head–neck array coil was demonstrated in human volunteers using high‐resolution accelerated imaging. Results: In the brain, the SNR matches the commercially available 32‐channel brain array and showed improvements in accelerated imaging capabilities. More importantly, the constructed coil array improved the SNR in the face area, neck area, and cervical spine by a factor of 1.5, 3.4, and 5.2, respectively, in regions not covered by 32‐channel brain arrays at 7 T. The interelement coupling of the 16‐channel transmit coil ranged from −14 to −44 dB (mean = −19 dB, adjacent elements <−18 dB). The parallel 16‐channel transmit coil greatly facilitates B1+ field shaping required for large FOV neuroimaging at 7 T. Conclusion: This new head–neck array coil is the first demonstration of a device of this nature used for combined full‐brain, head–neck, and cervical‐spine imaging at 7 T. The array coil is well suited to provide large FOV images, which potentially improves ultrahigh field neuroimaging applications for clinical settings. [ABSTRACT FROM AUTHOR]

Purpose: This work proposes a novel RF pulse design for parallel transmit (pTx) systems to obtain uniform saturation of semisolid magnetization for magnetization transfer (MT) contrast in the presence of transmit field B1+ inhomogeneities. The semisolid magnetization is usually modeled as being purely longitudinal, with the applied B1+ field saturating but not rotating its magnetization; thus, standard pTx pulse design methods do not apply. Theory and Methods: Pulse design for saturation homogeneity (PUSH) optimizes pTx RF pulses by considering uniformity of root‐mean squared B1+, B1rms, which relates to the rate of semisolid saturation. Here we considered designs consisting of a small number of spatially non‐selective sub‐pulses optimized over either a single 2D plane or 3D. Simulations and in vivo experiments on a 7T Terra system with an 8‐TX Nova head coil in five subjects were carried out to study the homogenization of B1rms and of the MT contrast by acquiring MT ratio maps. Results: Simulations and in vivo experiments showed up to six and two times more uniform B1rms compared to circular polarized (CP) mode for 2D and 3D optimizations, respectively. This translated into 4 and 1.25 times more uniform MT contrast, consistently for all subjects, where two sub‐pulses were enough for the implementation and coil used. Conclusion: The proposed PUSH method obtains more uniform and higher MT contrast than CP mode within the same specific absorption rate (SAR) budget. [ABSTRACT FROM AUTHOR]

Purpose: To enable whole-brain quantitative CEST MRI at ultra-high magnetic field strengths (B0 ≥ 7T) within short acquisition times. Methods: Multiple interleaved mode saturation (MIMOSA) was combined with fast online-customized (FOCUS) parallel transmission (pTx) excitation pulses and B1+ correction to achieve homogenous whole-brain coverage. Examinations of 13 volunteers were performed on a 7T MRI system with 3 different types of pulse sequences: (1) saturation in circular polarized (CP) mode and CP mode readout, (2) MIMOSA and CP readout, and (3) MIMOSA and FOCUS readout. For comparison, the inverse magnetic transfer ratio metric for relayed nuclear Overhauser effect and amide proton transfer were calculated. To investigate the number of required acquisitions for a good B1+ correction, 4 volunteers were measured with 6 different B1 amplitudes. Finally, time point repeatability was investigated for 6 volunteers. Results: MIMOSA FOCUS sequence using B1+ correction, with both single and multiple points, reduced inhomogeneity of the CEST contrasts around the occipital lobe and cerebellum. Results indicate that the most stable inter-subject coefficient of variation was achieved using the MIMOSA FOCUS sequence. Time point repeatability of MIMOSA FOCUS with single-point B1+ correction showed a maximum coefficient of variation below 8% for 3 measurements in a single volunteer. Conclusion: A combination of MIMOSA FOCUS with a single-point B1+ correction can be used to achieve quantitative CEST measurements at ultra-high magnetic field strengths. Compared to previous B1+ correction methods, acquisition time can be reduced as additional scans required for B1+ correction can be omitted. [ABSTRACT FROM AUTHOR]

Purpose: To mitigate spatial flip angle (FA) variations under strict specific absorption rate (SAR) constraints for ultra‐high field MRI using a combination of universal parallel transmit (pTx) pulses and fast subject‐specific optimization. Methods: Data sets consisting of B0, B1+ maps, and virtual observation point (VOP) data were acquired from 72 subjects (study groups of 48/12 healthy Europeans/Asians and 12 Europeans with pathological or incidental findings) using an 8Tx/32Rx head coil on a 7T whole‐body MR system. Combined optimization values (COV) were defined as combination of spiral‐nonselective (SPINS) trajectory parameters and an energy regularization weight. A set of COV was optimized universally by simulating the individual RF pulse optimizations of 12 training data sets (healthy Europeans). Subsequently, corresponding universal pulses (UPs) were calculated. Using COV and UPs, individually optimized pulses (IOPs) were calculated during the sequence preparation phase (maximum 15 s). Two different UPs and IOPs were evaluated by calculating their normalized root‐mean‐square error (NRMSE) of the FA and SAR in simulations of all data sets. Seven additional subjects were examined using an MPRAGE sequence that uses the designed pTx excitation pulses and a conventional adiabatic inversion. Results: All pTx pulses resulted in decreased mean NRMSE compared to a circularly polarized (CP) pulse (CP = ~28%, UPs = ~17%, and IOPs = ~12%). UPs and IOPs improved homogeneity for all subjects. Differences in NRMSE between study groups were much lower than differences between different pulse types. Conclusion: UPs can be used to generate fast online‐customized (FOCUS) pulses gaining lower NRMSE and/or lower SAR values. [ABSTRACT FROM AUTHOR]

