title: 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, B₁ 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 B₁ 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 B₁ inhomogeneity in 19 normal subjects. However, since B₁ map acquisition takes up precious scanning time and most retrospective studies do not have B₁ map, we designed a template-based correction strategy. B₁ 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 B₁ correction based on a set of features was designed. The optimum set of weighting factors for those features was computed. The best available B₁ correction was detected in almost all subjects while corrections comparable to the T₁ map corrected using the B₁ map from the same subject were detected in the others.
authors.affiliation: Department of Radiology and Imaging Sciences (NIH-DR&IS), National Institutes of Health, Bethesda, MD 20892, USA. castroma@cc.nih.gov
authors.forename: Marcelo A
authors.initials: MA
authors.lastname: Castro
authors.forename: Jianhua
authors.initials: J
authors.lastname: Yao
authors.forename: Yuxi
authors.initials: Y
authors.lastname: Pang
authors.forename: Christabel
authors.initials: C
authors.lastname: Lee
authors.forename: Eva
authors.initials: E
authors.lastname: Baker
authors.forename: John
authors.initials: J
authors.lastname: Butman
authors.forename: Iordanis E
authors.initials: IE
authors.lastname: Evangelou
authors.forename: David
authors.initials: D
authors.lastname: Thomasson
mesh_terms: D000465:Algorithms; D016477:Artifacts; D001921:Brain / Q000033:anatomy & histology*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D010363:Pattern Recognition, Automated / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013382:Subtraction Technique