Assessment of Pulse Wave Velocity in the Aorta by using 4D Flow MRI

Abstract: The purpose of this master thesis was to evaluate the estimation of pulse wave velocity (PWV) in the aorta using 4D flow MRI. PWV is the velocity of the pressure wave generated by the heart during systole and is a marker of arterial stiffness and a predictor of cardiovascular disease (CVD). PWV can in principle be estimated based on the time (travel-time) it takes for the pulse wave to travel a fixed distance (travel-distance), or based on the distance the pulse wave travels during a fixed time. In the commonly used time-to-travel-a-fixed-distance approach, planes are placed at two or more locations along the aorta. The travel-time is found by studying velocity waveforms at these pre-defined locations over time and thereby by estimating the time-difference for the pressure wave to reach each of these locations. In the distance-travelled-in-a-fixed-time approach, the pulse wave is located by studying at the velocity along the aorta at pre-defined instances in time. The travel-distance for the pulse wave between two instances in time is set as the difference in location of the pulse wave, where the location is identified as the location when the velocity has reached a predefined baseline. The specific aims of this thesis was to investigate the effect of using multiple locations as well as the effects of temporal and spatial resolution in the time-to-travel-a-fixed-distance approach, and to evaluate the possibility of using the distance-travelled-in-a-fixed-time approach. Additionally, the possibility of combining the two approaches was investigated. The study of using multiple locations revealed that more planes reduces the uncertainty of PWV estimation. Temporal resolution was found to have a major impact on PWV estimation, whereas spatial resolution had a more minor effect. A method for estimating PWV using 4D flow MRI using the distance-travelled-in-a-fixed-time approach was presented. Values obtained were compared favourably against previous findings and reference values, in the case of healthy young volunteers. The combination of the time-to-travel-a-fixed-distance and distance-travelled-in-a-fixed-time approaches appears feasible.