Objective

To comparatively assess the performance of highly selective pulses computed with the SLR algorithm in fast-spin echo (FSE) within the current radiofrequency safety limits using a metamaterial-based coil for wrist magnetic resonance imaging.

Methods

Apodized SINC pulses commonly used for clinical FSE sequences were considered as a reference. Selective SLR pulses with a time-bandwidth product of four were constructed in the MATPULSE program. Slice selection profiles in conventional T1-weighted and PD-weighted FSE wrist imaging pulse sequences were modeled using a Bloch equations simulator. Signal evolution was assessed in three samples with relaxation times equivalent to those in musculoskeletal tissues at 1.5T. Regular and SLR-based FSE pulse sequences were tested in a phantom experiment in a multi-slice mode with different gaps between slices and the direct saturation effect was investigated.

Results

As compared to the regular FSEs with a conventional transmit coil, combining the utilization of the metadevice with SLR-based FSEs provided a 23 times lower energy deposition in a duty cycle. When the slice gap was decreased from 100 to 0%, the “slice cross-talk” effect reduced the signal intensity by 15.9–17.6% in the SLR-based and by 22.9–32.3% in the regular T1-weighted FSE; and by 0.0–6.4% in the SLR-based and by 0.3–9.3% in the regular PD-weighted FSE.

Discussion and conclusion

SLR-based FSE together with the metadevice allowed to increase the slice selectivity while still being within the safe SAR limits. The “slice cross-talk” effects were conditioned by the number of echoes in the echo train, the repetition time, and T1 relaxation times. The approach was more beneficial for T1-weighted SLR-based FSE as compared to PD-weighted. The combination of the metadevice and SLR-based FSE offers a promising alternative for MR investigations that require scanning in a “Low-SAR” regime such as those for children, pregnant women, and patients with implanted devices.

中文翻译：

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使用基于超材料的 MRI 对连续多切片快速自旋回波成像的 SINC 形和 SLR 脉冲性能进行比较分析

客观的

使用基于超材料的线圈进行腕部磁共振成像，比较评估在当前射频安全限制内使用 SLR 算法计算的高选择性脉冲在快速自旋回波 (FSE) 中的性能。

方法

通常用于临床 FSE 序列的切趾 SINC 脉冲被视为参考。在 MATPULSE 程序中构建了时间带宽积为 4 的选择性 SLR 脉冲。使用 Bloch 方程模拟器对传统 T1 加权和 PD 加权 FSE 腕部成像脉冲序列中的切片选择配置文件进行建模。在三个样品中评估了信号演变，其弛豫时间相当于 1.5T 肌肉骨骼组织中的弛豫时间。常规和基于 SLR 的 FSE 脉冲序列在具有不同切片间隙的多切片模式的幻象实验中进行了测试，并研究了直接饱和效应。

结果

与具有传统发射线圈的常规 FSE 相比，将元设备的使用与基于 SLR 的 FSE 相结合，可在占空比中降低 23 倍的能量沉积。当切片间隙从 100% 降低到 0% 时，“切片串扰”效应使基于 SLR 的信号强度降低了 15.9-17.6%，在常规 T1 加权 FSE 中降低了 22.9-32.3%；基于 SLR 的 0.0-6.4% 和常规 PD 加权 FSE 的 0.3-9.3%。

讨论与结论

基于 SLR 的 FSE 与元设备一起允许增加切片选择性，同时仍处于安全的 SAR 限制范围内。“切片串扰”效应受回波序列中回波数量、重复时间和 T1 弛豫时间的影响。与 PD 加权相比，该方法对基于 T1 加权 SLR 的 FSE 更有利。元设备和基于 SLR 的 FSE 的组合为 MR 研究提供了一种有希望的替代方案，这些研究需要在“低 SAR”方案中进行扫描，例如针对儿童、孕妇和植入设备的患者。