Linear Bloch-Siegert phase-encoded low-field MRI: RF coils, pulse sequence, and image reconstruction.

Conventional B 0 $$ {B}_0 $$ gradient systems have several weaknesses including high cost and bulk. As a step towards addressing these while providing new degrees of freedom for spatial encoding and system design in Magnetic Resonance Imaging (MRI), a radio frequency (RF) gradient encoding system and pulse sequence for phase encoding using the Bloch-Siegert (BS) shift were developed. Optimized BS spatial encoding coils with bucking windings (counter-wound loops) were designed and constructed, along with compatible homogeneous imaging coils for excitation and signal reception. Two coil systems were developed: one for phantom imaging and a second for human wrist imaging. BS phase-encoded imaging and BS RF pulse simulations were performed. Pulse sequences were designed for linear stepping in k-space and implemented on a 47.5-mT scanner to image resolution phantoms in both coil setups. Reconstructions were performed using both the full B 1 + $$ {B}_1^{+} $$ -based encoding fields for each BS pulse amplitude and using inverse discrete Fourier transforms. A B 0 $$ {B}_0 $$ gradient was used for frequency encoding during signal readout, and the third axis was projected. Specific absorption ratio (SAR) calculations were performed for the wrist coil to determine the safety of BS-based RF encoding for B 0 $$ {B}_0 $$ fields in the low field MRI regime. The optimized RF spatial encoding coils resulted in higher linearity ( R 2 = 0.9981 $$ {R}^2=0.9981 $$ and 0.9921 in the phantom and wrist coils, respectively) than coils used in previous work. The phantom and wrist imaging coils were validated in simulations and experimentally to produce a peak B 1 + = 1.35 $$ {B}_1^{+}=1.35 $$ G and 0.8 G with 12-W input power, respectively, in the field-of-view (length = 11 cm) used for imaging. Nominal imaging resolutions of 5.22 and 7.21 mm were, respectively, achieved by the two-coil systems in the RF phase-encoded dimension. Coil systems, pulse sequences, and image reconstructions were developed for linear RF phase encoding using the BS shift and validated using a 47.5-mT open low field scanner, establishing a key component required for B 0 $$ {B}_0 $$  gradient-free imaging at low B 0 $$ {B}_0 $$  field strengths.

Preparation-based B 1 + mapping in the heart using Bloch-Siegert shifts.

PURPOSE: To develop and evaluate a robust cardiac B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping sequence at 3 T, using Bloch-Siegert shift (BSS)-based preparations. METHODS: A longitudinal magnetization preparation module was designed to encode | B 1 + | $$ \mid {\mathrm{B}}_1^{+}\mid $$ . After magnetization tip-down, off-resonant Fermi pulses, placed symmetrically around two refocusing pulses, induced BSS, followed by tipping back of the magnetization. Bloch simulations were used to optimize refocusing pulse parameters and to assess the mapping sensitivity. Relaxation-induced B 1 + $$ {\mathrm{B}}_1^{+} $$ error was simulated for various T 1 $$ {\mathrm{T}}_1 $$ / T 2 $$ {\mathrm{T}}_2 $$ times. The effective mapping range was determined in phantom experiments, and | B 1 + | $$ \mid {\mathrm{B}}_1^{+}\mid $$ maps were compared to the conventional BSS method and subadiabatic hyperbolic-secant 8 (HS8) pulse-sensitized method. Cardiac B 1 + $$ {\mathrm{B}}_1^{+} $$ maps were acquired in healthy subjects, and evaluated for repeatability and imaging plane intersection consistency. The technique was modified for three-dimensional (3D) acquisition of the whole heart in a single breath-hold, and compared to two-dimensional (2D) acquisition. RESULTS: Simulations indicate that the proposed preparation can be tailored to achieve high mapping sensitivity across various B 1 + $$ {\mathrm{B}}_1^{+} $$ ranges, with maximum sensitivity at the upper B 1 + $$ {\mathrm{B}}_1^{+} $$ range. T 1 $$ {\mathrm{T}}_1 $$ / T 2 $$ {\mathrm{T}}_2 $$ -induced bias did not exceed 5.2 % $$ \% $$ . Experimentally reproduced B 1 + $$ {\mathrm{B}}_1^{+} $$ sensitization closely matched simulations for B 1 + ≥ 0 . 3 B 1 , max + $$ {\mathrm{B}}_1^{+}\ge 0.3{\mathrm{B}}_{1,\max}^{+} $$ (mean difference 0.031 ± $$ \pm $$ 0.022, compared to 0.018 ± $$ \pm $$ 0.025 in the HS8-sensitized method), and showed 20-fold reduction in the standard deviation of repeated scans, compared with conventional BSS B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping, and an equivalent 2-fold reduction compared with HS8-sensitization. Robust cardiac B 1 + $$ {\mathrm{B}}_1^{+} $$ map quality was obtained, with an average test-retest variability of 0.027 ± $$ \pm $$ 0.043 relative to normalized B 1 + $$ {\mathrm{B}}_1^{+} $$ magnitude, and plane intersection bias of 0.052 ± $$ \pm $$ 0.031. 3D acquisitions showed good agreement with 2D scans (mean absolute deviation 0.055 ± $$ \pm $$ 0.061). CONCLUSION: BSS-based preparations enable robust and tailorable 2D/3D cardiac B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping at 3 T in a single breath-hold.

