Quantitative MRI mOOCT2 Mapping · Exercise 3
Myelin water fraction and echo time
Using Eq. 3.8 and the MW/IEW T2 values estimated in Exercise 2, calculate the myelin water fraction (MWF) values at the following TEs: 25, 50, 75, 100, 200, 300. Assume that there is 50% MW and 50% IEW.
$$\textit{MWF} = \frac{S_{MW}e^{-TE/T_{2,MW}}}{S_{MW}e^{-TE/T_{2,MW}} + S_{IEW}e^{-TE/T_{2,IEW}}}$$
MWF at six echo times
| T2,MW (ms) | |
| T2,IEW (ms) | |
| MW fraction (%) | |
| TEs (ms) |
The T2 values of Exercise 2c.
- The voxel is 50% MW at every TE, yet Eq. 3.8 drops: it is MW’s share of the signal at that TE, and MW decays 6× faster.
- Only TE → 0 gives the true 50%: MWF fits extrapolate to TE = 0, so they need short echoes.
Exercise 3: answers
| TE (ms) | 25 | 50 | 75 | 100 | 200 | 300 |
|---|---|---|---|---|---|---|
| MWF (Eq. 3.8) | 25% | 9.8% | 3.5% | 1.2% | 0.014% | 0.00017% |
With T2,MW = 19 ms and T2,IEW = 121 ms. The true 50% only comes out at TE = 0.
Questions: github.com/qMRLab/mooc/discussions