Assessment of intravoxel incoherent motion MRI with an artificial capillary network: analysis of biexponential and phase-distribution models

Purpose: To systematically analyze intravoxel incoherent motion (IVIM) MRI in a perfusable capillary phantom closely matching the geometry of capillary beds in vivo and to compare the validity of the biexponential pseudo-diffusion and the recently introduced phase-distribution IVIM model.

Methods: IVIM-MRI was performed at 12 different flow rates ( $0.2\cdots 2.4mL/\min$ ) in a capillary phantom using 4 different DW-MRI sequences (2 with monopolar and 2 with flow-compensated diffusion-gradient schemes, with up to $16$$b$ values between $0$ and $800s/{\mathrm{mm}}^2$ ). Resulting parameters from the assessed IVIM models were compared to results from optical microscopy.

Results: The acquired data were best described by a static and a flowing compartment modeled by the phase-distribution approach. The estimated signal fraction $f$ of the flowing compartment stayed approximately constant over the applied flow rates, with an average of $f=0.451\pm 0.023$ in excellent agreement with optical microscopy ( $f=0.454\pm 0.002$ ). The estimated average particle flow speeds $v=0.25\cdots 2.7\mathrm{mm}/s$ showed a highly significant linear correlation to the applied flow. The estimated capillary segment length of approximately $189um$ agreed well with optical microscopy measurements. Using the biexponential model, the signal fraction $f$ was substantially underestimated and displayed a strong dependence on the applied flow rate.

Conclusion: The constructed phantom facilitated the detailed investigation of IVIM-MRI methods. The results demonstrate that the phase-distribution method is capable of accurately characterizing fluid flow inside a capillary network. Parameters estimated using the biexponential model, specifically the perfusion fraction $f$ , showed a substantial bias because the model assumptions were not met by the underlying flow pattern.