Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biomedical Engineering

Supervisor

Dr Brian Rutt

Abstract

In the ongoing quest to extract more information from MRI images, there has arisen a need to rapidly map the flip angle. This has been primarily driven by the shift to stronger main field strengths, which bring with them improved SNR, but also new difficulties. In particular, the radio frequency field used to excite the magnetization can no longer be assumed uniform at field strengths of 3 Tesla and above. New rapid quantitative imaging techniques, such as DESPOT1 and DESPOT2, rely on accurate knowledge of the flip angle, and while this could safely be assumed to be the prescribed value at 1.5 Tesla, the same can no longer be said at 3 Tesla and above.

In this thesis, a new technique for mapping the flip angle in 3D will be introduced. This technique is based on a highly time efficient T1 mapping technique, known as Look-Locker, which was adapted to map the flip angle using either a double angle (DALL) or dual repetition time (DτLL) approach. The technique introduced is capable of producing 3D maps of the flip angle in less than one minute.

As part of the development of this flip angle mapping technique, a theoretical framework was developed to allow the optimization of the imaging parameters to achieve the best possible measurement of the flip angle. This framework also made it possible to compare the performance of the Look-Locker approaches to other techniques currently in use, which confirmed that, for imaging times less than one minute, the Look-Locker approach was optimal.

This fast imaging led to the development of a new approach to spoiling. While conventional spoiling approaches are optimized for imaging in the steady state, they can lead to significant errors and artefacts if used when sampling the transients of the Look-Locker method, especially with the short repetition times and large flip angles used. A newly developed spoiling technique using randomized RF phase and gradient crusher amplitudes was shown to significantly reduce the deviations from ideally spoiled transients, and thereby reduce the errors associated with the quantitative values derived from them.

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