Small-tip-angle spokes pulse design using interleaved greedy and local optimization methods.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency Pulse Design
• Medical Physics

Background:

• Current spokes pulse design methods rely on sparse approximation or local optimization.
• Sparse approximation methods are efficient but limited in handling off-resonance effects and multifrequency designs.
• Local optimization methods accommodate off-resonance effects but are prone to initialization issues and local minima.

Purpose of the Study:

• To introduce a new method for designing spokes pulses that combines the strengths of existing approaches.
• To improve flip angle homogeneity in MRI radiofrequency pulses.
• To address limitations of current sparse approximation and local optimization methods.

Main Methods:

• A hybrid method interleaving local optimization iterations with a greedy location selection strategy.
• Optimization of radiofrequency pulses, target phase patterns, and spatial frequency locations.
• Simulations and experimental validation at 3 T and 7 T.

Main Results:

• The proposed method consistently produced spokes pulses with lower flip angle inhomogeneity.
• Demonstrated superior performance for both single- and multifrequency designs.
• Outperformed current state-of-the-art methods in simulations and experiments.

Conclusions:

• The interleaved local and greedy method offers a robust approach to spokes pulse design.
• This technique enhances radiofrequency pulse performance, leading to improved image quality.
• The method is effective across different field strengths (3 T and 7 T).