Enhanced and robust contrast in CEST MRI: Saturation pulse shape design via optimal control

Area of Science:

• Magnetic Resonance Imaging
• Biomedical Engineering
• Physical Chemistry

Background:

• Chemical Exchange Saturation Transfer (CEST) is a valuable MRI technique for detecting low-concentration metabolites.
• CEST imaging is sensitive to radiofrequency (RF) pulse imperfections and magnetic field (B0) inhomogeneities.
• Developing robust and efficient CEST saturation pulses is crucial for accurate imaging.

Purpose of the Study:

• To design Chemical Exchange Saturation Transfer (CEST) saturation pulses using optimal control theory.
• To maximize image contrast and stability against B0 inhomogeneities in CEST imaging.
• To compare the performance of optimal control-designed pulses against conventional methods.

Main Methods:

• An optimal control framework was used to design CEST saturation pulse shapes.
• The cost functional minimized pulse energy and spectral discrepancy, subject to hardware constraints.
• Optimized pulses were compared to continuous-wave, Gaussian, block, and adiabatic pulses on a 7T preclinical scanner and in 3T phantom measurements.

Main Results:

• Optimal control pulses achieved saturation levels comparable to continuous-wave and exceeded Gaussian saturation by up to 50%.
• The optimized pulses demonstrated superior CEST contrast and stability against B0 inhomogeneities in phantom studies.
• Block pulse saturation resulted in artifacts, which were explained by Bloch-McConnell simulations, highlighting the robustness of optimized pulses.

Conclusions:

• Optimal control principles offer substantial improvements for pulsed saturation CEST imaging.
• This approach effectively mitigates sensitivity to B0 inhomogeneities.
• CEST contrast close to continuous-wave saturation can be achieved with improved pulsed saturation methods.