Limitations of skipping echoes for exponential T<sub>2</sub> fitting
Bibliographic record
Abstract
Background Exponential fitting of multiecho spin echo sequences with skipped echoes is still commonly used for quantification of transverse relaxation (T 2 ). Purpose To examine the efficacy of skipped echo methods for T 2 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 2 ] = 68–270 mM). Field Strength/Sequence 1.5T and 4.7T; multiple‐echo spin echo. Assessment Exponential fitting for T 2 using all echoes, skipping the first echo or skipping all odd echoes, compared with Bloch simulations. Resulting T 2 values were examined over a range of T 2 (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 2 ranged from 23–39% for exponential fitting of all echoes, or 15–32% for skipped echo methods. In vivo, T 2 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 2 , and ETL. In vivo, skipped echo T 2 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 2 <50 msec and ETL = 8, with potential to reduce T 2 errors to 10%, depending on refocusing angle and T 2 . Data Conclusion Skipping echoes is insufficient for avoiding stimulated echo contamination. Resulting T 2 errors depend on a complicated interplay of T 2 , 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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.022 | 0.074 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.002 | 0.002 |
| Open science | 0.002 | 0.002 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.004 | 0.003 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".