Limitations of skipping echoes for exponential T2 fitting.

ABSTRACT

BACKGROUND: Exponential fitting of multiecho spin echo sequences with skipped echoes is still commonly used for quantification of transverse relaxation (T2 ). PURPOSE: To examine the efficacy of skipped echo methods for T2 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 ([MnCl2 ] = 68-270 mM). FIELD STRENGTH/SEQUENCE 1.5T and 4.7T; multiple-echo spin echo. ASSESSMENT Exponential fitting for T2 using all echoes, skipping the first echo or skipping all odd echoes, compared with Bloch simulations. Resulting T2 values were examined over a range of T2 (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 T2 ranged from 23-39% for exponential fitting of all echoes, or 15-32% for skipped echo methods. In vivo, T2 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, T2 , and ETL. In vivo, skipped echo T2 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 T2 <50 msec and ETL = 8, with potential to reduce T2 errors to 10%, depending on refocusing angle and T2 . DATA CONCLUSION: Skipping echoes is insufficient for avoiding stimulated echo contamination. Resulting T2 errors depend on a complicated interplay of T2 , refocusing angle, and ETL. Modeling of the multiecho sequence is recommended. LEVEL OF EVIDENCE 2 Technical Efficacy Stage 1 J. Magn. Reson. Imaging 2018;471432-1440.