The double-angle method (DAM) is commonly used as a reference standard in radiofrequency field (B ) mapping studies. This study explored two aspects of DAM: (i) use of small flip angle pairs to reduce the repetition time (TR) needed for adequate longitudinal relaxation (T); and (ii) the effect of using different flip angle ratios for B mapping. Results of phantom studies show that B correction using small flip angle pairs ≤ 60° with TR  =  5000 ms can allow for accurate estimation of T up to about 1500 ms; and that increasing the ratio of the two flip angles used for B correction resulted in more accurate estimation of T . These modifications allow 3-dimensional (3D) B mapping to be consistently performed with the same 3D spoiled gradient echo sequence used for T mapping in dynamic contrast-enhanced MRI.

Objective. The variable flip angle (VFA) method for longitudinal relaxation time (T1) measurement is inherently sensitive to inaccuracies in the radiofrequency transmit field (B1) and incomplete spoiling of transverse magnetization. The objective of this study is to devise a computational method that addresses the problems of incomplete spoiling and B1 inhomogeneity in the estimation of T1 using VFA method. Approach. Using an analytical expression of the gradient echo signal with account of incomplete spoiling, we first showed that ill-posedness in the simultaneous estimation of B1 and T1 can be lifted with the use of flip angles larger than the Ernst angle. We then devised a nonlinear optimization method based on this signal model of incomplete spoiling for simultaneous estimation of B1 and T1. Main Results. We evaluated the proposed method on a graded-concentration phantom to show that the derived T1 estimates offers an improvement over the regular VFA method and compares well with reference values measured by inversion recovery. Reduction of the number of flip angles from 17 to 5 yielded consistent results indicating that the proposed method is numerically stable. T1 estimates derived from in-vivo brain imaging were consistent with literature values for gray and white matter tissues. Significance. Contrary to the common notion that B1 correction in the VFA method for T1 mapping should be performed separately, we show that combined estimation of B1 and T1 is feasible by the proposed method simply with the acquisition of 5 flip angles, as demonstrated on both phantom and in-vivo imaging data.