Quantitative ultrashort T₂ components imaging in the brain using variable flip angle balanced steady state free...

This study aimed to develop a new method for quantitative measurement of ultrashort T₂ (uT₂) components in the brain that are associated with myelin, providing reliable and high-quality fitting results of uT₂ fraction.

Four healthy volunteers and four multiple sclerosis (MS) patients were recruited. A modified dual-echo balanced steady state free precession (bSSFP) ultrashort echo time (UTE) sequence was applied for acquisitions at six flip angles. The signals originating from uT₂ components (e.g., myelin) were extracted and fitted by analyzing the differences between TE₁ = 50 μs and TE₂ = 2.2 ms signal curves along flip angles. A two-sample t-test was used for statistical comparison between uT₂ fractions in white matter (WM), gray matter (GM), and MS lesions.

The differences between two TEs' signal curves along flip angles showed clear evidence of a uT₂ component with a shape that matched the simulated results. Significantly higher uT₂ fraction values were found in total WM than total GM for all individuals (around 6 %-7 % vs. 2 %, P < 0.001). In healthy volunteers, the uT₂ fraction was consistent between repeated scans. In addition, the uT₂ fraction in MS lesions was quantified with significantly reduced values than surrounding normal appearing WM by this method (4.84 %±0.49 % vs. 7.96 %±0.94 %, P < 0.001).

An isotropic sub-millimeter resolution uT₂ fraction map was achieved with a simplified fitting model, and corresponded to expected patterns of myelination. There was a significant uT₂ fraction reduction in MS patients' lesions. The repeatability of the quantification was verified by the consistency between two separate scans.