Quantitative MRI mOOCT2 Mapping · Exercise 2
Myelin water and intra/extracellular water
- a. Using Figure 3.5, determine the echo time at which the signal myelin water (MW) signal has reduced by: 50%, 63%, and 90%.
- b. Using the signal curve for intra/extra-cellular (IEW) water presented in the same figure, determine the echo time at which the signal has reduced by: 50%, 63%, and 90%.
- c. Without fitting, estimate the T2 value for MW and IEW in this figure.
- d. Determine the (erroneous) mono-exponential T2 value for voxels that would contain 50% MW and 50% IEW - is it closer to the T2 value of MW or IEW?
a. When has the MW signal dropped?
a. Determine the echo time at which the myelin water (MW) signal has reduced by 50%, 63%, and 90%.
Hover over the green curve; your readings carry over to the next slides.
| Drop | Signal left | TE (ms) |
|---|---|---|
| 50% | 0.50 | |
| 63% | 0.37 | |
| 90% | 0.10 |
Each drop is at a fixed multiple of T2: TE = −T2 ln(1 − drop) = 0.69, 0.99 and 2.3 T2.
b. When has the IEW signal dropped?
b. Using the signal curve for intra/extra-cellular water (IEW) in the same figure, determine the echo time at which the signal has reduced by 50%, 63%, and 90%.
| Drop | Signal left | TE (ms) |
|---|---|---|
| 50% | 0.50 | |
| 63% | 0.37 | |
| 90% | 0.10 |
Each drop takes about 6× longer than for MW: IEW decays that much more slowly.
c. T2 without fitting
c. Without fitting, estimate the T2 value for MW and IEW in this figure.
| TE (ms) at | 50% | 63% | 90% |
|---|---|---|---|
| MW | |||
| IEW |
T2,MW ≈ 19 ms and T2,IEW ≈ 121 ms: the TE at which each signal has dropped to 1/e ≈ 0.37.
d. A mono-exponential fit of a 50/50 voxel
d. Determine the (erroneous) mono-exponential T2 value for voxels that would contain 50% MW and 50% IEW. Is it closer to the T2 value of MW or IEW?
- T2 ≈ 78 ms, closer to IEW (121 ms) than to MW (19 ms).
- MW is mostly gone after about 40 ms: most echoes see only IEW.
- It depends on the echoes: TE ≥ 50 ms only gives 115 ms. Not a tissue property.
Exercise 2: answers
| Part | Answer |
|---|---|
| a | MW: TE ≈ 13, 19 and 44 ms for 50%, 63% and 90% drops |
| b | IEW: TE ≈ 84, 121 and 280 ms |
| c | T2,MW ≈ 19 ms, T2,IEW ≈ 121 ms: the TE of the 63% (1/e) drop |
| d | Mono-exponential T2 ≈ 78 ms, closer to IEW |
Questions: github.com/qMRLab/mooc/discussions