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Purpose: This study aims to map the transmit magnetic field (B1+$$ {B}_1^{+} $$) in the human body at 7T using MR fingerprinting (MRF), with a focus on achieving high accuracy and precision across a large dynamic range, particularly at low flip angles (FAs). Methods: A FLASH‐based MRF sequence (B1‐MRF) with high B1+$$ {B}_1^{+} $$ sensitivity was developed. Phantom and in vivo abdominal imaging were performed at 7T, and the results were compared with established reference methods, including a slow but precise preparation‐based method (PEX), saturated TurboFLASH (satTFL), actual flip angle imaging (AFI) and Bloch‐Siegert shift (BSS). Results: The MRF signal curve was highly sensitive to B1+$$ {B}_1^{+} $$, while T1 sensitivity was comparatively low. The phantom experiment showed good agreement of B1+$$ {B}_1^{+} $$ to PEX for a T1 range of 204–1691 ms evaluated at FAs from 0° to 70°. Compared to the references, a dynamic range increase larger than a factor of two was determined experimentally. In vivo liver scans showed a strong correlation between B1‐MRF, satTFL, and RPE‐AFI in a low body mass index (BMI) subject (18.1 kg/m2). However, in larger BMI subjects (≥25.5 kg/m2), inconsistencies were observed in low B1+$$ {B}_1^{+} $$ regions for satTFL and RPE‐AFI, while B1‐MRF still provided consistent results in these regions. Conclusion: B1‐MRF provides accurate and precise B1+$$ {B}_1^{+} $$ maps over a wide range of FAs, surpassing the capabilities of existing methods in the FA range < 60°. Its enhanced sensitivity at low FAs is advantageous for various applications requiring precise B1+$$ {B}_1^{+} $$ estimates, potentially advancing the frontiers of ultra‐high field (UHF) body imaging at 7T and beyond. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the impact of reduced k‐space sampling on B1+$$ {\mathrm{B}}_1^{+} $$ mapping and the resulting impact on phase shimming and dynamic/universal parallel transmit (pTx) RF pulse design. Methods: Channel‐wise 3D B1+$$ {\mathrm{B}}_1^{+} $$ maps were measured at 7 T in 35 and 23 healthy subjects for the heart and prostate region, respectively. With these B1+$$ {\mathrm{B}}_1^{+} $$ maps, universal phase shims optimizing homogeneity and B1+$$ {\mathrm{B}}_1^{+} $$ efficiency were designed for heart and prostate imaging. In addition, universal 4kT‐point pulses were designed for the heart. Subsequently, individual phase shims and individual 4kT‐pulses were designed based on B1+$$ {\mathrm{B}}_1^{+} $$ maps with different acceleration factors and tested on the original maps. The performance of the pulses was compared by evaluating their coefficients of variation (CoV), B1+$$ {\mathrm{B}}_1^{+} $$ efficiencies and specific energy doses (SED). Furthermore, validation measurements were carried out for one heart and one prostate subject. Results: For both organs, the universal phase shims showed significantly higher B1+$$ {\mathrm{B}}_1^{+} $$ efficiencies and lower CoVs compared to the vendor provided default shim, but could still be improved with individual phase shims based on accelerated B1+$$ {\mathrm{B}}_1^{+} $$ maps (acquisition time = 30 s). In the heart, the universal 4kT‐pulse achieved significantly lower CoVs than tailored phase shims. Tailored 4kT‐pulses based on accelerated B1+$$ {\mathrm{B}}_1^{+} $$ maps resulted in even further reduced CoVs or a 2.5‐fold reduction in SED at the same CoVs as the universal 4kT‐pulse. Conclusion: Accelerated B1+$$ {\mathrm{B}}_1^{+} $$ maps can be used for the design of tailored pTx pulses for prostate and cardiac imaging at 7 T, which further improve homogeneity, B1+$$ {\mathrm{B}}_1^{+} $$ efficiency, or SED compared to universal pulses. [ABSTRACT FROM AUTHOR]

