Myelination and long diffusion times alter diffusion-tensor-imaging contrast in myelin-deficient shiverer mice

Abstract

Diffusion tensor imaging (DTI) using variable diffusion times (t(diff)) was performed to investigate wild-type (wt) mice, myelin-deficient shiverer (shi) mutant mice and shi mice transplanted with wt neural precursor cells that differentiate and function as oligodendrocytes. At t(diff) = 30 ms, the diffusion anisotropy "volume ratio" (VR), diffusion perpendicular to the fibers (lambda( perpendicular)), and mean apparent diffusion coefficient (<D>) of the corpus callosum of shi mice were significantly higher than those of wt mice by 12 +/- 2%, 13 +/- 2%, and 10 +/- 1%, respectively; fractional anisotropy (FA) and relative anisotropy (RA) were lower by 10 +/- 1% and 11 +/- 3%, respectively. Diffusion parallel to the fibers (lambda(//)) was not statistically different between shi and wt mice. Normalized T(2)-weighted signal intensities showed obvious differences (27 +/- 4%) between wt and shi mice in the corpus callosum but surprisingly did not detect transplant-derived myelination. In contrast, diffusion anisotropy maps detected transplant-derived myelination in the corpus callosum and its spatial distribution was consistent with the donor-derived myelination determined by immunohistochemical staining. Anisotropy indices (except lambda(//)) in the corpus callosum showed strong t(diff) dependence (30-280 ms), and the differences in lambda( perpendicular) and VR between wt and shi mice became significantly larger at longer t(diff), indicative of improved DTI sensitivity at long t(diff). In contrast, anisotropy indices in the hippocampus showed very weak t(diff) dependence and were not significantly different between wt and shi mice across different t(diff). This study provides insights into the biological signal sources and measurement parameters influencing DTI contrast, which could lead to developing more sensitive techniques for detection of demyelinating diseases.

Figures

Fig. 1

Coronal T2-weighted images, fractional anisotropy (FA), volume ratio (VR), diffusion perpendicular (λ⊥) to the first Eigenvector obtained from a representative wild-type (wt), and a shiverer (shi) mouse brain at short (30 ms) diffusion time. T2-weighted images show striking differences in the corpus callosum between wt and shi mice (arrowheads).

Fig. 2

Group-average spatial profiles of (a) T2-weighted signal intensity normalized to cortical gray matter and (b) fractional anisotropy (FA) obtained crossing the corpus callosum of the wt (mean ± SEM, n = 7) and “shi + transplant control” (mean ± SEM, n = 8 for shi, and n = 3 for transplant control). The spatial profile starts at gray matter in the caudate putamen, crosses the corpus callosum, and ends with gray matter in the cortex, as shown in the inset. *P < 0.05, **P < 0.01, ***P < 0.001.

Fig. 3

(a) Spatial profile plot of fractional anisotropy (FA) obtained crossing the corpus callosum of the transplanted mice (n = 3, mean ± SEM). Group-average data of wt and “shi + transplant control” groups are re-plotted without error bars from Fig. 2 for comparison. (b) Spatial profile plots of FA and normalized T2-weighted signal intensities of transplant mouse #2. Group-average data of the wt and “shi + transplant control” group are re-plotted without error bars from Fig. 2 for comparison. The peak at pixel #15 arises from a white matter track projecting into the cortex and is not an artifact. (c) Histological sections showing locations of donor-derived green fluorescent protein (GFP) cells and myelin basic protein (MBP) immunoreactivity and the corresponding grayscale FA map and color-coded directional FA map from transplanted mouse #5. Donor-derived myelination by the transplanted precursor cells in the corpus callosum showed asymmetric distribution between the two hemispheres corresponding to FA maps (arrowheads).

Fig. 4

(a) Representative ROIs of the corpus callosum and the hippocampus overlaid on fractional anisotropy (FA) maps. Group-average (b) FA, (c) volume ratio (VR), and (d) diffusion perpendicular to the first Eigenvector (λ⊥) of the corpus callosum and hippocampus from wt and shi mice at different diffusion times. FA, VR, and λ⊥ of the corpus callosum show significantly stronger tdiff dependent relative to those of the hippocampus. Differences in VR and λ⊥ between wt and shi mice became significantly larger at longer tdiff in the corpus callosum. Values in plot are mean ± SD for n = 5. *P < 0.05, **P < 0.01.