Enhanced and Robust Contrast in CEST MRI: Saturation Pulse Shape Design via Optimal Control

Purpose: To employ optimal control for the numerical design of CEST saturation pulses to maximize contrast and stability against B0 inhomogeneities.
Theory and Methods: We applied an optimal control framework for the design pulse shapes for
CEST saturation pulse trains. The cost functional minimized both the pulse energy and the discrepancy between the corresponding CEST spectrum and the target spectrum based on a continuous
RF pulse. The optimization is subject to hardware limitations. In measurements on a 7 T preclinical scanner, the optimal control pulses were compared to continuous-wave and Gaussian saturation
methods. We conducted a comparison of the optimal control pulses were compared to with Gaussian, block pulse trains, and adiabatic spin-lock pulses.
Results: The optimal control pulse train demonstrated saturation levels comparable to continuouswave saturation and surpassed Gaussian saturation by up to 50 % in phantom measurements. In
phantom measurements at 3 T the optimized pulses not only showcased the highest CEST contrast, but also the highest stability against field inhomogeneities. In contrast, block pulse saturation
resulted in severe artifacts. Dynamic Bloch-McConnell simulations were employed to identify the
source of these artifacts, and underscore the B0 robustness of the optimized pulses.
Conclusion: In this work, it was shown that a substantial improvement in pulsed saturation CEST
imaging can be achieved by using Optimal Control design principles. It is possible to overcome the
sensitivity of saturation to B0 inhomogeneities while achieving CEST contrast close to continuous
wave saturation.