Template-Based B1 Inhomogeneity Correction in 3T MRI Brain Studies

Low noise, high resolution, fast and accurate T-1 maps from MRI images of the brain can be performed using a dual flip angle method. However, B-1 field inhomogeneity, which is particularly problematic at high field strengths ( e. g., 3T), limits the ability of the scanner to deliver the prescribed flip angle, introducing errors into the T-1 maps that limit the accuracy of quantitative analyses based on those maps. A dual repetition time method was used for acquiring a B-1 map to correct that inhomogeneity. Additional inaccuracies due to misregistration of the acquired T-1-weighted images were corrected by rigid registration, and the effects of misalignment on the T-1 maps were compared to those of B-1 inhomogeneity in 19 normal subjects. However, since B-1 map acquisition takes up precious scanning time and most retrospective studies do not have B-1 map, we designed a template-based correction strategy. B-1 maps from different subjects were aligned using a twelve-parameter affine registration. Recomputed T-1 maps showed an important improvement with respect to the noncorrected maps: histograms of all corrected maps exhibited two peaks corresponding to white and gray matter tissues, while unimodal histograms were observed in all uncorrected maps because of the inhomogeneity. A method to detect the best nonsubject-specific B-1 correction based on a set of features was designed. The optimum set of weighting factors for those features was computed. The best available B-1 correction was detected in almost all subjects while corrections comparable to the T-1 map corrected using the B-1 map from the same subject were detected in the others.