Lasič Samo, Just Nathalie, Nilsson Markus, Szczepankiewicz Filip, Budde Matthew, Lundell Henrik
Danish Research Centre for Magnetic Resonance, Department of Radiology and Nuclear Medicine, Copenhagen University Hospital-Amager and Hvidovre, Copenhagen, Denmark.
Department of Diagnostic Radiology, Lund University, Lund, Sweden.
Imaging Neurosci (Camb). 2025 Jun 11;3. doi: 10.1162/IMAG.a.35. eCollection 2025.
Tensor-valued encoding in diffusion MRI allows probing of microscopic anisotropy in tissue, however, time-dependent diffusion (TDD) can bias results unless b-tensors are carefully tuned to account for TDD. We propose two novel strategies for tuning b-tensors to enable accurate measurements without interference from TDD due to restricted diffusion. The first strategy involves identifying encoding tensor projections that yield equal mean diffusivities (MD), providing robust tuning across a wide range of diffusion spectra. The second strategy uses geometric averaging of signals, ensuring tuning regardless of the diffusion spectra. Importantly, the same encoding waveforms used for geometric averaging to probe microscopic anisotropy (µA) can also generate an independent contrast due to TDD. This is enabled by considering spectral anisotropy of encoding and defining the spectral principal axis system (SPAS), which unfolds TDD as an additional independent dimension in tensor-valued encoding. Projections of encoding waveforms along the SPAS axes allow for the simultaneous acquisition of independent contrasts due to both µA and TDD within a single multidimensional diffusion encoding protocol. Additionally, SPAS projections inherit useful properties from the reference tensor, such as optimized b-value, motion nulling, and minimal concomitant field effects. This framework is demonstrated through simulations of various restricted diffusion compartments. Experimental validation on perfusion-fixed andrat brains highlights the method's potential for enhanced microstructural specificity. In addition to mapping MD, fractional anisotropy, and unbiased microscopic fractional anisotropy, we propose a model-free approach to independently map µA and TDD. This approach uses a minimal yet highly specific protocol, enabling the identification of distinct µA-TDD contrasts across different brain regions, including details in cortical gray matter, choroid plexus, dentate gyrus of the hippocampus, and white matter.
扩散磁共振成像中的张量值编码能够探测组织中的微观各向异性，然而，时间依赖性扩散（TDD）会使结果产生偏差，除非对b张量进行仔细调整以考虑TDD。我们提出了两种新颖的策略来调整b张量，以实现不受受限扩散导致的TDD干扰的准确测量。第一种策略涉及识别产生相等平均扩散率（MD）的编码张量投影，从而在广泛的扩散谱范围内提供稳健的调整。第二种策略使用信号的几何平均，无论扩散谱如何都能确保调整。重要的是，用于几何平均以探测微观各向异性（µA）的相同编码波形，也会由于TDD产生独立的对比度。这是通过考虑编码的光谱各向异性并定义光谱主轴系统（SPAS）来实现的，该系统将TDD展开为张量值编码中的一个额外独立维度。沿着SPAS轴的编码波形投影允许在单个多维扩散编码协议中同时获取由于µA和TDD产生的独立对比度。此外，SPAS投影继承了参考张量的有用属性，如优化的b值、运动归零和最小的伴随场效应。通过对各种受限扩散隔室的模拟展示了该框架。对灌注固定大鼠脑的实验验证突出了该方法在增强微观结构特异性方面的潜力。除了绘制MD、分数各向异性和无偏微观分数各向异性外，我们还提出了一种无模型方法来独立绘制µA和TDD。这种方法使用了一个最小但高度特异的协议，能够识别不同脑区中独特的µA - TDD对比度，包括皮质灰质、脉络丛、海马齿状回和白质中的细节。
