Time-of-flight vs. phase contrast techniques for MRI velocimetry

Magnetic Resonance Imaging (MRI) is a non-invasive versatile tool for achieving full-field quantitative visualization. The MRI signal is a result of the interaction between radiofrequency (RF) pulses with nuclear spins exposed to a strong static magnetic field. The two main classes of techniques for MRI velocimetry are: spin-tagging techniques and phase contrast techniques. Spin-tagging techniques involve tagging and tracking a material volume of fluid. This allows a time-of-flight approach to the estimation of local velocity. Phase contrast is based on the difference in accumulated phase (time integral of angular frequency) in the signal arriving in the detector from moving and stationary spins exposed to magnetic field gradients. We compared velocity measurements with spin-tagging and phase contrast by probing the pressure-driven flow of water in a straight tube (Poiseuille flow). Profiles of the axial velocity along various cross sections were acquired for steady laminar flow with Reynolds numbers 170, 670, and 1000. Depending on the imaging sequence and Reynolds number, the velocity errors fell in the 0.5% - 3.2% range.