Robust T₂ estimation with balanced steady state free precession

Purpose: To develop a novel signal representation for balanced steady state free precession (bSSFP) displaying its T-2 independence on B-1 and on magnetization transfer (MT) effects. Methods: A signal model for bSSFP is developed that shows only an explicit dependence (up to a scaling factor) on E-2 (and, therefore, T-2) and a novel parameter c (with implicit dependence on the flip angle and E-1). Moreover, it is shown that MT effects, entering the bSSFP signal via a binary spin bath model, can be captured by a redefinition of T-1 and, therefore, leading to modification of E-1, resulting in the same signal model. Various sets of phase-cycled bSSFP brain scans (different flip angles, different TR, different RF pulse durations, and different number of phase cycles) were recorded at 3 T. The parameters T-2 (E-2) and c were estimated using a variable projection (VARPRO) method and Monte-Carlo simulations were performed to assess T-2 estimation precision. Results: Initial experiments confirmed the expected independence of T-2 on various protocol settings, such as TR, the flip angle, B-1 field inhomogeneity, and the RF pulse duration. Any variation (within the explored range) appears to directly affect the estimation of the parameter c only-in agreement with theory. Conclusion: BSSFP theory predicts an extraordinary feature that all MT and B-1-related variational aspects do not enter T-2 estimation, making it a potentially robust methodology for T-2 quantification, pending validation against existing standards.