Quantitative T2* Relaxation and Field Inhomogeneity Mapping Using Multi-Echo Radial FLASH Magnetic Resonance Imaging and Model-based Iterative Reconstruction

在多回波梯度回波磁共振成像(MRI)中，T2*弛豫和场不均匀的映射分别是指有效自旋-自旋弛豫时间(横向磁化强度的信号衰减)和非共振相位贡献的量化。一方面，T2*松弛是一种有价值的生物标志物，可用于临床诊断器官铁超载，以及通过注射造影剂后动态磁化率的变化来评估心肌氧合和组织血流灌注。另一方面，场的非均质性是组织敏感性图和化学位移成像(例如，水/脂肪分离)的基础。然而，当前的T2*松弛和场不均匀映射过程要么需要长时间的屏气和心电(ECG)门控，要么由于数据采样不足而受到图像伪影和不准确的影响。为了克服这些限制，我们开发了高度欠采样的多回波径向快速低角度激发(闪光)序列，它可以连续采样数据，而不需要屏气或心电门控。基于这种先进的采集技术，该项目的第一个目标是开发一种专用的基于模型的迭代重建技术，用于准确和精确的T2*弛豫和场不均匀映射。因此，第二个目标是动态研究人类心肌的氧合状态，并达到约60ms的时间分辨率和1 mm的空间分辨率。

Mapping of T2* relaxation and field inhomogeneity in multi-echo gradient-echo magnetic resonance imaging (MRI) refers to the quantification of effective spin-spin relaxation times (signal decay of the transversal magnetization) and off-resonance phase contributions, respectively. On the one hand, T2* relaxation is a valuable biomarker for clinical diagnosis of organ iron overload as well as for the assessment of myocardial oxygenation and tissue perfusion by dynamic susceptibility changes after contrast injection. On the other hand, field inhomogeneity is the basis for tissue susceptibility mapping and chemical shift imaging (e.g. water/fat separation). However, current procedures for T2* relaxation and field inhomogeneity mapping either require lengthy breath holding and electrocardiography (ECG) gating, or suffer from image artifacts and inaccuracy due to data undersampling. To overcome these limitations, we have developed a highly undersampled multi-echo radial fast low angle shot (FLASH) sequence, which continuously samples data without the need of breath holding or ECG gating. Based on this advanced acquisition technique, the first goal of this project is to develop a dedicated model-based iterative reconstruction technique for accurate and precise T2* relaxation and field inhomogeneity mapping. Accordingly, the second goal is to dynamically study the oxygenation state of the human myocardium and to achieve a temporal resolution of about 60 ms and a spatial resolution of 1 mm.