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

Magnetic resonance imaging (MRI) is a versatile noninvasive tool for achiev ing full-field quantitative visualization of complex fluid flows. The MRI s ignal results from the interaction of radio-frequency (RF) pulses with nucl ear spins exposed to a strong static magnetic field. The two main technique s of MRI velocimetry are time-of-flight and phase contrast techniques. Time -of- flight techniques involve tagging and tracking a material volume of fl uid, whereas phase contrast techniques use magnetic field gradients to enco de velocity information into the phase of the MRI signal. In this study, bo th 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 e xperiments show that the phase contrast method gives more accurate results, with 90% of the measurements within 10% of the local computational fluid d ynamics (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-f light experiments were not as accurate, they provide a good qualitative ima ge of the flow field. Sources of the discrepancies between the MRI data and the CFD results are also discussed, including acceleration and spin flow-t hrough artifacts.