Quantitative assessment of improved homogeneity using higher-order shims for spectroscopic imaging of the brain

Magnetic field homogeneity is of major concern for in vivo spectroscopy, and with the increased use of volumetric chemical shift imaging (CSI) techniques, the ability to shim over a large volume of tissue is now one of the primary limiting constraints in performing these studies. In vivo shimming is routinely performed using linear shim correction terms, and although many scanners are also equipped with additional resistive shim supplies that can provide second and third-order shim fields, they are often not used due to the additional effort and scan time required. The question as to how much improvement can be achieved using additional higher-order shims compared with the linear shims alone was quantitatively addressed. Performance measures for both intervoxel B(o) uniformity and intravoxel T₂* line broadening were evaluated for 15 normal volunteers. The analysis tools developed in this study, along with the summarized data, can be useful in deciding if a given application warrants the additional time, effort, and expense (if additional hardware needs to be purchased) of implementing higher-order shimming routines. For CSI studies of the brain, the use of the higher-order shims, compared with linear terms alone, yielded approximately 30% greater volume of brain tissue that could be shimmed within typical constraints for intervoxel B(o) shifts and intravoxel T₂* linebroadening. In addition, a regional analysis shows significant improvement in the homogeneity within specific areas of the brain, particularly those near the skull.