Multidimensional RF pulse design with consideration of concomitant field effects.

Area of Science:

• Magnetic Resonance Imaging
• Radiofrequency Pulse Design
• Gradient Imperfections

Background:

• Contemporary low-field MRI systems utilize high-performance gradients, which can introduce imperfections.
• Concomitant magnetic fields are a significant source of error in radiofrequency (RF) pulse design, particularly at lower field strengths.
• Existing k-space formalisms may not adequately account for these effects, leading to suboptimal excitation profiles.

Purpose of the Study:

• To develop a novel RF pulse design procedure for small-tip excitations.
• Incorporate linear time-invariant gradient imperfections and concomitant field effects into the pulse design.
• Improve excitation profile accuracy in low-field MRI with high-performance gradients.

Main Methods:

• Extended the small-tip excitation k-space formalism.
• Approximated concomitant fields as a Bloch-Siegert shift in the rotating frame.
• Evaluated using simulations across various field strengths (0.2T-7T) and experimental validation at 0.55T.

Main Results:

• The extended formalism significantly improved 2D excitation profiles compared to the original.
• The method effectively corrects concomitant field effects for B0 > 0.2T.
• Simulations and phantom experiments at 0.55T showed the proposed method yields sharper, more accurate profiles, especially at off-isocenter distances up to 15 cm.

Conclusions:

• Concomitant fields can be effectively modeled as a Bloch-Siegert shift for multidimensional RF pulse design.
• This approach leads to improved excitation profiles with sharp edges.
• Crucial for off-isocenter excitations and low-field MRI with strong gradients.