Purpose: To develop quantitative MRI-based algorithms for correction of signal polarity for measurement of T1. Methods: Two polarity maps of T1 are calculated according to the mixed-TSE acquisition, and repolarized on a voxel-wise basis. The two polarity maps are compared to thresholds for PD and T2. The correct polarity is selected based on which T1 more accurately describes the selected voxel. Maps of the T1/T2 ratio repolarize the remaining unpolarized tissue. Results: Repolarized T1 maps exhibit an accurate bimodal distribution in vivo. Conclusion: A qMRI-based repolarization technique allows T1 measurement from mixed-TSE magnitude data, toward studying the extremely preterm brain.