Effects of radiofrequency channel numbers on B1+ mapping using the Bloch-Siegert shift method.

PURPOSE: This paper aims to investigate the impact of the channel numbers on the performance of B1+ mapping, by using the Bloch-Siegert shift (BSS) method. B1+ mapping plays a crucial role in various brain imaging protocols. THEORY AND METHODS: We simulated the radiofrequency field of the human head model in six groups of multi-channel receive coil with a range of different channel numbers. MR signals were synthesized according to the standard BSS sequence, with quantified Gaussian added. Next, we combined the signals of each channel to reconstruct the B1+ map by weighted averaging and maximum likelihood estimation strategies and evaluate the bias by relative standard deviation of each coil. RESULTS: The simulation results revealed that the accuracy of B1+ maps improved with the increasing of channel numbers, meanwhile the per channel efficiency of B1+maps accuracy gradually decrease. Both trends slowed down when the channel numbers reached 12 or above. CONCLUSION: Our finding suggests that increasing the channel numbers can improve the accuracy of B1+map. However, a diminishing efficiency of per channel accuracy improvement was overserved, indicating that the relationship between quality of B1+ map and the channel numbers is nonlinear. Based on these findings, our study provides a reference for determining channel numbers to achieve a balance of coil selection and manufacturing cost. It also provides a theoretical basis for evaluating other B1+ mapping techniques.

Generating accurate tip angles for NMR outside the rotating-wave approximation.

The generation of accurate tip angles is critical for many applications of nuclear magnetic resonance. In low static field, with a linear rather than circular polarized rf field, the rotating-wave approximation may no longer hold and significant deviations from expected trajectories on the Bloch sphere can occur. For rectangular rf pulses, the effects depend strongly on the phase of the rf field and can be further compounded by transients at the start and end of the pulse. The desired terminus can be still be achieved, however, through the application of a phase-dependent Bloch-Siegert shift and appropriate consideration of pulse timings. For suitably shaped rf pulses, the Bloch-Siegert shift is largely phase independent, but its magnitude can vary significantly depending on details of the pulse shape as well as the characteristics of the rf coil circuit. We present numerical simulations and low-field NMR experiments with 1H and 3He that demonstrate several main consequences and accompanying strategies that one should consider when wanting to generate accurate tip angles outside the validity of the rotating-wave approximation and in low static field.

Selective excitation localized by the Bloch-Siegert shift and a B1+ gradient.

