Characterisation of microvessel blood velocity and segment length in the brain using multi-diffusion-time...

Multi-diffusion-time diffusion-weighted MRI can probe tissue microstructure, but the method has not been widely applied to the microvasculature. At long diffusion-times, blood flow in capillaries is in the diffusive regime, and signal attenuation is dependent on blood velocity ($v$) and capillary segment length ($l$). It is described by the pseudo-diffusion coefficient ($D^{*}=vl/6$) of intravoxel incoherent motion (IVIM). At shorter diffusion-times, blood flow is in the ballistic regime, and signal attenuation depends on $v$, and not $l$. In theory, $l$ could be estimated using $D^{*}$ and $v$. In this study, we compare the accuracy and repeatability of three approaches to estimating $v$, and therefore $l$: the IVIM ballistic model, the velocity autocorrelation model, and the ballistic approximation to the velocity autocorrelation model. Twenty-nine rat datasets from two strains were acquired at 7 T, with $b$-values between 0 and 1000 smm^-2 and diffusion times between 11.6 and 50 ms. Five rats were scanned twice to assess scan-rescan repeatability. Measurements of $l$ were validated using corrosion casting and micro-CT imaging. The ballistic approximation of the velocity autocorrelation model had lowest bias relative to corrosion cast estimates of $l$, and had highest repeatability.