Combined DTI and NODDI Scalars Improves Understanding of At‐Risk Tissue in and Around White Matter Hyperintensities.

Background: The novel diffusion‐based Neurite Orientation and Dispersion Imaging (NODDI) model may provide complementary information to Diffusion Tensor Imaging (DTI) to increase sensitivity in characterizing tissue microarchitecture. We examined if one or a combination of both approaches can improve our understanding of at‐risk tissue in and around white matter hyperintensities (WMH). Method: From ADNI3, we identified individuals [n = 54, age = 78 ± 7, 46% F) with concurrent measures of multi‐shell DTI, 3D FLAIR, and 3D MPRAGE. We segmented WMH from structural data using Lesion Segmentation Toolbox. We derived DTI (FA, MD) and NODDI parameters (NDI, ODI, Fiso) using QSIprep. We dilated up to 5 mm outside the WMH, applied an FA threshold of ≥ 0.2 to exclude gray matter, calculated DTI and NODDI parameters at 1 mm intervals within this region. We performed Pearson correlations between all parameters. Assuming normal white matter at 5 mm, we normalized these parameters with their respective values at 5 mm and plotted the spatial variation of each parameter in and outside the WMH. Result: Normalized DTI and NODDI parameters representing spatial variations within the 5 mm region are shown in Fig 1A. Table 1 shows p‐values comparing DTI and NODDI parameters between WMH the subsequent regions outside it. Compared to WMH, Fiso was significantly different at all distances. FA showed similar spatial characteristics with NDI and was negatively correlated with ODI. Fiso correlated very strongly with MD. Conclusion: In the WMHs and its immediate vicinity, low FA, NDI, ODI suggest lower cell density, while low Fiso and MD suggest low extracellular fluid volume. Together these observations may reflect cytotoxic edema, cell loss, and injury. Normal appearing white matter (3‐5 mm) showed high FA, NDI, ODI, and low MD consistent with high cell density and normal extracellular water. In the transition between WMH vicinity and normal white matter, low FA, NDI, ODI, high MD and Fiso suggest cell loss only. This transition region could be at risk of converting to WMH. Future work will include longitudinal evaluation of WMH growth in the at‐risk tissue. However, final validation of WMH pathology will need histological confirmation. [ABSTRACT FROM AUTHOR]