Transverse relaxometry with transmit field-constrained stimulated echo compensation

Purely exponential decay is rarely observed in conventional mono-exponential T₂ mapping due to transmit field inhomogeneity and calibration errors, which collectively introduce stimulated and indirect echo pathways. Stimulated echo correction (SEC) requires an additional fit parameter for the transmit field, resulting in greater uncertainty in T₂ relative to mono-exponential fitting. The aim of this study was to develop an accurate and precise method for T₂ mapping using SEC.

The proposed method, called two-step SEC (tSEC), leverages spatial correlations in the transmit field to reduce the number of fully independent fitting parameters from three to two. The method involves a two-pass fit: the first pass involves a fast but standard SEC fit. The initially estimated transmit field is smoothed and provided as a fixed input to the second pass.

Simulations and in vivo experiments demonstrated up to 38% and 27% decreases in relative T₂ variance with tSEC relative to SEC. Average T₂ values were unchanged between tSEC and SEC fits. The proposed method uses the same input data as SEC and exponential fits, so it is applicable to existing data.

The proposed method generates reliable and reproducible quantitative T₂ maps and should be considered for future relaxometry studies.