Evidence of the diffusion time dependence of intravoxel incoherent motion in the brain

Abstract

Purpose: To investigate the diffusion time (T_D ) dependence of intravoxel incoherent motion (IVIM) signals in the brain.

Methods: A 3-compartment IVIM model was proposed to characterize 2 types of microcirculatory flows in addition to tissue water in the brain: flows that cross multiple vascular segments (pseudo-diffusive) and flows that stay in 1 segment (ballistic) within T_D . The model was first evaluated using simulated flow signals. Experimentally, flow-compensated (FC) pulsed-gradient spin-echo (PGSE) and oscillating-gradient spin-echo (OGSE) sequences were tested using a flow phantom and then used to examine IVIM signals in the mouse brain with T_D ranging from ~2.5 ms to 40 ms on an 11.7T scanner.

Results: By fitting the model to simulated flow signals, we demonstrated the T_D dependency of the estimated fraction of pseudo-diffusive flow and the pseudo-diffusion coefficient (D*), which were dictated by the characteristic timescale of microcirculatory flow (τ). Flow phantom experiments validated that the OGSE and FC-PGSE sequences were not susceptible to the change in flow velocity. In vivo mouse brain data showed that both the estimated fraction of pseudo-diffusive flow and D* increased significantly as T_D increased.

Conclusion: We demonstrated that IVIM signals measured in the brain are T_D -dependent, potentially because more microcirculatory flows approach the pseudo-diffusive limit as T_D increases with respect to τ. Measuring the T_D dependency of IVIM signals may provide additional information on microvascular flows in the brain.

Keywords: compartments; diffusion time; intravoxel incoherent motion; microcirculation; oscillating gradient.