Rapid Parametric Mapping of the Longitudinal Relaxation Time T-1 Using Two-Dimensional Variable Flip Angle Magnetic Resonance Imaging at 1.5 Tesla, 3 Tesla, and 7 Tesla

Abstract

Introduction: Visual but subjective reading of longitudinal relaxation time (T-1) weighted magnetic resonance images is commonly used for the detection of brain pathologies. For this non-quantitative measure, diagnostic quality depends on hardware configuration, imaging parameters, radio frequency transmission field (B-1(+)) uniformity, as well as observer experience. Parametric quantification of the tissue T-1 relaxation parameter offsets the propensity for these effects, but is typically time consuming. For this reason, this study examines the feasibility of rapid 2D T-1 quantification using a variable flip angles (VFA) approach at magnetic field strengths of 1.5 Tesla, 3 Tesla, and 7 Tesla. These efforts include validation in phantom experiments and application for brain T-1 mapping. Methods: T-1 quantification included simulations of the Bloch equations to correct for slice profile imperfections, and a correction for B-1(+). Fast gradient echo acquisitions were conducted using three adjusted flip angles for the proposed T-1 quantification approach that was benchmarked against slice profile uncorrected 2D VFA and an inversion-recovery spin-echo based reference method. Brain T-1 mapping was performed in six healthy subjects, one multiple sclerosis patient, and one stroke patient. Results: Phantom experiments showed a mean T-1 estimation error of (-63 +/- 1.5)% for slice profile uncorrected 2D VFA and (0.2 +/- 1.4)% for the proposed approach compared to the reference method. Scan time for single slice T-1 mapping including B-1(+) mapping could be reduced to 5 seconds using an in-plane resolution of (2x2) mm(2), which equals a scan time reduction of more than 99% compared to the reference method. Conclusion: Our results demonstrate that rapid 2D T-1 quantification using a variable flip angle approach is feasible at 1.5T/3T/7T. It represents a valuable alternative for rapid T-1 mapping due to the gain in speed versus conventional approaches. This progress may serve to enhance the capabilities of parametric MR based lesion detection and brain tissue characterization.