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

Abstract

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 ((Formula presented.)) and capillary segment length ((Formula presented.)). It is described by the pseudo-diffusion coefficient ((Formula presented.)) of intravoxel incoherent motion (IVIM). At shorter diffusion-times, blood flow is in the ballistic regime, and signal attenuation depends on (Formula presented.), and not (Formula presented.). In theory, (Formula presented.) could be estimated using (Formula presented.) and (Formula presented.). In this study, we compare the accuracy and repeatability of three approaches to estimating (Formula presented.), and therefore (Formula presented.) : 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 (Formula presented.) -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 (Formula presented.) 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 (Formula presented.), and had highest repeatability.