Physical limits to human brain B0 shimming with spherical harmonics, engineering implications thereof.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biophysics
• Medical Physics

Background:

• Increasing MRI magnetic fields enhance signal-to-noise ratio but exacerbate susceptibility-induced B0-inhomogeneity and artifacts.
• Susceptibility differences between air and biological tissues are primary sources of magnetic field inhomogeneity.

Purpose of the Study:

• To explore the topological conditions for achieving perfect shimming within a Region of Interest (ROI), such as the human brain.
• To investigate the feasibility of perfect shimming considering only air-tissue susceptibility disparities.

Main Methods:

• Theoretical analysis of perfect shimming conditions.
• High-degree spherical harmonic (SH) shimming simulations using a 100-subject 3T brain fieldmap database.
• Simulations focused on whole brain and specific ROIs like the prefrontal cortex and temporal lobes.

Main Results:

• Perfect SH shimming is achievable only if the ROI can be isolated within a sphere devoid of magnetic field inhomogeneity sources.
• A hard shim limit of approximately 10 Hz inhomogeneity at 7T for whole-brain SH shimming is established, requiring shimming degrees over 90.
• 3D region-specific shimming significantly improves homogeneity in critical brain areas, even with limited power and SH degree.

Conclusions:

• The geometry of the ROI relative to air-tissue interfaces dictates the possibility of perfect shimming.
• High-performance shimming strategies are crucial for mitigating artifacts in advanced MRI applications.
• Region-specific shimming offers a practical approach to enhance image quality in targeted brain areas.