Mitigating the impact of flip angle and orientation dependence in single compartment R2* estimates via 2-pool modeling.

Giorgia Milotta , Nadège Corbin (1) , Christian Lambert , Antoine Lutti , Siawoosh Mohammadi , Martina F Callaghan

1 CRMSB - Centre de résonance magnétique des systèmes biologiques

The effective transverse relaxation rate ( R 2 * $$ {\mathrm{R}}_2^{\ast } $$ ) is influenced by biological features that make it a useful means of probing brain microstructure. However, confounding factors such as dependence on flip angle (α) and fiber orientation with respect to the main field ( θ $$ \uptheta $$ ) complicate interpretation. The α- and θ $$ \uptheta $$ -dependence stem from the existence of multiple sub-voxel micro-environments (e.g., myelin and non-myelin water compartments). Ordinarily, it is challenging to quantify these sub-compartments; therefore, neuroscientific studies commonly make the simplifying assumption of a mono-exponential decay obtaining a single R 2 * $$ {\mathrm{R}}_2^{\ast } $$ estimate per voxel. In this work, we investigated how the multi-compartment nature of tissue microstructure affects single compartment R 2 * $$ {\mathrm{R}}_2^{\ast } $$ estimates. We used 2-pool (myelin and non-myelin water) simulations to characterize the bias in single compartment R 2 * $$ {\mathrm{R}}_2^{\ast } $$ estimates. Based on our numeric observations, we introduced a linear model that partitions R 2 * $$ {\mathrm{R}}_2^{\ast } $$ into α-dependent and α-independent components and validated this in vivo at 7T. We investigated the dependence of both components on the sub-compartment properties and assessed their robustness, orientation dependence, and reproducibility empirically. R 2 * $$ {\mathrm{R}}_2^{\ast } $$ increased with myelin water fraction and residency time leading to a linear dependence on α. We observed excellent agreement between our numeric and empirical results. Furthermore, the α-independent component of the proposed linear model was robust to the choice of α and reduced dependence on fiber orientation, although it suffered from marginally higher noise sensitivity. We have demonstrated and validated a simple approach that mitigates flip angle and orientation biases in single-compartment R 2 * $$ {\mathrm{R}}_2^{\ast } $$ estimates.

Mots clés: Magnetic Resonance Imaging; Reproducibility of Results; Myelin Sheath; Brain; Water

Domaines: Médecine humaine et pathologie

Giorgia Milotta, Nadège Corbin, Christian Lambert, Antoine Lutti, Siawoosh Mohammadi, et al.. Mitigating the impact of flip angle and orientation dependence in single compartment R2* estimates via 2-pool modeling.. Magnetic Resonance in Medicine, 2023, 89 (1), pp.128-143. ⟨10.1002/mrm.29428⟩. ⟨hal-03864274⟩