Hybrid active and passive local shimming (HAPLS) for two-region MRI

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biomedical Engineering
• Physics

Background:

• Standard MRI scanners use global shim systems, limiting simultaneous shimming of multiple regions of interest (ROIs).
• Simultaneous shimming of two isolated ROIs is crucial for advanced MRI applications, including bilateral breast imaging and dyadic brain studies.
• Existing technologies struggle to achieve optimal shimming for two distinct areas concurrently.

Purpose of the Study:

• To develop and evaluate a hybrid active and passive local shimming technique for simultaneous shimming of two isolated ROIs.
• To address the limitations of current MRI systems in handling multi-ROI shimming requirements.
• To improve the efficiency and effectiveness of shimming in complex scan scenarios.

Main Methods:

• A local passive shimming system was designed using optimized bilateral ferromagnetic chip arrays to correct high-order B0 inhomogeneities.
• A 40-channel DC loop system powered by 64-channel current amplifiers was employed for active shimming.
• Optimized current distribution was used to correct residual low-order and subject-specific B0 inhomogeneities, and homogenize center frequencies of the two ROIs.

Main Results:

• The hybrid shimming technique reduced 95% peak-to-peak B0 inhomogeneity from 1.92 to 1.12 ppm (41.7% reduction).
• Root Mean Square (RMS) B0 inhomogeneity decreased from 0.473 to 0.255 ppm (46.1% reduction).
• The volume of MR voxels within a 0.5-ppm frequency span increased from 64.3% to 81.3% (26.3% increase).

Conclusions:

• The proposed hybrid active and passive local shimming technique effectively enables simultaneous shimming of two isolated areas.
• This technique addresses an unmet need in commercial MRI scanners for multi-ROI shimming.
• The method significantly improves magnetic field homogeneity, enhancing potential MRI applications.