Rapid high‐resolution T1 mapping by variable flip angles: Accurate and precise measurements in the presence of radiofrequency field inhomogeneity

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Abstract
Rapid 3D mapping of T1 relaxation times is valuable in diverse clinical applications. Recently, the variable flip angle (VFA) spoiled gradient recalled echo approach was shown to be a practical alternative to conventional methods, providing better precision and speed. However, the method is known to be sensitive to transmit field (B1+) inhomogeneity and can result in significant systematic errors in T1 estimates, especially at high field strengths. The main challenge is to improve the accuracy of the VFA approach without sacrificing speed. In this article, the VFA method was optimized for both accuracy and precision by considering the influence of imperfect transmit fields, noise bias, and selection of flip angles. An analytic solution was developed for systematic B1+-induced T1 errors and allows simple correction of T1 measurements acquired with any imaging parameters. A noise threshold was also identified and provided a guideline for avoiding T1 biases. Finally, it was shown that three flip angles were the most efficient for maintaining accuracy and high precision over large ranges of T1. A rapid B1+ mapping sequence was employed in all phantom experiments and high-field in vivo brain scans. Experimental results confirmed the theory and validated the accuracy of the proposed method. Magn Reson Med, 2006.