Purpose: To address the limitations of spinal cord imaging at ultra‐high field (UHF) due to time‐consuming parallel transmit (pTx) adjustments. This study introduces calibration‐free offline computed universal shim modes that can be applied seamlessly for different pTx RF coils and spinal cord target regions, substantially enhancing spinal cord imaging efficiency at UHF. Methods: A library of channel‐wise relative B1+$$ {B}_1^{+} $$ maps for the cervical spinal cord (six datasets) and thoracic and lumbar spinal cord (nine datasets) was constructed to optimize transmit homogeneity and efficiency for these regions. A tailored B0 shim was optimized for the cervical spine to enhance spatial magnetic field homogeneity further. The performance of the universal shims was validated using absolute saturation based B1+$$ {B}_1^{+} $$ mapping and high‐resolution 2D and 3D multi‐echo gradient‐recalled echo (GRE) data to assess the image quality. Results: The proposed universal shims demonstrated a 50% improvement in B1+$$ {B}_1^{+} $$ efficiency compared to the default (zero phase) shim mode. B1+$$ {B}_1^{+} $$ homogeneity was also improved by 20%. The optimized universal shims achieved performance comparable to subject‐specific pTx adjustments, while eliminating the need for lengthy pTx calibration times, saving about 10 min per experiment. Conclusion: The development of universal shims represents a significant advance by eliminating time‐consuming subject‐specific pTx adjustments. This approach is expected to make UHF spinal cord imaging more accessible and user‐friendly, particularly for non‐pTx experts. [ABSTRACT FROM AUTHOR]

Ultrahigh field magnetic resonance imaging (MRI) (≥ 7 T) has the potential to provide superior spatial resolution and unique image contrast. Apart from radiofrequency transmit inhomogeneities in the body at this field strength, imaging of the upper abdomen faces additional challenges associated with motion‐induced ghosting artifacts. To address these challenges, the goal of this work was to develop a technique for high‐resolution free‐breathing upper abdominal MRI at 7 T with a large field of view. Free‐breathing 3D gradient‐recalled echo (GRE) water‐excited radial stack‐of‐stars data were acquired in seven healthy volunteers (five males/two females, body mass index: 19.6–24.8 kg/m2) at 7 T using an eight‐channel transceive array coil. Two volunteers were also examined at 3 T. In each volunteer, the liver and kidney regions were scanned in two separate acquisitions. To homogenize signal excitation, the time‐interleaved acquisition of modes (TIAMO) method was used with personalized pairs of B1 shims, based on a 23‐s Cartesian fast low angle shot (FLASH) acquisition. Utilizing free‐induction decay navigator signals, respiratory‐gated images were reconstructed at a spatial resolution of 0.8 × 0.8 × 1.0 mm3. Two experienced radiologists rated the image quality and the impact of B1 inhomogeneity and motion‐related artifacts on multipoint scales. The images of all volunteers showcased effective water excitation and were accurately corrected for respiratory motion. The impact of B1 inhomogeneity on image quality was minimal, underscoring the efficacy of the multitransmit TIAMO shim. The high spatial resolution allowed excellent depiction of small structures such as the adrenal glands, the proximal ureter, the diaphragm, and small blood vessels, although some streaking artifacts persisted in liver image data. In direct comparisons with 3 T performed for two volunteers, 7‐T acquisitions demonstrated increases in signal‐to‐noise ratio of 77% and 58%. Overall, this work demonstrates the feasibility of free‐breathing MRI in the upper abdomen at submillimeter spatial resolution at a magnetic field strength of 7 T. [ABSTRACT FROM AUTHOR]

Purpose: To investigate safety and performance aspects of parallel‐transmit (pTx) RF control‐modes for a body coil at B0≤3T$$ {B}_0\le 3\mathrm{T} $$. Methods: Electromagnetic simulations of 11 human voxel models in cardiac imaging position were conducted for B0=0.5T$$ {B}_0=0.5\mathrm{T} $$, 1.5T$$ 1.5\mathrm{T} $$ and 3T$$ 3\mathrm{T} $$ and a body coil with a configurable number of transmit channels (1, 2, 4, 8, 16). Three safety modes were considered: the 'SAR‐controlled mode' (SCM), where specific absorption rate (SAR) is limited directly, a 'phase agnostic SAR‐controlled mode' (PASCM), where phase information is neglected, and a 'power‐controlled mode' (PCM), where the voltage amplitude for each channel is limited. For either mode, safety limits were established based on a set of 'anchor' simulations and then evaluated in 'target' simulations on previously unseen models. The comparison allowed to derive safety factors accounting for varying patient anatomies. All control modes were compared in terms of the B1+$$ {B}_1^{+} $$ amplitude and homogeneity they permit under their respective safety requirements. Results: Large safety factors (approximately five) are needed if only one or two anchor models are investigated but they shrink with increasing number of anchors. The achievable B1+$$ {B}_1^{+} $$ is highest for SCM but this advantage is reduced when the safety factor is included. PCM appears to be more robust against variations of subjects. PASCM performance is mostly in between SCM and PCM. Compared to standard circularly polarized (CP) excitation, pTx offers minor B1+$$ {B}_1^{+} $$ improvements if local SAR limits are always enforced. Conclusion: PTx body coils can safely be used at B0≤3T$$ {B}_0\le 3\mathrm{T} $$. Uncertainties in patient anatomy must be accounted for, however, by simulating many models. [ABSTRACT FROM AUTHOR]

