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.