Time optimal control‐based RF pulse design under gradient imperfections

Abstract

Purpose: This study incorporates a gradient system imperfection model into an optimal control framework for radio frequency (RF) pulse design. Theory and Methods: The joint design of minimum-time RF and slice selective gradient shapes is posed as an optimal control problem. Hardware limitations such as maximal amplitudes for RF and slice selective gradient or its slew rate are included as hard constraints to assure practical applicability of the optimized waveforms. In order to guarantee the performance of the optimized waveform with possible gradient system disturbances such as limited system bandwidth and eddy currents, a measured gradient impulse response function (GIRF) for a specific system is integrated into the optimization. Results: The method generates optimized RF and pre-distorted slice selective gradient shapes for refocusing that are able to fully compensate the modeled imperfections of the gradient system under investigation. The results nearly regenerate the optimal results of an idealized gradient system. The numerical Bloch simulations are validated by phantom and in-vivo experiments on 2 3T scanners. Conclusions: The presented design approach demonstrates the successful correction of gradient system imperfections within an optimal control framework for RF pulse design. K E Y W O R D Sgradient imperfections, gradient impulse response function, pulse design, simultaneous multi-slice excitation, time optimal control 562 | AIGNER Et Al.

Three‐dimensional static and dynamic parallel transmission of the human heart at 7 T

“…By contrast, it may be worth investigating the design of continuous RF pulses for a 3D k‐space trajectory 13 or joint design of excitation k‐space trajectory and RF pulses 42 to further improve pulse performance pulse duration, SAR and FA homogeneity. With time‐varying gradient shapes in place, gradient imperfections gain importance and should be considered as shown by an iterative correction for pTx 43 or directly in the RF pulse design for single transmission 44 . Multiple groups have proposed designing kT‐points in the large‐tip‐angle regime, including applications to the human head 45,46 .…”

“…With time-varying gradient shapes in place, gradient imperfections gain importance and should be considered as shown by an iterative correction for pTx 43 or directly in the RF pulse design for single transmission. 44 Multiple groups have proposed designing kT-points in the large-tip-angle regime, including applications to the human head. 45,46 Large FA pulses can be valuable for cardiac imaging to introduce tissue contrast, although B + 1 peak and RF power are limited in the body at UHF.…”

Assessment of measurement precision in single‐voxel spectroscopy at 7 T: Toward minimal detectable changes of metabolite concentrations in the human brain in vivo

“…28 This advantage of gradient-modulated pulses can be exploited specifically in applications in which the peak RF power or specific absorption rate are limiting factors. Nevertheless, the higher sensitivity to ΔB 0 and gradient imperfections, 57 as well as the gradient strength and slew rate limitations of the used system, need to be considered during the planning of an application study using gradient-modulated pulses, to avoid the nominal voxel size not being achieved.…”

DeepControl: 2DRF pulses facilitating inhomogeneity and B0 off‐resonance compensation in vivo at 7 T

“…RF pulse designers in general navigate between matters of pulse types; spatial (and/or spectral) target profiles; inhomogeneous and B 0 fields; underlying gradient waveforms; system imperfections 15 ; constraints (eg, power, 16 specific absorption rate [SAR], 17 and states 18,19 ); and finally, which pulse design tool and physics model to adopt for the task (eg, the Fourier‐based small tip angle regime, 20 a large tip angle optimal control type, 17,21,22 or a k‐space 23 or spatial domain algorithm) 24 …”