Microstructural Changes In White Matter Of Patients After Mild Traumatic Brain Injury:A Longitudinal Kurtosis And Diffusion Basis Spectrumimaging Study

Purpose:The integration of imaging distortion correction(unwarping),comparison of different models and their application on mild traumatic brain injury patients.Comparing tensor,kurtosis and diffusion basis spectrum imaging using the same acquisition unwarped with nonlinear registration methods to better understand the biological interpretability of the parameters generated by Diffusion Tensor imaging,Diffusion Kurtosis imaging(DKI)and Diffusion Basis Spectrum Imaging(DB SI)in healthy individuals and to investigate longitudinal changes of DTI,DKI and DBSI metrics in white matter of patients after mild traumatic brain inj ury(MTBI).Methods and materials:We compared reliability using two methods of unwarping:the first is nonlinear registration to a structural image using symmetric normalization implemented in Advanced Normalization Tools(ANTS),and the second is unwarping using an acquired field map.Then we performed this comparison in two different test-retest data sets acquired at differing sites(N=39 and N=32)and applied the one with higher reliability on our kurtosis data.We conducted an empirical survey of the inter-relation of DKI and DBSI models in 23 healthy volunteers using the same diffusion acquisition(unwarped with nonlinear registration).We manually delineated two regions of interest(ROIs)with anatomically known fiber structure.The first ROI is a coherent fiber bundle,deep in the genu of the corpus callosum(CC),The second ROI is an area of crossing fibers which is in the decussation of the corpus callosum and the inferior fronto-occipital fasciculus(IFOF).DKI data was obtained from 23 patients with MTBI within 10 days,1 month and 6 months’ post-injury and compared with age-matched control(n=23).DTI Fractional anisotropy(FA),mean diffusivity(MD),axial diffusion(AD),radial diffusion(RD),DKI mean kurtosis(MK),axial kurtosis(AK),radial kurtosis(RK)and DBSI fiber ratio(FR),water ratio(WR),restricted ratio(RR)hindered ratio(HR)were extracted from the genu of corpus callosum,bilateral genu of internal capsule and the decussation of the corpus callosum and the inferior fronto-occipital fasciculus(IFOF).Results:The nonlinear registration has higher test-retest reliability.We applied nonlinear registration method on our kurtosis data(control and MTBI)and compared DKI with DBSI model,we found that Mean Kurtosis(MK)is positively associated with the DBSI Fiber Ratio(FR),and negatively associated with the Hindered Ratio(HR).The axial kurtosis was strongly and specifically correlated with the Restricted Ratio.The j oint spatial distributions of DBSI and DKI parameters are tissue-dependent and stable across healthy individuals.In acute stage,FA,RK and FR in MTBI group decreased in several ROIs comparing to controls’.MD,MK,HR,WR and RR increased,respectively.In sub-acute stage,AK and RR elevated and RK descended in MTBI group.In chronic stage,FA,FR and RR is lower and RR,WR is higher that in control group.