This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Three‐dimensional (3D) human heart imaging at ultra‐high fields is highly challenging due to respiratory and cardiac motion‐induced artifacts as well as spatially heterogeneous B1+0.25em profiles. In this study, we investigate the feasibility of applying 3D flip angle (FA) homogenization targeting the whole heart via static phase‐only and dynamic kT‐point in vivo parallel transmission at 7 T. 3D B1+ maps of the thorax were acquired under free breathing in eight subjects to compute parallel transmission pulses that improve excitation homogeneity in the human heart. To analyze the number of kT‐points required, excitation homogeneity and radiofrequency (RF) power were compared using different regions of interest in six subjects with different body mass index (BMI) values of 20‐34 kg/m2 for a wide range of regularization parameters. One subset of the optimized subject‐specific pulses was applied in vivo on a 7 T scanner for six subjects in Cartesian 3D breath‐hold scans as well as in two subjects in a radial phase‐encoded 3D free‐breathing scan. Across all subjects, 3‐4 kT‐points achieved a good tradeoff between RF power and nominal FA homogeneity. For subjects with a BMI in the normal range, the 4 kT‐point pulses reliably improved the coefficient of variation by less than 10% compared with less than 25% achieved by static phase‐only parallel transmission. in vivo measurements on a 7 T scanner validated the B1+0.25em estimations and the pulse design, despite neglecting ΔB0 in the optimizations and Bloch simulations. This study demonstrates in vivo that kT‐point pTx pulses are highly suitable for mitigating nominal FA heterogeneities across the entire 3D heart volume at 7 T. Furthermore, 3‐4 kT‐points demonstrate a practical tradeoff between nominal FA heterogeneity mitigation and RF power.

This is an open access article under the terms of the Creat ive Commo ns Attri bution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Purpose Respiratory motion‐compensated (MC) 3D cardiac fat‐water imaging at 7T. Methods Free‐breathing bipolar 3D triple‐echo gradient‐recalled‐echo (GRE) data with radial phase‐encoding (RPE) trajectory were acquired in 11 healthy volunteers (7M\4F, 21–35 years, mean: 30 years) with a wide range of body mass index (BMI; 19.9–34.0 kg/m2) and volunteer tailored B1+ shimming. The bipolar‐corrected triple‐echo GRE‐RPE data were binned into different respiratory phases (self‐navigation) and were used for the estimation of non‐rigid motion vector fields (MF) and respiratory resolved (RR) maps of the main magnetic field deviations (ΔB0). RR ΔB0 maps and MC ΔB0 maps were compared to a reference respiratory phase to assess respiration‐induced changes. Subsequently, cardiac binned fat‐water images were obtained using a model‐based, respiratory motion‐corrected image reconstruction. Results The 3D cardiac fat‐water imaging at 7T was successfully demonstrated. Local respiration‐induced frequency shifts in MC ΔB0 maps are small compared to the chemical shifts used in the multi‐peak model. Compared to the reference exhale ΔB0 map these changes are in the order of 10 Hz on average. Cardiac binned MC fat‐water reconstruction reduced respiration induced blurring in the fat‐water images, and flow artifacts are reduced in the end‐diastolic fat‐water separated images. Conclusion This work demonstrates the feasibility of 3D fat‐water imaging at UHF for the entire human heart despite spatial and temporal B1+ and B0 variations, as well as respiratory and cardiac motion.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Subject-tailored parallel transmission pulses for ultra-high fields body applications are typically calculated based on subject-specific 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 B + 1 -maps from a single gradient echo localizer to overcome long calibration times.Methods: 126 channel-wise, complex, relative 2D 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 B + 1 -shimming was subsequently performed on the estimated 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 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 B + 1 -maps comparable to the acquired ground truth and anatomical scans reflect the resulting B + 1 -pattern using the deep learning-based maps. Conclusion:The feasibility of estimating 2D relative 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 B + 1 -maps, it offers high potential for advancing clinical body imaging at ultra-high fields.

Purpose Design, implementation, evaluation, and application of a 32‐channel Self‐Grounded Bow‐Tie (SGBT) transceiver array for cardiac MR (CMR) at 7.0T. Methods The array consists of 32 compact SGBT building blocks. Transmission field (B1+) shimming and radiofrequency safety assessment were performed with numerical simulations and benchmarked against phantom experiments. In vivo B1+ efficiency mapping was conducted with actual flip angle imaging. The array’s applicability for accelerated high spatial resolution 2D FLASH CINE imaging of the heart was examined in a volunteer study (n = 7). Results B1+ shimming provided a uniform field distribution suitable for female and male subjects. Phantom studies demonstrated an excellent agreement between simulated and measured B1+ efficiency maps (7% mean difference). The SGBT array afforded a spatial resolution of (0.8 × 0.8 × 2.5) mm3 for 2D CINE FLASH which is by a factor of 12 superior to standardized cardiovascular MR (CMR) protocols. The density of the SGBT array supports 1D acceleration of up to R = 4 (mean signal‐to‐noise ratio (whole heart) ≥ 16.7, mean contrast‐to‐noise ratio ≥ 13.5) without impairing image quality significantly. Conclusion The compact SGBT building block facilitates a modular high‐density array that supports accelerated and high spatial resolution CMR at 7.0T. The array provides a technological basis for future clinical assessment of parallel transmission techniques.

This work demonstrates the design and application of frequency robust, subject-tailored or universal, non-selective kT-point pulses to achieve a homogeneous flip-angle within the 3D human heart at 7T across a 1400Hz wide frequency range that includes water and six fat frequencies. Frequency robust universal pulses were computed offline based on 31 3D B1+-maps and could be used in time-critical situations for calibration-free 3D heart flip-angle homogenization. Experimental data at 7T validates the flip-angle predictions and demonstrates successful excitation of fat and water in the human heart at 7T.