Coherent flow effects on IVIM-based perfusion measurements: A phantom study in 3 T

Quantitative, non-invasive measurement of perfusion remains a challenge in MRI. The Intravoxel Incoherent Motion (IVIM) technique, based on Diffusion-Weighted Imaging (DWI), offers a method to estimate perfusion at the sub-voxel level by modeling blood flow as a pseudo-diffusive process within a microvascular network. Key IVIM-derived parameters include the perfusion fraction (f) and the pseudo-diffusion coefficient (D*). While under specific structural assumptions, IVIM can be used to estimate Cerebral Blood Flow (CBF), direct conversions from IVIM metrics to CBF are rarely validated. This study investigates the accuracy of the IVIM-based perfusion measures, exploring coherent flow influence on signal from voxel. To assess the relation between flow (ground truth CBF), IVIM parameters, and calculated CBF, a phantom, composed of agarose gel and tubes mimicking arterioles with regulated flow velocity, was constructed. Thus, a signal fraction from the "fast" compartment, described by D* in the IVIM model, is represented by flow in tubes occupying a fixed fraction of the imaged voxel, simulating desired Cerebral Blood Volume (CBV) and CBF values. The phantom was scanned using IVIM-optimized DWI protocol, and data were processed for the IVIM measures. Besides IVIM, we also propose a model based on extended Bloch-Torrey equations with coherent flow term. Results demonstrate that IVIM can yield reasonable estimates of CBF and CBV within physiologically relevant flow ranges (0.5-2 mm/s) observed in the brain vasculature. However, a consistent overestimation of flow (up to 200 %) was obscerved at higher velocities, especially in arterial or vein-like flow conditions.