On shimming approaches in 3T breast MRI

A comparative study is presented, analyzing quantitatively the impact of 15 shim strategies on the homogeneity of the main magnetic field over the three-dimensional breast region in 3T MRI. The results obtained in 12 female volunteers, spanning a wide range of body and breast types, indicate that the inclusion of the back and heart in the shim region of interest leads to considerable decrease in field homogeneity, and needs to be avoided. Comparison between shim strategies using volumetric B₀ maps, covering the entire breast region, and 1-6 plane B₀ maps indicate only minimally reduced performance for the latter. Interestingly, however, no single shim strategy relying on a limited number of B₀ maps as input was found to work best in all volunteers. This was attributed to the limited capability of a small number of B₀ maps to capture the B₀ variability existent within breast. On the average, a rectangular shim region of interest, encompassing the breast region alone, worked best for the cohort studied here.