Diffusion-weighted imaging with flow-compensated diffusion encodings and tri-exponential IVIM imaging

Denis le Bihan等人的体素内非相干运动（IVIM）将磁共振弥散加权成像在低b值下观察到的强信号衰减归因于含有血液作为流动流体的隔室的存在，该隔室与仅经历弥散运动的组织隔室分离。IVIM模型具有吸引力，因为它允许在本地检查中评估灌注参数。正在进行的关于人脑中钆沉积的讨论进一步增加了这种吸引力。这一建议建立在最近的两项研究发现的基础上。首先，当使用短流量补偿（FloCo）扩散编码时，IVIM效应消失，但对于持续时间较长的FloCo扩散编码，IVIM效应会恢复。使用Denis le Bihan等人的IVIM模型来解释这种行为，使得血流相关时间τ和血管段长度l的评估成为可能，这些参数在肿瘤血管生成的背景下是重要的。其次，通常假设的双指数信号衰减模型在低b值时失效，而三指数模型（tri-exp）通常看起来更合适。这种三指数行为的起源目前尚不清楚，但可以很容易地想象出有吸引力的解释，例如将静脉和动脉室归因于三指数模型的两个流动室。本提案旨在阐明产生FloCo和三阶IVIM对比的机制。在实验上，这将通过在不同的回波时间（TE）和场强（B0）下执行flo和三阶IVIM来实现。这将有助于识别不同的室室（如静脉，动脉，尿前肾）的贡献，利用这些室室的横向松弛时间不同，而且依赖于B0。由于这些IVIM考试在实验上具有挑战性，因此提案的一部分侧重于优化数据采集策略。这些实验将在健康志愿者中进行。

The intravoxel incoherent motion (IVIM) by Denis le Bihan et al. attributes the observed strong signal decay in magnetic resonance diffusion-weighted imaging at low b-values to the presence of a compartment containing blood as a flowing fluid, which is separated from a tissue compartment that only experiences diffusive motion. The IVIM model is attractive because it allows assessment of perfusion parameters in native exams. Ongoing discussions about Gadolinium depositions in the human brain have further increased this attractiveness. This proposal builds on two recent research findings. First, the IVIM effect vanishes when using short flow-compensated (FloCo) diffusion encodings, but returns for FloCo diffusion encodings of longer duration. Using the IVIM model by Denis le Bihan et al. for interpreting this behavior makes possible the assessment of blood flow correlation time τ and vessel segment length l, which represent parameters that are important in the context of angiogenesis in oncology. Second, the often assumed bi-exponential signal decay model breaks down at low b-values and a tri-exponential model (tri-exp) often appears to be more appropriate. The origin of this tri-exponential behavior is currently unclear, but appealing interpretations can easily be imagined such as attributing venous and arterial compartment to the two flow-compartments of the tri-exponential model. This proposal aims at elucidating the mechanisms that generate the FloCo and tri-exp IVIM contrast. Experimentally, this shall be achieved by performing FloCo and tri-exp IVIM with varying echo time (TE) and field strength (B0). This shall help identifying the contributions of different compartments (such as venous, arterial, pre-urine in kidney) by making use of the fact that transversal relaxation times of these compartments differ and moreover depend on B0. Since these IVIM exams are experimentally challenging, one part of the proposal is focused on optimized data acquisition strategies. The experiments will be conducted measuring healthy volunteers.