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Three‐dimensional static and dynamic parallel transmission of the human heart at 7 T
NMR in Biomedicine ( IF 2.7 ) Pub Date : 2020-12-16 , DOI: 10.1002/nbm.4450 Christoph Stefan Aigner 1 , Sebastian Dietrich 1 , Sebastian Schmitter 1, 2
NMR in Biomedicine ( IF 2.7 ) Pub Date : 2020-12-16 , DOI: 10.1002/nbm.4450 Christoph Stefan Aigner 1 , Sebastian Dietrich 1 , Sebastian Schmitter 1, 2
Affiliation
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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 
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 
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 
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.
中文翻译：
7T下人体心脏的三维静态和动态平行传输
由于呼吸和心脏运动引起的伪影以及空间异质性轮廓，超高场的三维 (3D) 人体心脏成像极具挑战性。在这项研究中，我们研究了通过仅静态相位和动态 kT 点体内平行传输在 7 T 下应用针对整个心脏的 3D 翻转角 (FA) 均质化的可行性。胸部的3D图是在自由呼吸下获得的八名受试者计算并行传输脉冲，以改善人类心脏中的激发均匀性。为了分析所需的 kT 点数，使用 6 名不同体重指数 (BMI) 值为 20-34 kg/m 2 的受试者的不同感兴趣区域来比较激发均匀性和射频 (RF) 功率用于广泛的正则化参数。在笛卡尔 3D 屏气扫描中对六名受试者以及在径向相位编码 3D 自由呼吸扫描中对两名受试者在 7 T 扫描仪上体内应用优化的受试者特定脉冲的一个子集。在所有受试者中，3-4 kT 点在 RF 功率和标称 FA 均匀性之间取得了良好的平衡。对于 BMI 在正常范围内的受试者，4 kT 点脉冲可靠地将变异系数提高了不到 10%，而静态仅相位并行传输实现的变异系数不到 25%。尽管忽略了 ΔB 0，但在 7 T 扫描仪上进行的体内测量验证了估计和脉冲设计在优化和 Bloch 模拟中。该研究在体内证明 kT 点 pTx 脉冲非常适合在 7 T 下减轻整个 3D 心脏体积的名义 FA 异质性。 此外，3-4 kT 点证明了名义 FA 异质性缓解和 RF 功率之间的实际权衡。
更新日期：2021-02-04
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 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 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.
中文翻译：
7T下人体心脏的三维静态和动态平行传输
由于呼吸和心脏运动引起的伪影以及空间异质性
轮廓，超高场的三维 (3D) 人体心脏成像极具挑战性。在这项研究中，我们研究了通过仅静态相位和动态 kT 点体内平行传输在 7 T 下应用针对整个心脏的 3D 翻转角 (FA) 均质化的可行性。胸部的3D图是在自由呼吸下获得的八名受试者计算并行传输脉冲，以改善人类心脏中的激发均匀性。为了分析所需的 kT 点数，使用 6 名不同体重指数 (BMI) 值为 20-34 kg/m 2 的受试者的不同感兴趣区域来比较激发均匀性和射频 (RF) 功率用于广泛的正则化参数。在笛卡尔 3D 屏气扫描中对六名受试者以及在径向相位编码 3D 自由呼吸扫描中对两名受试者在 7 T 扫描仪上体内应用优化的受试者特定脉冲的一个子集。在所有受试者中，3-4 kT 点在 RF 功率和标称 FA 均匀性之间取得了良好的平衡。对于 BMI 在正常范围内的受试者，4 kT 点脉冲可靠地将变异系数提高了不到 10%，而静态仅相位并行传输实现的变异系数不到 25%。尽管忽略了 ΔB 0，但在 7 T 扫描仪上进行的体内测量验证了估计和脉冲设计在优化和 Bloch 模拟中。该研究在体内证明 kT 点 pTx 脉冲非常适合在 7 T 下减轻整个 3D 心脏体积的名义 FA 异质性。 此外，3-4 kT 点证明了名义 FA 异质性缓解和 RF 功率之间的实际权衡。
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