Velocity Measurement of Fast Flows Inside Small Structures with Tagged MRI

Abstract

Velocity is one of the most important parameters to study in complex fluid flow systems. Magnetic resonance imaging (MRI) is a non-invasive tool for quantitative visualization of fast fluid flows. Many of the flow studies by MRI measure the velocity of a fluid media using either phase encoding or time of flight (TOF) measurements. Until now, motion-sensitized SPRITE is the best method for the velocity assessment of very fast flows when the structure is large enough compared to flow displacement during the encoding time ($$t_{{\text{p}}}$$). However, when displacement during the encoding time becomes greater than the pixel size, velocity misregistration takes place. To detect the velocity with accuracy, we need to reduce the encoding time. There is a restriction on the encoding time reduction in motion-sensitized SPRITE: both the velocity encoding, and spatial encoding times take place simultaneously, causing longer encoding time. If these two steps are separated, it is possible to reduce the encoding time. In this paper, we demonstrate how this can be achieved by applying a TOF approach where the fluid is labeled by a spatially periodic modulation of the magnetization. The modulation part is performed when the liquid is still moving slowly, and the imaging part does not require the bipolar gradient to be switched on and off. This leads to a reduction of the encoding time and greater accuracy in the detection of the fast flow velocity.