Fast and Accurate 4D Flow MRI for Cardiovascular Blood Flow Assessment
Sammanfattning: The investigation of blood flow is essential in understanding the physiology and pathophysiology of the cardiovascular system. Small irregularities in blood flow may over time evolve to more serious conditions. While the blood flow in a healthy human appears to be predominately laminar, turbulent or transitional blood flow is thought to be involved in the pathogenesis of several cardiovascular diseases. Wall shear stress (WSS) is the frictional force of blood on the vessel wall and has been linked to the pathogenesis of atherosclerosis and aneurysms. Despite the importance of hemodynamic factors, cardiovascular diagnostics largely relies on the indirect estimation of function based on morphological data.Time-resolved three-dimensional (3D) phase-contrast magnetic resonance imaging (MRI), often referred to as 4D flow MRI, is a versatile and non-invasive tool for cardiovascular blood flow assessment. The use of 4D flow MRI permits estimation of flow volumes, pressure losses, WSS, turbulence intensity and many other unique hemodynamic parameters. However, 4D flow MRI suffer from long scan times, sometimes over 40 minutes. Furthermore, the accuracy of the many different 4D flow MRI-based applications and estimates have not been thoroughly examined.In this thesis, the accuracy of PC-MRI turbulence mapping and MRI-based WSS estimation was investigated by using numerical simulations of MRI flow measurements. While the results from the turbulence mapping agreed well with reference values from computational fluid dynamics data, the accuracy of the MRI-based WSS estimates were found to be highly sensitive to different parameters, especially to spatial resolution, and WSS values over 5 N/m2 were not well resolved.To reduce the scan time, a 4D flow MRI sequence using spiral k-space trajectories was implemented and validated in-vivo and in-vitro. The scan time was reduced by more than two-fold compared to a conventional Cartesian acquisition already accelerated using SENSE factor 2, and the data quality was maintained. For a 4D flow scan of the human heart, the use of spiral k-space trajectories resulted in a scan time of around 13 min, compared to 30 min for the Cartesian acquisition. By combining parallel imaging and spiral trajectories, the total scan time for a 4D flow measurement of the whole heart may in a near future be reduced to less than 5 min. The scan time reduction may also be traded for higher spatial resolution.Numerical simulations of 4D flow measurements may act as an important tool for future optimization and validation of the spiral 4D flow sequence. The scan-time reductions offered by the spiral k-space trajectories can help to cut costs, save time, reduce discomfort for the patient as well as decrease the risk for motion artifacts. These benefits may facilitate an expanded clinical and investigative use of 4D flow MRI, including larger patient research studies.
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