Advanced design of MRI inversion pulses for inhomogeneous field conditions by optimal control

Abstract

Non-selective inversion pulses find widespread use in MRI applications, where requirements on them are increasingly demanding. With the use of high and ultrahigh field strength systems, robustness to ΔB 0 and B þ 1 inhomogeneities, while tackling SAR and hardware limitations, has rapidly become important. In this work, we propose a time-optimal control framework for the optimization of ΔB 0 -and1 -robust inversion pulses. Robustness is addressed by means of ensemble formulations, while allowing inclusion of hardware and energy limitations. The framework is flexible and performs excellently for various optimization goals. The optimization results are analyzed extensively in numerical experiments. Furthermore, they are validated, and compared with adiabatic RF pulses, in various phantom and in vivo measurements on a 3 T MRI system.

Robust dual‐angle T1 measurement in magnetization transfer spectroscopy by time‐optimal control

“…As a reference, T 1 values of an inversion recovery experiment are given. by Graf et al 30 In the formulation of the cost functionals, we incorporated the flexibility to achieve arbitrary excitation tip angles. The application of the optimized RF pulses to dual-angle T 1 mapping led to a significant enhancement in the accuracy of predicting T 1 values in numerical simulations, phantom measurements, and in vivo experiments.…”

“…To improve convergence of the nonconvex optimization problem (7), the state constraints (5) are replaced by an L p penalization with p ℕ, p even, p ! ∞ as we approach the optimizer; refer to the preceding work 30 for details. Then, the optimization problem transforms into min Tp > 0,r,φ…”

“…[25][26][27][28] Furthermore, RF pulses that exhibit robustness to field imperfections were designed by optimal control in two ways, by imposing adiabaticity directly 29 or by ensemble-based optimal control. [30][31][32][33] The primary aim of this work is to improve the precision of dual-angle T 1 mapping significantly by employing novel optimal control RF pulses within the context of magnetization transfer spectroscopy. Building upon the preceding work, 30 we extend the ensemble-based time-optimal control framework, originally derived for robust inversion pulse design, to encompass the design of excitation pulses with variable flip angles.…”

Hybrid adiabatic pulse with asymmetry (HAPY): An asymmetric adiabatic pulse with an application in pulsed arterial spin labeling at 7T

“…In the proposed framework, it is critical for the modified GOIA to be an energy‐efficient base pulse for HAPY. In previous works, GOIA RF pulse modulation 19,20,40‐42 is mostly controlled by the index, $$$ N $$$, of the time variable (i.e., $$$ T(t)={t}^N $$$). Besides reducing the peak power, 22,43 the current time‐resampling function (Equation ) provides more degrees of freedom in pulse shapes definition by mapping the linear time steps into fifth‐order polynomial shape.…”

Whole-Cerebrum distortion-free three-dimensional pseudo-continuous arterial spin labeling at 7T

“…The OPTIM pulse was designed by means of ensemble-based time optimal control ( Graf et al, 2022 ; Rund et al, 2018 ). Such a design alleviates the IE loss caused by the T2 relaxation during the pulse, and, thus, may achieve high performance at 7T.…”