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| Item Type: | Article |
|---|---|
| Title: | Fiber-orientation independent component of R(2)* obtained from single-orientation MRI measurements in simulations and a post-mortem human optic chiasm |
| Creators Name: | Fritz, F.J., Mordhorst, L., Ashtarayeh, M., Periquito, J., Pohlmann, A., Morawski, M., Jaeger, C., Niendorf, T., Pine, K.J., Callaghan, M.F., Weiskopf, N. and Mohammadi, S. |
| Abstract: | The effective transverse relaxation rate (R(2)*) is sensitive to the microstructure of the human brain like the g-ratio which characterises the relative myelination of axons. However, the fibre-orientation dependence of R(2)* degrades its reproducibility and any microstructural derivative measure. To estimate its orientation-independent part (R(2,iso)*) from single multi-echo gradient-recalled-echo (meGRE) measurements at arbitrary orientations, a second-order polynomial in time model (hereafter M2) can be used. Its linear time-dependent parameter, β(1), can be biophysically related to R(2,iso)* when neglecting the myelin water (MW) signal in the hollow cylinder fibre model (HCFM). Here, we examined the performance of M2 using experimental and simulated data with variable g-ratio and fibre dispersion. We found that the fitted β(1) can estimate R(2,iso)* using meGRE with long maximum-echo time (TE(max) ≈ 54 ms), but not accurately captures its microscopic dependence on the g-ratio (error 84%). We proposed a new heuristic expression for β(1) that reduced the error to 12% for ex vivo compartmental R(2) values. Using the new expression, we could estimate an MW fraction of 0.14 for fibres with negligible dispersion in a fixed human optic chiasm for the ex vivo compartmental R(2) values but not for the in vivo values. M2 and the HCFM-based simulations failed to explain the measured R(2)*-orientation-dependence around the magic angle for a typical in vivo meGRE protocol (with TE(max) ≈ 18 ms). In conclusion, further validation and the development of movement-robust in vivo meGRE protocols with TE(max) ≈ 54 ms are required before M2 can be used to estimate R(2,iso)* in subjects. |
| Keywords: | Effective Transverse Relaxation Rate, Biophysical Model, R(2)*, Orientation-Independent R(2)*, Myelin Water Fraction, G-Ratio, Fibre Dispersion, Multi-Echo Gradient Recalled Echo |
| Source: | Frontiers in Neuroscience |
| ISSN: | 1662-453X |
| Publisher: | Frontiers Media SA |
| Volume: | 17 |
| Page Range: | 1133086 |
| Date: | 25 August 2023 |
| Official Publication: | https://doi.org/10.3389/fnins.2023.1133086 |
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