Template-based B1 inhomogeneity correction in 3T MRI brain studies

Abstract

Low noise, high resolution, fast and accurate T1 maps from MRI images of the brain can be performed using a dual flip angle method. However, Β1 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 T1 maps that limit the accuracy of quantitative analyses based on those maps. A dual repetition time method was used for acquiring a Β1 map to correct that inhomogeneity. Additional inaccuracies due to misregistration of the acquired T1-weighted images were corrected by rigid registration, and the effects of misalignment on the T1 maps were compared to those of Β1 inhomogeneity in 19 normal subjects. However, since Β1 map acquisition takes up precious scanning time and most retrospective studies do not have Β1 map, we designed a template-based correction strategy. Β1 maps from different subjects were aligned using a twelve-parameter affine registration. Recomputed T1 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 Β1 correction based on a set of features was designed. The optimum set of weighting factors for those features was computed. The best available Β1 correction was detected in almost all subjects while corrections comparable to the T 1 map corrected using the Β1 map from the same subject were detected in the others.