Velocity measurements of flow through a step stenosis using Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is a versatile noninvasive tool for achieving full-field quantitative visualization of complex fluid flows. The MRI signal results from the interaction of radio-frequency (RF) pulses with nuclear spins exposed to a strong static magnetic field. The two main techniques of MRI velocimetry are time-of-flight and phase contrast techniques. Time-of- flight techniques involve tagging and tracking a material volume of fluid, whereas phase contrast techniques use magnetic field gradients to encode velocity information into the phase of the MRI signal. In this study, both techniques are used to probe the pressure-driven steady flow of water in a pipe with a step stenosis. The velocity measurements were then compared with computational results obtained using the FIDAP software package. The experiments show that the phase contrast method gives more accurate results, with 90% of the measurements within 10% of the local computational fluid dynamics (CFD) velocity predictions at Re = 100 and 94% of the measurements within 10% of the local CFD predictions at Re = 258. Although the time-of-flight experiments were not as accurate, they provide a good qualitative image of the flow field. Sources of the discrepancies between the MRI data and the CFD results are also discussed, including acceleration and spin flow-through artifacts.