Robust T2 estimation with balanced steady state free precession
Oliver Bieri, Claudia Weidensteiner, Carl Ganter
Abstract
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.