C-NODDI: a constrained NODDI model for axonal density and orientation determinations in cerebral white matter in normative aging

Purpose Neurite orientation dispersion and density imaging (NODDI) provides measures of neurite density and dispersion through computation of the neurite density index (NDI) and the orientation dispersion index (ODI). However, NODDI overestimates the cerebrospinal fluid water fraction in white matter (WM) and provides physiologically unrealistic high NDI values. Furthermore, derived NDI values are echo time (TE)-dependent. In this work, we propose a modification of NODDI, named constrained NODDI (C-NODDI), for NDI and ODI mapping in WM. Methods Using NODDI and C-NODDI, we investigated age-related alterations in WM in a cohort of 58 cognitively unimpaired adults. Further, NDI values derived using NODDI or C-NODDI were correlated with the neurofilament light chain (NfL) concentration levels, a plasma biomarker of axonal degeneration. Results ODI derived values using both approaches were virtually identical. We confirm the previous finding that NDI estimation using NODDI is TE-dependent. In contrast, C-NODDI-NDI exhibit lower susceptibility to TE. Further, C-NODDI-NDI values were lower than NODDI-NDI. Further, our results indicate a quadratic relationship between NDI and age suggesting that axonal maturation continues until middle age followed by a decrease. This quadratic association was notably significant in several WM regions using C-NODDI, while limited to a few regions using NODDI. ODI exhibited overall constant trends with age. Finally, C-NODDI-NDI values exhibited a stronger correlation with NfL concentration levels as compared NODDI-NDI, with lower NDI values correspond to higher levels of NfL. Conclusions C-NODDI provides a complementary method to NODDI for determination of NDI in white matter in normative aging. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work was funded by the National Institute on Agin of the NIH. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The protocol was approved by the MedStar Research Institute and the National Institutes of Health Intramural Ethics committees, and all examinations were performed in compliance with the standards established by the National Institutes of Health Institutional Review Board. All participants provided written informed consents. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors