Flip angle-optimized fast dynamic T1 mapping with a 3D gradient echo sequence.

PURPOSE To analyze the flip angle dependence and to optimize the statistical precision of a fast three-dimensional (3D) 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 step-wise 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 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.