High resolution optimal precision quantitative MRI at Ultrahigh Field

博士项目目标：利用超高频磁共振成像进行高分辨率定量神经成像利用先进的射频技术（并行传输，pTx）开发qMRI序列，目标是最大化整个大脑单位时间内实现的精度，量化脑组织中大分子的影响（MRI通常只关注液态水）项目描述/背景：MRI中的组织信号通常是许多因素的复杂函数，包括含水量，松弛时间（T1/T2）、大分子组成、大血管和微血管、脂肪含量、扩散特性等等。传统的磁共振成像使用标准方案，其组织对比度“加权”于一个或多个参数，放射科医生根据经验解释这些参数。定量MRI （qMRI）旨在直接测量这些重要参数，直接量化组织特性。这提供了在受试者之间进行定量比较或对同一受试者进行纵向比较的可能性，当与“大数据”方法的出现相结合时，可能会提高对大脑健康和疾病的理解。MRI的一个关键限制是可以实现的空间分辨率，通常在毫米范围内。新的超高场（UHF； 7T及以上）扫描仪可以潜在地实现更高的分辨率（低至100微米），新的7T MRI设备最近已在圣托马斯安装，目的是支持来自伦敦各地的临床和研究神经科学的广泛基础。然而，在7T下工作仍然存在特殊的挑战，包括高度空间不均匀的射频磁场（B1）以及严格的硬件和安全限制。B1不均匀性导致对比度的强烈变化，这可能是解释标准“加权”MRI的问题，并将导致qMRI可实现精度的巨大变化。特定吸收率（SAR）的限制意味着测量T2所需的方法（如平衡SSFP或自旋回波）是一个挑战。此外，先进的运动校正方法是必要的，以真正达到亚毫米的分辨率，因为即使是一个顺从的志愿者将在图像采集过程中不自主地移动在这个水平。“核磁共振指纹识别”（MRF）是qMRI领域的一项重要进展；通过使用一个不断变化的脉冲序列，不允许磁化达到一个稳定的状态，它已被证明是一个敏感的，有点运动容忍的方法。最近的工作主要集中在优化MRF，通过直接优化脉冲序列和图像重建来最大化估计精度。然而，现在人们普遍认识到，脑组织中水和大分子之间的“磁化转移”（MT）对于定量测量来说是一个很大的混淆，这包括传统的qMRI和MRF5。超高频下B1的高度不均匀性将使整个大脑的估计精度高度可变，并且由于MT效应与B12相关，因此效果将更强。

Aim of the PhD Project:Harness UHF MRI for high resolution quantitative neuroimagingDevelop qMRI sequences using advanced RF technology (parallel transmit, pTx) with the objective of maximising the achieved precision per unit time across the whole brainQuantify effects from macromolecules in brain tissue (MRI usually only looks at liquid water)Project Description / Background:The tissue signal in MRI is in general a complex function of many factors including water content, relaxation times (T1/T2), macromolecular composition, macro and microvasculature, fat content, diffusion properties and many more. Conventional MR imaging uses standard protocols whose tissue contrast is 'weighted' towards one or more parameter, and radiologists interpret these from experience. Quantitative MRI (qMRI) instead aims to directly measure many of these important parameters, to directly quantify tissue properties. This offers the possibility to make quantitative comparisons between subjects or longitudinally for the same subject, and when combined with the emergence of 'big data' methods could lead to improved understanding of the brain in health and disease.A key limitation for MRI is the spatial resolution that can be achieved, which is typically in the range of millimetres. New ultrahigh field (UHF; 7T and above) scanners can potentially achieve higher resolutions (down to 100s of microns) and a new 7T MRI facility has recently been installed at St.Thomas' with the objective of supporting a wide base of clinical and research neuroscience from across London. There are however still particular challenges for working at 7T, including highly spatially non-uniform radio frequency magnetic fields (B1) and stringent hardware and safety constraints. B1 non-uniformity leads to strong variations in contrast that can be a problem for interpretation of standard 'weighted' MRI, and which will cause large variations in achievable precision for qMRI. Limits on specific absorption rate (SAR) mean that methods needed for measurement of T2 (such as balanced SSFP or spin echo) are a challenge. Additionally, advanced motion correction methods are necessary to truly reach sub-millimetre resolution since even a compliant volunteer will move involuntarily at this level during image acquisition.'MR Fingerprinting' (MRF) is a significant recent development in qMRI; by using a constantly variable pulse sequence that does not allow magnetization to reach a steady state it has been shown to be a sensitive and somewhat motion tolerant approach. Recent work has focused on optimizing MRF to maximise estimation precision both by directly optimizing the pulse sequence and the image reconstruction. However it is now becoming widely acknowledged that 'magnetization transfer' (MT) between water and macromolecules in brain tissue is a strong confound for quantitative measurements4, and this includes both conventional qMRI and MRF5.The high degree of B1 non-uniformity at UHF will make estimation precision highly variable across the brain, and since MT effects are related to B12 the effect will be stronger.