Biomedical Engineering - Theses
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ItemContinuous wave nuclear magnetic resonance: estimation of spin-system properties from steady-state trajectories( 2017)Magnetic resonance imaging (MRI) is a powerful imaging modality, widely used in routine clinical practice and as an investigational tool in basic science. The contrast in MRI is related to both the underlying tissue properties, which undergo disease or injury related changes, and to the MRI method and sequence parameters used. It is the latter with which this thesis is concerned: the design and implementation of novel MRI acquisition paradigms and associated reconstruction methods. The majority of MRI methods excite the object of interest with a series of short RF pulses, varying the weaker spatial magnetic field using the gradients, and ensuring the RF transmitter is inactive while acquiring a series of decaying MR signals. This regime linearises the inherently nonlinear behaviour of a magnetic resonance spin-system, allowing the acquired signals to be considered in a spatial frequency space and an image to be reconstructed using the well known Fourier transform. It is our assertion that nonlinear behaviour of the magnetic spin signal will lead to advantageous attributes in future MR methods, just as moving beyond conventional linear spatial gradients to nonlinear encoding fields led to methods for accelerated imaging and variable spatial resolution. Reconstruction of spin-system properties from nonlinear MR signals requires algorithms beyond the Fourier transform. In this thesis we propose spectroscopy, radial projection imaging and relaxometry methods as optimisation problems which minimise the mismatch between experimental measurements and predictions from Bloch equation based signal models. The use of continuous wave (CW) excitation patterns allows the development of signal models which are computationally efficient as they rely on analytical solutions of the Bloch equations or matrix inversion via harmonic balancing, rather than numerical integration. Ultra-short relaxation methods have been applied to a range of applications and demonstrate that MRI is finding use in areas far beyond traditional soft-tissue imaging. Soft tissues have an easily observable long duration MR signal, whereas the signal decays rapidly for harder tissues such as bone, or in regions that distort the magnetic field due to magnetic susceptibility gradients, such as the lungs. Rabi modulated CW techniques operating in a fully continuous mode have the potential to measure ultra-short relaxation signals in a similar range to `true' zero echo time techniques. Work inspired by quantum optics has shown that exciting a spin-system with a long duration Rabi modulated RF field leads to a significant steady-state MR signal. The steady-state trajectory is highly nonlinear and can be expressed as a series of harmonics of the amplitude modulation frequency of the RF field. This harmonic response provides a natural decoupling of the excitation and measurement bandwidth, and the ability to maintain a steady-state response under low power excitation reduces the isolation requirements between hypothetical transmit and receive chains. Our experimental investigation of steady-state trajectories makes use of two pseudo-simultaneous excitation and measurement protocols. Whilst these methods were adequate to explore the proof-of-concept applications, hardware modifications are suggested to unlock the full potential of continuous wave excitation patterns. This thesis demonstrates that CW excitation patterns allow the construction of efficient prediction models and elicit an information-rich steady-state response from which underlying spin-system properties can be reconstructed. It is anticipated that further development of these concepts and related hardware modifications will lead to new continuous wave imaging paradigms.