Tendler Benjamin C
Wellcome Centre for Integrative Neuroimaging, FMRIB, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, UK.
Sci Rep. 2025 Jan 28;15(1):3580. doi: 10.1038/s41598-025-87377-x.
Diffusion MRI is a leading method to non-invasively characterise brain tissue microstructure across multiple domains and scales. Diffusion-weighted steady-state free precession (DW-SSFP) is an established imaging sequence for post-mortem MRI, addressing the challenging imaging environment of fixed tissue with short T and low diffusivities. However, a current limitation of DW-SSFP is signal interpretation: it is not clear what diffusion 'regime' the sequence probes and therefore its potential to characterise tissue microstructure. Building on Extended Phase Graphs (EPG), I establish two alternative representations of the DW-SSFP signal in terms of (1) conventional b-values (time-independent diffusion) and (2) encoding power-spectra (time-dependent diffusion). The proposed representations provide insights into how different parameter regimes and gradient waveforms impact the diffusion sensitivity of DW-SSFP. I subsequently introduce an approach to incorporate existing biophysical models into DW-SSFP without the requirement of extensive derivations, with time dependence estimated via a Gaussian phase approximation representation of the DW-SSFP signal. Investigations incorporating free-diffusion and tissue-relevant microscopic restrictions (cylinder of varying radius) give excellent agreement to complementary analytical models and Monte Carlo simulations. Experimentally, the time-independent representation is used to derive Tensor and proof-of-principle NODDI estimates in a whole human post-mortem brain. A final SNR-efficiency investigation demonstrates the theoretical potential of DW-SSFP for ultra-high field microstructural imaging.
扩散磁共振成像（Diffusion MRI）是一种用于跨多个领域和尺度对脑组织微观结构进行无创表征的主要方法。扩散加权稳态自由进动（DW-SSFP）是一种用于尸检磁共振成像的既定成像序列，可应对固定组织具有短T和低扩散率这一具有挑战性的成像环境。然而，DW-SSFP目前的一个局限性是信号解释：尚不清楚该序列探测的是什么扩散“机制”，因此也不清楚其表征组织微观结构的潜力。基于扩展相位图（EPG），我建立了DW-SSFP信号的两种替代表示形式，一种基于（1）传统b值（与时间无关的扩散），另一种基于（2）编码功率谱（与时间有关的扩散）。所提出的表示形式深入揭示了不同参数机制和梯度波形如何影响DW-SSFP的扩散敏感性。随后，我介绍了一种方法，可将现有的生物物理模型纳入DW-SSFP，而无需进行大量推导，通过DW-SSFP信号的高斯相位近似表示来估计时间依赖性。纳入自由扩散和与组织相关的微观限制（不同半径的圆柱体）的研究与互补的分析模型和蒙特卡罗模拟结果高度吻合。在实验中，与时间无关的表示形式被用于在完整的人类尸检大脑中推导张量和原理验证的神经突方向离散度与密度成像（NODDI）估计值。最后的信噪比效率研究证明了DW-SSFP在超高场微观结构成像方面的理论潜力。
