Piredda Gian Franco, Sleight Emilie, Yu Thomas, Klauser Antoine, Pato Montemayor Natalia, Philippe Jocelyn, Bacha Lina, Di Noto Tommaso, Maréchal Bénédicte, Liebig Patrick A, Nickel Dominik, Kober Tobias, Hilbert Tom, Heidemann Robin M, Herrler Jürgen
Swiss Innovation Hub, Siemens Healthineers International AG, Lausanne, Switzerland.
Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Lausanne, Switzerland.
Magn Reson Med. 2025 Oct;94(4):1616-1625. doi: 10.1002/mrm.30565. Epub 2025 May 24.
Although clinical 7T MRI offers various advantages compared to lower field strengths, achieving spatially uniform flip angle distributions remains a challenge. Sampling Perfection with Application optimized Contrast using different flip angle Evolution (SPACE) sequences employing a long train of refocusing pulses with varying flip angles pose a particular challenge in that regard. In this study, we investigate scalable dynamic parallel transmission (pTx) pulses to achieve homogeneous 3D high-resolution SPACE brain imaging at 7T.
Non-parametrized and scalable dynamic pTx pulses were designed for excitation, refocusing and inversion in SPACE sequences by using fast online customization (FOCUS). First, a database of B and multi-channel maps were used for optimizing universal pulses and parameters for flip angle homogeneity under strict specific absorption rate (SAR) constraints. During each new examination, B and maps were acquired as additional calibration step and pTx pulses were tailored to the subject. For scalability, a symmetry condition was enforced. T, T, fluid-attenuated inversion recovery (FLAIR) and double inversion recovery (DIR) SPACE images were acquired in five healthy subjects at 7T using the proposed FOCUS pulses and conventional circularly polarized (CP) pulses for comparison.
Improved SNR and better image homogeneity were observed in every image acquired with FOCUS pulses in comparison to CP. Quantitative analysis showed a significant reduction in the coefficient of variation (COV) of image intensities in the cerebellum, a region notably affected by inhomogeneities across all contrasts. FLAIR images, for example, exhibited a 46% COV reduction.
Individually optimized dynamic pTx pulses for 3D high-resolution SPACE imaging delivered clinically acceptable image homogeneity, enabling the application of widely used clinical contrasts at 7T.
尽管临床7T磁共振成像（MRI）与较低场强相比具有多种优势，但实现空间均匀的翻转角分布仍然是一项挑战。使用具有不同翻转角的长串重聚焦脉冲的应用优化对比度的采样完美（SPACE）序列在这方面带来了特别的挑战。在本研究中，我们研究了可扩展的动态并行传输（pTx）脉冲，以在7T下实现均匀的3D高分辨率SPACE脑成像。
通过使用快速在线定制（FOCUS），设计了非参数化且可扩展的动态pTx脉冲，用于SPACE序列中的激发、重聚焦和反转。首先，使用B和多通道图谱数据库在严格的特定吸收率（SAR）约束下优化通用脉冲和翻转角均匀性参数。在每次新检查期间，采集B和图谱作为额外的校准步骤，并根据受试者定制pTx脉冲。为了实现可扩展性，实施了对称条件。在7T下，对五名健康受试者使用所提出的FOCUS脉冲和传统圆极化（CP）脉冲采集T1、T2、液体衰减反转恢复（FLAIR）和双反转恢复（DIR）SPACE图像进行比较。
与CP相比，使用FOCUS脉冲采集的每张图像均观察到信噪比提高和图像均匀性改善。定量分析表明，小脑区域图像强度的变异系数（COV）显著降低，该区域在所有对比度下均明显受到不均匀性的影响。例如，FLAIR图像的COV降低了46%。
用于3D高分辨率SPACE成像的单独优化的动态pTx脉冲提供了临床可接受的图像均匀性，使得在7T下能够应用广泛使用的临床对比度。
