Development and Optimization of a NovelPassive Shim System for Human OrbitofrontalCortex Imaging at 4T MRI

Abstract Background: The orbitofrontal cortex (OFC) area plays a critical role in humanbrain functions. However, susceptibility differences between paranasal sinuses andnasal cavity tissues often cause local field variations in the OFC region duringMRI, resulting in image distortions and signal loss. This paper introduces asubject-friendly neck passive shim solution aimed at enhancing field homogeneityin the brain, specifically in the OFC region. Methods: Utilizing 3D gradient-echo pulse sequences, main magnetic field (B0)maps of four subjects’ brains were acquired at 4T MRI. Subsequently, these mapswere decomposed into spherical harmonic coefficients which were then averaged.Optimal positions for placing shim elements on a semi-cylindrical surface, whichwas positioned slightly above the neck and below the chin, were computed. Results: The findings indicate a substantial enhancement in B0 homogeneity, particularly within the OFC region, through the integration of thissemi-cylindrical passive shim system in conjunction with the first andsecond-order active shimming. Conclusion: Utilizing a local dipole passive shim field shows potential forimproving field homogeneity in the orbitofrontal region of human brain.