Improving fat saturation robustness in outer extremity MRI with a local shim coil insert

High-quality fat suppression is essential for various MRI applications. In musculoskeletal imaging, poor fat suppression caused by severe B₀ inhomogeneity can obscure important lesions, potentially leading to inaccurate diagnoses. This problem is particularly exacerbated in off-isocenter imaging, where conventional shimming using second-order spherical harmonic shim coils often proves inadequate due to elevated B₀ inhomogeneity. To address this challenge, we configured a simple local shim insert to provide additional localized B₀ shimming for off-isocenter regions, offering a practical hardware solution.

We designed and constructed a seven-channel shim coil and evaluated its performance in comparison to conventional second-order spherical harmonic shimming within a targeted volume near the scanner bore. The coil was tested with both phantom and in vivo studies using a clinical 3 T MRI scanner.

The improved B₀ homogeneity achieved with the local shim coil significantly enhanced fat saturation (fat-sat) uniformity across the imaged volumes. This improvement was particularly beneficial for areas far from the scanner isocenter, where B₀ inhomogeneity is most severe. Our results indicated a 40% reduction in RMS error of the B₀ field for elbow imaging and 35% for hand imaging, highlighting substantial improvements in B₀ field homogeneity. Additionally, the image quality score increased by 1 point for both hand and elbow images, reflecting enhanced fat-sat quality.

The simple local shim insert we configured improves fat-sat capability in both hand and elbow imaging. It offers the potential for improving off-isocenter musculoskeletal MRI.