Challenging the optimality of the pulse excitation in magnetic resonance imaging

摘要

The design of excitation signals for Magnetic Resonance Imaging (MRI) is cast as an optimal control problem. An appropriate cost criterion, the Signal Contrast Efficiency (SCE), is developed. It is to be optimised subject to dynamics expressed by the Bloch equation. The solution to the optimisation problem is potentially useful for all forms of MRI including structural and functional imaging. Here, we demonstrate that signals other than pulse excitations, which are ubiquitous in MRI, can provide adequate excitation, thus challenging the optimality and ubiquity of pulsed signals. A class of on-resonance piece-wise continuous amplitude modulated signals is introduced. It is shown that despite the bilinear nature of the Bloch equations, the optimisation problem is largely analytically tractable for this class of signals, using Galerkin approximation methods. Simulations demonstrate that this class of signals may provide an attractive alternative to pulsed excitation signals for MRI.