Variable velocity encoding in a three-dimensional, three-directional phase contrast sequence: Evaluation in phantom and volunteers.

PURPOSE: To evaluate accuracy and noise properties of a novel time-resolved, three-dimensional, three-directional phase contrast sequence with variable velocity encoding (denoted 4D-vPC) on a 3 Tesla MR system, and to investigate potential benefits and limitations of variable velocity encoding with respect to depicting blood flow patterns. MATERIALS AND METHODS: A 4D PC-MRI sequence was modified to allow variable velocity encoding (VENC) over the cardiac cycle in all three velocity directions independently. 4D-PC sequences with constant and variable VENC were compared in a rotating phantom with respect to measured velocities and noise levels. Additionally, comparison of flow patterns in the ascending aorta was performed in six healthy volunteers. RESULTS: Phantom measurements showed a linear relationship between velocity noise and velocity encoding. 4D-vPC MRI presented lower noise levels than 4D-PC both in phantom and in volunteer measurements, in agreement with theory. Volunteer comparisons revealed more consistent and detailed flow patterns in early diastole for the variable VENC sequences. CONCLUSION: Variable velocity encoding offers reduced noise levels compared with sequences with constant velocity encoding by optimizing the velocity-to-noise ratio (VNR) to the hemodynamic properties of the imaged area. Increased VNR ratios could be beneficial for blood flow visualizations of pathology in the cardiac cycle. J. Magn. Reson. Imaging 2012. © 2012 Wiley Periodicals, Inc