Evaluation of diffusion time-dependent changes in radial diffusivity as a surrogate for axon diameter

To experimentally evaluate the change in radial diffusivity with diffusion time ( $∆D_{\perp }$ ) as a simple estimate of axon diameter.

Ex vivo ferret spinal cords were imaged via MRI and scanning electron microscopy. Region-of-interest comparisons were made between $∆D_{\perp }$ and area-weighted mean axon diameter, $\left(d_{\mathrm{eff}}\right)$ , derived from scanning electron microscopy. Additional comparisons were made between $∆D_{\perp }$ and quantitative MRI myelin metrics.

A strong linear correlation was found between $∆D_{\perp }$ and $\left(d_{\mathrm{eff}}\right)$ . Negative correlations were found between myelin water fraction and $∆D_{\perp }$ as well as bound pool fraction and $∆D_{\perp }$ .

The value of $∆D_{\perp }$ is shown to be a good estimate of axon size in ex vivo spinal cords regardless of variations in myelin content, as indicated by quantitative MRI.