Robust T₂ estimation with balanced steady state free precession

To develop a novel signal representation for balanced steady state free precession (bSSFP) displaying its T₂ independence on B₁ and on magnetization transfer (MT) effects.

A signal model for bSSFP is developed that shows only an explicit dependence (up to a scaling factor) on E₂ (and, therefore, T₂) and a novel parameter c (with implicit dependence on the flip angle and E₁). Moreover, it is shown that MT effects, entering the bSSFP signal via a binary spin bath model, can be captured by a redefinition of T₁ and, therefore, leading to modification of E₁, 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 T₂ (E₂) and c were estimated using a variable projection (VARPRO) method and Monte-Carlo simulations were performed to assess T₂ estimation precision.

Initial experiments confirmed the expected independence of T₂ on various protocol settings, such as TR, the flip angle, B₁ 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.

BSSFP theory predicts an extraordinary feature that all MT and B₁-related variational aspects do not enter T₂ estimation, making it a potentially robust methodology for T₂ quantification, pending validation against existing standards.