High resolution optimal precision quantitative MRI at Ultrahigh Field

博士项目的目的：用于高分辨率定量神经成像的超高频MRI使用先进的RF技术（并行传输，pTx）开发qMRI序列，目的是最大限度地提高整个大脑每单位时间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，包括水含量，弛豫时间（T1/T2），大分子组成，宏观和微血管，脂肪含量，扩散特性等等。传统的MR成像使用标准协议，其组织对比度被“加权”到一个或多个参数，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中的许多参数，以直接量化组织特性。这提供了在受试者之间或对同一受试者进行纵向定量比较的可能性，并且当与“大数据”方法的出现相结合时，可以提高对健康和疾病中大脑的理解。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。新超高频（UHF; 7 T及以上）扫描仪可以潜在地实现更高的分辨率（低至100微米），并且最近在St.托马斯安装了一个新的7 T MRI设备，其目的是支持来自伦敦各地的广泛的临床和研究神经科学基础。然而，在7 T下工作仍然存在特殊的挑战，包括高度空间非均匀的射频磁场（B1）以及严格的硬件和安全限制。B1非均匀性导致对比度的强烈变化，这可能是标准“加权”MRI解释的问题，并且这将导致qMRI可实现精度的较大变化。对比吸收率（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，真正达到亚毫米分辨率，因为即使是一个顺从的志愿者将在图像采集过程中不自觉地移动在这个水平。“MR指纹”（MRF）是qMRI中的一项重要的最新发展;通过使用不允许磁化达到稳定状态的恒定可变脉冲序列，它已被证明是一种灵敏且有点运动容忍的方法。最近的工作集中在优化MRF，以最大限度地提高估计精度直接优化脉冲序列和图像重建。然而，现在人们普遍认为，脑组织中水和大分子之间的“磁化转移”（MT）是定量测量的一个强干扰因素4，这包括传统的qMRI和MRF 5。UHF下B1的高度非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，因此效应将更强。

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.