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 spectroscop y, and with the increased use of volumetric chemical shift imaging (CS I) techniques, the ability to shim over a large volume of tissue is no w 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 re sistive shim supplies that can provide second and third-order shim fie lds, they are often not used due to the additional effort and scan tim e required. The question as to how much improvement can be achieved us ing additional higher-order shims compared with the linear shims alone was quantitatively addressed. Performance measures for both intervoxe l B-0 uniformity and intravoxel T-2 line broadening were evaluated fo r 15 normal volunteers. The analysis tools developed in this study, al ong with the summarized data, can be useful in deciding if a given app lication warrants the additional time, effort, and expense (if additio nal hardware needs to be purchased) of implementing higher-order shimm ing routines. For CSI studies of the brain, the use of the higher-orde r shims, compared with linear terms alone, yielded approximately 30% g reater volume of brain tissue that could be shimmed within typical con straints for intervoxel B-0 shifts and intravoxel T-2 linebroadening. In addition, a regional analysis shows significant improvement in the homogeneity within specific areas of the brain, particularly those ne ar the skull.