Rapid Magnetic Resonance Imaging of Tissue Motion with Adaptive Velocity Sensitivity for Evaluating Cardiovascular Function



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The recognition that spin motion information can be encoded in the phase of a magnetic resonance imaging (MRI) signal led to the development of the so-called phase contrast MRI (PC-MRI) method. Despite the well-recognized benefits of PC-MRI, clinical adoption has lagged because these techniques suffer from velocity-to-noise ratio (VNR)/aliasing limitations and extended scan time. In this dissertation, we first investigated the possibility of a non-phase-dependent MR velocity mapping method, as such magnitude-dependent methods can address some of the problems currently faced by the conventional PC-MRI method. Secondly, we proposed and tested the feasibility of a dual-echo PC-MRI (DEPC) method in which each echo is acquired with a different velocity sensitivity within one repetition time (TR), as such scheme will lead to reduced scan time and improved VNR. We demonstrate a potential clinical application of DEPC in evaluating diastolic function from the simultaneous measurement of peak blood (E) and myocardial tissue (Em) velocities - which can differ by an order of magnitude. The response of the magnitude of an MR signal to tissue motion was investigated using theory, simulation and phantom experiments. Also, the proposed DEPC method was validated in flow phantom experiments and used to acquire velocity fields in vivo. Transmitral E/Em and deceleration time (DT) were estimated by using the proposed DEPC pulse sequence and compared with the conventional SEPC sequence in 14 human subjects. In both phantom and human studies, the velocities, E/Em and DT measured by using the SEPC and DEPC approaches were well correlated, and gave improved VNR with a minimum scan time reduction of about 20% for the DEPC approach. The MR signal magnitude showed a non-linear response to the bulk spin motion, with a time delay that is tissue and sequence-dependent. The proposed DEPC method is sensitive to two velocity regimes in a single TR, and can reduce the total scan time. Preliminary human studies indicate that this approach could be used for diastolic function analyses. The non-linear MR magnitude signal response to bulk spin motion gives motion information that must be interpreted with caution for quantitative velocity mapping.



Phase contrast magnetic resonance imaging, Simultaneous dual velocity encoding, Velocity-to-noise ratio, Diastolic function evaluation