Analysis of encoding efficiency in MR imaging of velocity magnitude and direction

The efficiency of balanced versus unbalanced techniques for phase‐angle‐based velocity magnitude and direction imaging is investigated. Methods having balanced flow‐encoding gradients (gradients in positive and negative directions with a zero center of gravity) are compared with unbalanced methods. For three‐dimensional imaging, a currently used balanced method is the six‐point technique having opposed gradients pairs for each orthogonal direction. A currently used unbalanced method is a four‐point null technique which has three orthogonal gradients and an additional acquisition having no specific flow encoding to correct the baseline (null) phase. In the gradient‐limited case of slow flow and perfusion. the balanced method is predicted to have higher velocity magnitude‐to‐noise ratio per time (SNR v ) by a factor of 1.63, with similar results for velocity direction. In the wraparound‐limited case of faster flows and motions, similar results are found when a null acquisition is added to the balanced method. This results in a seven‐point balanced method having an SNR v , 1.51 times that of the four‐point unbalanced method. If null phases are within the [–π/2, π/2] interval, this additional null acquisition is unnecessary. Other four‐point methods are also considered. These results indicate that, in general, balanced methods have advantages over unbalanced methods for velocity imaging. © 1992 Academic Press, Inc.