PURPOSE: To perform B1+$$ {B}_1^{+} $$ -selective excitation using the Bloch-Siegert shift for spatial localization. THEORY AND METHODS: A B1+$$ {B}_1^{+} $$ -selective excitation is produced by an radiofrequency (RF) pulse consisting of two summed component pulses: an off-resonant pulse that induces a B1+$$ {B}_1^{+} $$ -dependent Bloch-Siegert frequency shift and a frequency-selective excitation pulse. The passband of the pulse can be tailored by adjusting the frequency content of the frequency-selective pulse, as in conventional B0$$ {B}_0 $$ gradient-localized excitation. Fine magnetization profile control is achieved by using the Shinnar-Le Roux algorithm to design the frequency-selective excitation pulse. Simulations analyzed the pulses' robustness to off-resonance, their suitability for multi-echo spin echo pulse sequences, and how their performance compares to that of rotating-frame selective excitation pulses. The pulses were evaluated experimentally on a 47.5 mT MRI scanner using an RF gradient transmit coil. Multiphoton resonances produced by the pulses were characterized and their distribution across B1+$$ {B}_1^{+} $$ predicted. RESULTS: With correction for varying B1+$$ {B}_1^{+} $$ across the desired profile, the proposed pulses produced selective excitation with the specified profile characteristics. The pulses were robust against off-resonance and RF amplifier distortion, and suitable for multi-echo pulse sequences. Experimental profiles closely matched simulated patterns. CONCLUSION: The Bloch-Siegert shift can be used to perform B0$$ {B}_0 $$ -gradient-free selective excitation, enabling the excitation of slices or slabs in RF gradient-encoded MRI.

[Research on 7.0 T Magnetic Resonance Based Electrical Properties Tomography Based on Bloch-Siegert Shift].

Magnetic resonance based electrical properties tomography (MREPT) is a different method from proton density imaging, Bloch-Siegert shift (BSS) is used in this paper to reconstruct the radiofrequency (RF) field amplitude and calculate the distribution of the permittivity constant. The phase of the RF field is approximated by the phase component of the magnetization intensity, and the conductivity distribution is calculated. In the experiment, Bruker 7.0 T magnetic resonance device was used to image two water models and in vivo Balb/c mice to obtain the image of electrical characteristics. Experimental results show that the Bloch-siegert B1+ image is significantly more efficient than the double-angle B1+ image. The study can provide a reference for selecting appropriate B1 mapping technology for B1 field imaging of electrical characteristics organizations, and provide basic research support for promoting the practical application of magnetic resonance characteristics.

Demonstration and suppression of respiration-related artifacts in Bloch-Siegert shift-based B1+ maps of the human brain.

Respiration-induced movement of the chest wall and internal organs causes temporal B0 variations extending throughout the brain. This study demonstrates that these variations can cause significant artifacts in B1+ maps obtained at 7 T with the Bloch-Siegert shift (BSS) B1+ mapping technique. To suppress these artifacts, a navigator correction scheme was proposed. Two sets of experiments were performed. In the first set of experiments, phase shifts induced by respiration-related B0 variations were assessed for five subjects at 7 T by using a gradient echo (GRE) sequence without phase-encoding. In the second set of experiments, B1+ maps were acquired using a GRE-based BSS pulse sequence with navigator echoes. For this set, the measurements were consecutively repeated 16 times for the same imaging slice. These measurements were averaged to obtain the reference B1+ map. Due to the periodicity of respiration-related phase shifts, their effect on the reference B1+ map was assumed to be negligible through averaging. The individual B1+ maps of the 16 repetitions were calculated with and without using the proposed navigator scheme. These maps were compared with the B1+ reference map. The peak-to-peak value of respiration-related phase shifts varied between subjects. Without navigator correction, the interquartile range of percentage error in B1+ varied between 4.0% and 8.3% among subjects. When the proposed navigator scheme was used, these numbers were reduced to 2.5% and 2.9%, indicating an improvement in the precision of GRE-based BSS B1+ mapping at high magnetic fields.

Multiphoton magnetic resonance in imaging: A classical description and implementation.

PURPOSE: To develop a classical geometric interpretation of multiphoton excitation and apply it to MRI. To investigate ways in which multiphoton excitation can enable novel imaging techniques. THEORY AND METHODS: We present a fully geometric view of multiphoton excitation by taking a particular rotating frame transformation. In this rotating frame, we find that multiphoton excitations appear just like single-photon excitations again, and therefore, we can readily generalize concepts already explored in standard single-photon excitation. With a homebuilt low frequency coil, we execute a standard slice selective pulse sequence with all of its excitations replaced by their equivalent two-photon versions. In the case of no extra hardware, we use oscillating gradients as a source of extra photons for excitation. Finally, with the multiphoton interpretation of oscillating gradients, we present a novel way to transform a standard slice selective adiabatic inversion pulse into a multiband version without modifying the RF pulse itself. The addition of oscillating gradients creates multiphoton resonances at multiple spatial locations and allows for adiabatic inversions at each location. RESULTS: With Bloch-Siegert shift corrections, analytical multiphoton excitation expressions match with Bloch equation simulations. Two-photon gradient-echo images of a lemon and a pork rib match with their single-photon counterparts. Frequency-offset RF combined with oscillating gradients generate excitation where the RF alone does not. CONCLUSION: The multiphoton interpretation presents new flexibilities for imaging. Excitation needs not be bound to the Larmor frequency, which opens doors to RF pulse design beyond the usual filter design and the potential for further imaging innovations.

