High resolution optimal precision quantitative MRI at Ultrahigh Field

Lead Research Organisation: King's College London
Department Name: Imaging & Biomedical Engineering

Abstract

Aim of the PhD Project:

Harness UHF MRI for high resolution quantitative neuroimaging
Develop qMRI sequences using advanced RF technology (parallel transmit, pTx) with the objective of maximising the achieved precision per unit time across the whole brain
Quantify 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.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/S022104/1 01/10/2019 31/03/2028
2435136 Studentship EP/S022104/1 01/10/2020 30/09/2024 Felix Horger