| Summary: | 碩士 === 國立臺灣大學 === 醫療器材與醫學影像研究所 === 106 === This study is a systematic correlation analysis across different diffusion MRI models to better understand the new diffusion indices derived from Mean Apparent Propagator (MAP) MRI and the application to sex differentiation. MAP-MRI has been recently proposed to reconstruct diffusion indices from high angular resolution diffusion data including generalized fractional anisotropy (GFA), fractional anisotropy (FA), axial diffusivity (AD), radial diffusivity (RD), mean diffusivity (MD), mean apparent length (AML), effective diameter of mean apparent area (AMAD), effective diameter of mean apparent volume (AMVD), parallel non-Gaussianity (NGP), orthogonal non-Gaussianity (NGO), and non-Gaussianity (NG). As compared to the first 5 indices, the other indices are less understood. To understand these diffusion indices, this study has two parts. First, we performed a systematic correlation study to clarify the relationships between them (Experiment 1). Second, we applied these MAP-MRI indices to compare the brain microstructure differentiation between males and females (Experiment 2).
Diffusion models, including diffusion kurtosis imaging (DKI) and MAP-MRI, were evaluated using DSI datasets obtained in 122 healthy adults which were used to build a diffusion spectrum imaging template. We used whole brain tract-based automatic analysis to obtain profiles of the 14 diffusion indices along 76 tract bundles for each DSI dataset. In experiment 1, systematic correlation analysis was performed between each pairs of diffusion indices on each tract bundle. The definition of the correlation between two indices was the average of the z-values in each tract bundle. With the correlation between each pairs of diffusion indices, we also applied a hierarchical clustering analysis to group these indices. In experiment 2, we compared 61 female and 61 male participants using stepwise two-sample t-test on the MAP-MRI indices.
In experiment 1, we found the associations can be clustered into three groups. First, NG, NGO and NGP, second, AD, AML and axial kurtosis (AK), third, GFA, FA, RD, MD, AMAD, AMVD, radial kurtosis (RK) and mean kurtosis (MK). In experiment 2, men showed better microstructural property in the left uncinate fasciculus, bilateral medial lemnisci, the left thalamic radiation postcentral and callosal fibers of hippocampus, while women showed better microstructural property in the right thalamic radiation auditory and callosal fibers of the dorsal lateral prefrontal cortex.
According to the definition of these indices the relationships of AD and AML, RD and AMAD, MD and AMVD have met our expectation. The reason why diffusion kurtosis indices are less similar to non-Gaussianity might be because that the characterization of non-Gaussianity is different from diffusion kurtosis. The characterization of diffusion kurtosis indices combines information from both tensor and kurtosis parts, whereas non-Gaussianity only considers the non-Gaussian part. In experiment 2, according to the connected ROIs of the tract and the function of the tract, we speculate that males have greater coordination of body than females because of the better microstructural property in bilateral medial lemnisci, and females have greater hearing sensitivity than males because of the better microstructural property in the right auditory radiation.
This is the first study to clarify associations among different MAP-MRI derived diffusion indices in DSI datasets, and the first study to investigate the sexual differentiation using MAP-MRI derived diffusion indices obtained with DSI. With the knowledge of the correlation results from Experiment 1, we can select the indices of interest and avoid the problem of collinearity. Our results in Experiment 2 agree partially with previous studies on sexual dimorphism and provide a sound basis for further studies on behavioral and possible clinical correlates.
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