Using the heteronuclear Bloch-Siegert shift of protons for B1 calibration of insensitive nuclei not present in the sample.

Indirect rf field calibration using the heteronuclear Bloch-Siegert shift is presented. This method is useful for calibrating ω1 = -γB1 for the rf channels of small volume fast-spinning probes on which direct rf calibration is practically inconvenient or difficult for insensitive low-γ nuclei. Proton signals are observed for the rf calibration of the insensitive nuclei without requiring their presence in the sample. For a linearly modulated rf field, the heteronuclear Bloch-Siegert shift is given by, ΔωBS=ω0ω12/ω02-ωirr2, where ω0 and ωirr are the Larmor and irradiation frequencies, respectively. A short protocol using full-echo acquisition of protons is described for measurement of the phase change induced by the Bloch-Siegert shift. The calibration procedure is validated by a comparison with direct 13C calibration and demonstrated for 14N rf field measurement of a 0.75 mm 100 kHz triple-resonance magic-angle spinning probe.

Robust 3D Bloch-Siegert based B1+ mapping using multi-echo general linear modeling.

PURPOSE: Quantitative MRI applications, such as mapping the T1 time of tissue, puts high demands on the accuracy and precision of transmit field ( B1+ ) estimation. A candidate approach to satisfy these requirements exploits the difference in phase induced by the Bloch-Siegert frequency shift (BSS) of 2 acquisitions with opposite off-resonance frequency radiofrequency pulses. Interleaving these radiofrequency pulses ensures robustness to motion and scanner drifts; however, here we demonstrate that doing so also introduces a bias in the B1+ estimates. THEORY AND METHODS: It is shown here by means of simulation and experiments that the amplitude of the error depends on MR pulse sequence parameters, such as repetition time and radiofrequency spoiling increment, but more problematically, on the intrinsic properties, T1 and T2 , of the investigated tissue. To solve these problems, a new approach to BSS-based B1+ estimation that uses a multi-echo acquisition and a general linear model to estimate the correct BSS-induced phase is presented. RESULTS: In line with simulations, phantom and in vivo experiments confirmed that the general linear model-based method removed the dependency on tissue properties and pulse sequence settings. CONCLUSION: The general linear model-based method is recommended as a more accurate approach to BSS-based B1+ mapping.

Ultrafast 3D Bloch-Siegert B 1 + -mapping using variational modeling.

PURPOSE: Highly accelerated B 1 + -mapping based on the Bloch-Siegert shift to allow 3D acquisitions even within a brief period of a single breath-hold. THEORY AND METHODS: The B 1 + dependent Bloch-Siegert phase shift is measured within a highly subsampled 3D-volume and reconstructed using a two-step variational approach, exploiting the different spatial distribution of morphology and B 1 + -field. By appropriate variable substitution the basic non-convex optimization problem is transformed in a sequential solution of two convex optimization problems with a total generalized variation (TGV) regularization for the morphology part and a smoothness constraint for the B 1 + -field. The method is evaluated on 3D in vivo data with retro- and prospective subsampling. The reconstructed B 1 + -maps are compared to a zero-padded low resolution reconstruction and a fully sampled reference. RESULTS: The reconstructed B 1 + -field maps are in high accordance to the reference for all measurements with a mean error below 1% and a maximum of about 4% for acceleration factors up to 100. The minimal error for different sampling patterns was achieved by sampling a dense region in k-space center with acquisition times of around 10-12 s for 3D-acquistions. CONCLUSIONS: The proposed variational approach enables highly accelerated 3D acquisitions of Bloch-Siegert data and thus full liver coverage in a single breath hold.

Enhanced accuracy of the microwave field strength measurement in a CW-EPR by pulsed modulation technique.

