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Emerging Imaging

High resolution optimal precision quantitative MRI at Ultrahigh Field

Project ID: 2020_003

1st Supervisor: Shaihan Malik, King’s College London
2nd Supervisor: Jo Hajnal, King’s College London
Clinical Champion: Alexander Hammers, King’s College London
Industry Supervisor: Raphael Tomi-Tricot

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 µm) 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)1 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 sequence2 and the image reconstruction3. 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.

This project seeks to address these challenges, to produce a robust and high precision high resolution quantitative neuro examination for 7T MRI, using parallel transmit (pTx) technology as an additional degree of freedom in sequence design.

References:

  1. Ma, D. et al. Magnetic resonance fingerprinting. Nature 495, 187–192 (2013).
  2. Assländer, J., Lattanzi, R., Sodickson, D. K. & Cloos, M. A. Optimized Quantification of Spin Relaxation Times in the Hybrid State. (2019). doi:10.1017/CBO9781107415324.004
  3. Assländer, J. et al. Low rank alternating direction method of multipliers reconstruction for MR fingerprinting. Magn. Reson. Med. 79, 83–96 (2018).
  4. A.G. Teixeira, R. P., Malik, S. J. & Hajnal, J. V. Fast quantitative MRI using controlled saturation magnetization transfer. Magn. Reson. Med. 81, 907–920 (2019).
  5. Malik, S. J., Teixeira, R. P. A. G. & Hajnal, J. V. Extended phase graph formalism for systems with magnetization transfer and exchange. Magn. Reson. Med. 80, 767–779 (2018).
Figure 1: T1 mapping provides a quantitative measure of brain tissue properties that can be used for studying the brain health and disease. 7T MRI could be used to achieve much higher spatial resolution but the radiofrequency magnetic field varies strongly in the brain at this field strength. The result is highly variable precision in T1 estimation. This project seeks to create high precision high resolution quantitative maps in the brain

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