Abstract 19843: Myocardial Microcirculation Induces Anisotropic Diffusion-like Magnetic Resonance Contrast

Abstract

Introduction: The magnetic resonance (MR) signal in the presence of a magnetic field gradient is highly sensitive to the aggregate translational molecular motion of water. In highly vascularized tissues, diffusion-like contrast induced by microcirculation has been used to characterize blood flow in heart. The effect in tissues with random capillary orientations has been modeled as intravoxel incoherent motion (IVIM), but is not completely understood in organized tissues. Hypothesis: We hypothesize that blood flow in organized tissues will manifest as speed- and orientation- dependent “pseudo” diffusion coefficient D*. Methods: For Gaussian-distributed flow with mean speed vm in parallel capillaries as a function of the relative encoding angle θ, it can be shown that D* is proportional to the square power of both vm and cos(θ), which is then incorporated into a generalized formulation of anisotropic tissue diffusion and flow with a tensor entity D* to characterize the flow pseudo diffusivity in 3D space using the familiar diffusion tensor imaging (DTI) notation. Validation experiments were conducted on isolated, perfused, guinea pig hearts (n=7) in a Langendorff setup and imaged with Bruker (7T) MRI scanner under normal and low flow conditions. Statistical analyses were conducted via 1-way ANOVA with Bonferroni post-hoc multiple-comparison correction (P<0.05 considered statistically significant). Results: Figure 1A-C show representative diffusion-weighted MR images encoded at perpendicular directions, and Fig 1D shows the flow-speed and angular dependence (or anisotropy) of the fitted D* component at high flow and low flow. D* significantly decreased by 63% only in the parallel direction to myocytes (or capillaries) (P<0.05). Conclusions: These findings suggest that the DTI formalism of anisotropic spin motion can be incorporated into the classical IVIM theory to describe the MR signal arising from diffusion and microcirculation in organized tissues.