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 (vv<math><mi>v</mi></math>) and capillary segment length (ll<math><mi>l</mi></math>). It is described by the pseudo-diffusion coefficient (D*=vl/6D*=vl/6<math><msup><mrow><mi>D</mi></mrow><mrow><mi>*</mi></mrow></msup><mo>=</mo><mi>v</mi><mi>l</mi><mo>/</mo><mn>6</mn></math>) of intravoxel incoherent motion (IVIM). At shorter diffusion-times, blood flow is in the ballistic regime, and signal attenuation depends on vv<math><mi>v</mi></math>, and not ll<math><mi>l</mi></math>. In theory, ll<math><mi>l</mi></math> could be estimated using D*D*<math><msup><mrow><mi>D</mi></mrow><mrow><mi>*</mi></mrow></msup></math> and vv<math><mi>v</mi></math>. In this study, we compare the accuracy and repeatability of three approaches to estimating vv<math><mi>v</mi></math>, and therefore ll<math><mi>l</mi></math>: 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 bb<math><mi>b</mi></math>-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 ll<math><mi>l</mi></math> 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 ll<math><mi>l</mi></math>, and had highest repeatability.