Comparison of FASTMAP and B0 Field Map Shimming at 4T: Magnetic Field Mapping Using a Gradient-Echo Pulse Sequence

Abstract

Local susceptibility variations result in B0 field inhomogeneities, causing distortions and signal losses in MR imaging. Susceptibility variations become stronger with increasing B0 magnetic field strength. Active shimming is used to generate corrective magnetic fields, which can be used to improve B0 field homogeneity. FASTMAP is an effective shimming technique for computing optimal coil currents, which uses data from six projection directions (or columns): this technique is routinely used for shimming cubic volumes of interest (VOIs). In this paper, we propose several improvements to FASTMAP at 4T. For each shim coil, using a modified 3D gradient-echo pulse sequence, we compute B0 inhomogeneity maps and project them onto eight 1st and 2nd order spherical harmonic functions. This process is repeated for shim currents between -15,000 to 15,000 with increments of 5000 Digital to Analog Converter (DAC) units, and is used to compute the gradient between spherical harmonic coefficients and DAC values for all 8 shim coils—along with the R2 values of linear fits. A method is proposed (based on R2 values) to further refine optimal shim currents in respective coils. We present an analysis that is numerically robust and completely flexible in the selection of the VOIs for shimming. Performance analyses, phantom results, and in vivo results of a human brain are presented, comparing our methods with the FASTMAP method.