Generalization of Optimal Control Saturation Pulse Design for Robust and High CEST Contrast

Area of Science:

• Magnetic Resonance Imaging
• Biomedical Engineering
• Spectroscopy

Background:

• Chemical Exchange Saturation Transfer (CEST) is a powerful MRI technique for detecting low-concentration metabolites.
• Conventional CEST saturation pulses often lack flexibility and robustness, limiting their clinical applicability.
• Developing advanced pulse sequences is crucial for optimizing CEST performance and expanding its diagnostic potential.

Purpose of the Study:

• To design a single, generalized optimal control (OC) pulse for chemical exchange saturation transfer (CEST) that is flexible and robust across various imaging parameters.
• To achieve high saturation efficiency and improved image contrast compared to existing CEST saturation methods.
• To ensure the designed pulse is adaptable for different duty cycles, saturation durations, and magnetic field strengths.

Main Methods:

• An optimal control (OC) framework was employed to design a novel single-pulse shape for CEST saturation.
• The pulse design targeted a continuous-wave (CW) spectrum but was adapted for broader applicability.
• Performance was evaluated using simulations, phantom studies, and in vivo 3 T MRI, comparing against Gaussian, Fermi, and adiabatic spin-lock (aSL) pulses.

Main Results:

• The generalized OC pulse demonstrated contrast comparable to CW saturation and maintained performance under field inhomogeneities.
• Low-pass filtering effectively suppressed artifacts, enabling generalization across different field strengths.
• Phantom experiments showed superior contrast with the OC pulse compared to Gaussian, Fermi, and aSL pulses for various CEST agents.
• In vivo imaging revealed significantly enhanced CEST contrast for creatine/phosphocreatine in muscle and amide proton transfer (APT) in the brain.

Conclusions:

• The generalized OC pulse offers a robust and flexible alternative to conventional CEST saturation strategies.
• This OC pulse design enhances MRI sensitivity for key metabolites.
• Integration into the open-source Pulseq-CEST framework facilitates reproducibility and vendor-independent implementation.