Siemens Healthcare Ltd, Rosh Ha'ayin, Israel.
Department of Brain Sciences, Weizmann Institute of Science, Rehovot, Israel.
Sci Rep. 2022 Aug 18;12(1):14088. doi: 10.1038/s41598-022-17607-z.
Magnetic resonance imaging (MRI) is a powerful and versatile technique that offers a range of physiological, diagnostic, structural, and functional measurements. One of the most widely used basic contrasts in MRI diagnostics is transverse relaxation time (T)-weighted imaging, but it provides only qualitative information. Realizing quantitative high-resolution T mapping is imperative for the development of personalized medicine, as it can enable the characterization of diseases progression. While ultra-high-field (≥ 7 T) MRI offers the means to gain new insights by increasing the spatial resolution, implementing fast quantitative T mapping cannot be achieved without overcoming the increased power deposition and radio frequency (RF) field inhomogeneity at ultra-high-fields. A recent study has demonstrated a new phase-based T mapping approach based on fast steady-state acquisitions. We extend this new approach to ultra-high field MRI, achieving quantitative high-resolution 3D T mapping at 7 T while addressing RF field inhomogeneity and utilizing low flip angle pulses; overcoming two main ultra-high field challenges. The method is based on controlling the coherent transverse magnetization in a steady-state gradient echo acquisition; achieved by utilizing low flip angles, a specific phase increment for the RF pulses, and short repetition times. This approach simultaneously extracts both T and RF field maps from the phase of the signal. Prior to in vivo experiments, the method was assessed using a 3D head-shaped phantom that was designed to model the RF field distribution in the brain. Our approach delivers fast 3D whole brain images with submillimeter resolution without requiring special hardware, such as multi-channel transmit coil, thus promoting high usability of the ultra-high field MRI in clinical practice.
磁共振成像（MRI）是一种强大且多功能的技术，提供了一系列生理、诊断、结构和功能测量方法。MRI 诊断中最广泛使用的基本对比之一是横向弛豫时间（T）加权成像，但它仅提供定性信息。实现定量高分辨率 T 映射对于个性化医疗的发展至关重要，因为它可以实现疾病进展的特征描述。虽然超高场（≥7T）MRI 通过提高空间分辨率提供了获得新见解的手段，但如果不克服超高场中增加的功率沉积和射频（RF）场不均匀性，就无法实现快速定量 T 映射。最近的一项研究展示了一种基于快速稳态采集的新基于相位的 T 映射方法。我们将这种新方法扩展到超高场 MRI 中，在 7T 时实现了定量高分辨率 3D T 映射，同时解决了 RF 场不均匀性问题，并利用低翻转角脉冲；克服了两个主要的超高场挑战。该方法基于在稳态梯度回波采集过程中控制相干横向磁化；通过利用低翻转角、RF 脉冲的特定相增量和短重复时间来实现。该方法可以从信号的相位中同时提取 T 和 RF 场图。在进行体内实验之前，该方法使用设计用于模拟大脑中 RF 场分布的 3D 头形体模进行了评估。我们的方法可以在不使用特殊硬件（如多通道发射线圈）的情况下，以亚毫米分辨率快速获取整个大脑的 3D 图像，从而提高超高场 MRI 在临床实践中的高可用性。
