Generalization of Optimal Control Saturation Pulse Design for Robust and High CEST Contrast
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Purpose
Optimal Control (OC) CEST pulse design for singular pulses that can be used flexibly and robustly with high saturation at different duty cycles, saturation durations and magnetic field strengths.
Theory and Methods
An OC framework was developed to design a single pulse shape that can be flexibly applied for arbitrary pulse train parameters and outperform typically used CEST saturation pulses shapes. The pulse design was developed primarily with a continuous wave spectrum (CW) as the optimization target, but can be easily adapted to specific scenarios. The generalized OC pulse was evaluated through simulations, phantom, and in vivo measurements on a 3 T clinical scanner. Performance was assessed in terms of contrast, robustness to field inhomogeneities, and resilience against artifacts such as Rabi oscillations and sidebands, compared to established saturation techniques.
Results
Investigations showed that the generalized OC pulse achieved a contrast matching CW saturation and also functioned well under field inhomogeneities. Low-pass filtering of the optimized pulse shape effectively suppressed artifacts outside the initial optimization frequency range, enabling generalization across different field strengths. Phantom experiments consistently showed higher contrast than Gaussian, Fermi, and adiabatic spin-lock pulses for various CEST agents covering most clinically relevant regimes. In vivo imaging demonstrated substantially enhanced CEST contrast for both creatine/phosphocreatine in muscle and Amide Proton Transfer (APT) in brain compared to Gaussian saturation.
Conclusion
The generalized OC pulse provides a robust and flexible alternative to conventional CEST saturation strategies. Its integration into the open-source Pulseq-CEST framework supports simple reproducibility and a vendor-independent implementation.
