Multidirectional high-moment encoding in phase contrast MRI

Abstract

The use of phase contrast MRI to measure vascular flow provides a unique method for acquiring quantitative estimates of flow as well as morphological imaging. The quantitative aspects of phase contrast magnetic resonance angiography (PC-MRA) provide unique relationships between measurement parameters and resulting signal to noise ratio of the velocity measurements. This article introduces a new method to exploit these relationships providing increased efficiency, and therefore, higher vessel conspicuity. Signal to noise ratio gains in high-moment PC-MRA are limited by the ability to unalias phase measurements that fall outside the -π to π interval. Unaliasing phase on a per pixel basis is limited by errors in the measurements due to noise and intravoxel flow distributions. Current dual-VENC methods have been shown to be robust to these errors and provide high velocity to noise ratio gains, however, the collection of a required high-VENC set can be inefficient. The presented method provides more time efficient gains in velocity to noise ratio compared to a dual-VENC approach by eliminating the high-VENC acquisitions and using shared information between nonorthogonal measurements. Simulations, phantom, and in vivo angiography are used to characterize the noise performance of each method. The velocity to noise ratio efficiency of the proposed method is shown to be ∼1.7 times greater than the dual-VENC method at the same gradient moment.