Velocity‐selective excitation: Principles and applications

Abstract

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 analyze practical VS excitation pulse sequences that yield sinusoidal or sincshaped velocity profiles. Then we demonstrate how these two types of VS excitation can be used as magnetization preparation for clinical applications, including saturation-or inversion-based arterial spin labeling and black-or bright-blood angiography. We also discuss 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.

Spatially and velocity‐selective magnetization preparation for noncontrast‐enhanced peripheral MR angiography

“…Field error‐resistant VS excitation pulse sequences can be designed by concatenating multiple excitation steps, each of which is a combination of a hard RF pulse, unipolar or bipolar gradient pulses, and composite refocusing RF pulses. This pulse sequence can be analyzed theoretically using the extended excitation k‐space formalism, which includes the effects of refocusing pulses in the k‐space framework (see the Appendix for details) 21 . Figure 1A shows an example VS pulse design, which uses two refocusing pulses in each excitation step (see the design details in 2.2).…”

“…This pulse sequence can be analyzed theoretically using the extended excitation k-space formalism, which includes the effects of refocusing pulses in the k-space framework (see the Appendix for details). 21 Figure 1A shows an example VS pulse design, which uses two refocusing pulses in each excitation step (see the design details in 2.2).…”

“…According to recent studies, this refocusing-free counterpart can be obtained by flipping the sign of all the gradient pulses that experience an odd number of refocusing pulses until the end of the pulse sequence. 14,21 Figure 7A,B show the equivalent pulse sequences for the VS and SVS preparations shown in Figure 1A,B, respectively. From the excitation kspace perspective, the equivalent VS preparation sequence (Figure 7A) deposits the multiple hard RF pulses in a form of sampled rectangular function along the k v axis (Figure 7C), which is Fourier-transformed to aliased sinc-shaped M xy response and results in an aliased notch-shaped M z response over velocity.…”