Rapid in vivo proton shimming.

A rapid and completely automated method of adjusting the magnetic field (B0) homogeneity for in vivo proton spectroscopy and imaging is described. B0 inhomogeneity maps are generated by a gradient-recalled echo pulse sequence in which the frequency dispersion is chosen to eliminate the effects of the fat/water chemical shift. Low-order shim values are derived by magnitude-weighted least-squares fits to the B0 maps and automatically applied as DC offsets to the X, Y, and Z gradient amplifiers. Imaging with chemical shift selective saturation is used as a measure of the efficacy of the technique. Results indicate that AUTOSHIM improves the overall homogeneity: however, local high-order field distortions which cannot be corrected by linear gradients are generated by certain air/tissue and bone/tissue morphology. In such cases a "Zoom SHIM" may be applied over a limited region of interest for local homogeneity improvement at the expense of other regions. It is suggested that such scans are a necessity for recording the homogeneity during clinical MR spectroscopy.