A METHOD TO IMPROVE THE B-0 HOMOGENEITY OF THE HEART IN-VIVO

A homogeneous static (B-0) magnetic field is required for many NMR exp eriments such as echo planar imaging, localized spectroscopy, and spir al scan imaging, Although semi-automated techniques have been describe d to improve the B-0 field homogeneity, none has been applied to the i n vivo heart, The acquisition of cardiac field maps is complicated by motion, blood flow, and chemical shift artifact from epicardial fat, T o overcome these problems, an ungated three-dimensional (3D) chemical shift image (CSI) was collected to generate a time and motion-averaged B-0 field map, B-0 heterogeneity in the heart was minimized by using a previous algorithm that solves for the optimal shim coil currents fo r an input field map, using up to third-order current-bounded shims (1 ). The method improved the B-0 homogeneity of the heart in all 11 norm al volunteers studied, After application of the algorithm to the un-sh immed cardiac field maps, the standard deviation of proton frequency d ecreased by 43%, the magnitude H-1 spectral linewidth decreased by 24% , and the peak-peak gradient decreased by 35%. Simulations of the high -order (second- and third-order) shims in B-0 field correction of the heart show that high order shims are important, resulting for nearly h alf of the improvement in homogeneity for several subjects. The T-2 o f the left ventricular anterior wall before and after field correction was determined at 4.0 Tesla. Finally, results show that cardiac shimm ing is of benefit in cardiac P-31 NMR spectroscopy and cardiac echo pl anar imaging.