Fast measurements of blood flow using magnetic resonance

grantor: University of TorontoThis thesis investigates the non-invasive measurement of blood flow in large arteries using magnetic resonance (MR). Functional information can be obtained from such measurements, including stenosis grade, tissue perfusion, and vessel-wall elasticity. Unfortunately, traditional MR measurements of blood flow are relatively slow, and must combine data acquired from different cardiac cycles. Measurement errors may be introduced by this procedure if the flow is not purely periodic (e.g., due to respiration or arrhythmia). A solution to this problem is to collect an independent flow measurement each heartbeat. This thesis develops MR methods for this purpose using volume-selective excitations with reduced spatial encoding, with the goal of measuring aortic stiffness from data collected in one heartbeat. This work is divided into three incremental stages. The first develops a method to track the one-dimensional (1D) displacement of an excited bolus of blood. Displacement is derived from the frequency of the MR signal, which changes as the bolus moves through a magnetic-field gradient. The second stage extends this work by exciting blood at multiple positions along the vessel, and recording their displacements simultaneously using rapid 1D projections. From these 1D measurements, an expression for the velocity spectrum of the blood is also derived. The influence of specific excitation profiles and velocity profiles on these measurements is explored. The final stage applies the measurement to pulsatile aortic flow. An algorithm that detects the onset of systolic flow at each vessel position is developed and used to measure the aortic pulse-wave velocity (PWV), a parameter related to vessel stiffness. Measurements of the PWV in a single heartbeat are presented. This thesis demonstrates that volume-localization can be used to obtain independent blood-flow measurements each heartbeat. This is significant because it enables measurement of blood motion despite potential beat-to-beat fluctuations in the flow. The research culminates in a non-invasive measurement of the pulse-wave velocity that could facilitate the study of vascular mechanics. Use of this tool could lead to a greater understanding of cardiovascular physiology, particularly the mechanism through which disease processes and therapeutic interventions affect the mechanical properties of the aortic wall.Ph.D