Purpose: To mitigate B1+ inhomogeneity in quantitative CEST MRI at ultra‐high magnetic field strengths (B0 ≥ 7 Tesla) using a parallel transmit system. Methods: Multiple interleaved mode saturation employs interleaving of 2 complementary phase sets during the saturation pulse train. Phase differences of 45° (first mode) and 90° (second mode) between 2 adjacent transmitter coil channels are used. The influence of the new saturation scheme on the CEST contrast was analyzed using Bloch‐McConnell simulations. The presented method was verified in phantom and in vivo measurements of the healthy human brain. The relayed nuclear Overhauser effect was evaluated, and the inverse magnetic transfer ratio metric was calculated. Results were compared to a published B1+ correction method. All measurements were conducted on a whole‐body 7 Tesla MRI system using an 8 transmitter and 32 receiver channel head coil. Results: Simulations showed that the inverse magnetic transfer ratio metric contrast of relayed nuclear Overhauser effect shows a smaller dependency on the relative amplitudes of the 2 different modes than the contrasts of Cr and amide proton transfer. Measurements of an egg white phantom showed markedly improved homogeneity compared to the uncorrected inverse magnetic transfer ratio metric (relayed nuclear Overhauser effect) images and slightly improved results compared to B1+ corrected images. In vivo multiple interleaved mode saturation images showed similar contrast compared to B1+ corrected images. Conclusion: Multiple interleaved mode saturation can be used as a simple method to mitigate B1+ inhomogeneity effects in CEST MRI at ultra‐high magnetic field strengths. Compared to previous B1+ correction methods, acquisition time can be reduced because an additional scan, usually required for B1+ correction, can be omitted. [ABSTRACT FROM AUTHOR]

Purpose: To enable whole-brain quantitative CEST MRI at ultra-high magnetic field strengths (B 0 ≥ 7T) within short acquisition times., Methods: Multiple interleaved mode saturation (MIMOSA) was combined with fast online-customized (FOCUS) parallel transmission (pTx) excitation pulses and B 1 + correction to achieve homogenous whole-brain coverage. Examinations of 13 volunteers were performed on a 7T MRI system with 3 different types of pulse sequences: (1) saturation in circular polarized (CP) mode and CP mode readout, (2) MIMOSA and CP readout, and (3) MIMOSA and FOCUS readout. For comparison, the inverse magnetic transfer ratio metric for relayed nuclear Overhauser effect and amide proton transfer were calculated. To investigate the number of required acquisitions for a good B 1 + correction, 4 volunteers were measured with 6 different B 1 amplitudes. Finally, time point repeatability was investigated for 6 volunteers., Results: MIMOSA FOCUS sequence using B 1 + correction, with both single and multiple points, reduced inhomogeneity of the CEST contrasts around the occipital lobe and cerebellum. Results indicate that the most stable inter-subject coefficient of variation was achieved using the MIMOSA FOCUS sequence. Time point repeatability of MIMOSA FOCUS with single-point B 1 + correction showed a maximum coefficient of variation below 8% for 3 measurements in a single volunteer., Conclusion: A combination of MIMOSA FOCUS with a single-point B 1 + correction can be used to achieve quantitative CEST measurements at ultra-high magnetic field strengths. Compared to previous B 1 + correction methods, acquisition time can be reduced as additional scans required for B 1 + correction can be omitted., (© 2021 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.)

Purpose: To mitigate B 1 + inhomogeneity in quantitative CEST MRI at ultra-high magnetic field strengths (B 0 ≥ 7 Tesla) using a parallel transmit system., Methods: Multiple interleaved mode saturation employs interleaving of 2 complementary phase sets during the saturation pulse train. Phase differences of 45° (first mode) and 90° (second mode) between 2 adjacent transmitter coil channels are used. The influence of the new saturation scheme on the CEST contrast was analyzed using Bloch-McConnell simulations. The presented method was verified in phantom and in vivo measurements of the healthy human brain. The relayed nuclear Overhauser effect was evaluated, and the inverse magnetic transfer ratio metric was calculated. Results were compared to a published B 1 + correction method. All measurements were conducted on a whole-body 7 Tesla MRI system using an 8 transmitter and 32 receiver channel head coil., Results: Simulations showed that the inverse magnetic transfer ratio metric contrast of relayed nuclear Overhauser effect shows a smaller dependency on the relative amplitudes of the 2 different modes than the contrasts of Cr and amide proton transfer. Measurements of an egg white phantom showed markedly improved homogeneity compared to the uncorrected inverse magnetic transfer ratio metric (relayed nuclear Overhauser effect) images and slightly improved results compared to B 1 + corrected images. In vivo multiple interleaved mode saturation images showed similar contrast compared to B 1 + corrected images., Conclusion: Multiple interleaved mode saturation can be used as a simple method to mitigate B 1 + inhomogeneity effects in CEST MRI at ultra-high magnetic field strengths. Compared to previous B 1 + correction methods, acquisition time can be reduced because an additional scan, usually required for B 1 + correction, can be omitted., (© 2019 International Society for Magnetic Resonance in Medicine.)