Velocity‐selective RF pulses in MRI

A family of velocity‐selective pulses consisting of a series of RF hard pulses followed by bipolar gradients was designed. The succession of required pulses was deduced using a k ‐space approach within a small tip‐angle approximation. Fourier transform of the desired velocity excitation determined the flip‐angle series, and the corresponding position in the generalized k ‐space identified the bipolar‐gradient first moments. Spins from any velocity class can be selected. To illustrate this approach we designed and experimentally tested a velocity‐slice selection that is analogous to standard spatial‐slice selection but involves excitation of spins moving at a chosen velocity (velocity‐slice center) and within a given interval (velocity‐slice thickness). The assumed approximation does not limit the design to small angles, because velocity selection still holds for angles up to 90°. Velocity slices were experimentally selected, centered on velocities ranging from −1 m s −1 to 1 m s −1 with a velocity‐slice thickness of 0.4 m s −1 . The experimental velocity‐slice profile was assessed and the flow was quantified. Magn Reson Med, 2006. © 2005 Wiley‐Liss, Inc.