Flip-angle-optimized fast dynamic T1 mapping

Purpose: To analyze the flip-angle dependence and to optimize the statistical precision of a fast three-dimensional T1-mapping technique based on the variable-flip-angle (VFA) method. The proposed single-flip-angle (1FA) approach acquires only a single 3D spoiled gradient-echo data set for each time point of the dynamical series in combination with a longer baseline measurement. Theory and Methods: The optimal flip angle for the dynamic series can be calculated as αdyn,opt=arccos[(2E1–1)/(2–E1)] (with E1 = exp(–TR/T1)) by minimizing the statistical error of T1. T1 maps of a liquid phantom with stepwise increasing concentrations of contrast-agent were measured using a saturation-recovery (SR) and a VFA/1FA technique with 11 flip angles. The standard deviation of the parameter maps was defined as statistical error of the 1FA measurement. Results: The measured statistical error of the 1FA technique as a function of αdyn agrees well with the derived theoretical curve. The optimal flip angle increases from about 5° for T1=2629 ms to 30° for T1=137 ms. The relative deviation between 1FA and SR measurements varies between –2.9 % and +10.3 %. Measurements in vivo confirm the expression for the optimal flip angle. Conclusion: The proposed flip-angle-optimized 1FA technique optimizes the precision of T1 values in dynamic phantom measurements.