Velocity-selective excitation: principles and applications.

Overview

Velocity-selective (VS) excitation is a relatively new type of excitation that can be useful for generating image contrast based on spin's motion. This review aims to explain the principles of VS excitation and their utilization for clinical applications. We first review the generalized excitation k-space formalism, which reveals a Fourier relationship between sequence parameters and excitation profiles for spins with arbitrary spatial location, off-resonance and velocity. Based on the k-space framework, we analyzed practical velocity-selective excitation pulse sequences, which yield sinusoidal or sinc-shaped velocity profiles. Then we demonstrated how these two types of velocity-selective excitation can be used as magnetization preparation for clinical applications, including saturation-based or inversion-based arterial spin labeling and black or bright blood angiography. We also discussed practical considerations and issues for each application, including the determination of design parameters and the effects of MR system errors, such as magnetic field offsets and eddy currents.