Three‐dimensional static and dynamic parallel transmission of the human heart at 7 T

• Published in: NMR in biomedicine Vol. 34; no. 3; pp. e4450 - n/a
• Main Authors: Aigner, Christoph Stefan, Dietrich, Sebastian, Schmitter, Sebastian
• Format: Journal Article
• Language: English
• Published: England Wiley Subscription Services, Inc 01.03.2021
• Subjects: Biological products; Body mass; Body mass index; Body size; Breathing; Cartesian coordinates; Coefficient of variation; Design optimization; Excitation; Feasibility studies; Heart; heart, kT‐points, parallel transmission, 7 T; Heterogeneity; Homogeneity; In vivo methods and tests; Mitigation; Radio frequency; Regularization; Respiration; Thorax; Tradeoffs; heart, kT-points, parallel transmission, 7 T
• ISSN: 0952-3480; 1099-1492; 1099-1492
• DOI: 10.1002/nbm.4450

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+ 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+ 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 study demonstrates the feasibility and benefits of subject‐specific spatially non‐selective kT‐point pulses to achieve 3D FA homogenization across the entire human heart at 7 T. The optimized kT‐point pulses were compared to static phase‐only B1+ shimming and were tested via various simulations for multiple kT‐points optimized for subject‐specific 3D B1+ maps. 3D GRE data acquired with radial phase encoding (RPE) in free breathing with subject‐specific dynamic kT‐point pulses validated the B1+ prediction.