A 32-channel combined RF and B0 shim array for 3T brain imaging

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency (RF) Engineering
• Biomedical Engineering

Background:

• B0 field inhomogeneity is a major challenge in MRI, particularly for brain imaging.
• Conventional B0 shimming methods have limitations in achieving optimal field uniformity.
• Parallel imaging techniques enhance MRI speed but can be sensitive to field distortions.

Purpose of the Study:

• To introduce user-controllable direct currents (DC) to individual elements of a radio-frequency (RF) receive array.
• To enable B0 shimming capabilities within the RF array itself.
• To maintain the array's reception sensitivity and parallel imaging performance.

Main Methods:

• Simulated B0 shim performance using constrained DC current for brain arrays (8-128 elements).
• Fabricated a 32-channel, 3-tesla brain array using inductive chokes to integrate DC currents.
• Assessed RF and B0 shimming performance through bench and imaging measurements.

Main Results:

• Successfully added DC currents to the 32-channel RF array with minimal impact on RF performance and no adverse effects.
• Demonstrated B0 shimming performance superior to third-order spherical harmonic (SH) shimming, validated by simulations and measurements.
• Reduced frontal lobe susceptibility-induced fields and improved echo planar imaging geometric distortion.
• Simulations suggest potential for further shimming improvements with larger arrays (up to 6th-order SH equivalent).

Conclusions:

• Integrating user-controlled shim currents into conventional parallel brain array coils is feasible.
• Modest DC current levels are sufficient to achieve improved B0 shimming performance.
• The proposed method offers superior B0 shimming compared to standard second-order SH shimming.