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 degrees. 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.