AUTHOR=Fritz Francisco J. , Mordhorst Laurin , Ashtarayeh Mohammad , Periquito Joao , Pohlmann Andreas , Morawski Markus , Jaeger Carsten , Niendorf Thoralf , Pine Kerrin J. , Callaghan Martina F. , Weiskopf Nikolaus , Mohammadi Siawoosh 

TITLE=Fiber-orientation independent component of R2* obtained from single-orientation MRI measurements in simulations and a post-mortem human optic chiasm

JOURNAL=Frontiers in Neuroscience

VOLUME=17

YEAR=2023

URL=https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2023.1133086

DOI=10.3389/fnins.2023.1133086

ISSN=1662-453X

ABSTRACT=<p>The effective transverse relaxation rate (R<sub>2</sub>*) 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<sub>2</sub>* degrades its reproducibility and any microstructural derivative measure. To estimate its orientation-independent part (R<sub>2,iso</sub>*) 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, <italic>β</italic><sub>1</sub>, can be biophysically related to R<sub>2,iso</sub>* 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 <italic>β</italic><sub>1</sub> can estimate R<sub>2,iso</sub>* using meGRE with long maximum-echo time (TE<sub>max</sub> ≈ 54 ms), but not accurately captures its microscopic dependence on the g-ratio (error 84%). We proposed a new heuristic expression for <italic>β</italic><sub>1</sub> that reduced the error to 12% for <italic>ex vivo</italic> compartmental R<sub>2</sub> 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 <italic>ex vivo</italic> compartmental R<sub>2</sub> values but not for the <italic>in vivo</italic> values. M2 and the HCFM-based simulations failed to explain the measured R<sub>2</sub>*-orientation-dependence around the magic angle for a typical <italic>in vivo</italic> meGRE protocol (with TE<sub>max</sub> ≈ 18 ms). In conclusion, further validation and the development of movement-robust <italic>in vivo</italic> meGRE protocols with TE<sub>max</sub> ≈ 54 ms are required before M2 can be used to estimate R<sub>2,iso</sub>* in subjects.</p>