Effect of head motion on MRI B₀ field distribution

Area of Science:

• Medical Physics
• Biomedical Imaging
• Neuroimaging

Background:

• Head motion is a significant source of artifacts in Magnetic Resonance Imaging (MRI).
• B0 field inhomogeneity can lead to image distortions and signal loss, particularly in multi-shot T2*-weighted sequences.
• Understanding the origins of B0 field fluctuations is crucial for improving image acquisition and interpretation.

Purpose of the Study:

• To identify and characterize the sources of B0 field changes induced by head motion in human brain MRI.
• To investigate the impact of head and torso susceptibility on B0 field variations.
• To reduce motion sensitivity in brain MRI scans.

Main Methods:

• Measured B0 fields in 5 healthy volunteers across various head poses at 7 Tesla.
• Calculated subject-specific magnetic fields by subtracting external fields.
• Developed subject-specific susceptibility models for head and torso contributions.
• Analyzed spatial complexity of field changes using spherical harmonic expansion.

Main Results:

• Minor head movements (approx. 5° rotation, 5mm translation) cause substantial and complex B0 field changes (SD ~10 Hz).
• The torso significantly contributes to subject-associated B0 field changes (SD ~5 Hz).
• Subject-associated B0 field changes induce phase errors in multi-shot T2*-weighted MRI acquisitions.

Conclusions:

• B0 field changes from head motion are detrimental to multi-shot T2*-weighted MRI.
• Characterizing these field changes offers insights into mitigation strategies.
• Future strategies may involve individualized predictive field models and real-time monitoring/correction.