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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. and Mordhorst, L. and Ashtarayeh, M. and Periquito, J. and Pohlmann, A. and Morawski, M. and Jaeger, C. and Niendorf, T. and Pine, K.J. and Callaghan, M.F. and 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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