Transition from transient-optimized rf phase cycling to steady-state with fixed Ψ in FLASH

Determine how to transition in gradient- and rf-spoiled gradient echo (FLASH) sequences from a transient-state regime using individually optimized radiofrequency (rf) phase cycling to a steady-state regime employing a fixed quadratic rf phase increment Ψ; specifically, establish whether optimizing only an initial subset of excitations (e.g., the first 200 events) before switching to fixed-Ψ quadratic phase cycling is an appropriate strategy.

Background

Quadratic rf phase cycling with a fixed increment Ψ is historically derived for steady-state signal behavior and is widely used in practice, including transient FLASH acquisitions, despite lacking theoretical justification for the transient regime. In this work, the authors optimize individual rf phases (and flip angles) during the transient to better approximate the ideally spoiled Look-Locker decay.

Because many acquisitions eventually aim for a steady-state, a practical challenge arises: how to bridge the optimized transient behavior to subsequent steady-state acquisition with a fixed Ψ. The authors suggest a possible approach—optimizing only the initial portion of the rf pulse train before switching to fixed quadratic phase cycling—but leave the exact handling as an open question. They also note evidence that for typical low-flip-angle transient sequences the steady-state may be nearly independent of Ψ, potentially reducing the need for a special transition, yet a general method remains to be established.