Optimal Pulse Sequences for Magnetic Resonance Elastography
thesisposted on 28.10.2014 by Temel K. Yasar
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Magnetic Resonance Elastography (MRE) is a non-invasive phase contrast MR imaging method that captures the three-dimensional harmonic wave propagation introduced into subject by external actuators. This wave propagation vector field is processed into stiffness maps of various kinds that are used to assess the pathological changes that cannot be detected otherwise with non-invasive imaging methods. As in all other MR imaging methods, long acquisition duration is one of the important limiting factors for MRE. There are different approaches to reduce the scan time, such as as reduced motion encoding MRE or fractional multi-frequency MRE; however, these methods are all at the cost of the reduced signal to noise ratio (SNR) or reduced phase to noise ratio (PNR). Recently we have introduced two accelerated MRE methods, which do not compromise SNR or PNR while reducing the acquisition time by a factor of three compared to the conventional MRE methods. The first one is Selective Spectral Displacement Projection (SDP) MRE method that can encode a mechanical motion of multiple frequency components at once. The second one is SampLe Interval Modulation (SLIM) MRE which can encode the mono-frequency motion in multiple directions concurrently. In this dissertation, I propose a final optimal method that integrates the technique developed in SLIM MRE into SDP MRE, namely Unified sampLing Time Interval ModulATion (ULTIMATe) MRE. This method is the optimal MRE method in the sense that it can reach the limit of time efficiency without sacrificing SNR and PNR. A new mathematical framework was introduced to accommodate all three methods while preventing any ambiguity which might otherwise can occur with the existing MRE notation.