SEMANTIC CLASSIFICATION OF PICTURES & WORDS: REPETITION PRIMING FMRI: BILINGUAL

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Introduction: Magnetic field homogeneity is of major concern for spectroscopy as well as other imaging applications. In vivo shimming is routinely performed using linear shim correction terms, and while many scanners are also equipped with additional resistive shim coils that can provide second and third order shim fields, they are often not used due to the additonal effort and scan time required. Methods and Results: We measured the homogeneity improvement achieveable using additional higher order shims as compared to the linear shims alone for the specific application of chemical shift imaging of the human brain. 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. 15 normal volunteers were studied on a 1.5T GE Signa scanner, and the B0 homogeneity analysis results were obtained. As demonstrated, a significant improvement in overall field homogeneity was obtained using the additional non-linear shims. For example, the fraction of brain voxels with center frequencies within 1 0.1 ppm increased by approximately one third with the use of the linear + higher order shims as compared to the linear shim corrections alone. Intravoxel line broadening was also reduced using higher order shimming, however the effect was less pronounced. As expected, intravovel linebroadening effects are more severe as the spectroscopic voxel size increases. Conclusions: For applications such as volumetric CSI studies, the benefits of using additional non-linear shim terms are substantial, and plots can be used to decide if a given application warrants the effort of implementing higher order shimming.