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

Abstract

Purpose

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.

Methods

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.

Results

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.

Conclusion

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.