Template-Based $B _{1}$ 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.