Limitations of Skipping Echoes for Exponential T Fitting

Background: Exponential fitting of multiecho spin echo sequences with skipped echoes is still commonly used for quantification of transverse relaxation (T ).

Purpose: To examine the efficacy of skipped echo methods for T quantification against computational modeling of the exact signal decay.

Study Type: Prospective comparison of methods.

Subjects/phantom: Eight volunteers were imaged at 4.7T, six volunteers at 1.5T, and phantoms ([MnCl ] = 68-270 mM).

Field Strength/sequence: 1.5T and 4.7T; multiple-echo spin echo.

Assessment: Exponential fitting for T using all echoes, skipping the first echo or skipping all odd echoes, compared with Bloch simulations. Resulting T values were examined over a range of T (10-150 msec), refocusing flip angles (90-270°), and echo train lengths (ETL = 6-32).

Statistical Tests: Shapiro-Wilk tests and Q-Q plots were used to check for normality of data. Paired sample t-tests and Wilcoxon rank tests were used to compare fitting models using α = 0.05. Multiple comparisons were accounted for with Bonferroni correction.

Results: In examined regions of interest, typical incorrect estimation of T ranged from 23-39% for exponential fitting of all echoes, or 15-32% for skipped echo methods. In vivo, T estimation error was reduced to as little as 10% with skipped echo methods using 180° refocusing and ETL = 8, although error varied due to refocusing angle, T , and ETL. In vivo, skipped echo T values were significantly different than all echo exponential fitting (P < 0.004), but also were significantly different from reference values (P < 0.002, except frontal white matter). Simulations showed skipping the first echo was the most effective form of exponential fitting, in particular for T <50 msec and ETL = 8, with potential to reduce T errors to 10%, depending on refocusing angle and T .

Data Conclusion: Skipping echoes is insufficient for avoiding stimulated echo contamination. Resulting T errors depend on a complicated interplay of T , refocusing angle, and ETL. Modeling of the multiecho sequence is recommended.

Level Of Evidence: 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:1432-1440.