Purpose: Depicting the stiffness of biological soft tissues, MR elastography (MRE) has a wide range of diagnostic applications. The purpose of this study was to improve the temporal resolution of 2D hepatic MRE in order to provide more rapid feedback on the quality of the wavefield and ensure better temporal sampling of respiration‐induced stiffness changes. Methods: We developed a rapid MRE sequence that uses 2D segmented gradient‐echo spiral readout to encode 40 Hz harmonic vibrations and generate stiffness maps within 625 ms. We demonstrate the use of this technique as a rapid test for shear wave amplitudes and overall MRE image quality and as a method for monitoring respiration‐induced stiffness changes in the liver in comparison to 3D MRE and ultrasound‐based time‐harmonic elastography. Results: Subsecond MRE allowed monitoring of increasing shear wave amplitudes in the liver with increasing levels of external stimulation within a single breath‐hold. Furthermore, the technique detected respiration‐induced changes in liver stiffness with peak values (1.83 ± 0.22 m/s) at end‐inspiration, followed by softer values during forced abdominal pressure (1.60 ± 0.22 m/s) and end‐expiration (1.49 ± 0.22 m/s). The effects of inspiration and expiration were confirmed by time‐harmonic elastography. Conclusion: Our results suggest that subsecond MRE of the liver is useful for checking MRE driver settings and monitoring breathing‐induced changes in liver stiffness in near real time. [ABSTRACT FROM AUTHOR]

Purpose: Subject‐tailored parallel transmission pulses for ultra‐high fields body applications are typically calculated based on subject‐specific B1+$$ {\mathrm{B}}_1^{+} $$‐maps of all transmit channels, which require lengthy adjustment times. This study investigates the feasibility of using deep learning to estimate complex, channel‐wise, relative 2D B1+$$ {\mathrm{B}}_1^{+} $$‐maps from a single gradient echo localizer to overcome long calibration times. Methods: 126 channel‐wise, complex, relative 2D B1+$$ {\mathrm{B}}_1^{+} $$‐maps of the human heart from 44 subjects were acquired at 7T using a Cartesian, cardiac gradient‐echo sequence obtained under breath‐hold to create a library for network training and cross‐validation. The deep learning predicted maps were qualitatively compared to the ground truth. Phase‐only B1+$$ {\mathrm{B}}_1^{+} $$‐shimming was subsequently performed on the estimated B1+$$ {\mathrm{B}}_1^{+} $$‐maps for a region of interest covering the heart. The proposed network was applied at 7T to 3 unseen test subjects. Results: The deep learning‐based B1+$$ {\mathrm{B}}_1^{+} $$‐maps, derived in approximately 0.2 seconds, match the ground truth for the magnitude and phase. The static, phase‐only pulse design performs best when maximizing the mean transmission efficiency. In‐vivo application of the proposed network to unseen subjects demonstrates the feasibility of this approach: the network yields predicted B1+$$ {\mathrm{B}}_1^{+} $$‐maps comparable to the acquired ground truth and anatomical scans reflect the resulting B1+$$ {\mathrm{B}}_1^{+} $$‐pattern using the deep learning‐based maps. Conclusion: The feasibility of estimating 2D relative B1+$$ {\mathrm{B}}_1^{+} $$‐maps from initial localizer scans of the human heart at 7T using deep learning is successfully demonstrated. Because the technique requires only sub‐seconds to derive channel‐wise B1+$$ {\mathrm{B}}_1^{+} $$‐maps, it offers high potential for advancing clinical body imaging at ultra‐high fields. [ABSTRACT FROM AUTHOR]