Regularized higher‐order in vivo shimming

A regularized algorithm is presented for localized in vivo shimming. The technique uses first‐ ( X , Y , Z ), second‐ ( Z 2 , ZX , ZY , X 2 ‐ Y 2 , XY ), and third‐order ( Z 3 ) shim coils, and is robust when applied to arbitrarily‐shaped, as well as off‐center, regions of interest (ROIs). A single‐shot spiral pulse sequence is used for rapid field map acquisition, and a least‐squares calculation of the shim currents is performed to minimize the root‐mean‐square (RMS) value of the B 0 inhomogeneity over a user‐selected ROI. The use of a singular value decomposition (SVD) in combination with a regularization algorithm significantly improves the numerical stability of the least‐squares fitting procedure. The fully automated shimming package is implemented on a 3 T GE Signa system and its robust performance is demonstrated in phantom and in vivo studies. Magn Reson Med 48:715–722, 2002. © 2002 Wiley‐Liss, Inc.