Techniques and Methods Session 4: Rethinking Imaging of Function, Flow, Perfusion, and Morphology (2020)-Fundamentals of Flow Imaging and Processing

Video Transcription

Video Summary

The talk introduces the fundamentals of phase-contrast MRI and 4D flow imaging. MRI normally measures only magnitude, but phase contrast uses bipolar gradients to encode velocity in moving blood or tissue. Because phase offsets can also come from system imperfections, at least two acquisitions are needed: a reference scan and a flow-sensitive scan. ECG gating synchronizes measurements with the cardiac cycle.  <br /><br />The speaker explains 2D through-plane flow imaging and then 4D flow MRI, which adds three-directional velocity encoding and 3D volume coverage, usually requiring respiratory control and acceleration methods. The “VENC” setting must balance aliasing against noise: too low causes wraparound artifacts, too high reduces signal from slow flow.  <br /><br />Processing steps include correction for phase offsets, anti-aliasing, noise masking, angiogram creation, and vessel segmentation. The data can then be visualized with streamlines/pathlines and quantified for peak velocity, wall shear stress, and pressure gradients. Applications include congenital heart disease, bicuspid aortic valve, and aortic coarctation.

Keywords

phase-contrast MRI

4D flow imaging

velocity encoding

ECG gating

wall shear stress