Modeling the Intermediate Flow Regime in Flow-Compensated Intravoxel Incoherent Motion MRI

The intravoxel incoherent motion (IVIM) model is commonly used to separate the effects of motion related to diffusion and blood microcirculation (perfusion) on the MR signal. Depending on the encoding time (T), it is possible to probe the different temporal regimes of blood motion, which resemble a ballistic flow at short T and a pseudo-diffusion at long T. The purpose of this work was to derive an encoding-time-dependent analytical model for flow-compensated IVIM and to estimate the corresponding microvascular IVIM parameters in healthy brain.

An encoding-time-dependent analytical IVIM model was derived for flow-compensated/non-flow-compensated (FC/NC) double diffusion encoding (DDE) from the Langevin equation and validated using simulations. Eleven healthy participants were scanned to estimate microvascular IVIM parameters (blood velocity ν and blood correlation time τ) in healthy brain using the proposed model, with T = 50-100 ms.

The IVIM parameters were estimated to be τ = 123.1 ± 50 ms, ν = 1.51 ± 0.76 mm/s, perfusion fraction f = 4.75 ± 1.94%, and tissue diffusion coefficient D = 0.91 ± 0.32 μm²/ms in the healthy human brain, although simulations indicate a positive bias for τ. For very short/long T, the proposed model approaches established models for the ballistic/diffusive regimes. Pseudocode for the derivation of the analytical model is presented to facilitate a transfer to other gradient waveforms or pulse sequences.

An encoding-time-dependent analytical IVIM model is presented for FC/NC DDE. In vivo results and simulations indicate that IVIM experiments with encoding times typical for clinical MRI scanners probe an intermediate to ballistic blood flow regime in the brain.