The microwave magnetic field strength, B1, in the cavity of a conventional continuous wave electron paramagnetic resonance, CW-EPR, spectrometer was measured by employing modulation sidebands, MS, in the EPR spectrum. MS spectrum in CW-EPR is produced by applying the modulation frequency, ωrf, which exceeds the linewidth, δB, given in frequency units. An amplitude-modulated CW-EPR, AM-CW-EPR, was selected as detection method. Theoretical description of AM-CW-EPR spectrum was modified by adding Bloch-Siegert-like shift obtained by taking into account the cumulative effect of the non-resonant interactions between the driving fields and the spin system. This approach enables to enhance the precision of B1 measurement. In order to increase the sensitivity of the method when saturation effects, due to higher intensity of B1, decrease the resolution of AM-CW-EPR spectrum, detection at the second harmonic of CW-EPR has been employed.

B1 mapping for bias-correction in quantitative T1 imaging of the brain at 3T using standard pulse sequences.

PURPOSE: B1 mapping is important for many quantitative imaging protocols, particularly those that include whole-brain T1 mapping using the variable flip angle (VFA) technique. However, B1 mapping sequences are not typically available on many magnetic resonance imaging (MRI) scanners. The aim of this work was to demonstrate that B1 mapping implemented using standard scanner product pulse sequences can produce B1 (and VFA T1 ) maps comparable in quality and acquisition time to advanced techniques. MATERIALS AND METHODS: Six healthy subjects were scanned at 3.0T. An interleaved multislice spin-echo echo planar imaging double-angle (EPI-DA) B1 mapping protocol, using a standard product pulse sequence, was compared to two alternative methods (actual flip angle imaging, AFI, and Bloch-Siegert shift, BS). Single-slice spin-echo DA B1 maps were used as a reference for comparison (Ref. DA). VFA flip angles were scaled using each B1 map prior to fitting T1 ; the nominal flip angle case was also compared. RESULTS: The pooled-subject voxelwise correlation (ρ) for B1 maps (BS/AFI/EPI-DA) relative to the reference B1 scan (Ref. DA) were ρ = 0.92/0.95/0.98. VFA T1 correlations using these maps were ρ = 0.86/0.88/0.96, much better than without B1 correction (ρ = 0.53). The relative error for each B1 map (BS/AFI/EPI-DA/Nominal) had 95th percentiles of 5/4/3/13%. CONCLUSION: Our findings show that B1 mapping implemented using product pulse sequences can provide excellent quality B1 (and VFA T1 ) maps, comparable to other custom techniques. This fast whole-brain measurement (∼2 min) can serve as an excellent alternative for researchers without access to advanced B1 pulse sequences. LEVEL OF EVIDENCE: 1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2017;46:1673-1682.

Motion-robust cardiac B1+ mapping at 3T using interleaved bloch-siegert shifts.

PURPOSE: To develop and evaluate a robust motion-insensitive Bloch-Siegert shift based B1+ mapping method in the heart. METHODS: Cardiac Bloch-Siegert B1+ mapping was performed with interleaved positive and negative off-resonance shifts and diastolic spoiled gradient echo imaging in 12 heartbeats. Numerical simulations were performed to study the impact of respiratory motion. The method was compared with three-dimensional (3D) actual flip angle imaging (AFI) and two-dimensional (2D) saturated double angle method (SDAM) in phantom scans. Cardiac B1+ maps of three different views were acquired in six healthy volunteers using Bloch-Siegert and SDAM during breath-hold and free breathing. In vivo maps were evaluated for inter-view consistency using the correlation coefficients of the B1+ profiles along the lines of intersection between the views. RESULTS: For the Bloch-Siegert sequence, numerical simulations indicated high similarity between breath-hold and free breathing scans, and phantom results indicated low deviation from the 3D AFI reference (normalized root mean square error [NRMSE] = 2.0%). Increased deviation was observed with 2D SDAM (NRMSE = 5.0%) due to underestimation caused by imperfect excitation slice profiles. Breath-hold and free breathing Bloch-Siegert in vivo B1+ maps were visually comparable with no significant difference in the inter-view consistency (P > 0.36). SDAM showed strongly impaired B1+ map quality during free breathing. Inter-view consistency was significantly lower than with the Bloch-Siegert method (breath-hold: P = 0.014, free breathing: P < 0.0001). CONCLUSION: The proposed interleaved Bloch-Siegert sequence enables cardiac B1+ mapping with improved inter-view consistency and high resilience to respiratory motion. Magn Reson Med 78:670-677, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Motion-insensitive determination of B1+ amplitudes based on the bloch-siegert shift in single voxels of moving organs including the human heart.

