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

TL;DR

This paper introduces an optimal control-based method for designing CEST MRI saturation pulses that significantly improve contrast and robustness against B0 inhomogeneities, outperforming traditional pulse shapes in phantom experiments.

Contribution

The study develops a novel optimal control framework for CEST pulse design, enhancing contrast and stability compared to existing methods.

Findings

Optimal control pulses achieve saturation comparable to continuous-wave methods.

Optimized pulses outperform Gaussian saturation by up to 50% in contrast.

Enhanced B0 inhomogeneity robustness demonstrated in phantom experiments.

Abstract

Purpose: To employ optimal control for the numerical design of CEST saturation pulses to maximize contrast and stability against $B_0$ 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 continuous-wave 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 $B_0$ 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.