Limitations of skipping echoes for exponential T₂ fitting.

Area of Science:

• Magnetic Resonance Imaging
• Biomedical Engineering
• Quantitative Imaging

Background:

• Exponential fitting is commonly used for T2 quantification in multiecho spin echo sequences.
• Skipped echo methods are frequently employed, but their accuracy is debated.

Purpose of the Study:

• To evaluate the accuracy of skipped echo methods for T2 quantification.
• To compare these methods against computational modeling of exact signal decay.

Main Methods:

• Prospective comparison of exponential fitting with all echoes versus skipped echo methods (first echo or odd echoes).
• Utilized Bloch simulations and analyzed data from 1.5T and 4.7T MRI scans of volunteers and phantoms.
• Assessed T2 values across a range of T2 (10-150 msec), flip angles (90-270°), and echo train lengths (ETL=6-32).

Main Results:

• T2 estimation errors ranged from 23-39% for all echoes and 15-32% for skipped echo methods in regions of interest.
• In vivo, T2 error was reduced to 10% with skipped echo methods (180° refocusing, ETL=8), but varied with parameters.
• Skipping the first echo was most effective for T2 <50 msec and ETL=8, potentially reducing errors to 10%.

Conclusions:

• Skipped echo methods are insufficient to avoid stimulated echo contamination, leading to T2 errors.
• T2 errors result from a complex interplay of T2 value, refocusing angle, and ETL.
• Accurate modeling of the multiecho sequence is recommended for precise T2 quantification.