PURPOSE: To reliably determine the amplitude of the transmit radiofrequency ( B1+) field in moving organs like the liver and heart, where most current techniques are usually not feasible. METHODS: B1+ field measurement based on the Bloch-Siegert shift induced by a pair of Fermi pulses in a double-triggered modified Point RESolved Spectroscopy (PRESS) sequence with motion-compensated crusher gradients has been developed. Performance of the sequence was tested in moving phantoms and in muscle, liver, and heart of six healthy volunteers each, using different arrangements of transmit/receive coils. RESULTS: B1+ determination in a moving phantom was almost independent of type and amplitude of the motion and agreed well with theory. In vivo, repeated measurements led to very small coefficients of variance (CV) if the amplitude of the Fermi pulse was chosen above an appropriate level (CV in muscle 0.6%, liver 1.6%, heart 2.3% with moderate amplitude of the Fermi pulses and 1.2% with stronger Fermi pulses). CONCLUSION: The proposed sequence shows a very robust determination of B1+ in a single voxel even under challenging conditions (transmission with a surface coil or measurements in the heart without breath-hold).

Approximate Fourier domain expression for Bloch-Siegert shift.

PURPOSE: In this study, a new simple Fourier domain-based analytical expression for the Bloch-Siegert (BS) shift-based B1 mapping method is proposed to obtain |B1+| more accurately while using short BS pulse durations and small off-resonance frequencies. THEORY AND METHODS: A new simple analytical expression for the BS shift is derived by simplifying the Bloch equations. In this expression, the phase is calculated in terms of the Fourier transform of the radiofrequency pulse envelope, and thus making the off- and on-resonance effects more easily understandable. To verify the accuracy of the proposed expression, Bloch simulations and MR experiments are performed for the hard, Fermi, and Shinner-Le Roux pulse shapes. RESULTS: Analyses of the BS phase shift-based B1 mapping method in terms of radiofrequency pulse shape, pulse duration, and off-resonance frequency show that |B1+| can be obtained more accurately with the aid of this new expression. CONCLUSIONS: In this study, a new simple frequency domain analytical expression is proposed for the BS shift. Using this expression, |B1+| values can be predicted from the phase data using the frequency spectrum of the radiofrequency pulse. This method works well even for short pulse durations and small offset frequencies.

Fast REDOR with CPMG multiple-echo acquisition.

Rotational-Echo Double Resonance (REDOR) is a widely used experiment for distance measurements in solids. The conventional REDOR experiment measures the signal dephasing from hetero-nuclear recoupling under magic-angle spinning (MAS) in a point by point manner. A modified Carr-Purcell Meiboom-Gill (CPMG) multiple-echo scheme is introduced for fast REDOR measurement. REDOR curves are measured from the CPMG echo amplitude modulation under dipolar recoupling. The real time CPMG-REDOR experiment can speed up the measurement by an order of magnitude. The effects from hetero-nuclear recoupling, the Bloch-Siegert shift and echo truncation to the signal acquisition are discussed and demonstrated.

Adiabatic RF pulse design for Bloch-Siegert B1+ mapping.

The Bloch-Siegert (B-S) B1+ mapping method has been shown to be fast and accurate, yet it suffers from high Specific Absorption Rate (SAR) and moderately long echo time. An adiabatic RF pulse design is introduced here for optimizing the off-resonant B-S RF pulse to achieve more B-S B1+ measurement sensitivity for a given pulse width. The extra sensitivity can be used for higher angle-to-noise ratio B1+ maps or traded off for faster scans. Using numerical simulations and phantom experiments, it is shown that a numerically optimized 2-ms adiabatic B-S pulse is 2.5 times more efficient than a conventional 6-ms Fermi-shaped B-S pulse. The adiabatic B-S pulse performance is validated in a phantom, and in vivo brain B1+ mapping at 3T and 7T are shown.